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Equipment Discussions >> ATM, Optics and DIY Forum

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
9.25 inch refractor project
      #5626874 - 01/16/13 07:49 PM

Hello all who are curious as to what has come of the lens I have been working on. Truth of the matter is that it was shelved for a while due to lots of other more critical projects. I have tested the lens and found the crown to have a turned edge that just won't come out with just polishing. Also, while inserting the flint element into its cell the flint was cocked a bit and got stuck. In the process of getting it unstuck a small flake broke loose at the edge about 3/4 inch diameter. I have resolved to going back to fine grinding to remove the turned edge and most if not all of the flake defect in the flint. I believe the turned edge is a result of not properly supporting the lens during grinding and polishing. I have a better support setup now and should get it done right this time. I want this lens -objective to turn out perfect as possible like the 6 inch lens I made years ago.

I spent a lot of time polishing to try removing the turned edge but have seen it was not getting it done in a timely manner, I'd be polishing for hundreds of hours at the rate it was going. After popping off the flake on the flint , that pretty much cinched the decision to go back to fine grinding.


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Sean Cunneen
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Reged: 08/01/07

Loc: Blue Island Illinois
Re: 9.25 inch refractor project new [Re: jimegger]
      #5626952 - 01/16/13 08:26 PM

That is too bad, I applaud your perseverance, a 9.25" refractor you've made yourself will be a "forever" scope and well worth the effort now! I am cheering for ya!

Edited by Sean Cunneen (01/16/13 08:27 PM)


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: 9.25 inch refractor project new [Re: jimegger]
      #5627054 - 01/16/13 09:31 PM

I feel your pain. A number of years ago I had a very similar experience with a 6" lens I had made. I lost 3 or 4 dime sized pieces off the face when I tried to remove it from the cell when it got cocked. I had to go all the way back to rough grinding to clean it up. In the end, all worked out just as fine and I still have that old glass somewhere.

dan


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: dan_h]
      #5628021 - 01/17/13 12:41 PM

In the bigger scheme of things this all is a minor setback and one easily dealt with. From what I have seen with actually putting the lens in the cell and looking through it earlier , it will be well worth the extra effort to get it right. I am certainly not the first nor the last to have such experiences.

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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 9.25 inch refractor project new [Re: jimegger]
      #5628034 - 01/17/13 12:45 PM

Thanks for the post . I know the views will be great. very cool .Hope the flat works out .

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KenScharf
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Reged: 12/16/12

Loc: South Fla
Re: 9.25 inch refractor project new [Re: kfrederick]
      #5628709 - 01/17/13 07:24 PM

May Alvin Clark be looking down on you smiling.

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mikey cee
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Reged: 01/18/07

Loc: bellevue ne.
Re: 9.25 inch refractor project new [Re: KenScharf]
      #5628747 - 01/17/13 07:50 PM

You mean Alvan correct? Alvin as "Ziggy" would say was the chipmunk. Mike

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: mikey cee]
      #5631653 - 01/19/13 02:13 PM Attachment (182 downloads)

Here is the chip in the edge of the flint. The rumpled area under the chip is from the silicone I used to support the lens. The flake that came off was very thin except right at the edge which doesn't matter once in the lens cell. Some more grinding with 120 grit will get rid of most of it if not all of it. I used a plaster matrix for a base the squirted silicone sealant on it covered by Saran wrap to support the lens from below. The silicone sealant made a perfect match to the curve and won't scratch the lens.

Last time I ground the R2 and R3 curves on the objective by crown on flint since they are the same curves. This time I am going to use the glass tools all the way on all the surfaces just in case i need to go back and fine grind again. I am also going to use them for support during polishing by using the silicone again. The edges will be taped to prevent grinding compound from getting between the support base and lens. The lens will be supported over its entirety that way in order to avoid the turned edge due to flexure.


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Mike I. Jones
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Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5631678 - 01/19/13 02:34 PM

Ouch! Hate them oysters.

As you grind, monitor both your radii and center thicknesses, and I can keep nudging the design back to optimum with ZEMAX as things change. Watch out to keep your wedge <0.001" on both elements.

Glad to help,
Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5631723 - 01/19/13 03:06 PM

Mike, you have been a great help through this whole thing.

I'll be a lot more delicate with the lens materials this time around !

I printed out your program you ran for the different lens thicknesses and did some interpolating from that as to what values to get on the spherometer to keep the objective design on target. It turns out that it doesn't change much per .02 inches thickness change. The chip is 3/4 inch diameter. It really only affects the aesthetics of the lens more than anything but I am going to grind away on the flint anyway and get rid of most of if not all of the "oyster". It is the crown that has the turned edge and needs the fine grinding. The focal length was within .3 inches of what your program predicted and more careful control of the radii will get me closer this time. The color correction was excellent just the sharpness was off due to the turned edge.

Oh, and the edge thickness variation was less than .001 inches on both lenses.

Edited by jimegger (01/19/13 09:46 PM)


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Ed Jones
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Reged: 04/06/04

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5633280 - 01/20/13 01:53 PM Attachment (120 downloads)

It looks like I'm in the same boat as you. I had finished my 7 inch Jones-Medial but when working on the baffels the OTA slipped off the mount and slammed onto my concrete basement floor from about 3 feet early this week. I thought it survived but it has a crack in the side. The images were great too.

Fortunately I have extra pieces of glass and I've already generated a replacement lens, lapped on side and getting ready to start on the second side.


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Ed Jones]
      #5635268 - 01/21/13 04:06 PM Attachment (96 downloads)

Ed, You must have been wincing when it rolled off !!!

I started the re-grinding process to help remove some of the chip and get back to a sphere on 3 of the 4 surfaces. The design is one where the R2 and R3 are equal in radius. Because I used the flint to grind on the crown from 120 grit on down, the glass tools for those 2 surfaces needed 120 grit to work them down to the final grit size. Therefore I had to start out with 120 grit on R2 and R3 even though it did not need to go that coarse for the purpose of getting rid of the turned edges. R1 was taken down to very fine so I do not need to go with 120 on it, I'll start with 320 grit there. The only 3 surfaces I am working are R1, R2 and R3. I know the crown was the lens element with the turned edges from testing with the flat. This time I will be using the glass tools as support on R1 and R2 to eliminate flexure.

Here is where I am at now after regrinding the flint R3. The edge chip has greatly diminished after .02 inches of glass thickness removed. The numbers on the edges are thousandths of inches variation in edge thickness. This now varies by .002 inches max. I'll take care of that in the 220 grit phase.


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5635280 - 01/21/13 04:12 PM Attachment (100 downloads)

Here is the crown before re-grinding. Thickness is at 1.046 inches.

So now Mr. Mike Jones, my question to you is what should the different radii be with the flint at around .550 thickness and the crown at around 1 inch thickness ? Do you still have the data for this BK7 and F2 lens combination for the f/12 design (106.3 inch focal length)


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5635297 - 01/21/13 04:22 PM

Here is the data for these lenses from page 84 previously.
Well - well - well, I finally got the melt data for the new BK7 glass. Are you ready Mike ? !

The new BK7 is listed as the following;
d-line = n,d/e 1.51660
V/de = 64.12

e-line = n,d/e 1.51852
V/de = 63.91

n/x
g line = 1.52649
F line = 1.52218
F' line = 1.52263
C line = 1.51412
C' line = 1.51452

The flint element is the same as before and so has not changed.
the flint is;

The F2 flint;
nd= 1.61966
ne= 1.62371
nF= 1.63172
nC= 1.61465
ng= 1.64167
Vd= 36.30
Ve= 36.04

Can you get me a new printout of the f/12 design as it relates to the new values Mike ? I will fabricate the objective to those and we will see how it turns out. I am excited to see how this all turns out. Thanks Mike for all your help !

Edited by jimegger (01/21/13 04:31 PM)


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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5635310 - 01/21/13 04:35 PM

OK - got the glass fitted and verified over wavelength. First of all, which radii will not be changing during the re-grinding? I assume 1 or 2 of the four will not be touched and will remain the same.

Standing by...
Mike


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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5635627 - 01/21/13 08:01 PM Attachment (68 downloads)

Using 1.0" for the BK7 CT and 0.55" for the F2, and the new BK7 melt data, here's the nominal prescription:

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5635710 - 01/21/13 09:00 PM

The R4 was the one radius I was not going to change - for now anyway Mike. I was planning to stay as close to the other radii as possible on R1, R2 and R3.

This all looks good Mike and I will adjust accordingly.
Thanks for the update !


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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5635800 - 01/21/13 09:58 PM Attachment (65 downloads)

OK, here's the updated nominal prescription with R4 being your current value. The system EFL is 107.47" with these nominal radii. Same deal, as you converge on a final CT for each element, I'll generate a lookup table of center thickness ranges.
Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5641664 - 01/24/13 10:34 PM Attachment (94 downloads)

The lenses are at the final fine grinding stage with 5 micron grit. By using the glass tools as support for the lenses I have had real good luck at achieving curves that appear to be very accurate as noticed from the smooth action of the grinding in these later stages. By putting the lenses on the tools with the properly matched curve separated by Saran Wrap then taped together around the edge virtually all flexure seems to have been eliminated. Polishing will be done in the same manner only using matched plaster backing as the tools will be used as pitch laps and not available as backing. The plaster can be poured on the proper surfaces and made to fit perfectly.

The sphereometer has indicated that the radii are as close as possible to calculated values as they can get using the 1/10,000 inch marks.

The small chip in the flint is quite diminished in size now and will not be a factor in the mounted objectives light path in the scope. It is just a matter of being careful now not to ding them anymore !!! Polishing will commence in about a week after my Gugolz pitch comes in.

Edited by jimegger (01/24/13 10:40 PM)


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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5641727 - 01/24/13 11:25 PM

Good progress! Question: what is your chamfer width around the edge? Needs to be 45 deg. by 0.05" wide, or even a little wider, up to maybe 0.08" wide, as long as it is uniform in width around the edge. Sharp edges will oyster-chip on you with the slightest impact; chamfered and/or rounded edges are at least less likely to chip. I still have my little piece of brass sheet from 40 years ago for rounding over chamfers, using 30 grit Al2O3.

Post your final ground CT's and I'll tweak the design. Hopefully your radii won't have to change from nominal.

Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5642503 - 01/25/13 12:38 PM

As soon as the final grinding is done Mike , I will post the center thickness. At that point of course, the radii will not change but the spacing of the elements can. I believe the lenses are very close to the .550 inch thick for the flint and the 1 inch crown thickness. I noticed that the radius of curvature does not change much with a change in thickness but the spacing does.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5642873 - 01/25/13 04:03 PM

The measured thickness on the flint is at .550 and the crown is 1.02 inches respectively. I should be right on at the last set of values you gave me Mike. The .02 inches difference with the crown is nothing for the calculated values of the radii but the spacing would be more critical from what I am seeing in the numbers changing.

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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5643076 - 01/25/13 06:09 PM

My flight's about to take off, and it has WiFi, so I'll reply in the air. Tell me your four radii again, as closely as possible, so there's no mistake.
Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5643615 - 01/26/13 12:00 AM

The radii are ;

R1= 62.5"
R2 and R3 = 38.76
R4 = 171.135"

These are as measured with the spherometer Mike to the nearest .0001 inches.


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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5646184 - 01/27/13 02:40 PM Attachment (59 downloads)

OK, with that data, here's your finished lens prescription and EFL. I added the longitudinal color curves to show you how I corrected the color. I picked 0.5461um (green) and 0.588um (yellow) to span the peak photopic eye response, and minimized spherical aberration at those two wavelengths. I chose to intersect the 0.656um (red) and 0.486um (blue) colors at the 90% zone to minimize the visual blur circle. The coma is essentially zero. That's about as good as it can get with these two particular glass types.

Can't wait to hear about the imagery!
Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5666313 - 02/07/13 12:25 AM

Polishing of the lens elements has commenced - hopefully for the final time ! I have 6 hours on the flint with probably at least 2 more to go with the Cerium Oxide and Gugloz pitch. I can really feel the difference this time in the smoothness of action that I hope is an indication of a much closer spherical surface than the first go around. I have the flint element on a plaster backing and taped to it in order to keep it centered and firmly attached with the R4 surface protected (it is already polished). I'll have pictures soon. The crown will be started on later in the day after a lap is prepared. R2 will be polished first with R1 supported on its tool with a Saran Wrap protective film between the tool and lens glass surfaces. The tool and lens will be taped together with electrical tape to seal it off and prevent movement while being polished. This should give adequate support so as to eliminate the lens acquiring the shape it had from the first go around of a hill in the middle and a turned edge.

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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project new [Re: jimegger]
      #5666828 - 02/07/13 10:50 AM

I had a thought. In the cell design, why not make the cell perhaps 1/8" oversize diameter-wise, and place something on the edge of each lens to take up the room when placing in the cell? It would minimize chipping if it had plenty of room, which a typical thick ATM lens would have when tipping to locate in the cell..........?

**********
Also, design it with plenty of spacing to account for any radius variance when finished with polishing/grinding to final curve; which can be adjusted later on?
Just a thought,
M.


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mikey cee
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Loc: bellevue ne.
Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5666885 - 02/07/13 11:15 AM

That's what my 8" Brandt lens has. Strips of 1/4" vertical black foam that feel very firm to the touch. There are probably 6 or so spaced around the perimeter. The lens is 8" clear aperture but it measures a full 8-3/8". Mike

Edited by mikey cee (02/07/13 11:16 AM)


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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5666980 - 02/07/13 12:11 PM Attachment (33 downloads)

Jim, I used cement casts with wet newspaper padding for my Houghton corrector. Wet newspapers conform very well to the surface (in the past polishing laps used to be made out of paper) and won't scratch the glass.

Mladen


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5667038 - 02/07/13 12:52 PM

I take it that the wet news papers worked very well with the support of the cement !

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5667048 - 02/07/13 12:57 PM

For the lens cell , it wouldn't be a bad idea to leave more space and a foam type filler for edge support. I could still turn my cell on the lathe to accommodate such a support. The lens cell I have is made to spec using the math for the differential thermal properties giving proper clearance in the .02 inch range. One (me) just needs to be more careful when installing the elements in their respective cells !!!

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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5667211 - 02/07/13 02:39 PM

I think 1/8" might be too much. Check out the Artus shim stock at http://www.artuscorp.com/. 0.01" thick edge strips (black) might be more than enough.
Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5667298 - 02/07/13 03:28 PM Attachment (55 downloads)

Here is the flint element after 8 hours of polishing with Cerium Oxide and the Gugolz pitch. The flint is still taped to its plaster backing.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5667299 - 02/07/13 03:29 PM Attachment (44 downloads)

Another view of the flint on its backing.

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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5667710 - 02/07/13 07:25 PM Attachment (30 downloads)

Quote:

I take it that the wet news papers worked very well with the support of the cement !



They did.


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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: MKV]
      #5667738 - 02/07/13 07:46 PM Attachment (44 downloads)

Actually, the lens support was a little more elaborate. I used a brass ring to support the lens and a steel clamp to prevent it form sliding off.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5671140 - 02/09/13 06:05 PM Attachment (48 downloads)

Here is R2 on the crown element all polished out after 8 hours. The lens is still sitting on its glass tool backing with the Saran Wrap sticking out around the edge, hence the green color.
All that is left now is R1 and then time to install in the lens cell for a look see before doing any testing with the Foucault test.

Polishing was smooth as silk on R2 indicating a good surface as compared to the first time that had some fits and starts.

Edited by jimegger (02/09/13 06:06 PM)


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5673398 - 02/11/13 01:04 AM

It has been a polishing marathon here the past week on this lens in order to get caught up on lost time and come to completion. It is Sunday night as I write this with only 2 more hours of polishing left on the last of the unpolished surfaces for this objective. Tomorrow will be the acid test because when the polishing is done I'm sticking the objective in its cell and putting it to a visual test on Pioneer peak - weather permitting. I recall how the image was last time so any improvement will be readily apparent. I'll post the results with a picture as soon as it is accomplished. All of you out there who have made lenses know how much anticipation one feels right as the final polishing time has come to an end and it is time to have that first look see !!!

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Jeff B
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Reged: 12/30/06

Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5673655 - 02/11/13 09:15 AM

Quote:

OK, with that data, here's your finished lens prescription and EFL. I added the longitudinal color curves to show you how I corrected the color. I picked 0.5461um (green) and 0.588um (yellow) to span the peak photopic eye response, and minimized spherical aberration at those two wavelengths. I chose to intersect the 0.656um (red) and 0.486um (blue) colors at the 90% zone to minimize the visual blur circle. The coma is essentially zero. That's about as good as it can get with these two particular glass types.

Can't wait to hear about the imagery!
Mike




Mike, even from my limited knowledge, I appreciate that this is a fine, well balanced lens. Good job!


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Jeff B
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5673661 - 02/11/13 09:16 AM

Jim, I've really enjoyed this thread. Thanks!

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jimegger
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Re: 9.25 inch refractor project new [Re: Jeff B]
      #5675153 - 02/12/13 01:45 AM

well, the polishing of 8 hours per side is not enough after cleaning off all the lenses I saw a definite milky sheen so I put in 2 more hours per side. We will see if that does it but I may have to go another 2 hours per side. Gotta make the glass take on a good polish before trying it out !!!

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kfrederick
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5675381 - 02/12/13 08:12 AM

Jim Great thread thanks for posting .Are you folding the telescope . Could be mounted like a newt with the flat where the primary goes .Or a window setup with your big flat mounted ahead of the lens . Very cool how ever you do it .

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5675726 - 02/12/13 12:16 PM

How do you know you are done polishing? Is there a "super-duper" polish you could put on an objective? I know that Zambuto and Ziess polish a certain amount of time after they should be done, but I am curious as to your perspective?

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Re: 9.25 inch refractor project new [Re: Sean Cunneen]
      #5675732 - 02/12/13 12:21 PM

A severe test for polish is to shine a laser pointer through the glass at several different places. If you can see the beam spot on either surface, you ain't done polishing.
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5676816 - 02/12/13 11:37 PM

The laser test seems like a good one Mike , it makes sense, any pits would make the scattered light obvious.

I have 12 hours on both the crown sides now and 11on the flint. I am going to put them in the scope tomorrow for a look see at the mountain here. The polish looks close to done if not done but I have not done the laser pointer test on them. The first go around I had about 16 hours on the flint and 20 on the crown while attempting to correct the figure before I gave up and went back to fine grinding. All the surfaces looked pretty polished out at that time but I did notice that the 12 hour mark seemed to be the start of the full polish period. I only went to 5 micron grit for the final size in fine grinding but maybe next time I'll go to the 3 micron size to help expedite polishing.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5677057 - 02/13/13 06:07 AM Attachment (53 downloads)

Laser works good, and so doesn't holding it up with foreground light coming thru at different angles looking at a dark something-or-other with low contrast. Either is a dead give-away to a lack of polish. When I use a laser, I shoot it thru straight on to the surface. Greenies are even better with a divergent beam.
My 8" crown dialyte, I polished about 2 hours each side with pads on the grinding tool, then an hour each with pitch, NBK7. Completely transparent.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5677247 - 02/13/13 09:02 AM

Beautiful, and FAST there, Mr. Barkie! Sure sign of excellent work during fine grinding. Makes me wonder if Jim didn't smooth long enough earlier in grit stages, way before 5 micron, like even at 30 microns or larger. 12 hours of polishing on buttery-soft flint glass like F2 seems way overkill, and should have only taken a couple of hours or so. He may be trying to get out the bottoms of Carbo pits! I think I spent maybe 3 hrs/surface on my F4 flint for my 6" f/10 achromat.

But hey, Jim's a hard worker and if 12 hours does it, then GOOD! Go have fun with that lens!
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5677621 - 02/13/13 12:42 PM

I went 45 minutes per grade starting with 120 grit on the flint and crown. The 320 grade and 30 micron grit I followed it with are nearly the same size for a total of an hour and a half on that. It always polished out my pyrex mirrors well in less time than I have in this lens now. I am using my original pitch laps to which I had added some beeswax which may have slowed the polishing down somewhat. From what I see now at 12 hours it still needs some more time - but it is close ! I have to take a break here from all the polishing for a day or so to do some machine work but I may make new pith laps after that and continue on.

I did put the lens elements in their cells and will be taking a peek through the scope later on today. I see that the lenses need more polishing. A new lap may just be the ticket to finish up !


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5677778 - 02/13/13 02:23 PM



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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5678096 - 02/13/13 06:00 PM

The scope is together and a per-cursory peek shows a very good image even before the cool down of the lens. The elements do need more polishing time but they are getting closer. I believe a new series of laps may be the solution for more rapid polishing action. The old ones I am using are getting the job done but I suspect the constant use has allowed the wax to be not as polishing agent en training as new pitch will be. The spacing of the lens elements needs to be increased by .008 inches also to make it to your specs Mike. I am not going to do the double pass until polishing is complete as it would probably change the lens forms anyway. I can say with certainty though that the image I see is definitely sharper than the first go around so on that note I will stay the course.

As soon as the sun gets off to one side and the glare goes away , I'll attempt a picture through the scope and post it here.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5678174 - 02/13/13 06:54 PM

Great news, Jim! Can't wait to see your picture(s).

Just curious: what pitch and polishing compound are you using? Could either be slowing you down? I've been working for years out of a gallon jug of ZOX and really like it. CeO2 is fast but can sleek unless you ball-mill it. CeO2 is better for dummy shining mirror backs, sides, etc. Red rouge is SLOW!

Using Acculap or Gugolz 64, I find that if I slop on too much slurry the lap surface gets overcharged and the polishing also slows down, along with the figure getting hinky. Taking a wire brush to the lap occasionally to scrub the hard surface glaze off gets you back to native pitch.

Whatever, your perseverence is certainly paying off!

When you get to double-pass testing, test in green light like from a green LED, that spectrum should be narrow enough, and a properly corrected lens will give you nice straight fringes. Just a suggestion: before going to a figuring session, try a few different airspace shims first - the airspace I gave you assumes all measured data is exact.

Standing by,
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5678303 - 02/13/13 08:26 PM Attachment (54 downloads)

Mike, I do have a glaze on my lap that I suspect is not doing a good job with the Cerium Oxide. I bought some Gugolz 64 pitch which I aim to make into new laps if the wire brushing of the present laps doesn't speed things up. On the first go around I made some laps from Burgundy pitch and got a real nice polish at 14 hours with the Cerium Oxide. At any rate , it will get done if it takes me 20 hours! It is almost there.

Here is a picture I just took of Pioneer Peak on a windy cloudy day with my Canon 20 da. Focus may not be perfect but to my eye the images look good. Color correction seems very good as well.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5678567 - 02/13/13 11:15 PM Attachment (43 downloads)

Hmm, I'll take your word for it at this point - low contrast, no sense of scale, and unknown level of horizontal path turbulence, so it's hard to tell. Can't wait to see a clear day shot, followed by lunar terminator shots!

Here's a test suggestion for double-pass autocollimation testing (assuming you have like a 10" optical flat?). Get a Radio Shack 5mm Green LED, Model 276-022 , Catalog #: 276-022, for a whopping $1.99. Website is here. Spec sheets says its spectral centerline is 570nm, with half-power points at 550nm and 590nm. Perfect spectrum for double-pass testing with Ronchi and slitless knife-edge (do both!). I simulated your lens in double pass with these wavelengths and weightings.

It says the forward voltage is 3VDC, and operating current is 20 milliamps. You have to put a resistor in series with the LED or you'll burn it up. If you use a 9V battery, the resistor should be (9-3)/0.02 = 300 Ohms. A quarter-watt fixed resistor is plenty. To vary the LED brightness you can also put a little 0-1K pot in series with it as well. But the 300 ohm resistor must stay in the circuit path.

If you're a master electronics guru, I apologize!

Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5678618 - 02/13/13 11:49 PM Attachment (43 downloads)

I processed some on your picture to see if I could bring out any detail. Still can't tell scale, but you're right, it's still an RGB color picture, but I don't see much in the way of secondary spectrum in the fine details.

More pictures!
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5678708 - 02/14/13 02:10 AM

What i usually do Mike for comparison is look through my APO which I know to be excellent at the same power at the same time to get a better picture of how badly the atmosphere is behaving. I did not do it this time. The part of the peak you see here is about 5 miles distant and a Raven would be just about the size of that point you see at the very top. The lenses are still milky enough when used together to ruin the contrast as well as the sky being cloudy with a slight haze.

Tomorrow comes more polishing possibly with new laps. I am trying to weigh the time spent making a new lap to just using the old ones again for a bit. The glass is so close to being polished out another run of a couple hours on each side just may do it.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5680393 - 02/14/13 11:15 PM

Okay, so I wire brushed the laps and guess what.... it polished like a fiend !! Right now they appear fully polished but just to be certain I will be putting in 1 more hour on each lens surface. By wire brushing the pitch surface it made the lap "bite" much better - one could feel the lap working its magic with more of a grabbing feel during the stroke. Now the lenses look clear. If I had done this in the beginning I would have been done quite a ways back. I was not sure wire brushing the laps would be something one could do without hurting the laps until you told me you did it with no trouble Mike. Thanks for the advise !

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5680464 - 02/15/13 12:07 AM

Glad to help! Wire brushing shatters off the surface scab glaze and makes billions of little microchannels that the polishing slurry can more easily flow through. Be sure to brush lightly, and just enough to get rid of the glaze. And, you still have to press in a little afterwards, as the lap profile is definitely changed by brushing. Rinse the lap well in lukewarm to slightly warm water before pressing to get rid of the powderey brushed-off mess. Wire brushing is far superior to pressing petticoat material, scratching up with a razor blade, etc., but you have to develop a feel for how hard and long to brush, and how quickly the microchannels will press out while working the lap and it starts to get too "grippy" or "draggy".

You do know about pressing in after brushing with just clean water, no slurry, and a single layer of a polyethylene trash bag stretched between lap and glass, right? The freshly brushed pitch will re-form to the glass shape almost perfectly, and the two parts will never stick together. Using only water keeps the lap from charging up too much and needing another scrubbing. Another old cool optics shop trick I picked up along the way.

Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5680520 - 02/15/13 01:08 AM

The laps I have are pretty hard from use, being old and the fact that I added the beeswax in the very beginning. I cold press the laps overnight by letting the lenses sit on them with no weight to keep their shapes. The wire brushing was just enough to leave scratches on the hard pitch surface. It was just perfect for good polishing action. The other trick is to press a fine mesh into the pitch and get many small facets for increased action. The scratches were just enough to do a really good job in my case though. I use a fairly watered down slurry of Cerium Oxide for most polishing and at the end just use water as the lap leaves a smoother polish that way.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5680599 - 02/15/13 02:38 AM

Mike you're right - wire brushing is so much better than microfacets. Also quicker. I use brass wire brush only, but I imagine a very hard tooth brush may do the job too. Worth investigating. The bag and clean water pressing is another good shop technique. You can't overemphasize the need for such methods, along with the final figuring slurry concentration of 1:100, 1-minute figuring spells, and cold pressing after each.

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Re: 9.25 inch refractor project new [Re: MKV]
      #5681631 - 02/15/13 02:19 PM Attachment (46 downloads)

Here are a couple of pics of the final polishing results. The crown is on the polishing stand in this one.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5681636 - 02/15/13 02:19 PM Attachment (67 downloads)

Here is the flint element.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5684227 - 02/16/13 08:57 PM

I took the lenses and and put them in the cell then bolted it on the refractor tube for a look at Pioneer Peak with my ES 127 Apo at the same magnification for a visual comparison. The air was turbulent so neither instruments showed the best images. I can say with certainty however that the 9.25 inch and APO images differed little clarity-wise and the color in the 9.25 was excellent !

Before the 9.25 inch can be star tested a mounting must be made for it. Until I can make a German equatorial later on , the plan is to mount it on my 26 inch reflector with a dovetail for good tracking. In the meantime the objective will be undergoing the double pass auto collimation sometime later this week. It has not been Foucault tested yet and I imagine it needs some correcting. This time the image quality far surpasses what it gave the first time around. The focus actually seems pretty crisp for a long focus Achromat - I am pleased !

For the lunar limb shots , it will be a bit of time to get the scope setup.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5693897 - 02/22/13 01:18 AM

After trying to get some stable air to make a simple visual test of the objective today was the day. I had my ES 127 APO set up at the same magnification as the 9.25 inch scope looking at the same spot on the mountain 5 miles distant. I could actually see more detail in the 9.25 inch during moments of quiescence . There is definitely more range of focus in the bigger scope at f/12 but surprisingly tight for that f value. The color is excellent , not as good as the APO but not in the least bit distracting to my taste. I have yet to bench test the objective with a green LED in the Foucault test for fine tuning -if any is even necessary . From what it shows visually , it could very well be there now.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5694372 - 02/22/13 10:18 AM

Hi Jim any plans to get the lens coated? I bet the views will be great Thanks for posting.

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5694620 - 02/22/13 12:32 PM

The statement you made about focus being surpisingly tight is a very good sign! Double-pass autocollimation testing will be the true read on the lens figure. Can't wait to see the results.
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5697749 - 02/24/13 03:34 AM

When the flint was put under the Foucault test for the concave side I got a close sphere but have a pesky hill in the center that does not seem to respond to various strokes designed to get rid of it. It is very slight but we are seeking a perfect sphere here where it can be seen directly. It could be this pitch lap is too hard at this juncture. I did modify the lap to work more on the center and will give it some more time. The hill is so slight that it probably won't affect the wavefront very much but again... it should be gone on this surface. It is much smaller than when I first looked at it so the image should be improved from the pre-testing visual checks.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5697811 - 02/24/13 05:48 AM

Jim, fantastic project you got there! Hats off
Can't wait to see it pointing the sky.

Have you tryied 'W' strokes, 1/3D of course as Texereau describes? Those should get you a nice even surface.


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Re: 9.25 inch refractor project new [Re: neo]
      #5697899 - 02/24/13 08:49 AM

Hi Jim putting the concave on top might help fix the center . . Congrats

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5698333 - 02/24/13 02:04 PM

Subdiamter tool, watery, thin polishing suspsnsion, light pressure, very short spells.

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Re: 9.25 inch refractor project new [Re: MKV]
      #5698815 - 02/24/13 07:30 PM

-NO.-
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5699188 - 02/25/13 12:38 AM

What I did do was to try and get the normal tool to work instead of going to a sub-diameter tool as it is my experience that the sub diameter tools are a bit harder to blend in just right. I used those on my 26 inch mirrors with great success but have had better luck on the smaller stuff with modifying strokes or modifying the full sized lap. In this case I have had great success bringing down the hill by putting a couple of extra layers of paper between the backing tool and the center of the lens about the same size as the hill - 2 inches in this case. I have been using the lens with backing tool on top. Now the extra layers of paper have put a bit more pressure on the center of the relatively thin flint forcing the center of it to get more polishing action. I believe the flexure in the flint has allowed me now to get the bulk of the hill out. It is very nearly gone now with an excellent cutoff on the Foucault test. It can't be more than a small per cent of a wavelength off from a sphere now. Now I am merely trying to get a smoother figure by putting less pressure on the polishing strokes. Today should be the final day for figuring it. It will go back into the lens cell for another look see to see how much the image improved. Afterwards the lens cell will be put into the double pass auto-collimation test apparatus with the green LED for a look at the whole assemblies performance.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5699424 - 02/25/13 08:48 AM

"What I did do was to try and get the normal tool to work instead of going to a sub-diameter tool as it is my experience that the sub diameter tools are a bit harder to blend in just right."
********
Precisely! from the tryout earlier, you're really close. A sub-tool at this close to being finished on a flint can make a lot of trouble, and resultant do-overs. I use sub-tools only when they -AREN'T- the last step, if at all.(particularly with a softer glass) Nice way to shim the center, btw!
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5699479 - 02/25/13 09:21 AM

The reason for having the concave on top of a full sized tool is that will work the center more . Like in fine grinding if you wont the curve deeper put the concave on top . Many good ways to happy .

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5699905 - 02/25/13 02:03 PM

Yeah Kevin, I have been using the lens on top throughout the whole polishing process with the typical center over center 1/3 stroke as well as blending strokes towards the end. I believe my lap is a bit to hard for best results with the added bees wax. With the center of the flint shimmed out it is working better. It is almost there now.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5700192 - 02/25/13 04:29 PM Attachment (43 downloads)

Here is the setup for getting rid of the hill on the flint concave side. The flint R4 is resting on the R4 tool with Saran Wrap and a paper towel between it. I put 2 more layers of paper towel in the center where the hill is to put added pressure on it during polishing. I have a collar of Mylar taped to the R4 tool so the flint can "float" on its paper towel bed. It is loose in there in that it can be taken in and out but does not move back and forth to any appreciable extent. IT WORKED !!! The hill is gone !

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5700196 - 02/25/13 04:30 PM Attachment (38 downloads)

Here is the lens in its cell ready for mounting on the tube assembly.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5700199 - 02/25/13 04:30 PM Attachment (41 downloads)

Mounted on the tube .

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5700204 - 02/25/13 04:31 PM Attachment (49 downloads)

here is the whole scope at about 106 inches focal length.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5700210 - 02/25/13 04:35 PM Attachment (45 downloads)

This is a shot with a hand held camera before the lens has cooled down through a 50 mm eyepiece at about 53 X of a mountainside 5 miles distant with a lot of heat waves from the sun causing image deterioration. Visually I noticed a distinct improvement from what it was before addressing the hill on the flint the focus is even more crisp and the image sharper in spite of the heat effects. Once it settles down this evening and the lens reaches equilibrium I'll give it another look see.

Edited by jimegger (02/25/13 04:41 PM)


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5701110 - 02/26/13 07:44 AM

Plan to leave it uncoated, or coat?
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5701687 - 02/26/13 01:31 PM

Not sure yet whether to coat or not. I am leaning towards getting it coated at a later time, certainly not until I am fully satisfied with its performance.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5701798 - 02/26/13 02:36 PM

Food for thought-
The coater should have a really good track record. Not so much the crown, but the finished flint is susceptible to cold drafts. One bad habit when taking out of the chamber, and you could wind up with pieces.
If you leave it uncoated, tweaking later on is handy.
fwiw,
M.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5701807 - 02/26/13 02:41 PM

And hope to God it doesn't affect it's figure for what seems to be a non consensus on the pros and cons. That's exactly why I lived for 30 plus years with an uncoated R. E. Brandt 8" f/13.3! Probably why outfits like D&G take forever to make a coated achromat. Too many do overs! Anyways Jim I've enjoyed watching this post. Very informative and entertaining too. Mike

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5701833 - 02/26/13 02:57 PM

Jim, could you post the same picture of that mountaintop taken with your 9.25" and with your APO, at the same scale, for comparison? Thanks.

Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5702002 - 02/26/13 04:27 PM Attachment (89 downloads)

For comparison here are 2 shots of the same mountain scene taken with the APO and the ACHRO. Here is the APO shot first.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5702004 - 02/26/13 04:28 PM Attachment (81 downloads)

Here is the image taken at the same setting through the same dirty eyepiece at close to the same scale only this time through the 9.25 inch achro.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5702027 - 02/26/13 04:35 PM

I notice the contrast is much better on the APO. This is probably due to the fact that the sun is shining into the maw of the larger 9.25 inch and causing glare on the still not yet blackened lens cell. The sun shade on the 127 Apo is much better protected that way. To be a better test as such it would be best to wait until the air is more stable and the sun not a factor. The sun angle is very low still here in Alaska and is only about 20 degrees away from where this shot is being made.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5702847 - 02/27/13 02:43 AM

I managed to get the scope on a really undersized mount I have just for a look at Jupiter tonight. I also set up the 127 APO for comparison. At the same power the APO gave a more crisp image but not much more than the big scope. The lens needs some more work but is very close as the stars were points and I did not notice any color around Jupiter that stood right out. I was pleasantly surprised about the lack of violet. Now all of this is being done before the double pass auto collimation test with the green LED. This will be done soon and these visual tests are a way of gauging improvement after the auto-collimation test and correction.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5702897 - 02/27/13 04:57 AM

Thanks Jim. The glare might be a factor for the difference. Very telling, indeed. I am sure all of us will be looking for your update on the green laser DPAC test results, and perhaps comparative shots of the mountaintops when you blacken the tube inside. Also, I think a true test of color correction is best made against a bright star. A simple achromat can only do so much.

Thanks for sharing your project with all of us, Jim.

Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5705297 - 02/28/13 12:52 PM

I looked at the auto-collimation green LED test yesterday and got an objective that shows small amounts of spherical aberration such as would be demonstrated by one that has a longer center focus than edge focus giving a look similar to a mirror showing a parabolic shape. Now the trick is to find which surface or surfaces need the attention. I suspect it is the R4 surface as it was not redone during this session. The plan now is to test only the crown and see what it looks like first and do any touch ups needed there. If that needs no work then it is onto R4. I'll try to get a focal gram and post it here.
All in all the lens is not too far off but from my experience with the 6 inch objective I made that little bit makes all the difference in the world when it comes to crisp images !


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5705599 - 02/28/13 03:41 PM

Jim, it seems to me that if the center focus is longer than the edge the wavefront is oblate and undercorrected.

Mladen

Edited by MKV (02/28/13 03:41 PM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5705779 - 02/28/13 05:22 PM

That is what I get from the test results. The shadows look like a paraboloid mirror but on the lens it would amount to an oblate spheroid. So I am back to doing some more polishing now on the crown as it tested out to have that particular shape. I am using some blending strokes in an effort to bring the center up to a sphere.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5707675 - 03/01/13 07:29 PM

The old laps for the crown were just not getting the job done so I made 2 more laps from the Gugolz 64 pitch with no additive beeswax this time. From what has been happening it appears that the pitch needs to flow better in order to facilitate making the lens surfaces into the sphere they need to be. Also the fresh pitch should hold the Cerium Oxide better than the waxy surface on the old laps. I am just finishing up on the R2 lap. Polishing will commence tomorrow or possibly later on today and it will be interesting to see the difference if there is any between how these new laps do as compared to the old ones.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5708275 - 03/02/13 07:45 AM

Just me, I find invariably that a good hard 73 will make beautiful spheres.
I fiddled around with the old OSLO design of your achro, and found that shortening R3 by just .015" pushed the green line from the supposed long focus center to about a perfect null @ 586nm. Focus for the green line moved a total of around .006"~. Seems that you could deepen the curve on that flint with less than an hour of polishing, perhaps. How long did the center focus?
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5711062 - 03/03/13 05:32 PM

I have not measured the focus difference for the various wavelengths. I did do a star test on the objective last night and found it to be under-corrected the crown being an oblate spheroid. It seems that the figure is improving with the new pitch laps but it will take some time to get it done with a regular lap so I am planning to make a convexing lap and work in conjunction with the regular laps to get the crown surfaces to a sphere. It seems my extra hard laps with the 1/3 stroke was not able to produce a spherical surface - at least the way I did it. The convexing tool will help speed up the process.
Color looks excellent and the inside focus image as well as the extra focal image showed well blended colors in the proper proportions for a doublet refractor design !


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5711846 - 03/04/13 01:25 AM

Quote:

I did do a star test on the objective last night and found it to be under-corrected the crown being an oblate spheroid.



Jim, just curious: why star test the scope when you already did a double-pass autocollimation test, which is more sensitive than a single-pass the star test, and found the wavefront undercorrected?

I don't recall you mentioning it but you should be able to determine if your concave surfaces are truly spherical by the Foucault knife-edge test at their ROCs. Since R2 and R3 are the same, you can then test your R2 against R3 by contact interference method and figure R2 until you get straight fringes. Then you can be sure R2 is also spherical.

Once you know R2 through R4 are spherical then any residual spherical aberration comes from R1.

If this is the case, and you already know R2-R4 are spherical, and you are getting an undercorrected wavefront (i.e. central focus longer than edge focus) then R1 is oblate, (its center is too flat relative to the edge). Either way, 1/3 stroke should bring it to a sphere. Just work on it until you get a clean null using a knife-edge and your autocollimation flat.

Mladen

Edited by MKV (03/04/13 01:54 AM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5712074 - 03/04/13 08:33 AM

Flat center on a convex:
= a short stroke which whacks the center lower than the rest of the lens-
Or you are standing in one position while doing the "W"s for too long a time.
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In short, the overall wear pattern is simply not random enough. Generally this comes from being overcautious. The shorter radius on a convex's edge has the same analogy as a concave with TDE. It has the same cause.
(this is what I've found out with my meager efforts!)
fwiw,
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5712125 - 03/04/13 09:08 AM

Jim are the inter surfaces matching RC ? Looks like you are getting close . Did it focus the stars nice? Be a fun ATM project Having the zemax help [Mike ]made your telescope much better . I like the idea of a folded refractor mounted like a dob with the lens at the top /flat in the bottom.Mounted like John Halls 30inch .A 12inch might not be too bad wonder how much the glass is for big lens? I used the 30 inch refractor at the Allegheny Observatory once . Makes me wonder how big they could now AS the 30 was made over 100years back . Congrads Jim on your work

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5713756 - 03/05/13 01:42 AM

Ok , here is the skinny on the lens so far. Mladen , I did the star test to double check that the flat I am using is giving me the correct information. Yes Kevin , the flat appears to be doing its job correctly as the star test validates the results from the flat. The star test also gives me an idea how far off the lens actually is as well as a good test for astigmatism. The lens looks excellent for both color and astigmatism but undercorrected enough that I believe the crown will need considerable more polishing. I believe my probably fast short strokes were the cause of the flat center so BINGO Mark! I am now taking longer strokes with my regular lap with a narrow w motion to try and bring the center up. I will give it a couple of hours if need be doing that but if it seems to be taking too long then I'll go with the convexing lap to speed things up. Ive used one before and it worked great. The lens seems to be improving with every polishing session now so it is a matter of time. Now that I know the flat is doing its job correctly the auto collimation tests will continue as will a couple of star tests to see the real results.
I really do not want to use the flint as a test plate for the crown for fear of scratching and I am not sure if they are an EXACT radius match. The sphereometer says they are but that is only accurate down to 1/10,000 ths of an inch. My flints concave is a sphere now by Foucault testing. The crown on the auto collimation test shows the under correction quite clearly. It is polish..... polish .... polish for now....


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5713864 - 03/05/13 04:58 AM

Jim, you can test R2 on R3 if you use thin shims to avoid scratching. The two do not have to be in physical contact. If R2 is also oblate, you may actually ruin the lens by working only on R1 and assuming that R2 is spherical.

As for precision, that depends on the calibration of your spherometer, to include the precision of the feet separations as well, and not just the dial indicator's stated accuracy. Every spherometer has an inherent error built into it. In other words when you read a sagitta of say 0.0505 it doesn't mean it's 0.0505, but .0505XXXX, where XXXX is your internal error that has to be calculated.

None of this should prevent you from obtaining interference fringes and determining if the R2 surface is spherical.

You can read more on this test method from this source.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5713873 - 03/05/13 05:24 AM

By the way Jim, you can also test the accuracy of your flat using the Ritchey-Common test. You will need a good sphere of a rather slow focal ratio (f/10 or so) and a bright pinhole a few microns in dimeter and a knife-edge. Make sure the mirror is a really good sphere.

You can also use a small stainless steel precision ball bearing in place of a pinhole. Just shine a super bright LED on it from a couple of feet. The image of the LED in the ball bearing should be close to or even less than the Airy disc size for the focal ratio. You want your light source small for maximum sensitivity of the test.

The other method is to use a small laser pointer < then 5 mW output, and a small lens (~ 10 mm fl) what will focus the 3 mm laser beam to a sharp point and onto a small pinhole. The waves emerging from the pinhole should be perfectly spherical and centered on the pinhole if the pinhole is small enough..

Also, it would be a good idea to measure your longitudinal spherical aberration in green light. From that parameter it is easy to figure out the wavefront error (optical path difference, OPD) and from that the surface error on R1 (provided R1 is the only culprit). Knowing how much surface needs to be removed would help you gauge how much polishing you need and not overcorrect.


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Re: 9.25 inch refractor project new [Re: MKV]
      #5713883 - 03/05/13 05:32 AM

In polishing the convex surface, TOT is the usual method. Size of the polisher should be kept down around 3/4 size; and in this way, nice long smooth strokes can be used, and it keeps the edge up. With full size, strokes can't be anywhere near as long and still maintain the same radius.
So it can be seen that with full size tool, the same effect on a convex surface can exist with long stroking, as short stroking relatively fast over the center. (!!!) Thought you might want to keep this in mind???

All the time I ever was in the mfr of optics, in contact testing, we never used shims. They were more of a headache than a solution. Just keep in mind- glass breaks and scratches, handle accordingly. With big pieces such as you have, I always tested by direct contact, but offset so that I had a good hunk of edge to grab/manipulate. Contact testing also teaches one how to -CLEAN- the glass to do the test correctly. Too bad you didn't have a concave testplate about 5" diameter- testing could be done in about 10 seconds.
Contact testing regardless, takes some time to get the hang of. But it's a very handy asset.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5714320 - 03/05/13 12:08 PM

Another way of testing to see which surface in the double convex crown is off spheroid is to put a collar around it and fill the surface with Acto oil which has a very close index of refraction to BK7 glass. Then you can do a direct Foucault test on the curve and see which one is bad. Of course you need to do the Foucault test either horizontally through a flat that is optically good or vertically looking down on the lens.
I am aware of the fringe testing but have never done it and am not sure I would be competent enough to evaluate the results accurately without more study. On my 6 inch objective I tried both sides and evaluated which one was giving me the results I wanted and it worked out. This lens will not suffer from more polishing action at this point and it has shown movement in the right direction.

On my six inch objective I turned my 12.5 inch mirror into a collimator and used a red pinhole to get a null as I had no optical flat at that time.

I am using the tool on top and have been all along , I was short stroking it all the time though and believe that is why I have an oblate spheroid as a result.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5714649 - 03/05/13 03:00 PM

I just spent 1-1/2 hours polishing long narrow w strokes on the R2 curve and have seen significant improvement in the auto-collimation test. The figure is slowly working towards a sphere.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5714894 - 03/05/13 05:00 PM

Would love to see DPAC Ronchigrams of your progress. Just put the camera on a firm mount with the lens behind the Ronchi grating, and focused on R4 of the lens.
Mike


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5715470 - 03/05/13 10:04 PM Attachment (36 downloads)

Quote:

Another way of testing to see which surface in the double convex crown is off spheroid is to put a collar around it and fill the surface with Acto oil which has a very close index of refraction to BK7 glass. Then you can do a direct Foucault test on the curve and see which one is bad.



I never heard of such a test for convex surfaces, and don't really understand how it's supposed to work. Here is a diagram of what you describe.

Mladen

Edited by MKV (03/05/13 10:39 PM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5716306 - 03/06/13 12:32 PM

By doing that with the oil you have effectively eliminated the upper surface curve and now can do a Foucault test on the concave back surface same as when testing any concave.

Mike , I do not have a Ronchi screen. What would I need for one - matched to this lens focal length ? I know Willman Bell sells them and they go by focal length for the mirrors. Is not the knife edge a sort of Ronchi test in itself ?


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5716342 - 03/06/13 01:00 PM

I like and use a 1" square straight-line Ronchi grating with 100 lines/inch, like Edmund Optics sells, for wavefronts slower than f/8 at COC (f/4 at infinity focus). I find the diffraction sidelobes to be annoying much above 100-150 lines/inch.

Be careful to get a straight-line Ronchi grating from either Willman Bell or Edmunds, not a curved-line inverse(Mobsby) grating! Try the W-B first; it is 10X cheaper than the Edmund, but the lines may not be as sharply defined.

Yep, the slitless KE acts as one Ronchi bar, and to me is better for showing surface smoothness and faint zonal shading. I personally like Ronchi's test for looking at the overall figure, and especially for locating breaks in curve slope when TDE is present. But these are just my personal experiences, not necessarily what works best for you.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5716456 - 03/06/13 01:51 PM

Thanks Jim. I take it you have tried this test with oil. It is not as simple or practical as it sounds. On the other hand, the contact fringe method is a proven and easily repeatable method used in industry. When you are "tweaking" a surface, you need a method that is quick, clean and easily repeatable very frequently. There is also plenty of information on the Internet about the method.

The oil method resembles the way one can test a Cassegrain secondary. It requires ray raytracing to determine the setup parameters. If you use oil, it would require making a damn for oil and refilling it with oil after each figuring spell, which in the final phases can mean couple of times an hour, allowing the lens to cool down after handling for a short time and short figuring spells of about one minute each.

The vertical position of the lens is almost a must in this case, to keep the oil surface flat, which only complicates the setup procedure, and makes the oil method too impractical for production purposes.

Once you know R2, R3, R4 are spherical, the fourth surface is worked on under DPAC until you get a clean null. No guesswork involved.

Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5718715 - 03/07/13 03:58 PM

I have not done this test myself Mladen but it is discussed in one of Ingalls Telescope making books. It can be set up so you do it horizontally by using an optical flat at one end of a tube and the lens at the other end with the fluid filling the tube. The other way is to view the lens looking down on it with the fluid surface rendered flat by gravity. It is a good way to see which surface is the worst or bad then work on it from there.
I ordered a Ronchi screen from Willman Bell but in the meantime I am polishing with the convexing tool that has the center area pitch thinned out gradually towards the edge so as to have less action on the center and allow for it to come up faster as the outside gets worn down. It appears to be doing its job okay so far and the knife edge is being used to check the curve until my Ronchi screen gets here. The center flat spot is shrinking in size.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5718999 - 03/07/13 06:23 PM Attachment (46 downloads)

Tanks Jim. No doubt the oil test is possible for finished optics. But it's not practical for the final figuring phase of completing an objective. Covering a lens with oil over and over every half hour or so and testing the figure is tedious, imprecise, and unnecessary work and requires very exacting setup which can be determined by raytracing.

Contact interference tests are clean, quick and require no calculations. They are practical and accurate. The only thing that's required is a good cleaning method and very careful handling.

I think you will find the DPAC Ronchi test to meet the frequent testing requirements very handy for a "quick" assessment. I would make sure the glass has completely thermally stabilized (mabye 15-30 minutes for a 9-10 inch lens) after handling it, before assessing the results. You can also use the grating as a knife-edge test for greater wavefront surface detail. Just remember that theRonchi test is not as accurate as the knife edge shadow test. The Ronchi will give you a good idea where you stand overall with the wavefront.

Using a single Ronchi band test is quite sensitive if you slowly pan the band across the illuminated disk and observe any changes. At this stage, a 133 lpi can actually give a hint of very small wavefront surface errors. The picture below shows a 6-inch f/4 spherical mirror tested at ROC with 133 lpi grating and a small "hill" or bump seen as a "bite" against the edge of the band, as shown by the arrow. In a knife-edge test, the bump was clearly visible, and interferometry placed it at approximately 1/20 wave elevation.

Mladen.

Edited by MKV (03/07/13 06:29 PM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5719904 - 03/08/13 06:46 AM

One line and space, subtends about .008" in the above pic.
********
Nowhere near sensitive enough. (a grating is not an IF) There is an edge issue as well. I hope it isn't coated yet?
(what do I know?)
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5722684 - 03/09/13 05:21 PM

After quite a bit more polishing with the convexing lap , there seems to be an obvious improvement in image quality while testing the scope on the mountain. I am suspicious of the auto collimation test due to uncertainties about the flat being up to snuff. After a few hours of polishing on both crown surfaces the Foucault test does not show as much change as I think it should. The star test showed under correction as I said earlier , but it wasn't bad. I am going to use my 12.5 inch as a collimator like was used with the six inch objective as I know it is excellent and gave me a top performing objective. That way I can double check the results from the flat as well.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5722801 - 03/09/13 06:43 PM

Nice way for a double-check.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5728860 - 03/12/13 07:32 PM

So here's the skinny on all this polishing action up to date.
The objective is VERY polished now after many extra hours of figuring trying to get it right in the double pass auto-collimation test. It turns out my optical flat isn't so flat !! Star testing has proved that. I started out with an under-corrected objective according to the flat and star testing, after using the convexing tools on the crown the flat showed still- an under correction while the star test shows a definite switch to over-correction now. So now I am discarding the flat for testing and am going to use my 12.5 inch mirror as a single pass collimator tester. It worked beautifully for the 6 inch refractor I made. It's simple to do. Just put a pinhole at the focus of the mirror , in this case a Mini Maglite with a perforated foil cover for the pinhole. You can tell when it is in perfect focus by using another telescope focused to infinity to examine the pinhole reflection at high power. When the pinhole is sharply defined you have it exactly right distance giving parallel rays off the mirror. Now all one needs to do is set up the objective to be tested in its tube assembly looking into the main mirror. I use a knife edge made to insert into the 2 inch focuser to do a Foucault test then also examine the pinhole image at high power to check for sharpness. It is amazing how well this setup works ! With the 6 inch objective the sudden sharp focus and image were suddenly much better after some touching up in polishing/figuring as backed up by a really good Foucault test. For those who have no access to a good flat but have a good parabolic mirror available of the proper size, this works out just fine.

I am now working to correct the over corrected nature of the 9.25 objective and am closing in on the target.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5728992 - 03/12/13 09:04 PM

Jim,
See PM from me.
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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5729948 - 03/13/13 09:30 AM

Jim,
The flat would have to be a 100 waves from flat to show an error in a double pass test. The problem I believe is that your reading the test like you would a mirror and for a lens every thing is backwards. What looks like hole on a mirror is a hill on a lens. So undercorrection is actually overcorrection when testing a lens. A double pass test has twice the sensitivity of a single pass test plus it has the added advantage that if the flat is smooth, the errors on it have little effect on the results but in a single pass test the error adds from the telescope producing the collimating beam, 1 to 1. A single pass test can also give you a false positive result. If your lens is 1/8 wave undercorrected and the telescope making the collimated beam is 1/8 overcorrected, they add and the test would show a perfect null. The same is true if both optical system were 1/8 under corrected, in a single pass test the result would be 1/4 undercorected. This doesn't happen in a double pass test.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5730352 - 03/13/13 01:20 PM

Dave /Jim flat is one I got off the shed I think Garys test uses one like it .I have not tested it. It was sold as 1/10 wave .Maybe test your good refractor lens with it .

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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5730518 - 03/13/13 03:01 PM

Quote:

Jim,
The flat would have to be a 100 waves from flat to show an error in a double pass test. The problem I believe is that your reading the test like you would a mirror and for a lens every thing is backwards. What looks like hole on a mirror is a hill on a lens. So undercorrection is actually overcorrection when testing a lens.



Initially, Jim reported the error to be such that the central focus was long and the edge or marginal focus was short, indicating the wavefront was oblate.

Since Jim also reported that his flint biconcave element had spherical surfaces, clearly, one or both sides of his biconvex crown element depart from the spherical shape.

I suggested that Jim test R2 against R3 by contact interference method in order to determine if R2 is spherical or not. The test is simple, quick and repeatable and easy to interpret and is quantitative. It is also used in industry all the time. A little reading about it, proper cleaning and handling avoids scratching the lenses.

Once he knows R2 through R4 are spherical, and the objective is still not corrected, the flaw is on R1. All he has to do is figure the front radius of curvature accordingly, and test the results by autcollimation until he reaches a clean null either by knife edge or by 100-133 lpi Ronchi grating. There is no guesswork in any of it.

As for his flat, he could have tested it by autocolliamting his 12 inch paraboloid, which according to him is good optics. If he got a clean null, the flat is good enough.

Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5730542 - 03/13/13 03:28 PM

Mladen,

A couple of very in-experienced questions:

1. How do you test R4 since it is a convex? Is there a way to test it from the front so it would appear concave? How would the differentiation be made between R3 and R4 then?

2. Once R2-4 are all good and the testing is being done on the remaining R1, do the elements have to be spaced properly (as per the design) during the test? Could any deviation from the design spec of any of the R2-4 or the spacing cause the reading of R1 to be incorrect or mistakenly interpreted?

Thanks,


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Re: 9.25 inch refractor project new [Re: Gord]
      #5730616 - 03/13/13 04:14 PM

Mladen, curious, I wasn't aware you actually worked in optics shops. Where all have you worked?
Mike


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Re: 9.25 inch refractor project new [Re: MKV]
      #5730633 - 03/13/13 04:20 PM Attachment (47 downloads)

The star tests will be a confirmation of whether or not the objective is under or over-corrected with a look at the in and out focused star images. Initially the inside focus image showed a distinct hole in the center and the outside focus showed a disk full of light with no hole as a matter of fact it had a bright point in the center indicating long focus central rays and shorter edge ray focus ... an under corrected objective according to my books. NOW ...I have the opposite after using the convexing tools for some polishing spells. Now the inside focus rays leave a light filled disk and the outside focus rays have a distinct hole in the star test. The flat I was using shows no change in the appearance of the Foucault test. It still shows the central rays as being long in focus and the edge rays as shorter. That is why I suspect the flat to have a significant curve to it. The star test cannot be denied as it is the eating of the pudding so to speak.

As far as using the 12.5 inch mirror as a collimator for a single pass test, it has proven itself with the production of a really excellent 6 inch lens by myself so I know it works very well. Once it is set up it is like doing a Foucault test at the focus of the scope on a star AND you can look at the pinhole with a very high power eyepiece without looking through all the atmosphere. I was amazed at how much improved the pinhole image was at high power when the objective showed a really good null in the Foucault test! It was like looking at the small pinhole through a microscope - the detail just jumped right out. After that I looked at Mars with 500 X and saw the most awesome detail on a particularly steady night. The stars are textbook diffraction images.

I have a week trip coming up here and when I get back it will be using the reflector mirror for a collimation test. In the meantime I am still star testing the objective since we are having clear weather here now. It is set up for a session tonight after another brief spell of polishing this morning. I am closing in on the perfect star test images at any rate - working back towards where I was from an over corrected state at present.Here is a shot of the focalgram I took of just the crown using the green LED with the flat in autocollimation. The light source is on the right from the knife edge. The light marks (squiggly lines) in the mirror are defects on it - not the lens.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730637 - 03/13/13 04:21 PM Attachment (46 downloads)

Here is a shot of the 9.25 inch on a really undersized wobbly mount for star testing purposes.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730645 - 03/13/13 04:22 PM Attachment (40 downloads)

Here are the 6 inch and 9.25 inch scopes set up looking at the mountain for visual comparison purposes.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730914 - 03/13/13 07:00 PM Attachment (35 downloads)

Here are the tools I have been using as of late. The convexing tools in the back and the normal tools in the fore.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730950 - 03/13/13 07:20 PM Attachment (30 downloads)

Here's what I use for polishers. It's a type we used to use on flats. I discovered they can aspherize surfaces just as well as those square types above, but far more predictably. Pitch on these was real old, perhaps 10 years or more, #73.
I never, never let mine get glazed- generally they're just as black and shiney as when first made. These 2 were for a 6" F/10 primary, the far one didn't work, the near one smoothed and corrected the mirror in 20 minutes. Minor differences, but those differences either made it work, or it didn't. This type of polisher can do down to around F/4.5 paraboloid without a problem.
M.

Edited by Mark Harry (03/13/13 07:23 PM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5730965 - 03/13/13 07:30 PM

I'm fairly CERTAIN the flint is not biconcave; but rather a negative meniscus.
M.


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Re: 9.25 inch refractor project new [Re: Gord]
      #5731460 - 03/14/13 12:29 AM

Gord, I think the best and most practical way to test convex lens surfaces is by the use of test plate interference method. The last surface to be figured can be tested by DPAC test agains a flat mirror, with a fully assembled objective, or using a spearate test plate.

Jim also mentioned an interesting oil test , which requires vertical testing. The oil would have to have a refractive index very close to that of the glass used. The test would require raytrace analysis for a proper test setup. However, cleaning and refilling the oil in the exact volume would make this method highly impracticsl for figuring purposes.

And yes, R4 in Jim's case is convex rather than concave, so he has three unknowns.
Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5731763 - 03/14/13 08:28 AM

Mladen, Yes the objective design has 3 convex surfaces, R1 , R2 and R4. As to the oil or any liquid for that matter , all you need is for one surface of the lens to be covered so as to prevent a flat surface to the knife edge. It would not matter if it was just covering or a couple of inches deep although just covering it would be best and all you would need. All you want to do is see the backside curve with the front side negated. At the very least it would be an easy way to tell which surface is the bad or worst one so you know which one to work on. The interference method is good if you are doing production work but for a hobbyist doing many different lens designs you would need a plethora of tooling. With a single pass collimation system such as I have using the 12.5 inch mirror You can accurately test many different designs with one setup. Same for the DPAC.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5731938 - 03/14/13 10:17 AM Attachment (26 downloads)

There is some misunderstanding on how the oil test is done. The oil test is done by having the lens enclosed in a tube that is as long as the radius of curvature of convex surface you want to test and the tube is filled with oil. The oil has a refractive index that matches that of the glass so what your doing is making a test tunnel that has replaced the air and filled it with oil to cancel out the glass of the lens. Your testing thru the lens and the convex surface now acts like a concave surface. If done this way then when the convex surface is spherical it will test just like a spherical concave mirror but your going to need enough volume of oil to fill the tube not just cover the lens.
If you just cover the one surface of the lens with enough oil to form a flat surface and by doing so make a plano convex lens and test thru the oil surface, you have to take into account the flat surface. If you use a fluid that has different refractive index then the glass you have to take that into account as well. So if you tested this way and figured the lens to show a null and assume that the convex surface is spherical you'll be way off. The convex surface won't be a sphere but a hyberbola.
Here are the results from OSLO showing what happens if only an enough oil is used to make a flat surface on one side of the lens and having a perfect sphere on the convex side. As one can see the spot diagram is very bad and the convex surface would look like it is strongly aspheric. So this is not a very good test for a convex spherical surface and a test plate using interference testing is much better. To do interference testing all you need to do is flash polish the concave tool used to grind the convex one and polish it spherical. When the interference fringes are straight then the convex surface is also spherical.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5731958 - 03/14/13 10:27 AM Attachment (26 downloads)

Here is what happens if you assume wrongly that if you just use enough oil to make a flat surface on the one side of lens and test thru this and think that if you figure the convex surface to show a null it will be sphere. In reality what you have done is put a very strong hyberbola on that surface and when you assembled both elements of the achromat and test it you would be wondering why the image is not very good.
Here is the spot diagram showing the results. The spots are very good and the lens would test like a perfect spherical mirror but again what has happen is that the convex surface is now a hyberbolic with conic of about -1.2 and not spherical.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5732496 - 03/14/13 04:14 PM

Had the same results when I tested a couple lenses. Though the convex is spherical, the test reveals an aspheric surface.
If you test the whole achro, (with a flat)you should get a null; which is what I did.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5732499 - 03/14/13 04:16 PM

I'm almost thinking that there might be an error between R2 and R3- which can give an indication of over/undercorrection. With polishers like that, I would think the R1 surface likely has a huge hill in the center.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5732854 - 03/14/13 08:00 PM

Quote:

To do interference testing all you need to do is flash polish the concave tool used to grind the convex one and polish it spherical. When the interference fringes are straight then the convex surface is also spherical.



Dave, the problem today is that most people don't use solid glass tools but rather ceramic tile tools which, obviously, cannot be used for contact interference. Purchasing extra glass disks just to turn them into matching test plates makes a one-time project really costly and wasteful, so it is no surprise ATMs shun away from such techniques and seek alternative ways to get the job done.

That's why picking a well-thought-through design from the start, one that matches your skills and tooling, is so critical. For ATMs, refracting objectives or compound correctors with as many matching surfaces as possible is the most economical approach. One such example is the Houghton corrector, where the rear element is biconcave and the front element biconvex where R1 = -R3, and R2 = -R4.

Almost symmetrical telescope objectives are also possible, especially with modern glass melts. If one can devise an objective where three out of four surfaces are known to be spherical, either by Foucault or by contact interference of matching surfaces, then the system can be nulled by DPAC by working on the front surface of an otherwise finished objective. Also, configurations with very long, almost flat R4 can be advantageous.

Unfortunately, Jim here is working in the dark, except for R2 (which can be contact tested against R3). Short of making two additional test plates his engima will not be easily solved. Assuming he determines R2 is spherical, then it would be a matter of judgment whether R1 or R4 or both are the culprits.

I would guess it's probably R1 because its relative power (shorter radius of curvature) is more likely to affect the overall wavefront error, than the weaker R4 surface, and is morel like to distort the wavefront even with smaller figure errors than R4.

I don't have time to do this right now, but this can be illustrated easily using a simple raytrace analysis of wavefront variation over different conic constant values for for R1 and R2. Maybe later on tonight I can find some time to do this analysis.

Mladen

Edited by MKV (03/14/13 09:17 PM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5733393 - 03/15/13 01:41 AM

I do know the problem is in the crown because while working ONLY on the crown the objective went from under-corrected to over-corrected while using the convexing tools. I over shot the mark because I was trusting the flat to be giving me an honest reading which it isn't. Even though it is over shot I am now working it slowly back to where it should be and checking it with star testing. I have put 1 hour of polishing on the crown on both R1 and R2 while watching the star tests get better each time. When I get back from a trip in a week I'll be setting up the 12.5 inch mirror for the single pass collimation test and doing both Foucault and star testing in the shop so I won't have to wait for stars to come out. It should speed the whole process up considerably. R3 is right on a sphere and R4 is a long radius curve so it won't have as much effect on the image. It really doesn't matter as long as the objective nulls out in the Foucault test.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5733701 - 03/15/13 09:30 AM

Jim,
Are you only testing the crown using double pass with the optical flat ?
- Dave


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5733952 - 03/15/13 11:38 AM

Jim, in similar configurations to yours, the R2 will contribute a lot more spherical aberration then R1 if it is not spherical. This is just another reason why it would pay to determine if R2 is spherical or not, and the only practical method I can think of with your tooling is the contact interference test against R3.

In MIJ's 4-inch f/15 configuration, with R2 = -R3, the LA will change 2.2 times more if R2 is given cc = 0.3 (oblate), rather then if R1 is given the same conic. The R2 will give LA = 0.0585, whereas R1 in LA = 0.0262.

Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5756435 - 03/25/13 03:31 PM

I have tested the crown only with the flat but also the whole objective assembly.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5756583 - 03/25/13 04:51 PM

Quote:

I have tested the crown only with the flat but also the whole objective assembly.




Jim,
What is purpose of testing the crown in combination with autocollimation flat ?

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5757075 - 03/25/13 09:46 PM

To see if it is or has spherical surfaces. Look for turned edges and spherical aberration.

Edited by jimegger (03/25/13 09:49 PM)


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5757238 - 03/25/13 11:29 PM

Jim,
The crown element by itself will show a huge amount of spherical aberration when tested by double or single pass autocollimation even if the surfaces were perfectly spherical. Only when the crown is combined with the flint will of the combination of the two shows a null when perfectly figured and you'll be able to determine if the surfaces are spherical. Just like the refractive index of the flint offsets the chromatic abberation of crown, the curves on the flint also offset the spherical aberation of the crown. It's the combination of both elements that correct for color and spherical aberration. Each by themselves has a large amount of both.
If you misuderstood this then this maybe the reason why you believe there is an issue with the flat. Since the crown element when tested by itself using double pass autocollimation would always show undercorrected spherical aberration but since it is a lens, all the errors are opposite of that of a mirror. So the picture you posted of the crown that looks like a parabolic mirror is not over correction but actually under correction. So if you believed that the test of crown was showing overcorrection and when you tested the assembled lens by a star test and it showed undercorrection, then that is were the confusion is coming from and misuderstanding that the quality of the flat maybe the cause.

- Dave


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5757293 - 03/26/13 12:19 AM

Quote:

To see if it is or has spherical surfaces. Look for turned edges and spherical aberration.



Jim, I am a little confused. Could you explain the rationale for testing a single biconvex lens by autocollimation?
Mladen


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5757298 - 03/26/13 12:27 AM

I did not realize that the crown could not be looked at alone to determine its aberrations. I have never read anywhere stating that the single lens elements could not be tested this way. One learns something new all the time ! Apart from that, I did also use the flat for both elements combined along with doing star testing. Originally the star testing showed under correction. After using the convexing laps it went to over correction according to the star tests. Now I am working on the crown to get it back gradually towards the proper amount of correction. I do understand the difference between mirror and lens Foucault appearance being essentially opposite and what needs to be done to correct it. Right now the lens is over corrected meaning the center has a shorter focal length than the edge so I am working to remove glass more from the center on the crown. It is gradually going towards the objective of equivalent star images on both sides of focus.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5757746 - 03/26/13 09:18 AM Attachment (23 downloads)

Hi Jim,
Here is an OSLO plot for just the crown element of your lens being tested via Double Pass with perfectly spherical surfaces. The wave front error is close to 250 waves !

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5757756 - 03/26/13 09:27 AM Attachment (22 downloads)

Jim,
Here is a drawing that might make it easier to understand why the zones on a lens are opposite of that of mirror. The drawing shows both a mirror and a lens with a hill on the surface. In the case of the mirror, the light is reflected off the convex surface of the hill and focuses long. In the case of the lens the light is refracted thru the hill and focuses short. You would see what looks like a hole when you tested the lens but it is actually a hill on the surface.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5758124 - 03/26/13 12:33 PM Attachment (16 downloads)

Dave, that is a very good pictorial example. The problem is that even a simple lens has two surfaces and both may have hills or holes. The illustration below shows a hypothetical situation where a hill on one surface and a whole on another results in complete correction!

But assume that the correction is not perfect. Trying to fix one surface will result in overcorreciton or undercorrection, so it's essential to know that all surfaces are spherical in order to know where exactly to work on. However that would require at least two tmeplates.


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Re: 9.25 inch refractor project new [Re: MKV]
      #5758253 - 03/26/13 01:28 PM

Mladen,
I agree that when both surfaces of the lens are unknown it is difficult tell which side or sides has the errors. As has been sugguested a good approach is to test the surfaces using interference against a test plate.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5758524 - 03/26/13 03:55 PM

Great project. Can you tell me how you make the pitch laps?
Do you use curved ceramic tool or curved glass tool as base?
I am wondering if the base can be flat.


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Re: 9.25 inch refractor project new [Re: saemark30]
      #5758948 - 03/26/13 07:35 PM

So..... a simple lens will not be able to be tested via Foucault test even using a single monochromatic wavelength of light .... because..... why is that ?

Mladen, the test plates are of course one way to see which surfaces are spherical ... but there must be others as well. It can not be that there is only one way.

From what I have read in ATM books, an objective will work just fine if a defect on one surface is compensated by a correcting defect on another surface such as the hill hole scenario Mladen did above.

I have the glass tools to make test plates from if it becomes necessary. For now I am going to keep working towards a better star test and Foucault test by more work on the crown with blending strokes to try and get the center down for a longer focal length there. It is so close now I can taste it !


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5759072 - 03/26/13 08:21 PM

Quote:

So..... a simple lens will not be able to be tested via Foucault test even using a single monochromatic wavelength of light .... because..... why is that ?





Jim,
A singlet, using spherical curves can be designed to have no spherical aberration at only one wavelength but in your case the curves on the crown element were choosen to have spherical aberration so the crown with not null when tested by itself. In an achromat it is the choice of the curves and the index of refractive of the elements working together to both correct for color and spherical aberration at the same time. In reality your objective is only fully corrected for spherical aberration at only one wavelength as well, that being in the green at 546 nm. All the other wavelengths are either slightly over or under corrected. This is called spherochromatism ie spherical aberration as a function of wavelength.
The new book by Smith, Ceragiolli and Berry "Telescope, Eyepieces and Astrographs" has two excellent chapters on achromats and apochromats and it explains how they work and what each element is doing.
I agree that a lens will work well when a defect on one surface is corrected by another, but right now you have 3 convex surface that are unknown. You need to get them close to, if not perfectly spherical to have a chance of correcting the assembled lens. So I agree with Mladen, your best bet is to use test plates to test and figure the convex surfaces. Once they are well figured to spherical surface you can test the assembled lens by double pass autocollimation and tweek a surface (usually R4) until the lens nulls perfectly Now you'll have a very well figured lens that most likely show no spherical aberration when star tested and will give jaw dropping images.
If you believe your close then test the assembled lens by double pass autocollimation. There is no guessing and it is a far more sensitive and much less error prone test then all your others test methods. If your close then the lens should show only faint zones and look nearly spherical.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759081 - 03/26/13 08:24 PM Attachment (34 downloads)

Jim,
Here are couple of images of achromats being tested via double pass autocollimation. It takes me about 5 minutes to set them and I can see exactly what is wrong with them.
Here is a picture of 4" f/15 Jaegers lens. Note the Ronchi bands are bowing and also not the evenly spaced top to bottom. The lens is undercorrected and has some astigmatism.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759085 - 03/26/13 08:25 PM Attachment (29 downloads)

Here is another Jaegers lens, this one is 5" f/5. Not great as well.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759094 - 03/26/13 08:28 PM Attachment (29 downloads)

Here is a "Zeiss" Telementor lens. We're pretty sure someone swapped out the original lens and put this one in it's place. It well undercorrected.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759100 - 03/26/13 08:31 PM Attachment (29 downloads)

Here is a real Zeiss lens. It is of excellent quality, notice how straight the bands are.

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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759114 - 03/26/13 08:34 PM Attachment (30 downloads)

Finally here is Selsi 80mm f/11 objective, also of excellent quality from the nice straight Ronchi bands.

- Dave


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5759344 - 03/26/13 10:47 PM Attachment (21 downloads)

Jim -

I have been following this project because I, too, have built refractors. Testing refractors is different from testing mirrors because the design of a refractor objective corrects chromatic aberration (as well as an achromat can), spherical aberration, and coma. This relies on the interplay between both the elements, the surfaces, the thicknesses, refractive indices, and spacing. For best results, it is not a willy nilly affair fixing a hole in one surface with a hill in another.

For best results, the radii have to be close to the design intent as well as the surface figures spherical. The recommended tolerance for R2 and R3 should be within +/- 1/20th % of their design value. R1 about 1/10th % and R4 fairly loose. Keep in mind there are three aberrations being corrected.

Autocollimation testing ONLY verifies that the spherical aberration is corrected, but the color and coma is in the radii.

for best results, a spherometer is not adequate to validate that the radius is correct. There should be some more precise method. Often used is measuring R3 with a knife edge and carefully measured rod. The results from this method should agree with calculation based on a spherometer by only a few thousandths.

Attached is a picture showing measuring R3 of one of my flint elements with rods.

Dick Parker

Edited by Dick Parker (03/26/13 11:20 PM)


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Re: 9.25 inch refractor project new [Re: Dick Parker]
      #5759356 - 03/26/13 10:54 PM Attachment (17 downloads)

Then if your design has R2 = R3 you can, and should test the two together to establish that the radii match. R3 can be validated for exact radius and figure by the knife edge test that we all know. Interpreting interference patterns can be a bit tricky, but is part of the process of making a refractor. If you are lucky, when R3 and R2 match exactly, and you test them againt each other by interference the interference fringes should be straight and parallel as show in the picture of one of my objectives. If they don't match exactly, you will end up with a bull's eye and rings. As you press the side of the top element in the interference test the bull's eye will move from the center to the side. If R2 is exactly spherical, the bull's eye will stay perfectly round. If it turns into a "D" shape or pear shape or anything but round, then you have zones in the surface.

With this test you can validate that R2 is the correct radius and spherical figure.

Dick Parker


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Re: 9.25 inch refractor project new [Re: Dick Parker]
      #5759370 - 03/26/13 11:05 PM Attachment (19 downloads)

You can get by making a refractor objective with only one other test plate, that being for R1. R1 should test against its test plate with the same result as discussed before. If I could pass on any advice it would be to go make a test plate for R1 being careful to validate that the test plate is the exact radius and a perfect sphere.

With three surfaces out of four validated, you can deduce that any further errors of the lens tested assembled will be in R4 and you can figure that surface.

Then you can assemble the lens and test with autocollimation. Shown is the autocollimation test of my 6 inch f/15.

Again I say, the autocollimation test only validates spherical aberration, not color correction or coma.

One thing I don't see mentioned is that optical glass has horrible thermal properties and you need to wait a long time after you work on a surface before you can test it. A way to minimize the wait is to have a bucket of room temperature water handy and immerse the element in it for about 1/2 hour after working on it before testing. This is big time. Don't overlook it. Ask me how I know.

Dick Parker

Edited by Dick Parker (03/26/13 11:11 PM)


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Re: 9.25 inch refractor project new [Re: Dick Parker]
      #5759387 - 03/26/13 11:19 PM Attachment (16 downloads)

Back to R2 against R3 by interference. Shown is the bull's eye and rings I described when R2 and R3 are not a perfect match.

Dick Parker


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Re: 9.25 inch refractor project new [Re: Dick Parker]
      #5759607 - 03/27/13 05:53 AM Attachment (20 downloads)

Thanks DAVIDG and Dick Parker for your input. I believe this will at least give Jim some idea of what to do if his current attempts to correct the lens by trial and error don't work to his satisfaction.

On the measurement of radii, radius bars are the method of choice to for many optical applications. The picture below shows a setup where precise light source was to be positioned relative to a concave surface of a lens for a knife-edge test. The radius bar used consists of sections that screw into each other to form a rod. the ends have panhaead nylon screws that contact the glass surface and the light source at one point. The length of the rod is then measured either with calipers (I have 24 inch calipers) or on a lathe bed. Even a tape measure can determine the radius to within a mm. The nylon screws serve as fine adjustments for "tweaking" the radius bar length precisely to the desired distance.


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Re: 9.25 inch refractor project new [Re: saemark30]
      #5759631 - 03/27/13 06:42 AM

Short answer, no; not if you don't want to saddle yourself with a lot of work.
M.


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Re: 9.25 inch refractor project new [Re: MKV]
      #5759639 - 03/27/13 06:58 AM

I can add one more thing to Dicks excellent posts. If the radii in the IF are not equal, and show a bullseye pattern,

DO NOT VIEW SUCH A PATTERN THRU THE BACK SIDE OF THE FLINT!!!
A round pattern will most likely show up as being flattened somewhat on the side approaching the bevel, IIRC; giving an indication of a ripped edge. Whenever making an IF test, an effort should be made to look thru a positive power; ideally at the focus of the piece.
********
A lot of the angst can be alleviated, if you make a reasonable testplate that's around 1/2D or a bit more. Such a plate, if ground along with the full size surfaces only takes a few minutes of additional effort. And it doesn't have to be completely polished; only spherical and precisely measured.
My 2 millicents worth,
M.


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759776 - 03/27/13 08:53 AM Attachment (32 downloads)

Quote:

Finally here is Selsi 80mm f/11 objective, also of excellent quality from the nice straight Ronchi bands.




Very telling and indicatuive of the value of autocllimation. Even though these are Ronchi an not IF fringes, the test is a double precision test at the focus, so perfect null even with a 133 lpi Rinchi is smomwhere around 1/8 to 1/10 wave PV if not better.

I would like to add something DAVIDG alerted me to previously, namely that it is very important the lens is properly oriented front to back when installed in a cell. It can inadvertently happen.

This image illustrates the effect of reversing the front to back as opposed to the correct orientation. The reversed orientation showed the lens to be severely undercorrected, but when it was flipped around the lens showed clean straight bands indicating a reasonably well corrected optic (this was a $30 Surplus Shed 80 mm, f/5 air spaced lens)

Mladen

NB The light source was not fully diffused

Edited by MKV (03/27/13 08:55 AM)


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Re: 9.25 inch refractor project new [Re: MKV]
      #5760069 - 03/27/13 11:50 AM Attachment (24 downloads)

Autocollimation can be used to check the color correction of an objective as well. As Dick pointed out the color correction is a function of the radii and thickness of the elements along with the refractive index. In the design for most classic achromats the red and blue wavelengths come to focus at the same or nearly the same point. So to check the color correction you set up the lens in double pass mode and use a narrow band light source to produce either red or blue light and find the exact focal position. Then without moving the knife edge or ronchi screen switch to other wavelength. If the color correction is right, there should be no are very little difference in were the focal plane is located at the other wavelength. I use narrow band interference filters and look thru them to selection the wavelength I want to check the color correction with a white light source. I also have a selection of fairly narrow wavelength LEDs that I can quickly change on my tester.
Here is a graph of the longitudinal spherical aberration of a typical achromat. It's hard to see but the green image is perfectly straight and on the y-axis which is the focal plane. The red and blue images focus behind the green but nearly on top of each other. So again one can take advantage of this and use double pass autocollimation to check this. If a radius is off, you still can get the green image to show a null but the red and blue images will no longer focus at the same position, they will move apart from each other. So as Dick pointed out it is critical to get the radii and thickness as close as possible to the design parameters.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5760203 - 03/27/13 12:56 PM

All of these discussions are very informative so thanks to all of you. I am certainly no professional optician and my knowledge base is from what I have read and experienced over the years. All of these discussions have pointed out some gaps in that for me to which I am most appreciative.

That being said, My crown has one heck of a polish ! Also after star testing it last night I have found that it has now gone ever so slightly under corrected again. I had polished on R1 for 45 minutes with the regular lap and it did the job of removing glass from the center but just a hair too much - and I mean a hair ! I was using Sirius as the test star and I was impressed by the color correction as well as the closeness of the optics to putting out an excellent star test. The images are round as well so good news on the astigmatism front as well. Today I am going to put 10 - 15 minutes on the convexing lap again to try and push it back towards a shorter center focus. I believe this may just do it.

Apart from the star test I used the scope to look at detail on the mountain as before. The sharpness of the image it provided was much better than previously noticed. Even with a burbling air mass I could clearly see details on the small pointed rock I have been using for comparison purposes all along. Furthermore there was a diminution of color noticeable along the edges of objects to where it seemed there wasn't any against a bright blue sky.

One other thing I noticed was the lens cell had shrunk up in diameter to where it was starting to squeeze the crown a bit too tight due to pretty cold temperature at around 10 above. So the calculations I did for differential thermal properties was off by a hair as well and need to be remedied by machining the cell diameter out a bit more.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5760295 - 03/27/13 01:31 PM

Jim,
Can you discribe what your looking for when you do a star test and how you do it ?

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5761047 - 03/27/13 06:56 PM

I find a fairly bright star and look at the image inside focus and outside focus by racking in and out. The images should show identical diffraction patterns on both side of focus with even spaced and same width diffraction rings.

If the diffraction pattern on the inside of focus shows a hole in the center but the outside focus image is full of light with a bright spot in the center then the objective is under corrected / the center has a longer focus than the edge.

If the diffraction pattern shows inside focus, a bright center and flooded with light in the diffraction rings but the outside focus image shows a diffraction pattern of rings surrounding a hole in the center then the objective is over corrected. The over corrected objective has a shorter center focus than edge focus.

My objective now shows a very small hole in the middle of the inside focus but the center with a bright spot outside of focus. It is now ever so slightly under corrected. The rings are all round and symmetrical for good signs of little if any astigmatism.

The diffraction rings should be round and symmetrical to show no astigmatism. They should be of even brightness and width to show no zonal errors.

The in focus star image should be a small bright dot with a couple of diffraction rings the first one brightest and the second quite a bit dimmer on a steady seeing night. My six inch shows perfect diffraction patterns on both sides of focus to a textbook "T". This 9.25 inch is very close to that now.

Edited by jimegger (03/27/13 06:59 PM)


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5762236 - 03/28/13 11:37 AM

Jim,
Sound good. If your not already doing so I would sugguest that you use a green filter and also an eyepiece whose focal length is same or close to the f-ratio of your scope.
Refractor can be difficult to star test in white light because of the chromatic aberration. A great book on the subject is "Testing and Adjusting Telescopic Objectives " by Cooke. It is an old book but the theory is the same today. You can read an on-line copy from here. http://archive.org/details/onadjustmentand00taylgoog

I also highly recommend that you test your fully assembled lens by double pass autocollimation with the objective facing the flat just like it would in the telescope. It will be a cross check of your star test results. If your lens is as close as you think to the proper figure then it should show a smooth figure using double pass and very close to a null. If it doesn't I would recommend investaging both the star test result and the double pass result to find out why they differ. As I have said, the flat would have to be many waves off before it adds any real amount of error to the test results and it is double pass test so it is very sensitive to errors in the objective. I have seen many amateurs believe they are doing the star test correctly but in reality they were not and getting a false impression that their optics were much better then they really were. I personnally know of a number of refractors were the image was bragged about as being excellent but when I tested them I found the objective was backwards in the cell!


Both Dick and I teach optical fabrication and we have won multiple awards for our optics. I'm not bragging, just saying that their is independent data from others to back our test procedures and methods. We were just at the 13th annual Delmarva mirror making class were we helped null figure around 20 mirrors, one as large as 20 f/6 in a few days. Over the years we both have seen optics that the owners stated were "1/10 wave" or better turn out to be no better then 1/4 wave or worse when tested correctly. The more data you have the better the finally results will be.

- Dave


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5762315 - 03/28/13 12:30 PM

Seems like you are grinding and polishing by hand?!

May I ask where you got the glass blanks? I would consider such a project in the future.


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jimegger
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Re: 9.25 inch refractor project new [Re: saemark30]
      #5762366 - 03/28/13 01:04 PM

David, I do have that book "Testing and Adjusting Telescope Objectives" by Cooke. It is excellent !
Right now I do not trust my flat so I am going to have it tested that is why the star tests are being done. When I think I have it in the bag the 12.5 inch reflector will be used for a single pass auto collimation test for a null in the Foucault test and a look with the Ronchi grating as well.

When I did the six inch lens some years back I used the 12.5 inch reflector as a single pass collimator and got excellent results to the point of having a nice null in the Foucault test and the star test confirmed the objective was good. Star images are text book and Mars at 500X shows exquisite detail. That being said , until I know how good the flat is I must rely on what I know works. The images now are really sharp. It clouded over last night so the objective was not able to be star tested after the last polishing run. Tonight may clear up and if so I'll test it again.

I read a neat story about a fellow in "Telescope Making Magazine" some years back when it was still being published about a fellow I believe was in Colorado who was making an 11 inch refractor. He had no flat and would literally test by driving to the side of a mountain where he could look at a light some 3 miles distant. He would do a star / Foucault test after polishing using that light. It took a bit but in the end he got a really good objective. There are many ways to skin a cat !

These blanks were obtained from 2 sources, Newport Glass works in California and Schott Glass works back east. They are F2 flint and BK7 crown. The color correction is really good from what I am experiencing.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5762493 - 03/28/13 02:17 PM

WOW....great read. Don't understand everything but feel like I'm taking a crash course using the Socratic teaching method.

Thanks...


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Gord
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Re: 9.25 inch refractor project new [Re: Mr. Bill]
      #5762523 - 03/28/13 02:45 PM

Agreed, this is an awesome thread, truly showcasing CloudyNights at its best! Learning so much and really appreciate the detailed explanations, images, and commentary.

It's making me want to go a try it!

Clear skies!


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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5762532 - 03/28/13 02:51 PM

Since both internal radii are the same you could oil and tape them together and autocollimate them. This would largely desensitize these internal surfaces. This will introduce overcorrection, however, but you can use a Ross lens to correct this SA.

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Re: 9.25 inch refractor project new [Re: Gord]
      #5762534 - 03/28/13 02:51 PM

Quote:


It's making me want to go a try it!

Clear skies!




Uhhh...I wouldn't go that far. I'm happy to build my stuff with finished optics but it's nice to have some understanding of what's going on.



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MKV
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5762572 - 03/28/13 03:12 PM Attachment (20 downloads)

Jim, you can test your flat by autocllimation using the 12 inch mirror, which you say is a good parabola. If you are absolutely sure the 12 inch is a good parabola then you can test your flat with it, as shown below.

You should be able to observe straight Ronchi bands or a clean Foucualt null with the knife edge. Anything else (i.e. bowed Ronchi bands) will confirm that the flat is unacceptbale.

Double pass autcollimation is a superior test to asingle-pass method you're proposing.


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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5762875 - 03/28/13 06:19 PM Attachment (15 downloads)

Jim,
FYI I ran a quick ZEMAX macro to scan across the green spectrum, optimize BFL at each wavelength increment, and plot P-V, RMS wavefront error, and Strehl ratio. Looks like your best DPAC test LED spectrum would be in the 0.55-0.57um range.
Mike


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jimegger
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Re: 9.25 inch refractor project new [Re: MKV]
      #5763119 - 03/28/13 08:23 PM

The bottom line in all this is to have a scope that does what you want it to do - period. People who are perfectionists can be anal about having everything just perfect when in reality nothing we make ever is, there are just varying degrees of acceptable imperfection the levels of which depend on your own personal taste. For me perfection is having a scope that will show me the belts on Jupiter in good detail as well as the markings on Mars etc. The challenge of crafting my own objective with my own hands to that point is invigorating to the max. To look through a large refractor with unobstructed optics is a thrill in itself as all who have done so will testify. All of you have been a great deal of help through all of this and I thank you all from the bottom of my heart for it. My advise to anyone who wants to give it a try is - go for it ! At the very least you will learn an awful lot and with all this good help failure would be unlikely. When I did my 6 inch lens about 16 years ago I was pretty hesitant because I thought it was harder than it actually is. It is more work because there are 4 surfaces and you cannot directly test them all but there are ways to get it done.

I am right there at the finish line with this objective for what I want with it. There will be more coming as I have 4 more sets of blanks. That will be down the road some since this one still needs a mount etc.

Not having been able to star test it since the last polishing session makes me uncertain as to whether it is there yet but I can say this, Looking at the mountain today the image is every bit as sharp as the 6 inch with even more details visible in the 9.25 inch probably due to its greater resolving power. For my experience , the color correction is excellent as well ! Kudos to you Mike for a great design !!!


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5763188 - 03/28/13 09:03 PM

Jim,
I'm glad your happy with your results but those that make optics both amateurs and professionals and use Double Pass understand that this is one of the best ways to test with the least amount of errors.
You referenced an article about the construction of a 11" refractor, were the author tested his lens using distant a street light. I believe the article your referring to is in the March 1978 Sky and Tel "A California Amateur's 11 inch Refractor". Here is a quote from that article about the author testing his lens, "A large optical flat was unavailable for autocollimation so for a source of nearly parallel light I settled on a distant streetlight". So clearly the author would have preferred to use the much better test of autocollimation but settled for one that wasn't as good. He goes on to say that his lens turned out well but doesn't perform at the Dawes limit.
So I believe that what all of us are saying is that if one is lucky enough to have a much better test available one should take advantage of it and not dismiss it just because one is more comfortable with other test means. The out come could be a much better performing lens for a lot less work and a new found understanding in optical testing.

All the Best,
- Dave


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Dick Parker
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5763363 - 03/28/13 10:58 PM

And, Jim, We wish for your wonderful project to be the best it can be, that's all. Good luck and thank you for sharing your project with us. We chimed in because we love advanced optical projects and the old long focus achromats. Please keep us posted on your progress and we sure would like to see a photo or two of your completed system.

cheers
Dick Parker


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Re: 9.25 inch refractor project new [Re: Dick Parker]
      #5763535 - 03/29/13 01:50 AM

I certainly did not have any intonations that double pass auto collimation , laser interferometry or any other methods of lens testing should be in any way shape or form discounted or dismissed. I absolutely understand how double pass auto collimation is better than single pass or star testing or any other form of testing. If I knew my flat was good I would be using it now for the DPAC. I am sending it off to get it checked. In the meantime the alternative methods of old such as single pass testing with the 12.5 inch mirror plus star testing will have to suffice. The six inch lens made this way on a night of good seeing easily reaches Dawes limit and was tested this way so I know it can be done with this lens. I'm not done yet and will keep posting. There is more to do such as yet getting this objective right on the money and painting the tube and fitting it up on a mount etc. I'll be posting pics as time goes by.As for the mount , I run Aeroquest Machining so will be making my own drives and the whole mount assembly probably over summer.
When this objective is done it will be giving results at Dawes limit (or it won't be done). I sure hope you guys with all the testing advise didn't think I was knocking any of it because I certainly didn't mean it that way. What I was getting at was some people need 1/20th wave to be happy when 1/8 wave would be more than sufficient. 1/20th wave would certainly be better than 1/8th wave but hardly noticeable is what I was getting at.

I love the views through these doublet refractors and it has been a fun time so far building this one. When I can I'll take some pictures through it and post them. I did buy a Baader IR-UV filter that might help in some shooting with the DSLR once I get the scope mounted.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5763545 - 03/29/13 02:10 AM

Jim,
First, thanks for the compliment, you are most welcome!

Second, and just a thought: you could try DPAC-testing your lens with your 12.5" scope if you put a small high-precision flat mirror (like a Newt diagonal) at the focus of your lens and ran the light backwards through the lens and 12.5" scope. Worth a try anyway.

Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5763642 - 03/29/13 04:53 AM Attachment (38 downloads)

Why do you need a diagonal mirror? Could you draw a picture of that setup? Is this what you are proposing:

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Re: 9.25 inch refractor project new [Re: MKV]
      #5764139 - 03/29/13 10:27 AM

I will, but read what I wrote again and try thinking it through.
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5764570 - 03/29/13 12:41 PM

What I do Mike when I use the 12.5 inch mirror is to put the pinhole at the focus of the 12.5 inch mirror turning the mirror into a searchlight of parallel light. The way I determine the pinhole is at exact focus is to use a telescope focused at infinity to observe the pinhole image until it is sharp by looking into the mirror and even doing a Foucault test if necessary. After seeing that the pinhole is dead on focus I put the objective I want to test in the parallel light path and do both a visual and Foucault test on the objective. It is not any different than looking at parallel light from a star except that the star in this case is a pinhole and has some angular size.

For testing of the six inch lens it worked excellent as the results show in its use on the stars. It is truly diffraction limited. I also noticed that using a very high power eyepiece in the telescope when set up this way gave a microscope type view of the pinhole and you could evaluate the qualitative value of the pinhole as well. I was surprised how sensitive this was and how little polishing action could dramatically change the quality. I actually made a knife edge tool that inserts into the focuser of the telescope to do the Foucault test. Once I did that I'd take the knife edge tool out and put in the eyepiece and examine the image at best focus for the quality.

Being a single pass test it did not show the sensitivity of the DPAC but it is entirely sufficient for getting an objective to work at its theoretical limit. For those people who have a larger Newtonian mirror of excellent quality than the objective they want to test it is an effective substitute for using an optical flat they probably do not have handy.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5764684 - 03/29/13 01:25 PM Attachment (31 downloads)

This is the test I was proposing that uses your 12.5" and a small return flat to give a DPAC test. The flat can be a small Newtonian diagonal, placed at the focus of your refractor.

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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5764690 - 03/29/13 01:27 PM Attachment (21 downloads)

and this is the DPAC test result for either interferometry or Foucault testing at 0.578m. Pretty much perfect double-pass test, assuming your Newtonian primary has the figure smoothness needed for DPAC testing.
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5765278 - 03/29/13 04:58 PM

Mike,

Would it be possible to replace the Newt secondary with the small flat and perform the DPAC test on the refractor just as if the newtonian primary and small flat were working as a flat? There would be the same limitations on the quality of the mirror(s) and it would require a modified setup versus a newtonian on it's own.

One benefit I could see to this alternate approach (if it would work...) is that you could use a smaller secondary (flat) for less obstruction. For example, there are high quality round flats available from Surplus Shed that are only about 1" diameter and 1/20th wave.

Thanks,


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5765294 - 03/29/13 05:02 PM

Now I see how you set it up Mike, I was under then impression that you were somehow using only 1 small flat but in fact you are utilizing the Newtonian's normal flat as well. It makes sense.

When I did the single pass testing the Newtonian's diagonal was removed and a small led light with a pinhole was placed out in front of the mirror at its focus. It was supported on a thin blade so the whole assembly does not take up much central area making a much smaller silhouette there.

The setup for DPAC using the mirror is pretty slick Mike but a bit more involved than doing a single pass test.


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5765674 - 03/29/13 08:21 PM

Gord, yes, you could reverse the test by putting an even smaller flat, like 1/2" diameter, at the Newt focal point. The slitless KE tester would be located at the lens focal point, using a green LED for illumination. Again, the test is only as good as the Newt primary.

The angle of the flat is critical; if it is tilted, the expanding cone of light reflected from it is tilted twice as much. This causes vignetting of the lens pupil. Fortunately, it's not hard to get good alignment. The flat surface needs to be clean, as any dust on it is sharply imaged at the DPAC focal plane.

DPAC with a truly smooth, flat mirror is clearly THE best method. But in Jim's case, the figure quality of his return flat is highly suspect, and the Newt collimator test setup is his cheapest and quickest fallback method.

Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5765692 - 03/29/13 08:32 PM

Mike I sold Jim a surplushed flat that was to be 1/10wave big enough to test it . But I do not know if it is good enough . Jim if the telescope works good and you are happy then that is the best test .Congrads . What is next a 12inch??

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5770632 - 04/01/13 11:54 AM

Jim, do you recall the size of the BK7 and F2 blanks you started with in your great 9.25" refractor including the thickness for ordering? (In case someone wants to duplicate this project).

Edited by saemark30 (04/01/13 11:54 AM)


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Re: 9.25 inch refractor project new [Re: saemark30]
      #5772948 - 04/02/13 01:20 PM

The blanks started out at 1.1 inches thick for both the flint and crown. I had them made to the maximum diameter they had in stock at 9.25 inches. They were ground flat on both sides as well. I remember the crown being around $275.00 and the flint about $450.00. I got one set from Schott and one from Newport glass works. I agree with Mark Harry in getting the curves generated beforehand , it will save glass thickness and a lot of time. Newport Glass actually has 6 and 8 inch sets they sell with curves generated and all the tooling ready for fine grinding. The last time I looked they had an oiled set and an air spaced set with different glass types for the lens materials and for different f values. They list BK7 and F4 glass types with f/10, f/14.1 and f/15 kits in the astronomy glass section.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5775613 - 04/03/13 01:42 PM Attachment (47 downloads)

After some more figuring on R1 using the convexing lap to bring the center up, the star images are looking nearly perfect and the inside focus and outside focus diffraction patterns are very close in having identical appearance. I picked a bright star and looked at it with about 70X magnification. The colors look excellent. I do not see a turned edge at all. The only thing I did notice was the star image is slightly out of alignment (collimation) but not bad enough to get a good feel for the lens performance.

Then I looked at Jupiter just as one of its moons was coming into view from either in front of or behind it. It was obvious in the refractor appearing as a small dot looking very star like. The air was fairly steady so seeing was good. I did notice a ghosting of Jupiter due it seems to the internal reflections of the lens and being off collimation it was in the same direction as the star images when out of focus but it would go away when I slightly de-centered my eye at the eyepiece. The image of Jupiter itself was sharp with clearly defined bands that had excellent contrast. I put my 12.5 inch reflector on Jupiter at the same time for comparison of image quality and could see little difference other than the ghosting. This all leads me to believe that it is on the verge of being finished. Today I did one more 15 minute figuring session with the convexing lap on R1 since that amount of time seems to take the objective in baby steps towards ideal diffraction images. It was as described above very very slightly under corrected.

Here is a shot with my Canon 20 da through it at the mountain. I did my best to focus but with my old man eyes it is hard to get it right on. At any rate the color looks excellent and with the eyepiece in it everything looks razor sharp ! Here's the pic.

The point at the very top had a Magpie sitting on it one time and it was about one fourth the size of that pointy rock. This is 5 plus miles distant for an idea of scale.

Edited by jimegger (04/03/13 02:09 PM)


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5775956 - 04/03/13 04:42 PM

Not too bad- shoot the 'crow' off that rock!
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5775976 - 04/03/13 04:50 PM

Looks like Mikeys numbers were good . Congrats Jim Great thread .

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5779700 - 04/05/13 02:04 PM

I found the spacer I had machined into the lens cell was not of uniform thickness by putting a dial gauge on it while chucked up in the lathe. Total variation was .014 inches from one side to the other so I machined off the difference to make the R2 and R3 surfaces parallel to each other within .001 inch. Last night I did more star testing after machining the cell and after doing 10 minutes of corrective polishing on the crown. Results were , the ghosting disappeared and the colors look even better. The inside and outside focus star images are even closer in looking alike but still not exactly the same. There is still remaining a tiny bit of under correction. The objective was able to quite easily split Caster in spite of moderate seeing conditions. Today it gets another round of corrective polishing action and more star testing tonight if it stays clear.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5779712 - 04/05/13 02:12 PM

Good on you for getting the airspace wedge down to 0.001", that is critical. You are getting so close that you might also try marking the sides of the lenses and rotating the crown with respect to the flint to minimize the effects of any residual wedge in the elements themselves.
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5780850 - 04/05/13 11:07 PM

How much does the objective weigh?

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Re: 9.25 inch refractor project new [Re: Old Will]
      #5780942 - 04/06/13 12:14 AM Attachment (33 downloads)

I'll have to weigh that yet - as of now I am not sure how much it weighs.

After doing a 10 minute correction session this morning and assembling the objective back into its cell, back on the scope, I took this picture through it. I hand held my Canon camera up to the eyepiece looking at the mountain after the air had settled down enough to give decent images without all the boiling air. This is what I got at around 53X magnification. This is the sharpest the mountain has looked yet when using my eyes at the eyepiece. The glinting of the sun off the snow gave tiny sharp star-like glows. It was a type of star test and if it is any indication of the objectives state I would say it is right where it should be. Anyway, here is the pic.....


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5780970 - 04/06/13 12:34 AM Attachment (24 downloads)

The effect of having a 0.014" wedge across your airspacer ring diameter is pretty bad, as shown in the plots. The airspace wedge angle is ATAN(0.014/9.25) = 0.08672. The image should have shown at least some perceptible coma, which I think it had and you saw, but you mis-interpreted as collimation error. Good that you got it machined down to 0.001" or so, for obvious reasons.
Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5780974 - 04/06/13 12:42 AM Attachment (55 downloads)

Here's the OSLO file that includes the wedged airspace, in case you wanted to doodle with it.

Jim, that's one fine lookin' refractor image. Let's see some Moon shots as soon as you are able to!

It was an honor to work with you on this project. You're an A+ optical worker for sure.

Mike


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5780977 - 04/06/13 12:44 AM Attachment (69 downloads)

I did see that Mike in the star images during star testing.

This is your design Mike, come to fruition. As soon as the moon comes into view again I'll get some shots of it, maybe even Jupiter. After a week of sunny clear weather we have gone into the usual Alaskan cloudy stuff with snow on the way. We only have until April 24rth at the latest for night skies then it is just too light except for the moon and bright planets. By this coming fall I should have the scope mounted up and ready for some real use as well as imaging.

Here are a few of the scopes I have available to me along with the 127 APO, a six inch refractor I made and the 9.25 inch refractor. The blue scope is my 12.5 inch reflector which has awesome optics and is equipped with a 5 inch Surplus Shed refractor, a 90 mm refractor and a 50 mm finder. It takes excellent images and is easy to set up for imaging. I am really spoiled here ! The 26 inch reflector is in the background and also has excellent optics which produce great images when the atmosphere allows it. I made all of the 26 inch components except for the diagonal mirror as well as all of the 12.5 inch components except for the lenses and the diagonal. There is something about working with glass that I just really enjoy.

Edited by jimegger (04/06/13 02:39 AM)


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5781180 - 04/06/13 06:17 AM

Jim, what a difference in the last 2 mountain pics!!!
I'm impressed!!! Nice work.
(quite a stable of scopes there, too.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5781305 - 04/06/13 08:49 AM

Man, lookit them polar axis angles almost pointin' straight up! You could almost just put a rock under the south side of a Dobson and be equatorial!
Mike


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Old Will
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Re: 9.25 inch refractor project new [Re: Old Will]
      #5782439 - 04/06/13 07:16 PM

I weighed my 9,5" achromat blank set ,, it is 12.6 lbs
The set is SK18A & SF5 11/16" thick. When I make it I am thinking of f/20 = 180". I am going to tri-fold it down to a 60" OTA. First though I am going to use my other achromat sets, a 6" & an 8" to gain experience. I'm also wanting to avoid excessive moment arm torque,, cheers. Will


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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5782446 - 04/06/13 07:19 PM

Mike can you give me the radii multipliers for 4" achromat using SK51 & DF2 ,, is DF2 different than F2? I have three sets, they were gifts

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Re: 9.25 inch refractor project new [Re: Old Will]
      #5787286 - 04/09/13 02:25 AM

I weighed the objective in its cell and it came to 15 lbs.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5787805 - 04/09/13 10:53 AM

Dude, you deserve some kind of an award!

Incredible!


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Re: 9.25 inch refractor project new [Re: herrointment]
      #5789658 - 04/10/13 06:42 AM

Jim, how long has this project taken?
2 years?
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5790725 - 04/10/13 05:02 PM

Mark, it took a year and a half. A lot of that was time off from it though and I had a couple of glitches with chipping blanks etc. Actual labor time in the glass itself is about 90 hours. It was well worth it !

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5790907 - 04/10/13 05:51 PM

You mean, so far, or are you DONE??!!

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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5791484 - 04/10/13 10:36 PM

Mike , I may be done after this next test but it is so close that it is not going to be much longer. It is giving nearly identical star test images inside and outside focus. Star images are very tight to where it easily splits Castor and has improved even more with the last test. I am still awaiting clear skies to evaluate the last 15 minute correction session.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5791818 - 04/11/13 07:01 AM

Quote:

I may be done after this next test but it is so close that it is not going to be much longer. It is giving nearly identical star test images inside and outside focus. Star images are very tight to where it easily splits Castor and has improved even more with the last test. I am still awaiting clear skies to evaluate the last 15 minute correction session.



Jim, Castor is a whopping 5 arc seconds apart these days -- hardly a "test" for a 9.25 inch aperture. If you're testing the theoretical limit of your 9.25 inch objective, then the distance ten times smaller (i.e. 0.5 arc seconds) may present a real challenege. Find a 0.5 arc second double in the Almanac and, on a good night, see how close the scope comes to its theoretical limit.

One thins is certain: your objective should be able to cleanly separate 1 arc second doubles, i.e. stars five times closer than Castor on any average night.

Even way back in the 1960s and 1970s, Castor was a tighter pair, separated by 1.8 arc seconds, which was a good test for a 3-inch objective, but certainly never for a 9.25
inch lens!

Reagrds,
Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5791865 - 04/11/13 07:51 AM

http://www.windows2universe.org/our_solar_system/moons_table.html Jim you might be able to see some of Jupiter other moons . Congrats

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5791937 - 04/11/13 08:54 AM

Finding atmospheric conditions where .5 arc second rez is possible, is ludicrous where there's a lot of weather related changes- my 2 cents.
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5792238 - 04/11/13 12:14 PM

When I mentioned Castor as being EASILY separated with this objective , it was done with not the best seeing. It was done with the objective still in the process of being figured which it still is but indicates it is going in the right direction. Once I get to where the star is a point with 2 or so diffraction rings around that point like in my six inch will I consider it done. It is still inching towards that moment - more work to be done ! The weather up here is the clincher as it generally is bad seeing.

Edited by jimegger (04/11/13 12:16 PM)


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5792241 - 04/11/13 12:15 PM

Did you start with pregenerated curves on the blanks Jim?
I gather you did all the grinding and figuring by hand.
If so, you deserve an extra round of applause.


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Re: 9.25 inch refractor project new [Re: saemark30]
      #5792253 - 04/11/13 12:19 PM

I tried generating curves myself with a diamond tool but chipped the blank (crown) so I ended up doing it all by hand. I would certainly recommend getting the supplier to generate curves as it will save a ton of time grinding. If you are using glass tooling then you need them made with matching curves as well of course.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5803838 - 04/17/13 01:27 AM Attachment (36 downloads)

Okay Mike, here's your lunar limb shot taken with a hand held Canon digital camera through a Meade 50 mm wide angle 2 inch eyepiece when the sky is still somewhat light in the evening.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5804012 - 04/17/13 07:07 AM

I'd say that's pretty darn good!!! Moon's so bright there should be a bit of color anyhow. Doesn't look objectionable, and usually a small camera's shot misses a lot of what can actually be seen.
(My 2 cents)
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5804277 - 04/17/13 10:27 AM

Nice job Jim. Even my large objective shows a scosh of yellowish green along the limb....plus a tad of purple but that is only around 410 and above. Mike

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Re: 9.25 inch refractor project new [Re: mikey cee]
      #5804632 - 04/17/13 02:12 PM

If i move my eye around the eyepiece the color on the limb goes from purple to greenish but is not hugely so on either account. I have noticed the eyepiece itself has some small color issues when used on my reflectors so that may account for some of the color as well. This objective is still a work in progress as the figuring is progressing toward the goal of diffraction limited performance. It is very close now. After doing star testing and getting it in the ball park I used the first surface mirror I was suspicious of to check the objective and it seems to be okay since it gave the same basic shape on the objective as the star tests showed. Now I am going for the DPAC straight Ronchi lines and Foucault cutoff. It is very close according to that test. Visually the moon was showing very nice detail and the stars at 200 X were tight diffraction patterns but too many bright rings yet and the bad seeing wasn't helping. Baby steps..... baby steps...

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mikey cee
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5804706 - 04/17/13 02:49 PM

Jim you've done a bang up job there. I still can't get my head around the fact you can turn clear blanks into frosted glass and back into crystal clear and highly polished "paper weights"! That's exactly what I would call them after I got done that's for damn sure! Still can't believe my eyes! Super job! Mike

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saemark30
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Re: 9.25 inch refractor project new [Re: mikey cee]
      #5804735 - 04/17/13 03:19 PM

Could you take an image of Polaris (as it doesn't move much) showing the bright rings?

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Re: 9.25 inch refractor project new [Re: saemark30]
      #5804807 - 04/17/13 03:53 PM

Jim, that looks pretty good, but it's hard to tell with all the extra optics (eyepiece, camera lens) in the path, and the relatively low image contrast. Do you have a DSLR that you could just plug directly in and take photos, directly onto the focal plane? That will be the true telling of performance. Also, the Polaris idea is good.
Mike


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jimegger
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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5805466 - 04/17/13 08:59 PM

I do have a Canon 20 da so I'll give it a shot when I can.

Apart from that I went back to the flat I have after getting the star tests to where the images were pretty good and comparing the star test data to what I was getting from the flat. It seemed to be reasonably in the ball park so I decided to call the flat good and trust it for the DPAC test. I have been looking at the Ronchi lines and it shows what the star test indicates which is under correction with a good edge. I would say (not having a wire) that the under correction is about 3/4 wave, that is the Ronchi lines bend to 3/4 ths of their width in the DPAC. They are curved smoothly so that is good news. Now I am trying to work on R2 and try getting the straight Ronchi lines. I am assuming the DPAC test with a 3/4 width bend is 3/4 wave or is it half that ? I'm not sure how that is read as such.
This is the finesse part of figuring now.


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MKV
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5806258 - 04/18/13 08:37 AM

Quote:

I am assuming the DPAC test with a 3/4 width bend is 3/4 wave or is it half that ?



Jim, the Ronchi test is not quantitative. In other words, you can't judge the wavefront error by the width of the bands and the amount of their curvature. Using a 100-133 lpi screen, you can be reaosnably sure that the optics are good quality if in a double-pass autocllimation test the bands are smooth and straight to the edge.

Another way of judging good optics is by how much its image can be enlarged and still remain sharp and reveal more detail. One such striking example is this Nikon image of a library. If you click on the image of the far windows, you will see intricate detail of the window's smaller componenets, as well as the mosaic decoration on the walls.

The image was taken with a 15 mm fl, lens at f/8.

Nikon hi res image of library


Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5807596 - 04/18/13 08:10 PM

You all are going to have a good laugh at what I had been doing wrong here in the DPAC testing. I had the objective turned the wrong way for the test. Instead of having the objectives front element facing the mirror I had it facing the light source ! In going back to my books I found out that fact and it accounted for the hard time I have been having in figuring the lens to perfection. It was showing the objective to be undercorrected with it turned toward the light source but when installed the proper orientation , it showed over correction. So as a result I went back to work on it with my regular laps and now have it finished to near perfection with straight Ronchi lines on both sides of focus and a nice sharp Foucault cutoff. It is finally finished !!! The only thing that I notice on the figure is a very very very slight small hole that is very faint in the Foucault test right in the center of the objective. It dies not affect the Foucault cutoff and is probably on the concave flint element which had a slight hill in the middle that I had to work on to get rid of during the direct testing of the concave surface. It will not amount to anything when in use from what I can see. One comment I want to make about all the books I have read on testing refractor objectives and that is that none of them I read made it very clear as to which way the Objective faces during DPAC testing or if it really made a difference .... well it sure does !!!! I finally saw a picture of an objective set up for DPAC in Ingall's book with the objective crown facing the mirror which makes sense since that is the way the parallel beam from the stars enters it in the first place. Any way, now the lens is ready for putting under the stars for some serious star testing to see if all is well that way.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5807639 - 04/18/13 08:31 PM

BRAVO Jim! Well done! Your viewing should be nothing less than spectacular!
Mike


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Dave O
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5807758 - 04/18/13 09:24 PM

Quote:

It is finally finished !!!




Wow. It has been a long road, with many lessons. Congratulations! I've enjoyed this thread immensely, thanks for sharing.


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MKV
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5807768 - 04/18/13 09:34 PM

Quote:

One comment I want to make about all the books I have read on testing refractor objectives and that is that none of them I read made it very clear as to which way the Objective faces during DPAC testing or if it really made a difference ....well it sure does !!!!



Hmmm, Eureka Jim! This was brought up just three weeks ago on this forum. Apparently it went by unnoticed. Maybe the pictures should have been bigger...

take a look



And, btw, when you turn it around you don't get an overcorrection...

Mladen


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Re: 9.25 inch refractor project new [Re: MKV]
      #5807811 - 04/18/13 10:14 PM

What I got before turning the objective around showed under correction, when I turned it around facing the proper way was some over correction which took a small amount of time to correct. It has been a long road on this project with a lot of mistakes which means I learned an awful lot along the way. It is like Mark Twain once said that a kid who carries a cat home by the tail will learn 100 times what a kid who doesn't carry one home by the tail will.

I will like to say that I have received a lot of very useful help along the way which has made this journey a very interesting one and I truly feel humbled by all of it. There is nothing like learning by doing and I have to say to anyone who wants to undertake a project like this is - go for it. I have truly seen that when you know what you are doing it is not hard at all - just a bit of elbow grease and if there is something you do not know - there is someone out there who has the knowledge to steer you down the right path and are willing to help. To be honest , I would go bigger than 9.25 inches as it wouldn't be any harder than what I did , just cost a bit more. If I could have gotten a 12 inch lens blank set without special castings I would have. This objective appears to give awesome images even before it was completely done and I see the reasons people have for using and making large refractors. I have 4 more sets of glass to make into lenses/ objectives of 9.25 inches so after a bit of a break here, maybe this winter will see some more grinding action with new knowledge of what NOT to do !


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Re: 9.25 inch refractor project new [Re: jimegger]
      #5808076 - 04/19/13 02:34 AM Attachment (34 downloads)

Here are a couple of shots of the moon taken tonight at prime focus of the refractor at 107 nominal inches focal length with the Canon 20 da. I focused with my eye with some burbling atmosphere but not too bad at ISO 200 1/125th of a second shots.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5808077 - 04/19/13 02:35 AM Attachment (29 downloads)

More shots.

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jimegger
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5808079 - 04/19/13 02:36 AM Attachment (38 downloads)

Still more!

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Mark Harry
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5808267 - 04/19/13 08:03 AM

Those are best yet. I don't see much of anything for color fringing.
****
I think of an AC flat, as a chunk of artificial sky; so the scope faces it just as you would use it when viewing; whether Newt or refractor, etc.
And congratulations!
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5808377 - 04/19/13 09:40 AM

Jim,
Congratulations. The image look very much improved and I'm sure that you now see the power of DPAC.
If you looking for next challenge and want a perfectly achromatic refractor try a Schupmann refractor which is perfectly color corrected but only made from two pieces of cheap BK-7. I bet you'll find it easier and cheaper to make then an achromat and with much better optical performance.

- Dave


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jimegger
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5808616 - 04/19/13 12:53 PM

In looking at the moon visually the mountains around Mare Imbrium were spectacularly crystal clear during the fleeting moments of good seeing. The detail visible is impressive. Jupiter had good detail visible at times as well with the expected bit of violet haze around it but not anything that one would call excessive by any means for an achromat. The details on Jupiter were sharp during moments of atmospheric quiescence as were the disks of its moons but it is so low in the western sky now that a good test of the objectives abilities on it are not probable. I got up at 3 AM and picked out a star in Leo - Delta Leo and put the scope on that. The air was still unsettled but it formed a nice tight diffraction pattern jostling around with a small bright core like a BB bouncing around inside. I couldn't have expected much better under the conditions. One would have to conclude that the objective appears to be at its diffraction limited performance goal. I celebrated all this with a couple of glasses of wine - it is like a relief valve had released all the doubts about ever being able to get to this point. I think anyone who has done this sort of project knows exactly what I am talking about.

Again, a big thanks to a;ll you who helped me along in this project !

Edited by jimegger (04/19/13 12:54 PM)


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niteskystargazer
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5808675 - 04/19/13 01:22 PM

Jim,

Congratulations.

CS,KLU,

,

Tom


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Jeff B
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5810234 - 04/20/13 08:22 AM

Jim, thank you so much for sharing this thread with us. It covers a lot of ATM ground, especially testing. I've REALLY enjoyed it.

So what mount will you be using?

Jeff


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Re: 9.25 inch refractor project new [Re: Jeff B]
      #5811045 - 04/20/13 02:31 PM Attachment (63 downloads)

Some more comparisons of moon shots. First the 9.25 inch refractor.

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jimegger
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5811048 - 04/20/13 02:32 PM Attachment (47 downloads)

Now the 12.5 inch reflector shot.

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Mark Harry
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5811350 - 04/20/13 04:28 PM

Not bad!
M.
The refractor image looks just a touch soft. Conditions?
Does the contrast help?
M.


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jimegger
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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5811484 - 04/20/13 06:17 PM

Conditions were good for Alaska which means bad for your neck of the woods . It was the usual high frequency vibrations. The reflector has a focal length of 66 inches vs. 106.5 for the refractor. Look at the edge of the moon in the refractor image and how warbled it is. I am not sure if my focus was dead on in either shots to boot , visually the refractor images looked every bit as crisp if not more so than the reflector during those fleeting moments of good seeing. Our summer is coming fast and the lighting conditions at night are rapidly approaching the point where the sun will wash out the night sky. In a week it will be done for any type of serious imaging for certain.
Next step is to build a mount to hold this beast !


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mikey cee
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5811950 - 04/20/13 10:14 PM

Looks like you captured an asteroid transit on the right limb of the moon! Mike

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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5812004 - 04/20/13 10:43 PM Attachment (45 downloads)

Jim,
There is way more detail in your pictures than you're showing. All I did to your picture in post 5808077 above was boost the contrast and edge sharpen. Sharpening does nothing to increase the resolution of a picture, other than enhance contrast to detail that's already there. It doesn't create detail where there is none. I would have posted the whole picture but it was nearly 500Kb and CN wouldn't accept it. But I can email it to you if you like.

Hope you get many more nights this good and better!
Mike


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jimegger
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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5812082 - 04/20/13 11:41 PM

That looks really excellent Mike ! Send it on !

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Mark Harry
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5812448 - 04/21/13 08:09 AM

-POW!-
amazing!
M.


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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5812805 - 04/21/13 11:15 AM

Jim,
Have you thought of going back to your 6" refractor and DPAC testing it as well? It may have some slight figure errors you could remove as well, especially while your skills are at peak coming off the 9".
Mike


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kfrederick
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Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5812913 - 04/21/13 11:58 AM

Mike Congrats on a great design . Jim this post was great . Great ATM project .

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Re: 9.25 inch refractor project new [Re: kfrederick]
      #5813284 - 04/21/13 02:59 PM Attachment (25 downloads)

Fantastic end to a great project Jim !

I've pulled some sliders in Iris astro processing software on your image from above and looks really good for a single frame. As Mike said al I've done was to bring in front the details that were already there. Fantastic!


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Re: 9.25 inch refractor project new [Re: neo]
      #5813324 - 04/21/13 03:14 PM Attachment (20 downloads)

..and one more

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Re: 9.25 inch refractor project new [Re: neo]
      #5813511 - 04/21/13 04:48 PM Attachment (29 downloads)

Since the scope has been studying the clarity of moon shots I decided to try putting a focal reduction lens on the camera for possible use in future astro imaging and here is what I got with some image processing.

From the image size it appears this would make the f/11.8 objective about equivalent to an f/6 or slightly less.

Edited by jimegger (04/21/13 04:53 PM)


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saemark30
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5815466 - 04/22/13 02:44 PM

Looks great!
How does the 9.25" refractor compare with your 12.5" Newtonian and did you make the mirror also?
What did you use for a focal reducer, it's even sharper looking.


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Re: 9.25 inch refractor project new [Re: saemark30]
      #5815670 - 04/22/13 04:40 PM

I think using the moon as a measure of optical quality is not only theoretically dishonest, but is ultimately unfair to Jim's refractor! A casual search on the Internet of similar shots made by 8 to 10 inch reflectors immediately makes that obvious, as even commercial reflectors of that size make "darn good" moon shots!

such as this one!


Mladen


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Mark Harry
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Re: 9.25 inch refractor project new [Re: MKV]
      #5816126 - 04/22/13 07:03 PM

Yes, but if you had stick time with real scopes, and were that proficient, you might be able to tell--- the precise degradation from color-smear.
My opinion, totally, of course. But what do I know?
I have generally 3 competing airmasses colliding over my region-
But you have the advantage of Coral Gables, FL and suggest half/second conditions which simply don't exist where either Jim, or I observe from. Do you know how to judge high contrast targets and make a reasonable error evaluation of a refractor??? ( thought as much)
M.


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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5816137 - 04/22/13 07:06 PM

I can get the example's resolution you posted with a 70mm refractor.
FYI,
M.


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Re: 9.25 inch refractor project new [Re: MKV]
      #5817129 - 04/23/13 10:20 AM

The link I provided simply shows that prime focus moon shots are not really a good comparative test of optical quality.

Standard test of optical quality available to amateurs are Ddouble-pass autocllimation (knife-edge or Ronchi, showing 2 or 3 bands max) test and/or interfergrams.

Edited by MKV (04/23/13 10:24 AM)


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jimegger
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Re: 9.25 inch refractor project new [Re: saemark30]
      #5818954 - 04/23/13 11:53 PM

That focal reducer used is one from Surplus Shed made by Wollensak as a 2 inch focal reducer for astrophotography. It uses a lot of back focal length , I am guesstimating about 6 inches on the 9.25 inch scope.

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Re: 9.25 inch refractor project new [Re: jimegger]
      #5819162 - 04/24/13 05:48 AM Attachment (21 downloads)

I don't suppose many on here run Linux but there is a very easy to use photo processing package available for it called Fotoxx. It's mainly aimed at normal photography and designed to do 99% of what people are likely to want to do quickly.

Taking one of the moon shots. I tone mapped it - contrast level sensitive enhancement, very slight unsharp mask as reduced jpg's always benefit from a bit, brightness expand, and lastly a slight even brightness increase. Total time couple of mins. It's free too. Most packages that will do this sort of thing use layers. Fotoxx does too but creates and destroys them automatically. Any of it's enhancements can be applied locally with a mouse too. I hardly ever use Gimp since finding it.

John
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5858504 - 05/13/13 08:36 PM

Hey Jim. Saw you on TV yesterday on Assignment Alaska. Awesome!

Assignment Alaska Butte Telescope Builder


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mikey cee
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Re: 9.25 inch refractor project new [Re: Tak North]
      #5858794 - 05/13/13 10:45 PM

Where???? I don't see any link there at all!

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Re: 9.25 inch refractor project new [Re: mikey cee]
      #5858843 - 05/13/13 11:03 PM

Go back about 3 sets of videos and you will find it there.
Jim, very cool, actually looks very cold!


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