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9.25 inch refractor project

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#1 jimegger

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Posted 16 January 2013 - 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.

#2 Sean Cunneen

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Posted 16 January 2013 - 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!

#3 dan_h

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Posted 16 January 2013 - 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.

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#4 jimegger

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Posted 17 January 2013 - 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.

#5 kfrederick

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Posted 17 January 2013 - 12:45 PM

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

#6 KenScharf

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Posted 17 January 2013 - 07:24 PM

May Alvin Clark be looking down on you smiling.

#7 mikey cee

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Posted 17 January 2013 - 07:50 PM

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

#8 jimegger

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Posted 19 January 2013 - 02:13 PM

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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#9 Mike I. Jones

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Posted 19 January 2013 - 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

#10 jimegger

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Posted 19 January 2013 - 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.

#11 Ed Jones

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Posted 20 January 2013 - 01:53 PM

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. :nonono: 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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#12 jimegger

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Posted 21 January 2013 - 04:06 PM

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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#13 jimegger

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Posted 21 January 2013 - 04:12 PM

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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#14 jimegger

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Posted 21 January 2013 - 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 !

#15 Mike I. Jones

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Posted 21 January 2013 - 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


#16 Mike I. Jones

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Posted 21 January 2013 - 08:01 PM

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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#17 jimegger

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Posted 21 January 2013 - 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 !

#18 Mike I. Jones

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Posted 21 January 2013 - 09:58 PM

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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#19 jimegger

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Posted 24 January 2013 - 10:34 PM

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.

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#20 Mike I. Jones

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Posted 24 January 2013 - 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

#21 jimegger

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Posted 25 January 2013 - 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.

#22 jimegger

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Posted 25 January 2013 - 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.

#23 Mike I. Jones

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Posted 25 January 2013 - 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

#24 jimegger

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Posted 26 January 2013 - 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.

#25 Mike I. Jones

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Posted 27 January 2013 - 02:40 PM

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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