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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
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
Vendor
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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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Reged: 01/18/07

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
Carpal Tunnel
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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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Reged: 07/02/06

Loc: Fort Worth TX
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
Carpal Tunnel
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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
Carpal Tunnel
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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 [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
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [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
Carpal Tunnel
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Reged: 12/30/06

Re: 9.25 inch refractor project [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
Carpal Tunnel
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Reged: 12/30/06

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

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

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