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Have a 12 inch Maksutov corrector...what to do?

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#1 PrestonE

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Posted 02 February 2013 - 04:19 PM

Hello Everyone,

Having purchased all sorts of optical glass over the years, and going through it to see what might make a nice next project I find a lovely fully finished 12 inch Maksutov corrector lens.

No details on the type of glass. I could measure the curves.

Would it be possible and is it worth the time to reverse design something from this lovely piece of glass :question:

If so, what would be the best use and design?

There is no central perforation.

Thanks in advance for you kind ideas and suggestions.

Like I need another project, but it has been 4 years since finishing the 20" RC and we have very clear dark sky's here in this part of Mexico.

Very Best Regards,

Preston

#2 ccaissie

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Posted 02 February 2013 - 04:30 PM

If applying it to a Mak design, need accurate measurement on R1,R2 and the thickness. The glass type that was commonly used in the Mak club days was usually BSC-2, and someone from that era could provide the info on that. Sounds like a 14" Sphere is the mating R3 element.

Otherwise I can't guess it's usefulness. This is about the best place to post such a query.

#3 ed_turco

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Posted 02 February 2013 - 06:04 PM

BSC2 had a refractive index of 1.517 and an Abbe Number (V) of 64.5. In optical parlance, it would have a number of 517645. Hope this helps.

Schott glass is listed as 517641, and frankly, that ain't much of a difference.

#4 gatorengineer

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Posted 02 February 2013 - 06:17 PM

A fast mak newt would complement the 20 RC nicely.... Would be interesting to see what could be reverse engineered outta that glass.....

#5 Mike I. Jones

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Posted 03 February 2013 - 01:37 AM

Hi Preston,
By all means measure the radii and central thickness, and clear apertures for R1 and R2 as well. The steepness of the curves will help narrow down whether to do a Mak-Newt, Mak-Cass, or even Mak-Gregorian, and if not a Newt, whether the secondary mirror can be a spot on R2, or has to be a separate optical element.

One thing to do early on is get a dummy shine on both sides and test for strain with crossed polarizing filters. If zero to very little color birefringence is observed, the blank is suitable for use in a telescope. If you see dazzling stress rainbows, Maltese crosses, etc., it should either be re-annealed by someone that has the right equipment, or set up for a few rounds of fun practice target shooting.

Mike

#6 PrestonE

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Posted 03 February 2013 - 12:45 PM

Thanks Guys,

This is the first time the lense has ever been totally unwrapped as far as I can remember.

Kindly check the following information below:

R1 radius 12.17 inches
R2 radius 12.703 inches
R1 clear diameter 11.9375 inches
R2 clear diameter 12.254 inches
Edge thickness .885 inches
center thickness .895
Hole diameter 2.996 inches

I did not know that it had a hole in the center :bow:

There is no strain at all :cool:

I purchased this lens in 2006 from a well known ATM in hopes of making a nice planetary scope.

Let's see what the best use will be and thanks in advance for your kind help and suggestions.

Very Best Regards,

Preston

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#7 PrestonE

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Posted 03 February 2013 - 12:46 PM

And the top view.

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#8 JohnH

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Posted 03 February 2013 - 01:09 PM

I would appear to be polished so you can o a strain test right off.

With the hole already drilled, it will be more difficult to do any more polishing or apherizing. This also suggests it was planned to have the secondary mounted separately. Cassegrain or Newtonian though is currently anyones guess.

#9 ed_turco

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Posted 03 February 2013 - 02:56 PM

I don't see that hole as much of a problem in polishing or figuring. You just use a pitch lap with a hole trimmed away to a diameter slightly larger than the hole diameter. Have done this many times.

You might get a turned up edge around that hole but that is so easy to correct.

#10 dan_h

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Posted 04 February 2013 - 09:29 AM

I don't see that hole as much of a problem in polishing or figuring. You just use a pitch lap with a hole trimmed away to a diameter slightly larger than the hole diameter. Have done this many times.


I did a 6" f2 like this back in the 80's. Nobody told me I couldn't so I did it. I don't recall any issues at all.

dan

#11 Jeff B

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Posted 04 February 2013 - 09:45 AM

With the hole in the middle, it would make a really cool and trendy bathroom sink.

#12 Mike I. Jones

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Posted 04 February 2013 - 10:41 AM

Preston - I'm not coming up with anything (yet) that can use those radii without changing them and still have good optical performance. Can you test R1 to see if it's spherical, or if any aspherization has been done to it?

Not giving up, will post some results soon.
Mike

#13 PrestonE

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Posted 04 February 2013 - 12:20 PM

Mike, I will have to dig around the ATM stuff to find the knife edge or Ronchi tester and set it up.

May be a couple of days :foreheadslap: as there are still 50 to 75 boxes that have yet to be up packed!!!!

Not near the space here as the 3 shops and attic in Houston use to have.

Best Regards,

Preston

#14 Ed Jones

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Posted 04 February 2013 - 02:47 PM

Preston,
My guess is that it is BK7 and I come up with this design. You could measure it's specific gravity to help identify it. The corrector R1 is aspherized as well as the secondary.

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#15 PrestonE

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Posted 04 February 2013 - 04:45 PM

Hi Ed,

I will source a container that I can do that with in the next few days and get that done.

How much aspherize will be needed on R1?

What is the final F ratio?

Will the primary need to be larger than the corrector?

Best Regards,

Preston

#16 Ed Jones

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Posted 04 February 2013 - 05:37 PM

Preston,
Looks to me like ablout 7.6 fringes @.633 nm. The F ratio is f/10 and I think a 12.5 inch blank is what it was intended to be used with.

Ed

#17 Mike I. Jones

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Posted 05 February 2013 - 09:43 AM

I'm guessing Preston's corrector is purely spherical, and probably very good spheres at that. Assuming spherical surfaces, I tried several configurations that did not require aspherization of R1 or R2. This design was particularly interesting: the unmodified, all-spherical, BSC-2 517645 shell matched with a weakly aspherized primary mirror and all-spherical BK7 Mangin gives very sharp spots. Light passes twice through both the corrector shell and Mangin. I've never seen a Mak shell used quite like this. The shell and primary mirror are mounted on the same spool that holds the primary baffle tube. I tried reversing the shell, and it wasn't as good as in this direction. The primary is figured with only about 6.5 waves departure from the nearest fit sphere. It's a weak higher order asphere, but you would just simply figure it from a sphere to give a double-pass system null, without realizing you're making an 8th or 10th order asphere. EFL is 122.8", f/10.4, field is 1/2" diameter.
Mike

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#18 Ed Jones

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Posted 05 February 2013 - 04:36 PM

That's totally unique Mike, very interesting!

#19 PrestonE

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Posted 05 February 2013 - 05:51 PM

Now that is really thinking outside of the box :jump:

I really like this very different design a lot Mike.

What more do you need from me at this point :question:

I will be digging around all the boxes in the next few days to try and discover just where all of the ATM goodies are including the testers.

Very Best Regards,

Preston

#20 Mike I. Jones

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Posted 05 February 2013 - 06:14 PM

Well, one thing for sure, if you build it you'd have the only one like that bubba on the block! Or in Mexico. Or Earth.

For any design you end up using, just confirm the radii values and sphericity, the hole ID to 0.001", and the thickness at the edge of the hole to 0.001" (so we can calculate what the center thickness would be). Also, get in contact with that "well known ATM" to see if he has working logs for it that document all the final dimensions.

I can put up the prescription now (it's too many surfaces for OSLO-EDU), and if your own measurements or the ATM fellow's data are different from what you said above, we can tweak the design to match.

Ed and/or I can hunt for a near-null test geometry for the separate corrector/primary assembly, and the Mangin by itself. Nice to reduce risk by getting good clean nulls on subassembly components before you put them together.

I'll also dimension the baffle tubes for you, once the design is frozen. You got good at makin' them conical baffles for sure.

It would be fun to see an unconventional wack-job design like this one built. Trying the corrector in that position was totally an experiment on my part, one I was surprised that worked so well.

Mike

#21 PrestonE

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Posted 05 February 2013 - 06:25 PM

Hay Mike, you know I am in for different always.

Looks fun to build and by mounting the corrector on the stalk of the primary I will not have to make a cell for that beast :roflmao:

I start to work on getting all the details measured very accurately.

Thanks So Much for this very different design an use of
this beautiful lens.

Very Best Regards,

Preston

#22 jcham21

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Posted 05 February 2013 - 09:31 PM

How about an F/4.25 astrograph? This design looks like a Companar but with an aspheric primary and secondary. The field lens was added to correct lateral color, but can be removed if you re-grind one surface of the Mak corrector. AR coatings will most likely be needed on the refractive elements. Secondary linear obscuration of 50% should be OK for astrophotography. Corrected for a full-frame camera.

The F/2 primary would be fun to figure :grin:

James

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#23 jcham21

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Posted 05 February 2013 - 09:37 PM

Spots showing correction from 0.38 to 1 micron.

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#24 jcham21

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Posted 05 February 2013 - 09:39 PM

MTF, Field Curvature, & Distortion.

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#25 PrestonE

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Posted 06 February 2013 - 11:39 AM

Hi James,

I had seen this Companar design before and it is interesting.

We have a 20"RC that is robotic and will hopefully be installed just after the Texas Star Party this May.

My current thoughts are something more for planetary imaging
similar to what Mike and Ed have recommended.

Also, I'm not really wanting to have to change the corrector too much as it is polished out quite nicely.

Very Best Regards,

Preston






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