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#1 wfj

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Posted 20 October 2013 - 02:18 PM

Seems like a good classics weekend for all!

This year the theme for me has been mysteries - scopes, mounts, optics, eps - all sorts. Even an Nihon Seiko (posted earlier). I haven't caught up on most of them ... so here are a few more, for your consideration.

This one is a real oddball a 7 3/4" x 26" Cassegrain OTA, that looks like a kit build. I put it on an Edmund/Anchor/Jaegers 1" GEM to star test it - astigmatic - probably a pinched mirror/corrector.

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#2 wfj

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Posted 20 October 2013 - 02:34 PM

Speaking of the corrector, look at the size of the aluminized spot on the corrector (clear aperture 5.5", spot 2.37").

Yet the corrector is a thin flat plate, not a thick meniscus.

It's a mystery to me how this even can optically work, as it looks like an optical window with an aluminized spot.

The primary mirror is larger in diameter, fills the tube ID so I'd estimate 6.5".

It could be a very weak negative lens like a Schmidt corrector, but still the spot is hard to figure. The primary seems to have a ROC of 96" or so thus f/8?

Could this be a coaxial folded "newt"? If so I've never seen such, much less a kit for same.

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#3 wfj

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Posted 20 October 2013 - 02:38 PM

The back story on this is said to be a retired Lockheed machinist who worked on satellites. The focuser does seem to bear that out ...

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#4 davidmcgo

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Posted 20 October 2013 - 04:36 PM

Any relay lenses in the baffle tube?

Dave

#5 terraclarke

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Posted 20 October 2013 - 05:59 PM

Neat scope!

#6 wfj

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Posted 20 October 2013 - 06:50 PM

Any relay lenses in the baffle tube?

Dave

Nope. Nor a baffle tube - I made one out of a core of paper towels :)

No lens glued to corrector either.

#7 wfj

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Posted 20 October 2013 - 06:54 PM

Neat scope!

Yeah - it was fairly competently executed. It looks like a kit build, but I thought I'd see all of the Dall Kirkham kits from Coulter and others, and the few Tinsley "classical cass" kits. And the german SCT kits too.

Still trying to figure out how to remove the corrector ...

#8 wfj

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Posted 20 October 2013 - 08:30 PM

... and here's the eyepieces that came with it ... one on right has no marks, other has a 1" .

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#9 wfj

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Posted 20 October 2013 - 08:31 PM

... field lens side ...

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#10 terraclarke

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Posted 21 October 2013 - 06:15 AM

The 1 inch looks like a Huygens. I bet the other is a 1/2 inch Ramsden.

#11 wfj

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Posted 21 October 2013 - 01:30 PM

Terra,
You are exactly, perfectly correct in both cases. Clearly no mystery for you. A gold star!

Here's a harder one - the unmounted finder. The eerie thing about this piece, is that I swear I used one identical to it on a 12" Dynascope in the early 70's:

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#12 wfj

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Posted 21 October 2013 - 01:36 PM

It's a 50mm finder with a 10mm exit pupil (5x) and a FOV about the size of the great square of Pegasus. The thumb screw allows you to separately adjust the reticule focus plane.

The other end has a tank eyepiece:

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#13 DAVIDG

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Posted 21 October 2013 - 03:06 PM

The finder is an old Edmund unit. I have one. It was made with a WWII "Erfle" eyepiece.
As for the optical design of the telescope, it is most likely either a folded Newtonian or a Maksutov. In a Mak the corrector can have shallow curves when it is used with a slow primary.

- Dave

#14 wfj

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Posted 22 October 2013 - 01:41 PM

The finder is an old Edmund unit. I have one. It was made with a WWII "Erfle" eyepiece.

Oh yeah - "Edmund Giant Finder" in the catalog. Thanks, I knew I remembered it from the past, that nails it.

As for the optical design of the telescope, it is most likely either a folded Newtonian or a Maksutov. In a Mak the corrector can have shallow curves when it is used with a slow primary.

- Dave

... which is why I'm taking it apart, which it looks like the corrector bezel is glued on, so the "corrector" cell slides out the mirror end.

By the way, the star test showed a stretched circle with perhaps a weaker single inner ring, with a sharp disruption of the outer ring, as if two back to back D's in mirror image of slightly differing size - never seen anything like it. Similar pattern inside focus. No center dot in pattern.

So I'm first "debugging" the optics, attempting to find the source of the diffraction pattern. I did notice on the moon a combination of issues - very sharp crater focus/resolution but only in certain positions, uneven focal plane, and fogging/glare. The last is due to baffling issues. I'm not yet certain about any SA yet, can't determine degree of correction. Could split sub microsecond fairly even magnitude doubles with a small Airy disk though.

So I need to find the culprit for the astigmatism and the feature. Strain on the mirror, crack, or a pinched corrector - what do you think it is?

Here's the primary in a machined brass cell on top of a particle board end cap - looks like a 6" dob repurposed item:

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#15 wfj

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Posted 22 October 2013 - 01:45 PM

Note - four top screws and four side screws above.

Is the mirror under tension, possibly bound as if pressure inserted like a bushing into a casting?

The mirror is sealed in - view from below:

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#16 DAVIDG

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Posted 22 October 2013 - 02:14 PM

Since you have the mirror out of the OTA, I would bench test it. In a few minutes you'll know if it's something close to a parabola or spherical. If spherical then the front glass would need to be a menicus for the system to have a chance of working. If it looks like the mirror is parabolic then the front glass needs to be a high quality optical window. You also know if the usual diffraction pattern is coming from the mirror or the corrector. If it coming from the mirror then you can take it out the cell and see if it goes away.
I've seen a couple of homemade "telescopes" made from military optical parts that don't belong together and just won't work. This has that look about it. My guess is that the mirror might be Ok but the window/corrector is not a precision piece of optics.

- Dave

#17 Darren Drake

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Posted 22 October 2013 - 06:10 PM

The back of this scope looks exactly like my Meade 8 inch Starfinder I used to own. I even drilled the 3 holes exactly like the ones shown here. Interesting.

#18 Jon Isaacs

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Posted 23 October 2013 - 04:11 AM

The back of this scope looks exactly like my Meade 8 inch Starfinder I used to own. I even drilled the 3 holes exactly like the ones shown here. Interesting.


Interesting.. With the particle board and sonotube construction, it definitely screams 1980's construction techniques.

Jon

#19 wfj

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Posted 23 October 2013 - 07:49 PM

Since you have the mirror out of the OTA, I would bench test it. In a few minutes you'll know if it's something close to a parabola or spherical.


Took me a while to find a flashlight to convert to tester:

3-line ronchi at 96" : significant convex - parabola not sphere.

If spherical then the front glass would need to be a menicus for the system to have a chance of working. If it looks like the mirror is parabolic then the front glass needs to be a high quality optical window.

Working on getting out the "corrector" so I can test it separately.

You also know if the usual diffraction pattern is coming from the mirror or the corrector. If it coming from the mirror then you can take it out the cell and see if it goes away.


Still have it in the cell, and the lines look more "bent" near the edge, as if its a "turned edge". Could you get such from a 1" thick slab somehow warped under tension/compression ?

I've seen a couple of homemade "telescopes" made from military optical parts that don't belong together and just won't work. This has that look about it.


It does at that.

My guess is that the mirror might be Ok but the window/corrector is not a precision piece of optics.


- Dave

Like a piece of window glass that he had aluminized a spot on.

Here's another part of the mystery - another pair of similar optics:

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#20 wfj

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Posted 23 October 2013 - 08:01 PM

The back of this scope looks exactly like my Meade 8 inch Starfinder I used to own. I even drilled the 3 holes exactly like the ones shown here. Interesting.


Thanks for that - never had one of those. Did have a similar Orion DS-12 inch with a particle board, glued on mirror (three enormous blobs of RTV), but it had wingnuts out the rear of the cell.

So its likely a Starfinder cell/tube - did it have the same front bezel?

#21 wfj

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Posted 23 October 2013 - 08:10 PM

The back of this scope looks exactly like my Meade 8 inch Starfinder I used to own. I even drilled the 3 holes exactly like the ones shown here. Interesting.


Interesting.. With the particle board and sonotube construction, it definitely screams 1980's construction techniques.

Jon

The original owner was in his 90's ... so in the 1980's he'd be reaching retirement - fits.

I'm beginning to suspect this was a never completed project. To get any kind of image, I needed to fake up a baffle tube that there was no provision for ...

By the way, the alternate mirror looks to be a 6" f/4 sphere - lines look flat to me. Not cored. Plate glass.

Now if I can just get off the tape some genius stuck to the aluminized spot to hold on the tissue paper to protect it, maybe I can see if I can get an interference pattern off the four surfaces, to see what they are ...

Oh, and the glass of the alternative corrector is "water white" not plate. The edge is carefully ground and beveled.

#22 wfj

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Posted 24 October 2013 - 02:55 PM

As to the cells, the screws were in so tight that the allen heads were rounded.

In my experience with satellite fixtures, [thank you Ernie Baldarama and my Dad] they have tightly machined geometries, tightly fit together, with tightened bolts. Because otherwise on the "shaker" table tests that simulate launch vibrations, they come loose and parts foul up other systems post encapsulation.

So both mirror and "corrector" are pinched.

My current hypothesis - someone trying to create own SCT incrementally, by incrementally fabricating components, getting it working in a particle board "boilerplate" or mockup, then the second pass was to make the new optics in the made cells fit into a finished final fabrication.

It may have never made it to step 2 because the missing baffles meant he never found a useful image due to lack of understanding of optics. Past that, mounting optics would have been the next block - tight to survive launch by always being in tension.

What do you think?

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#23 DAVIDG

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Posted 24 October 2013 - 03:14 PM

If the ronchi lines show a fair amount of curvature then the mirror is not parabolic but hyberbolic or an oblate spheroid and maybe strongly so. A 6" f/8 parabola would show only slightly curving Ronchi lines and if one wasn't careful could mistake them for straight. You said that the lines were convex in curvature. Were you inside or outside of focus when they showed this pattern since this would determine if the mirror was an oblated spheroid vs a ellipse/parabola or hyberbola ?
If the other 6" mirror looks to be a sphere you can use it to test the corrector by placing it in front of the mirror while your testing the mirror. If it is an optical window with no optical power the ronchi lines will stay unchanged.

- Dave

#24 wfj

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Posted 10 November 2013 - 03:37 PM

If the ronchi lines show a fair amount of curvature then the mirror is not parabolic but hyberbolic or an oblate spheroid and maybe strongly so. A 6" f/8 parabola would show only slightly curving Ronchi lines and if one wasn't careful could mistake them for straight. You said that the lines were convex in curvature. Were you inside or outside of focus when they showed this pattern since this would determine if the mirror was an oblated spheroid vs a ellipse/parabola or hyberbola ?


Correct. In other words, I should really do a Foucalt test, and measure the radius of curvature of the radial distances ("zones"), to determine the conic section most appropriate.

If the other 6" mirror looks to be a sphere you can use it to test the corrector by placing it in front of the mirror while your testing the mirror. If it is an optical window with no optical power the ronchi lines will stay unchanged.

- Dave

Well, Dave, thank you for your gentle prodding to "get the numbers" on all the mirrors/correctors.

This required resurrecting the 6" Foucalt tester my dad made for me when we were doing a telescope (http://jolitz.telemu.../benjamin/scope). I've carried it along with me since, and it was completely rust encrusted. Took a lot of work and PB Blaster.

The result - both 6" f/4 and f/8 are on the numbers (24.5" and 50.25") exactly as parabolas. Very different results from my sloppy ronchi tests. Next step is to confirm by star testing with a diagonal mirror.

As to the "correctors", putting them in front of the mirror(s) under test (again, not spheres), revealed different results.

The one that had been mounted is striated - a bright line slightly off center is prominent, there are other lines more ghostly, a wavery pattern overall, and a "wedge" on one side of the line. Perhaps the symmetrical "D" back to back of slightly different diameters, is now explained, as with it the selective fogging?

The other one is worse - larger blotchy-ness, roughly in irregular highly off centered rings like a weak lens.

There is no way these optics work this way, with these fittings.

Oh, and in the center of the f/4, there's a ... hole. Right where you'd either cut a physical one, or have a secondary shadow. Almost like a "drill here" mark ...

Here's the tester in action with the f/4 mirror:

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#25 wfj

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Posted 10 November 2013 - 03:47 PM

Am thinking the only way to finish this as a cass, is to make a 1.5" secondary with a 36" seperation (new tube or 10" tube extension sort of like Zephram Cochrane's:
http://images1.wikia...en/images/3/...
:)

Around f/28, depending on amount of BFL required. Fairly shallow hyperbola.






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