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C-14 Prototype that was never completed

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#126 tim53

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Posted 11 October 2021 - 10:39 AM

Yep.  Next time I have the chance to work on my test stand, I'll first get the ronchi eyepiece/led mounted more securely so I can use the xyz stage it's on to get right on axis.  Better yet would be to put it in the visual back and adjust collimation to bring the reflection back through the eyepiece.  I'll do that all visually.  Once everything is aligned, I'll set up the camera to image the results for reporting here.

 

Remember, a primary reason for the secondary shadow being off-center is that the OTA is aimed off center to the flat, which is 2" less in diameter than the SCT, and why I'd want to rig this up with an oil flat (unless I can find a bigger flat), if refiguring is called for.



#127 tim53

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Posted 11 October 2021 - 10:44 AM

When I set up this time, I used my laser collimator to confirm that the flat is perpendicular to the OTA's optic axis, as the hologram is concentric with the secondary hub on it's return to the OTA from the flat.



#128 tim53

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Posted 11 October 2021 - 06:45 PM

Okay. Squoze off some quality time this afternoon for the c-14. 
 

as you can see, I adjusted the flat and the secondary so that I could put the ronchi eyepiece in the visual back and get collimation much better than before. 
 

The ronchis are taken inside and outside focus, but by turning the focus knob, not moving the eyepiece back and forth. 
 

the secondary shadow is off center because it’s a 12” flat and a 14” aperture that’s offset from center on the flat. The led reflection off the secondary is a lot closer to centered on the shadow than it was before. 
 

the ronchis are a little blown out, particularly the outside one. But I think I was better able to image them with my iPhone than with the Olympus. 
 

whadaya think?  Should I try different separations by turning the focus knob with the eyepiece at different locations?  Could the bands be straighter at whatever the optimal separation Is?  Or will it matter?

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#129 tim53

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Posted 11 October 2021 - 06:55 PM

I just moved the eyepiece out about an inch and refocused.  I think I can almost convince myself that the bands straightned out a bit.  If this thing was designed to have the same back focus as a stock c-14, maybe I should use a 2" star diagonal to pull the eyepiece farther out?



#130 tim53

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Posted 11 October 2021 - 07:14 PM

So I couldn’t stand the suspense and put an old prism 2” diagonal in. The bands are indeed straighter. I’ll try to image them in a bit, but might get called to feed any minute now. 
 

thought I’d uploaded the message, but I hadn’t. 
 

here are the innie and outie images. With iPhone and better focus. 

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#131 tim53

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Posted 11 October 2021 - 07:23 PM

Well, on screen they look similar to the ones without the diagonal.  I just put a 2" extension tube in, and they still look about like above.

 

Too hip, gotta go!


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#132 macdonjh

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Posted 11 October 2021 - 10:14 PM

If I remember correctly, the optical path length of a 2" mirror diagonal is approximately 100mm...


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#133 tim53

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Posted 13 October 2021 - 12:25 PM

Not that I NEED to make a decision about this scope soon, but I am thinking, in the back of my mind, about the projects I have on the various back burners, and which I should push to the front, once I've made a decision about this one.

 

First, about this one:  whether I refigure the secondary or not, this thing was clearly intended for wide field imaging, but SCTs don't have flat, coma-free fields over large areas (particularly a problem with detectors with small pixels).  Does anybody make an SCT focal reducer/field flattener that's ~3" (or more) in clear aperture?  Because I'm not sure I'm up to aspherizing the primary and secondary (though the secondary is probably already somewhat hyperbolic, per stock SCT designs).  If not, and/or if I elect not to do any work on these optics and just put it together and use it, it's certainly not a bad C-14 considering the investment so far.

 

As for back-burner things I need to do.  Obvious first:

 

1.  Make a cell for the 8" f/8 mirror I refigured and have coated (still in shipping box from L&L), and make a new counterweight for the Schafer 8".  Shouldn't take me more than a few hours to complete that scope.

2.  Take the MK63 apart again and get the primary that I refigured sent out for coating, put it together and use it.  Pretty straightforward.

3.  Finish the elliptical secondary for my 6" f/30 Gregorian.  I finished the primary at the last Delmarva mirror class a few years back.  Just need to finish coring it and get it coated.  The secondary is elliptical, but I need to dpac the whole system to finialize it.  Then get it coated with the primary.

4.  I have 2 DX8s.  I was inspired by Dave's description of his polishing the flat side into a smooth flat surface, and tweaking the ashperic surface to remove the stig.  I need to test mine and pick which one to rework.  The most complete has nice mechanicals, and does decently on planets up to about 150x, falling apart above that.  It'd be fun to make that into a good planetary scope, and would give me experience with SCT correctors.

5.  The holy grail:  fine grind, polish, and figure the corrector for the 16" Mak Cass optics I bought several years ago.  I have the mount for it in my observatory at Cosmic Acres (Tak EM-500 with Sitech goto).

 

So, whadaya think?  Like I said before, if I'm going to refigure anything of the C-14, I'm going to build an oil flat test jig that allows me to swap the secondary in and out while testing so that I can quickly see what I've accomplished.  it'll also be the first time I've worked a convex mirror, so there's that. But if there's a large aperture FF/FR or COTS lenses therefore that I can use to make it into a decent wide-field imager without refiguring stuff, I'd like to know about that, too.

 

-Tim



#134 davidc135

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Posted 13 October 2021 - 02:04 PM

A nice selection of projects; from the straightforward to the fiendish.

 

I'd go for assessing the C-14 firstly with a star test and a look at the moon/planets if the weather is kind. The latest Ronchi suggested that the optics are reasonably smooth and if that is so it would be worth figuring the 2ndry if the optics are, say, under-corrected by 1/2wave.

 

However, if the correction is only off by 1/4 wave but there is noticeable roughness there would be less point in doing the work.

Celestron altered its method I think in the 90s, fine grinding and polishing the front surface flat so subsequent correctors can probably be smoothed

 

I'd put the DX-8s to the back of the queue. They'll drive you up the wall.

 

David


Edited by davidc135, 13 October 2021 - 02:04 PM.

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#135 tim53

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Posted 13 October 2021 - 02:21 PM

A nice selection of projects; from the straightforward to the fiendish.

 

I'd go for assessing the C-14 firstly with a star test and a look at the moon/planets if the weather is kind. The latest Ronchi suggested that the optics are reasonably smooth and if that is so it would be worth figuring the 2ndry if the optics are, say, under-corrected by 1/2wave.

 

However, if the correction is only off by 1/4 wave but there is noticeable roughness there would be less point in doing the work.

Celestron altered its method I think in the 90s, fine grinding and polishing the front surface flat so subsequent correctors can probably be smoothed

 

I'd put the DX-8s to the back of the queue. They'll drive you up the wall.

 

David

I like that "fiendish".  grin.gif

 

Visually, the ronchigrams look better than the images I last posted - the curvature isn't as obvious.  I have looked at Jupiter with the scope.  It was in the "nearly collimated" state at the time, and the views were nice up to about 400x.  If I do refigure the secondary, I want to have an oil flat setup so I can see the whole 'gram at once, and be able to swap the secondary in and out quickly for adjustments to the figure.  So yeah, I'm hoping to get some thoughts on the current quality before I go to that trouble.

 

The corrector is allegedly from an Edge HD.  In any case, not decades old.  Probably newer than the rest of it by a few years?

 

I like your comment re the DX8s.  I'm reckoning that, if I can make something with one of them and still have most of my hair at the end, then I shouldn't fear anything, right?  wink.gif

 

-Tim


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#136 PETER DREW

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Posted 13 October 2021 - 02:54 PM

Depends on where you are up to mentally on a project.  Sometimes if it gets bogged down it is helpful to select a doable project that can be completed from the backburner list.  Success on a lesser project can get the adrenaline going again. 


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#137 coinboy1

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Posted 21 October 2021 - 09:21 PM

Hi Tim,

 

Here is a large format SCT corrector for your scope made by Starizona. It is designed to cover full frame sensor. Interesting project!

 

https://starizona.co...-coma-corrector


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#138 Augustus

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Posted 21 October 2021 - 10:12 PM

I would get to those first 5 projects first and leave this alone. It would seem you have at least a decent example of a C14 if perhaps not perfect (though I wouldn't be surprised if most are on this level of quality) and have fun observing with this. Maybe try the Starizona corrector.



#139 tim53

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Posted 21 October 2021 - 10:38 PM

Hi Tim,

 

Here is a large format SCT corrector for your scope made by Starizona. It is designed to cover full frame sensor. Interesting project!

 

https://starizona.co...-coma-corrector

Neat, but not big enough.  The ID of my baffle is 3 1//4" - the same as the OD of a stock C-14.  I'd like something that could fill a medium format camera, or maybe a cooled camera with a 16803 KAI chip in it (assuming I can afford one of those one of these days).  



#140 coinboy1

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Posted 22 October 2021 - 05:24 AM

Try the 3" Optec SCT corrector. Designed for a 16" SCT it could work for your 14". It will cover a 52mm diagonal. 

 

Other than that it will have to be custom. I don't think there is anything else out there on the market. 

 

 


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#141 tim53

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Posted 22 October 2021 - 01:07 PM

Kind of hard to find the price for that puppy.  Looks expensive, though, and may require also purchasing a 3" focuser, unless I can machine a 3" visual back with the right backfocus requirements.

 

So nothing else out there?  What do owners of Meade 16" LX200s do?

 

-Tim.




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