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Using a Hyperstar Schmidt Cass without a corrector

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#1 abordage

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Posted 02 January 2013 - 08:18 AM

Hi;

Can someone tell me if there is a way to make a hyperstar system work with a schmidt cass scope that does not have a corrector plate?
Is there a lens that could be added to the mirror facing end of the hyperstar assembly that would do the job of the missing corrector lens?
I was thinking of using a simple 3 or 4 vane newt type of secondary holder to mount the hyperstar unit to the front end of the schmidt cass ota.

Any thoughts?

Adrien

#2 D_talley

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Posted 02 January 2013 - 09:52 AM

It would seem to me the corrector is very specific to the mirror in the SCT. I have never seen anything else sold that would replace it.

#3 rmollise

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Posted 02 January 2013 - 11:05 AM

Hi;

Can someone tell me if there is a way to make a hyperstar system work with a schmidt cass scope that does not have a corrector plate?
Is there a lens that could be added to the mirror facing end of the hyperstar assembly that would do the job of the missing corrector lens?
I was thinking of using a simple 3 or 4 vane newt type of secondary holder to mount the hyperstar unit to the front end of the schmidt cass ota.

Any thoughts?

Adrien


The only way to make this work is by replacing the corrector. If you can find a surplus one, it should work OK. That is the only way; you must have a corrector.

#4 abordage

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Posted 02 January 2013 - 03:34 PM

Rod;

I was thinking that since a corrector plate in a schmidt cass ota basically corrects for spherical abberation created by the primary that introducing a lens that corrects for that spherical abberation in front of the hyperstar assembly would create the same end result.
I have an opportunity of buying an 8 inch sct ota without corrector for about $50 locally, and thought it would make a great dedicated astrophoto scope for a very good price.

Adrien

#5 rmollise

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Posted 02 January 2013 - 04:00 PM

No. The Hyperstar is designed to work with the corrector plate, and you must have that for satisfactory results. 50 bucks might be OK as a hangar queeen/parts source, but unless you have a source for a corrector, it's not a practical way to get a working C8.

#6 GlennLeDrew

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Posted 02 January 2013 - 05:21 PM

I suppose a suitably aspherized f/2 primary *might* be one way to compensate for a missing Schmidt corrector. A sub-aperture lens-based corrector would almost certainly require at least two elements, the number depending on whether a surface is aspherized, and to what extent if so.

#7 Geo.

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Posted 02 January 2013 - 05:42 PM

The Vixen line of modified Cassegrains are something like this. Instead of the Schmidt corrector they use a corrector in the optical path between the primary and secondary. In this way they can use spherical mirrors rather than a parabolic primary and hyperbolic secondary. There has been some discussion of corrector-less Schmidt cameras in the ATM, Optics and DIY Forum forum. Basically, a mask is used to obstruct some of the path and eliminate the spherical abberations. All based on some of Schmidt's writings. A C8 might make a an interesting subject for a project like this.

See: Lensless Schmidt camera #2

#8 abordage

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Posted 03 January 2013 - 12:53 PM

Rod;

Okay thanks for the info. I may get the ota anyhow for spare parts.

Adrien

#9 Eddgie

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Posted 04 January 2013 - 04:42 PM

I have a slightly different take.

When Hubble was first placed in orbit, it had 1.7 waves of SA. That is quite severe.

And yet, the pictures were still better than many ground based instruments. And that was with chips that had giant pixel sizes.

The amount of spherical abberation at f/2 I would think would be very large, but the question is at f/2, considering that the image scale is incredibly small, I am just not sure the damage would be all that horrible.

Even if the SA was on the order of one full wave, the Airy pattern grows very large, but at F/2, the angular width of the entire pattern would still be quite small.

I would think that you could compute the diameter of the Airy Pattern out to 5 or 6 rings (use Abberator to simulate it) and compare that to the pixel size on your chip.

You would not want to image planets with it, but at the same time, at f/2, depending on your pixel size, the damage may not be all that great.

Will if give acceptable results? Well, I tuess that depends on you. Large scale targets like large Nebula will still be captured and will show decent extension, but the structure inside will be obscured.

Look at some "Before/After" pictures of Hubble. It is actualy quite surprising how good some of them were and Hubble was used that way for several years before it was corrected.

Here is model of 1.7 waves of SA on a 203mm telescope with an image of Jupiter and the SA applied to the result.


Now considering that Jupiter is less than one angular degree, and that at f/2 it would be tiny on the focal plane, it seems to me that most very large scale targets would still be recorded with enough fidelity to see the major structural features.

So, I would not be to sure that the instrument would be unusable, but fine structure would be lost.

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#10 Eddgie

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Posted 04 January 2013 - 04:48 PM

Here is a link that shows the Before and After pictures taken by Hubble. While the damage is clearly bad as compared to the "After" picture, the fact remains that a lot of the structure has been captured.

http://hubblesite.or...lbum/pr1995049e

I think the same thing would happen in the C8.

The question though is how big the Airy pattern will extend on the chip. At f/2, will stars still look more or less like points, or will they spread beyond that.

#11 Namlak

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Posted 04 January 2013 - 08:41 PM

I had read that the Hubble flaw spurred on a lot of work in image processing, possibly advancing it 10 years ahead of where it would have been otherwise. Then when both the digital work *and* the corrected optics were put in place, it exceeded design expectations.

Broken eggs to omlette :^)

#12 rmollise

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Posted 05 January 2013 - 08:25 AM

Here is a link that shows the Before and After pictures taken by Hubble. While the damage is clearly bad as compared to the "After" picture, the fact remains that a lot of the structure has been captured.

http://hubblesite.or...lbum/pr1995049e

I think the same thing would happen in the C8.

The question though is how big the Airy pattern will extend on the chip. At f/2, will stars still look more or less like points, or will they spread beyond that.


What does anything look like in an SCT without a corrector plate? Something that came out of the wrong end of my old Aunt Lulu's poodle dog. ;)

#13 *skyguy*

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Posted 05 January 2013 - 11:54 AM

It would certainly be fun and interesting to experiment getting an acceptable image using a Hyperstar lens on a C8 missing the corrector plate ... perhaps masking the primary mirror would help? However, spending the time and effort on a project most likely to fail would be too much for me. ;)

#14 Alph

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Posted 05 January 2013 - 12:05 PM

at f/2, considering that the image scale is incredibly small,


Opposite, incredibly large.
The scale of atoms is very small.
The scale of stars is very large.
Hope this helps to memorize it.

#15 GlennLeDrew

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Posted 05 January 2013 - 07:09 PM

At the prime 'focus' (ha ha) of an f/2 sphere, the spherical aberration would be measured as closer to tens of waves. It would be not much worse than imaging with a cheap hand magnifier, missing only the lovely rainbow colors.






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