Author Topic: Coma correctors for fast Newtonians  (Read 397 times)

brodsandbacksosp

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Coma correctors for fast Newtonians
« on: December 24, 2017, 07:24:03 PM »
The Paracorr Type II seems to be the de-facto benchmark for correcting coma on quickly Newtonian optics.  But it is pretty spend-y.  As I was searching around on Black Friday I noticed there were some more affordable choices for coma correction, but I truly don't know if they're intended for f/3 or quicker optics.  I know that even the P2 is not, technically, rated for sub-f/3 and it performs very well (so I hear).  My question, then, is are there workable less expensive choices for managing rapid optics, or do I simply must bite the bullet and find a P2?



highdanmyne

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Re: Coma correctors for fast Newtonians
« Reply #1 on: December 28, 2017, 05:33:34 AM »
I doubt anything other than the TV PC2 would work @ F3.
The Baader MPCC III works down to F3.5.

Mike

Ronald Saldana

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Re: Coma correctors for fast Newtonians
« Reply #2 on: December 28, 2017, 11:08:26 AM »
The light cone angles at f/3 demand a large aperture, steep curves, multilayer AR coatings, and multiple elements for any kind of corrective lens. No way does that kind of optic come cheap.

Keith Pennington

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Re: Coma correctors for fast Newtonians
« Reply #3 on: December 29, 2017, 05:17:36 PM »
High Point says f3-f6 optimized for f4.5.

Eric Castillo

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Re: Coma correctors for fast Newtonians
« Reply #4 on: January 08, 2018, 12:03:53 AM »
If you plan to have any other fast telescopes in the future, it is time to invest in the Paracorr 2, or, better yet, a SIPS.

You get what you pay for, and you only buy it once.

I can say from personal experience that the P2 works superbly at F/3.0, and personally I won't use anything else.

Tim Massey

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Re: Coma correctors for fast Newtonians
« Reply #5 on: January 08, 2018, 02:53:06 AM »

settmagganen

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Re: Coma correctors for fast Newtonians
« Reply #6 on: January 08, 2018, 04:27:31 AM »
Quote
The Paracorr Type II seems to be the de-facto standard for correcting coma on fast Newtonian optics. But it is pretty spend-y. As I was looking around on Black Friday, I noticed there were some cheaper options for coma correction, but I really don't know if they are intended for f/3 or faster optics. I know that even the P2 is not, technically, rated for sub-f/3 and yet it performs really well (so I hear). My question, then, is are there viable less expensive options for dealing with fast optics, or do I simply need to bite the bullet and get a P2?

It's a function of eyepiece used and the size of the coma corrected field.
The original Paracorr completely corrected coma in a 40mm field down to f/5.
Below that, the coma corrected field shrank, but it left only a residual coma at the edge at, say, f/4--visible, but usually tolerable.
The Paracorr Type 2 corrects coma completely in a 40mm field down to f/3.5.
That doesn't mean it won't work below that. It will not completely correct coma in that wide a field at f/3, but most f/3 scope users
will not use eyepieces with field stops larger than about 36mm anyway, so coma correction at f/3 will be fine.

Other coma correctors provide decent correction in an f/4 scope, but the Paracorr stands alone at f/3 (or faster).

The Explore Scientific HRCC, Baader MPCC Mk.III, and GSO will all do just fine at short f/ratios if the eyepiece fieldstops are kept smaller.
Each has its own foibles.

hluhsubshoona

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Re: Coma correctors for fast Newtonians
« Reply #7 on: January 15, 2018, 01:09:56 PM »
There is a relevant thread over on the Reflector forum now:

http://www.cloudynig...tor-compariosn/

I was hopeful the MPCC would work out since it does not have a barlow effect. Not a big deal for visual, but disappointing for NV or imaging to get a fast optic and hobble it with a 15% speed penalty.

Eric Castro

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Re: Coma correctors for fast Newtonians
« Reply #8 on: January 23, 2018, 02:38:20 PM »
That 15% is an important issue if you have never used a coma corrector
Once you use one, it kind of disappears.
My low power eyepiece drops from a 1.30° field to a 1.14° field, but that difference isn't important when the outer 1/3 of the field is mush
without it and sharp to the very edge with it.
I'd rather have 1.14° of sharp star images than 0.8 degrees (or less) without it.
And if you use the coma corrector with all eyepieces, as I do, I just assume my scope has a longer focal length and plan accordingly.

Michael Litvack

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Re: Coma correctors for fast Newtonians
« Reply #9 on: January 26, 2018, 01:59:00 AM »
Quote
That 15% is an important issue if you have never used a coma corrector
Once you use one, it kind of disappears.
My low power eyepiece drops from a 1.30° field to a 1.14° field, but that difference isn't important when the outer 1/3 of the field is mush
without it and sharp to the very edge with it.
I'd rather have 1.14° of sharp star images than 0.8 degrees (or less) without it.
And if you use the coma corrector with all eyepieces, as I do, I just assume my scope has a longer focal length and plan accordingly.


For night vision and other electronic devices the issue is brightness.

If I did the math right, f/3 is about 32% brighter than f/3 with a 15% barlow effect. (3.45^2/3^2).

32% is kind of a lot to lose.

A zero-power coma corrector would be a much more desirable item for imagers and NV observers. Perhaps conventional observers might like it too if it actually corrected coma?

It appears that the MPCC only reduces coma when used with a paraboloid. Looking over at Ed Jones current thread, perhaps there is a possible path for the ATM.

http://www.cloudynig...coma-corrector/

A lot of work, but a big payoff.

Anton Balderrama

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Re: Coma correctors for fast Newtonians
« Reply #10 on: January 30, 2018, 12:09:11 PM »
If you want full coma correction (vs. partial suppression) for paraboloids you'll need at least a 3-element Wynne type corrector. Two-element correctors only suppress coma and usually have considerable chromatic residual -- unless expensive exotic glasses are used.

Mladen

Chris Jiles

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Re: Coma correctors for fast Newtonians
« Reply #11 on: January 31, 2018, 08:31:41 AM »
Quote
If you want full coma correction (vs. partial suppression) for paraboloids you'll need at least a 3-element Wynne type corrector. Two-element correctors only suppress coma and usually have considerable chromatic residual -- unless expensive exotic glasses are used.

Mladen

For this application I don't think extra elements would be a negative. Indeed, "whatever it takes".

Rather, I am surprised that with the growing popularity of electronic imaging there is not a solution on the market that offers good correction while maintaining the photographic speed of a paraboloid objective.

Maybe it is just not possible?

knigabretta

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Re: Coma correctors for fast Newtonians
« Reply #12 on: February 09, 2018, 05:24:45 AM »
No it's possible, SFAIK. Al Nagler told me the Barlow factor was mostly to allow the Paracorr to be used without having to modify scopes, because it gives the optics space to work in by extending the focal length. If you want to have a dedicated corrector it should be possible to have zero Barlow - or even focal reducers.

Wynne correctors are certainly available commercially with no Barlow.

http://www.teleskop-...cted-field.html

rennlispuring

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Re: Coma correctors for fast Newtonians
« Reply #13 on: February 09, 2018, 08:27:16 AM »
Quote
Quote

If you want full coma correction (vs. partial suppression) for paraboloids you'll need at least a 3-element Wynne type corrector. Two-element correctors only suppress coma and usually have considerable chromatic residual -- unless expensive exotic glasses are used.

Mladen

For this application I don't think extra elements would be a negative. Indeed, "whatever it takes".

Rather, I am surprised that with the growing popularity of electronic imaging there is not a solution on the market that offers good correction while maintaining the photographic speed of a paraboloid objective.

Maybe it is just not possible?

It's possible with multiple elements and exotic glasses but amateurs always want a cost-effective solution, which is much more difficult to achieve.

One reaosnable solution for small reflectors (up to 10-inches) is the Lurie anastigmat. It would require a full-aperture 2-element corrector which can be made with the cheap N-BK7 crown. It converts the Newtonian into essentially a visual Schmidt camera -- free of spherical, coma and astigmatism. The downside is of course, extra weight. However, the corrector is placed at or near the focus and not the RoC as in the Schmidt camera. This keeps the telescope compact. These systems with mirrors as fast as f/3 and cover wide fields. The corrector also has very lax tolerances which makes manufacturing a lot easier, especially for ATMs. Here's an example of a 10-inch f/4.2 covering a 2-degree filed (and could go much wider).
This configuration should not be confused with the more popular Houghton (aka Houghton-Lurie). Lurie anastigmats work only with aspheric mirrors, as opposed to the Houghton which works with spherical mirrors only, and is aplanatic (free of sphericla aberration and coma only), not anastigmatic (free of sphericla aberration, coma and astigmatism).

Mladen


Scott Rogers

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Re: Coma correctors for fast Newtonians
« Reply #14 on: February 09, 2018, 09:11:56 AM »
Quote
If you plan to have any other fast telescopes in the future, it is time to invest in the Paracorr 2, or, better yet, a SIPS.

You get what you pay for, and you only buy it once.

I can say from personal experience that the P2 works superbly at F/3.0, and personally I won't use anything else.

Thanks, everyone. Regarding the comment above, Mike, I had considered SIPS for my new scope, but wasn't sure if that would be my last fast one. So I opted for a non-SIPS focuser to allow the corrector to be moved between scopes. Having two (say) fast scopes with two SIPS is getting *really* spend-y (albeit very elegant).