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AlexxxAA

Coma Corrector for Visual???

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Hi guys...

Ive been enjoying my new lightbridge for the past month. Everything is great.

The only complaint i have is due to the coma caused by its fast optics (f/5).

I'm using an Explore Scientific 24mm 68-degree, a 5mm Nagler Type 6, and some Agena SuperWide 70 degree eyepieces: 20mm, 15mm, and 9mm. ( i totally recommend these eyepieces to you guys. Great price for good quality www.agenaastro.com)

I know that TeleVue tests their eyepieces to work down to fast scopes. And i don't seem to have noticed anything wrong on starfields with the Nagler. But that's why they are worth big bucks, and unfortunately upgrading all my eyepieces to TeleVue is impossible. The rest of my eyepieces give great, wide views. However, at about 25-40% of the radiaus, the stars start to become a little fuzzy and round instead of pointy and sharp like in the center.

So one cost effective solution would be to get a coma-corrector for my scope right?

However, i wanted to ask you guys if getting one for $200 is worth the trouble? Will it solve the visual problem? or is it mostly for astrophotography? Does anyone use one on a regular basis visually?

It just catches my attention more to spend only $200 on a coma-corrector, instead of a couple of thousand for a set of TeleVue's :p

What do you guys think?

Thanks in advance to everyone :grin:

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I have a Baader MPCC which I have used for visual and astrophotography with newts of F4 and it does correct the coma but the spacing to the camera sensor is critical and needs to be +- 2mm I think. It is equally as good for visual but each time you change the eyepiece you need to adjust the spacing ideally or it will not be 100% effective. The Paracorr is easier to adjust for visual observing and if you have $860 you can get a Feathertouch focuser with the Paracorr integrated so it needs no adjustment.

http://starlightinstruments.com/store/index.php?route=product/product&path=36&product_id=50

I haven't tried one but if I had the $860 lying around doing nothing.....

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The GSO corrector works well with a non-critical 75mm (+/- 10mm) spacing. It is available from Agena and Telescope Services, but not currently from any UK based supplier, so far as I know.

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With the exception of the Tele Vue Nagler, much of what you are seeing in the outer parts of the field of view could be astigmatism as much as coma. A coma corrector might improve things but won't cure the astigmatism. Buying more green and black eyepieces is the cure for that ! (but will cause an empty bank account, admittedly).

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Thank you guys for the help. Unfortunately upgrading my eyepieces is out of the question lol

So ill see if i can actually find a coma corrector somewhere here in the US. They seem to be back-ordered everywhere lol :tongue:

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With the exception of the Tele Vue Nagler, much of what you are seeing in the outer parts of the field of view could be astigmatism as much as coma. A coma corrector might improve things but won't cure the astigmatism. Buying more green and black eyepieces is the cure for that ! (but will cause an empty bank account, admittedly).

Take note of John, this is not just COMA. Not all eyepieces are equal when you are down at F5, I can start to see the edges go with very good eyepieces at F 5.2, I.E. Meade UWA and SWA. but the Tele-vues I have are still sharp!

Alan.

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I find it difficult to believe that Tele Vue would be selling a corrector if their eyepieces are immune to coma. In my (limited) experience, even quite modest eyepieces are cleaned up by a coma corrector,

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I find it difficult to believe that Tele Vue would be selling a corrector if their eyepieces are immune to coma. In my (limited) experience, even quite modest eyepieces are cleaned up by a coma corrector,

Their eyepieces enable you to actually see the coma that the scope optics produce rather than masking it with astigmatism. Thats probably why they developed the Parracor I reckon.

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Has anyone tried or read about the newer Rowe Coma Corrector by Baader? It seems to be a step up in optics compared to the MPCC, thus the slightly higher price. How does the Parracor compare? I think there is still a supplier that still sells the MPCC and the Parracor by itself.

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The advertisements for Baader RCC-I (Rowe Coma Corrector) say it is designed for Newtonian imaging systems (my emphasis). However I see no reason why it should not work for visua though spacing may be an issue and the connection at the eyepiece end is a T-thread which will limit the available true field of view. It does have a long back focus distance (from last lens to the new focal point) of 91.5mm. It is a more complex design than the MPCC.

The GSO coma corrector is also a complex design, designed by Roger Ceragioli with two doublet lenses. It connects via a 2" (48mm) filter thread so vignetting is less of an issue. Spacing can done with empty filters and/or Hyperion fine tuning rings. It has a back focus of 75mm, which is more than the MPCC but less than the RCC-I, and focus is moved in by about 10mm. The back focus is not critical with the designer saying that the spots look good with spacing from 65mm to 85mm, which is important if your eyepieces are not parfocal or if you use both 2" and 1.25".

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As John says above, there are two things at play when using fast newts (probably). they look different. coma is like little arrows, pointing to the centre of the mirror and getting increasingly large as you work out from centre. this starts closer to centre the faster the mirror. It is a mirror character not an eyepiece characyer. astigmatism looks more like a cross but could take many forms I exect and is generally an eyepiece character (although mirrors can be astigmatic (as can eyes) too).

astigmatism and coma working together is not a pleasant mix but many can 'stand it'. personally, I find it a little irritating. I use TV eyepieces and with my 16" f4, the view without Paracorr is just about bearable. with the paracorr it is like using an f8 scope with almost no coma. coma is generally not visible (or is less visible) at higher magnifications, affecting lower power wider fields more. the fact you see no issues with the Nagler does not surprise me for the reasons of power and quality.

I don't find that at f5 I need a cc but a f4 it's much better, in my f11 newt it makes no apparent difference at all.

I am not sure in your case a cc would help as it sounds like astigmatism in the eyepieces. In your position, I'd personally save for a bit and buy a used 24mm or 22mm Panoptic and sell the 24mm you have. I think this would provide more overall benefit than a coma corrector with your current scope.

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Newtonian telescopes are perfect on axis, but coma damages image quality at even a modest distance off axis. At the focal plane, about 1mm off axis, in a F/4.5 (1.4mm in an F/5), the image is just at the diffraction limit and the strehl of even a perfect mirror has fallen to 0.8. In a 250mm scope, this gives a coma free, sharp field of about 6 arc minutes across, about 1/5 of the apparent diameter of the moon. In a 250mm F/5, the area is a little bigger, but just 8.4 arc minutes across. The field stop of a 9mm orthoscopic eyepiece is about 6mm so only the central 1/3 (1/9 of the area) of the view is free of coma in an F/4.5 scope. In an F/5 this increases to nearly half the field (1/4 the area).

Of course no telescope is perfect and eyepieces add their own aberrations, but I do not think coma is to be ignored any more than, say, mirror figure, thermal control, collimation or light control through flocking. My coma corrector lives in the focuser, except on occasion for viewing planets with my smaller scope which is now driven. In passing, I note that Ian King is currently listing the Altair Astro coma corrector at £88, including carriage.

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wow thanks chris!!! That is a much better price than anything offered here. Now lets see how much it comes out to with shipping and everything :)

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wow thanks chris!!! That is a much better price than anything offered here. Now lets see how much it comes out to with shipping and everything :)

I think you will find your best value may be the GSO at Agena, but currently they list no stock...

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How much pfaffing is involved attaching a coma corrector to an eyepiece and does it alter the focal length like the Paracorr does?

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How much pfaffing is involved attaching a coma corrector to an eyepiece and does it alter the focal length like the Paracorr does?

The Astro-Tech/Altair Astro/GSO coma corrector increases effective focal length, by about 10%.

It was a lot of trouble to get into service at first as I had problems with focus and the correct spacing, but there is no problem now this is sorted. The unit does not come ready to use (and mine had no documentation!), because eyepiece adapter is to short to give the correct spacing of 75mm (+/- 10mm or so) to your eyepieces' focal point, so you need to space it with Hyperion fine tuning rings and/or empty 2" filters. You also have to sink the entire unit (about 70mm of it) into your focuser and the focal point is moved in by about 10mm.

I must do a proper write up and review with photos and will try to get it done in the next couple of days.

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Cheers Chris, its sure sounds like a lot of fiddling!

The troubles we go through for great views in our beautiful hobby :grin:

Worth it though...

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