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nitram100

Coma Corrector for visual

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Hi guys,

 

Does anyone recommend a coma corrector for visual use with my OO 250mm F4.8?

I have just acquired a 32mm Moonfish 82 degree eyepiece for cheap and was wondering if a coma corrector would eliminate softening at the edges?

Thanks,

Martin

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Hi Martin, I would recommend you look at the following post:

 

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A Paracorr would be a good option if affordable. The Moonfish eyepiece is not a particularly well corrected eyepiece for fast focal ratios.    ?

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The coma corrector will eliminate or reduce the coma but it can also make other aberrations which were masked by the coma more apparent. In other words, unless using good quality eyepieces (I think the Moonfish is reasonable rather than good but have not used one) you might swap one issue for another and see no difference in reality.

This is a good site explaining the different aberrations

http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

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I'd pick up a Revelation (GSO) coma corrector as a lower cost starter for your scope.  You'll just need to add a 15mm to 20mm spacer ring between the eyepiece holder and the optics element.  It corrects out most of the coma in that default configuration.

What you're seeing with the Moonfish is not coma, though.  It is a mixture of astigmatism and field curvature.  The CC will flatten the field, helping the field curvature, but their is still quite a bit of it left.  If you refocus for the edge right now without a CC, you'll see astigmatism is the main aberration.  Stars will switch between radial and tangential lines on either side of focus and appear as a cross or blob at best focus in the middle of the two.  I like to think of this eyepiece as a very sharp 40 degree eyepiece with another 20 degrees beyond that on all sides for framing.

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Thanks very much for all the info guys, will check out those links and have a little read!

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I used to use an Orion Optics 10" F/4.8 without a coma corrector and found the views pretty well corrected even with Ethos 100 degree eyepieces. Coma was present out by the field stop but not, to my eye, distracting.

The Moonfish 30mm 80 degree eyepieces (I've owned one) do show quite a lot of outer field astigmatism even with F/6 scopes so at F/4.8 it's going to be really quite apparent. I'm not convinced that a coma corrector can do much to help with that eyepiece. It might be better to put the £'s towards a better corrected 30mm eyepiece ?

 

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Thanks John, after doing some reading I think you're right. Better to spend some more cash and get a few good eyepieces. In all honesty, is Televue the only real option? For wide angle eyepieces with a f4.8 scope? Or is there an in between brand of eyepieces that would suffice. 

 

Thanks!

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1 hour ago, nitram100 said:

Thanks John, after doing some reading I think you're right. Better to spend some more cash and get a few good eyepieces. In all honesty, is Televue the only real option? For wide angle eyepieces with a f4.8 scope? Or is there an in between brand of eyepieces that would suffice. 

 

Thanks!

Tele Vue specialise in well corrected eyepieces for fast scopes and are individually tested down to F/4 but there are other options which also do pretty well well including Explore Scientific, Nirvana / UWAN's, some of the APM (Germany) eyepieces and Meade 5000 UWA's.

 

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I've got a Moonfish, I've never used to with my Baader c/c but I'd be surprised if it would be enough to correct the aberrations at f5.

 

 

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2 hours ago, nitram100 said:

Thanks John, after doing some reading I think you're right. Better to spend some more cash and get a few good eyepieces. In all honesty, is Televue the only real option? For wide angle eyepieces with a f4.8 scope? Or is there an in between brand of eyepieces that would suffice. 

 

Thanks!

There are other expensive options like the Nikon NAV-HW and NAV-SW and Pentax XW which also do well in faster scopes.  John lists some good mid-priced options that trade some edge correction, smoothness of polish, and attention to stray light control to keep prices down.  For general stargazing, you may never notice the difference if you never look through the higher-end offerings.  I would add the Baader Morpheus as one more line to look into.

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On 14/01/2019 at 11:02, nitram100 said:

32mm Moonfish 82 degree eyepiece......

My assumption here, the Moonfish is a 2" fitting, therefore you would need a 2" coma corrector.

I started to collect Delos eyepieces by TeleVue for this very reason, owning a much faster scope,  but decided the 8" Skyliner is staying longer, and I could not see any benefit of keeping and using the full  range of Delos EP's  on my f/6 when my Starguiders are doing  such a grand job.

One thing to note however..........for the shorter focal lengths, a good 2xBarlow will double your focal ratio, allowing  the shorter focal lengths to show some improved performance towards the edge of field.

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I have an OO 250 F4.8 and use a paracorr 2. My 26mm Nagler prompted me to get it as I couldn’t stand the coma with this eyepiece. It improves the view with my other televues but not to the same degree. I’m glad I have it.

Edited by Scooot
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The Paracorr would be best, and requires about 14mm of in-travel of the focuser.

The Explore Scientific is also good and requires 32mm of in travel of the focuser.

Both make all your eyepieces parfocal since, once the correct position of the coma corrector is found, differences in focal position are accommodated by the tunable tops.

You only need a couple millimeters travel in the focuser after that.

As has been mentioned, correcting the coma (and slightly flattening the field, which both of these do), may not correct the largest aberrations in the eyepiece.

If the eyepiece has a curved focal plane which causes stars at the edge to be a bit out of focus, it may not be corrected.

If the eyepiece has significant astigmatism (likely the main issue with the Moonfish), it will not be corrected.

Since, at that f/ratio, astigmatism in the eyepiece is likely to be a significantly larger problem than coma, I would first address the astigmatism issue (alas, astigmatism-free long focal length eyepieces

tend to be large, heavy, and expensive), then address the coma issue at a later time when you are looking for perfect star images. 

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For that scope, I’d spend the money on good eyepieces. The Explore Scientific 82° range will do well in that scope. Yes. Televue will do a little bit better, but ££££!!! You don’t need to jump straight into the 100° exotica. Try a couple of eyepieces in the 68° to 82° range. 50° makes star hopping more difficult than it needs to be.

Paul

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Thanks so much everyone, great info! Currently looking at the Skywatcher Panorama eyepieces on FLO, the 23mm looks like a nice all purpose eyepiece. Does anyone have any experience with the panorama eyepieces?

 

Thanks again everyone

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3 hours ago, nitram100 said:

Thanks so much everyone, great info! Currently looking at the Skywatcher Panorama eyepieces on FLO, the 23mm looks like a nice all purpose eyepiece. Does anyone have any experience with the panorama eyepieces?

 

Thanks again everyone

They are suspected to be the same as the Celestron Luminos eyepieces, so search for reviews and comments about them as well.

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