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Coma corrector for refractor??

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#1 oldtimer



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Posted 12 September 2013 - 08:11 AM

I have a 100mm F-5 achro. My 2" 30mm TMB Paragon is pretty sharp out to about 80% but then the stars get fuzzy/elongated. Will a coma corrector help?

#2 Eddgie


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Posted 12 September 2013 - 09:06 AM

An air spaced doublet will normally be coma free even at f/5.

If the edge of the field is not sharp, there are three possible causes.

The most likely is field curvature. A 100mm f/5 telescope will have a radius of curvature of the field (RC) equal to about 1/3rd of the focal length. This means that the RC of your scope is about -166mm.

Now considering that a C8 has an RC = 270mm and these scopes are considered to have meaningful field curvature, you can see that your scope is much worse by comparison.

Next is field curvature of the eyepiece. Here, it gets complicated. If the telescope has field curvature and the eyepiece has field curvature, the results depend on how these curves work with or against one another.

If the eyepiece has an opposite curve, the two can work together to cancel each other out. If the fields are curved the same way, they can work together to make it worse for you. If the field of the eyepiece is relatively flat (and in most cases, eyepieces have far flatter fields than the telescopes they are used in) then mostly what you see is coming from the telescope.

And what you are seeing is astigmatism from the eyepiece, but not because the eyepiece itself has astigmatism (and even the very very best eyepieces have some astigmatism but because you are seeing the blur of the eyepiece formed image out of focus, making it easier to resolve the astigmatic blur from the eyepiece.

What to do?

Field curvature can be counteracted most effectively by lowering magnification. The longer the focal length of the eyepiece for a given true field size, the harder it will be to see the curvature.

For example, if you used a 22mm Nagler, the magnification at the edge of the field would be higher than with a 27mm Panoptic, making it easier to see the blur of the astigmatism from the eyepiece (which is out of focus at the edge of the field) in the Nagler.

Both eyepeices give about the same size true field, but the lower magnification of the Panoptic makes it more difficult to see that the outside of the field is not in perfect focus. Simple. Lower magnification does not allow the blur to grow in size enough for your eye to see it.

If you are using 68 degree eyepeices, try going to a 58 degree eyepiece with the same size true field, again, reducing the magnification at the outside edge by lowering the power.

The next thing you can do is to see if you can visually accommodate some of the curvature. Try focusing half way out in the field. If you are a younger observer, this will reduce the size of the blur at the outside of the field making stars look sharper there, and your eye's own ability to refocus will allow it to refocus the center of the field automatically when you move your gaze from the outside do the center of the field.

Last, you can buy a field flattener.

Field curvature is to me a serious issue with smaller refractors because it makes it hard to get a field that is super sharp from one side to the other using modern ultra wide and super ultra wide eyepieces.

Anyway, not coma you are seeing. Air Spaced doublets are corrected for coma.

#3 Jon Isaacs

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Posted 12 September 2013 - 10:04 AM

Field curvature is to me a serious issue with smaller refractors because it makes it hard to get a field that is super sharp from one side to the other using modern ultra wide and super ultra wide eyepieces.

I agree that field curvature is part of what Gary is seeing. But I believe that at F/5, the eyepiece astigmatism, in focus or out of focus, of the 30mm Paragon is a serious contributor to the edge aberrations.

The possible choices for Gary that I see:

A better corrected eyepiece, the 31mm Nagler at F/5 is an admirable performer, but it's a lot of money and only the field curvature will be visible but the field curvature remains.. In my experience, a darn good view though not perfect.

- Switching to a lower power eyepiece, again that costs money and unless it is better performing (read more expensive) eyepiece in terms of off-axis astigmatism, the astigmatism will still be there. This solution also suffers because it's just hides field curvature, it similarly hides small objects.

- Accept it for what it is and enjoy the view anyway... This is my recommendation.

I am a "widefield" junky and I am lucky enough to own what is the ultimate, visual, 4 inch widefield refractor, the TeleVue NP-101. It is corrected for field curvature and chromatic aberration. Add the 31mm Nagler and the views are about as good as it gets. It performs admirably at any magnification I throw at it, this is the virtue of the NP-101, it does it all.

But I also own an Orion 100mm F/6 Achromat, a little longer focal length than the 102mm F/5 but it's quite similar, I have owned the 102mm F/5 as well. With a decent quality 30mm Widefield eyepiece, it's a good performer. It's not as perfect as the NP-101 but so what? There are plenty of nights when I haul out the 100mm F/6 and just enjoy the views, they enjoyable.. My goal is to enjoy the evening, that doesn't take a perfect scope with a perfect eyepiece...

That's my two cents.


#4 T1R2


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Posted 12 September 2013 - 02:20 PM

I agree with Jon and Eddgie, better ep's will make a BIG difference, if money is a factor get some ES68*, I don't think you'll disappointed with the performance at the edge of the field.

#5 john2233


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Posted 13 September 2013 - 01:41 AM

I have a Paragon 30mm with an f5 scope and I can confirm that the scope needs a better eyepiece. The scope has field curvature, but the Paragon 30mm really makes it stand out. I fixed the problem with a Nagler 22mm.

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