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exmedia
sage
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Reged: 05/26/13

Loc: Orange County, CA
Newtonian coma corrector?
      #5927219 - 06/18/13 10:43 AM

I have an f/5 C8-Newtonian that I would like to improve for my visual use. Is a coma corrector the accessory I'm looking for, or something else? I've seen field flatteners/reducers listed, but I'm not sure what they do and they appear to be for refractors. Muddling along here.

Any input will be appreciated.


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Don Trinko
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Reged: 07/05/09

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Re: Newtonian coma corrector? new [Re: exmedia]
      #5927287 - 06/18/13 11:15 AM

At f5 coma could appear but should no be to bad. most coma correctors are 2" and not cheap.
Try with different eye pieces. some show less coma than others. Don T.


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Tony Flanders
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Re: Newtonian coma corrector? new [Re: exmedia]
      #5927432 - 06/18/13 12:28 PM

Quote:

I have an f/5 C8-Newtonian that I would like to improve for my visual use. Is a coma corrector the accessory I'm looking for ...




Is there a specific problem that you're trying to correct?


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exmedia
sage
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Reged: 05/26/13

Loc: Orange County, CA
Re: Newtonian coma corrector? new [Re: Tony Flanders]
      #5927471 - 06/18/13 12:46 PM

Quote:

Is there a specific problem that you're trying to correct?




When it's accurately collimated it is very sharp in the center 30% or so of the field of view. I've recently been using a couple of used 65-75 deg ep's and I am very aware that the stars/objects near the edges of the fov aren't nearly as sharp and sometimes can see the hair-like coma effect. I would like to have the entire wide angle fov appear sharper.


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Achernar
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Re: Newtonian coma corrector? new [Re: exmedia]
      #5927485 - 06/18/13 12:56 PM

A coma corrector is a big help for a fast Newtonian because by removing the coma, star images away from the center of the field of view are tighter and sharper. You can see fainter stars, and the field of view is that much more pleasing, assuming your main optics are good, well collimated and the eyepiece is not subject to aberrations of it's own. Many less expensive and sophisticated eyepieces have astigmatism or curvature of field, flaws that coma can hide, but a coma corrector will render glaringly apparent. Coma increass very rapidy as the focal ratio decreases, an F/4 mirror has three times the coma an F/5 mirror does. If you intend to do imaging with the telescope, a coma corrector is mandatory if you want pinpoint stars to the edge of the field of view. If you have an F/5 or faster Newtonian or Dobsonian, a coma corrector is highly recommended, especially if you plan to use wide and ultra-wide angle eyepieces with it. I use two orginal TeleVue Paracorrs with my 10 and 15-inch Dobs for this reason. That is why they live in the focuser.

Taras


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Tony Flanders
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Re: Newtonian coma corrector? new [Re: exmedia]
      #5927491 - 06/18/13 12:58 PM

Quote:

When it's accurately collimated it is very sharp in the center 30% or so of the field of view. I've recently been using a couple of used 65-75 deg ep's and I am very aware that the stars/objects near the edges of the fov aren't nearly as sharp and sometimes can see the hair-like coma effect.




The eyepieces themselves are the first suspect. What kind of eyepieces are they?

Coma 1/3 of the way to the edge should be mighty small in an f/5 scope.


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SteveG
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Reged: 09/27/06

Loc: Seattle, WA
Re: Newtonian coma corrector? new [Re: Tony Flanders]
      #5927557 - 06/18/13 01:41 PM

Coma will show up in a newtonian scope, and might be objectionable at f5 and below, regardless of eyepieces used. I have an f5 scope and coma can be seen, especially at low-power using high-quality eyepieces. Like Taras states the Paracorr completely eliminates coma and greatly enhances the wide-field views - and even high-power views are improved. I purchased a type 1 tunable top Paracorr last year and it lives in the focuser of my f5 newt.

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Jon Isaacs
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Reged: 06/16/04

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Re: Newtonian coma corrector? new [Re: exmedia]
      #5927590 - 06/18/13 01:55 PM

Quote:

Quote:

Is there a specific problem that you're trying to correct?




When it's accurately collimated it is very sharp in the center 30% or so of the field of view. I've recently been using a couple of used 65-75 deg ep's and I am very aware that the stars/objects near the edges of the fov aren't nearly as sharp and sometimes can see the hair-like coma effect. I would like to have the entire wide angle fov appear sharper.




Richard:

As Tony said, the first question is what eyepieces are you using? In an F/5 telescope, most eyepieces are only sharp towards the center of the field of view, towards the edge, the stars are no longer round and clean.

Eyepieces that are "well corrected" for off-axis astigmatism are generally somewhat expensive, TeleVue has been the leader in the field of eyepieces that are free from Astigmatism in a fast scope but there are other more affordable eyepieces, that are good performers as well.
Unless one is using well corrected eyepieces, it will be difficult to see the coma because the astigmatism will overwhelm it.

The big question is always, what eyepieces are you using?

To try to identify what you are seeing, start with a magnitude 2-3 star in the center of the field, focused sharply and move it towards the edge.

Coma looks like a fan shaped tail pointing away from the center of the field. Ideally, astigmatism looks like a line that changes origination 90 degrees orientation as you move through focus. It's often mixed with small doses of other aberrations so it is not necessarily a line or even a blurry line. But viewed off-axis, near the edge, the change in orientation should be noticeable as you pass through focus. (Passing through focus looking at that star)

Field curvature is another aberration, the stars at the center focus sharply, the stars at the edge focus sharply, but the cannot be focused sharply simultaneously. Newtonians do not exhibit a great deal of field curvature but mix in a little field curvature, some astigmatism, some coma... it's not so easy to tell what is happening.

At F/5, many are not bothered by coma, I don't find it overly objectionable but I do notice it and since I have a Paracorr to correct the coma, I rarely use a F/5 Newtonian without one.

So...

If all that is confusing, I offer you this: I see you are in Orange County. If you ever get down my way to San Diego, I do have a Paracorr and a variety of eyepieces, I even have a 8 inch F/5 Newtonian... You could see what coma looks like, astigmatism, field curvature... I could see what I think is going on with your eyepieces, with your scope.

Jon


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exmedia
sage
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Reged: 05/26/13

Loc: Orange County, CA
Re: Newtonian coma corrector? new [Re: Jon Isaacs]
      #5927814 - 06/18/13 03:54 PM


Quote:

The big question is always, what eyepieces are you using?

So...

If all that is confusing, I offer you this: I see you are in Orange County. If you ever get down my way to San Diego, I do have a Paracorr and a variety of eyepieces, I even have a 8 inch F/5 Newtonian... You could see what coma looks like, astigmatism, field curvature... I could see what I think is going on with your eyepieces, with your scope.

Jon




Jon, thanks for all the info and and kind offer. It's difficult for me to get to S.D. so that's not an option. Based on what you and others wrote, I think what I need to do before anything else is to get some quality eyepieces. Tele Vue ep's and Paracorr coma correcter are definitely out of my price range. I've read good reviews about Explore Scientific 82 deg series and, at $99, are within my budget. Would be interested to hear what you (and anyone else who's reading) think.

Thanks, Richard


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David Knisely
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Re: Newtonian coma corrector? new [Re: Don Trinko]
      #5927858 - 06/18/13 04:16 PM

Quote:

At f5 coma could appear but should no be to bad. most coma correctors are 2" and not cheap.
Try with different eye pieces. some show less coma than others. Don T.




The Coma in a Newtonian is caused by the paraboloidal primary mirror and not by eyepieces. People often mistake eyepiece-induced astigmatism for coma, as it can mimic that effect depending on how the focus is adjusted. Astigmatism can make the stars in the outer portions of the field of view look like comets, seagulls, or oval blobs depending on where you have the focus adjusted. The less complex (and less expensive) eyepieces often have enough astigmatism in the outer field to completely cover up the coma of an f/5 Newtonian. At f/5, it is kind of a "maybe yes, maybe no" decision point where it comes to considering getting a coma corrector. Below f/5, the coma of the telescope itself tends to be enough of a problem that coma correctors really tend to be more useful. Clear skies to you.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Newtonian coma corrector? new [Re: exmedia]
      #5927864 - 06/18/13 04:19 PM

Quote:

I've read good reviews about Explore Scientific 82 deg series and, at $99, are within my budget. Would be interested to hear what you (and anyone else who's reading) think.

Thanks, Richard




Richard:

I have very little experience with the 82 degree Explore Scientific eyepieces but I hear lots of good things about them.

I do recommend contacting Don Pensack at EyepiecesEtc

Don is a long time observer, a long time member of Cloudy Nights. When it comes to eyepieces, Don is someone I trust more than I trust myself. He's in the L.A. area...

He can provide you with the guidance and eyepieces you need.

Jon


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SeattleScott
Pooh-Bah


Reged: 10/14/11

Re: Newtonian coma corrector? new [Re: Jon Isaacs]
      #5927888 - 06/18/13 04:31 PM

My 24mm ES 82 deg eyepiece does quite well in my F4.8 Newtonian. Is not perfect at the extreme edge like my vixen LVWs but still quite good. If you don't wear glasses when observing these work great. A bit heavy in the longer focallengths though.

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Newtonian coma corrector? new [Re: exmedia]
      #5927932 - 06/18/13 05:01 PM

Yes, as has been pointed out, the star images at the edge of the field exhibit a combination of the most common aberrations in eyepieces.
I had an eyepiece in my f/5 scope that seemed to exhibit horrible coma, and I thought it was entirely the scope so i bought a TeleVue Paracorr coma corrector. With the coma eliminated, I saw the eyepiece had field curvature which meant only the center was in focus and the stars at the edge were slightly out of focus. That defocusing caused coma to look larger and worse than it really was.
I was younger at the time, so I learned to focus about half-way from the center to the edge of the field in that eyepiece, and my eye's accommodation could focus the entire field, both center and edge.
Now, being older, I cannot accommodate such a large diopter difference, so I no longer have that eyepiece.

But, a coma corrector definitely improved the star images in the entire field--by eliminating coma and by applying a slight amount of field flattening.

It might be hard to justify a coma corrector, though, if its price is similar to what you paid for the entire scope. Will it work? Yes. Can you justify the expense? Well, that's up to you.


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Achernar
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Reged: 02/25/06

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Re: Newtonian coma corrector? new [Re: Jon Isaacs]
      #5928125 - 06/18/13 07:20 PM

I have all of them except for the 30mm, which I will probably buy sometime sooner than later. They are well built, and the image quality is very good. They are very sharp, have excellent contrast and color correction, and little field curvature that I can see. Flare and scatter are suppressed very well. They are indeed sealed and purged with nitrogen, or argon for the latest production which is nice when you are in a dewy, wet area like I am. Indeed, they come close to Naglers in performance, but at a much lower price. The main drawbacks are the longer focal length models are quite heavy, and that could be a problem for smaller telescopes. Also the eye-relief is too short for those who must wear glasses while observing. I wear them, but I take my glasses off while peering into my ES 82 degree eyepieces. For the price, they are an very good value, everyone I know who has tried them has a favorable impression of them. They work very well in F/5 and F/4.5 telescopes too.

Taras


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exmedia
sage
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Reged: 05/26/13

Loc: Orange County, CA
Re: Newtonian coma corrector? new [Re: Achernar]
      #5928443 - 06/18/13 11:35 PM

Quote:

...For the price, they are a very good value, everyone I know who has tried them has a favorable impression of them. They work very well in F/5 and F/4.5 telescopes too.

Taras




Taras,

Thanks for your first-hand impressions. This entire thread has been helpful in narrowing down my choices to the first step in the quest for better resolution: acquiring quality eyepieces, which for me means the ES 82 series.

-Richard


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Feidb
Pooh-Bah


Reged: 10/09/09

Loc: Nevada
Re: Newtonian coma corrector? new [Re: exmedia]
      #5928862 - 06/19/13 09:41 AM

Personal opinion from someone who's been at this for 47+ years, totally unnecessary. I use an f/4.5 and I use what's considered "low end" to "moderate" EP's, at best.

Since most of the objects I look for are small, faint fuzzies, I'm only concerned with the center of the field. However, even when it's an extended object, I don't find a little distortion at the edges objectionable. I've seen the amped up super flat and fully corrected fields and wasn't impressed. The tweaks gained weren't worth the extreme cost.

If you want to fork out maybe the price of your scope, or even more, just to get a slightly flatter field, more power to you.

That's just my two cents.


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Jon Isaacs
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Re: Newtonian coma corrector? new [Re: Feidb]
      #5928925 - 06/19/13 10:18 AM

Quote:

If you want to fork out maybe the price of your scope, or even more, just to get a slightly flatter field, more power to you.

That's just my two cents.




Field flatness/curvature is not the issue here,it's coma and astigmatism. Richard wants a sharper, cleaner field of view, it is possible achieve a reasonably sharp field of view without spending large amounts of money. The Explore Scientific 68 degree eyepieces are quite good and quite affordable. If one is willing to purchase used, the very similar Meade series 5000 SWAs are added to the mix.

There is value in a well corrected field of view even if the targets are small, particularly for those whose preferred method of navigation is starhopping. A sharper view means that small, faint objects near the edge of the field are more easily distinguished from a star.

Jon


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