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Coma Corrector Required?

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#1 daveyfitz

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Posted 01 February 2013 - 12:04 PM

Hi.
I am soon going to buy a 12" newt (XT12g) which has focal ratio 4.9.

I want to buy a set of eyepieces "once", i.e. "do it right the first time".
(Haha, has that ever happened?)

ONE burning question is "will I need a coma corrector?"

Or, if I buy superior eyepieces, will I be fine without one?
(Don't hesitate to recommend particular EPs that would do the trick)

Follow up question: WHICH coma corrector?
Paracorr VERY expensive, Baader, GSO, Highpoint offerings much less.
Should I spend triple the price for the paracorr?

Thanks so much to anyone willing to share their expertise.

Dave

#2 MikeBOKC

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Posted 01 February 2013 - 12:15 PM

Others will surely chime in but my understanding is that a coma corrector is basically not necessary above f/6 or so, useful down around f/5 and pretty close to essential (unless you have high tolerance for coma) below f/4.5.

#3 Jon Isaacs

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Posted 01 February 2013 - 12:18 PM

Hi.
I am soon going to buy a 12" newt (XT12g) which has focal ratio 4.9.

I want to buy a set of eyepieces "once", i.e. "do it right the first time".
(Haha, has that ever happened?)

ONE burning question is "will I need a coma corrector?"

Or, if I buy superior eyepieces, will I be fine without one?
(Don't hesitate to recommend particular EPs that would do the trick)

Follow up question: WHICH coma corrector?
Paracorr VERY expensive, Baader, GSO, Highpoint offerings much less.
Should I spend triple the price for the paracorr?

Thanks so much to anyone willing to share their expertise.

Dave


Dave:

I have several Newtonians in the F/4-F/5 range and a set of Naglers and Panoptics and a Paracorr or two. There is no doubt that if you want the edge stars to be clean and round, a coma corrector is necessary, eyepieces do not corrector for coma.

Used Paracorr type 1s work very well at F/5 and even F/4, they frequently popup on Astromart for around $200... they are definitely the most user friendly, transparent in operation and they work at all magnifications...

Jon

#4 csrlice12

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Posted 01 February 2013 - 12:21 PM

Will you "Need" one at f4.9? Only you will truely be able to answer that question. Paracorrs correct for coma; coma is something that you either notice and it bothers the cr@% out of you, or you just don't notice it and can view fine without it (and truely, at f4.9, there will be some coma, below f4.5 and you can get into some real coma). I got a used Paracorr type 1; they show up in the classifieds for about $200 used (over $500 for a new T2 Paracorr). Best to find someone around who has one you can try. That's really the only way you'll know for sure. Not familiar at all with the other brands, but the Paracorr does have a reputation and is the standard that the others will only hope to match.....

#5 stratocaster

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Posted 01 February 2013 - 01:20 PM

All good summaries above. I had the same question for my f5. At the end of the day you're just going to need to try it out. Borrow one if you can. If not, just get ready to jump on the used market for a type I - they sell quickly. Worst case you can resell it for little or no loss.

For me, at f5 a paracorr is worth it.

#6 coutleef

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Posted 01 February 2013 - 01:24 PM

i am using a scope similar to you and asked the same question . finally bougth a paracorr. for me, it is not essential but very useful. it now stays permanently in the focuser.

i like it but could live without it.

#7 turtle86

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Posted 01 February 2013 - 01:41 PM

All good summaries above. I had the same question for my f5. At the end of the day you're just going to need to try it out. Borrow one if you can. If not, just get ready to jump on the used market for a type I - they sell quickly. Worst case you can resell it for little or no loss.

For me, at f5 a paracorr is worth it.


+1 Nothing to lose by buying one used and trying it out.
I can tolerate coma at f/5 without a Paracorr but much prefer the view with one.

#8 KaStern

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Posted 01 February 2013 - 02:06 PM

Hi Dave,

you might find that article be helpful:

http://www.astromart...p?article_id=50

Cheers, Karsten

#9 Achernar

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Posted 01 February 2013 - 02:53 PM

F/5 is generally the borderline for many folks as to whether or not they want a coma corrector. The best thing you can do is to try one to see if the improvment to the views is great enough to justify purchasing one. They can cost up to $500.00, and therefore they are not cheap, at least when purchased new. You could snag an original Televue Paracorr on the used market for $200.00 that has a tunable top. The Paracorr, at least the original one I have, has two caveats. One is they increase the focal length 15 percent. Another is they require 15 to 20mm of additional in travel compared to using the same eyepieces without it. Not all coma correctors work the same way, but this is what you can expect from a Paracorr. If you're going to use eyepieces with apparent fields of view of 82 or 100 degrees, yes I would say a coma corrector is worthwhile. I use one in both my F/4.5 Dobs all the time. Strictly speaking, they are not required but they do enhance the views, pinpoint stars edge to edge when you're looking at the Double Cluster or Pleiades adds to the "spacewalk experience" ultra-wide angle eyepieces offer.

Taras

#10 dscarpa

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Posted 01 February 2013 - 06:26 PM

The Astro Tech coma corrector looks like it's the same as the GSO. On Agena's site the GSO got good reviews and at $130-same as the AT after CN discount-they're priced right. I've got a Teeter STS 11" F/5 on order and lots of 82*-100* eyepieces. I asked Rob if he could make it a F/6 but he said it would be unstable. David

#11 Jon Isaacs

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Posted 02 February 2013 - 06:53 AM

Strictly speaking, they are not required but they do enhance the views, pinpoint stars edge to edge when you're looking at the Double Cluster or Pleiades adds to the "spacewalk experience" ultra-wide angle eyepieces offer.



It seems most tend to think of coma as a problem with off-axis sharpness of stars in widefield eyepieces but there's more to it than nice, clean stars across the field of view. At higher magnifications coma limits the size of the "sweet spot", only the very center of the field is even "diffraction limited."

In a 12 inch F/4.9 scope, at 200x, only the center 20 degrees of the AFOV qualifies as "diffraction limited", that perfect mirror it's Strehl of 0.9999 is down to 0.8... The Paracorr increases the size of the sweet spot by a factor of 6, a big plus for undriven scopes.

Jon

#12 dscarpa

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Posted 02 February 2013 - 11:49 AM

I just decided to swing for the Feather Touch with the built in coma corrector. Not the one on Teeter's site which is a regular FT with the corrector as a add on which ups the cost quite a bit.. The one on FT's site goes for $860 with base. I was going to get the Moonlight so my extra expense is around $694. David

#13 northernontario

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Posted 02 February 2013 - 12:57 PM

After several years of tollerating coma in my Dob, I too am considering cleaning up my act and opting for a parracorr.

Coma bothered me last night. First time ever that I found myself getting annoyed. I was viewing the BeeHive cluster in the wee hours.

So at f/4.5, it can be bothersome.

jake

#14 daveyfitz

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Posted 03 February 2013 - 12:03 AM

After several years of tollerating coma in my Dob, I too am considering cleaning up my act and opting for a parracorr.

Coma bothered me last night. First time ever that I found myself getting annoyed. I was viewing the BeeHive cluster in the wee hours.

So at f/4.5, it can be bothersome.

jake


I notice that folks pretty much equate "coma corrector" with "paracor".

The coma correctors from Baader, Highpoint, GSO, are so much cheaper, but I'm not finding much in the way of reviews.

Surely Baader makes a quality product?
The Baader appears to be the only one that has no magnification.

Is anyone having success with any of these more economical alternatives?

#15 Jon Isaacs

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Posted 03 February 2013 - 12:04 AM

I just decided to swing for the Feather Touch with the built in coma corrector. Not the one on Teeter's site which is a regular FT with the corrector as a add on which ups the cost quite a bit.. The one on FT's site goes for $860 with base. I was going to get the Moonlight so my extra expense is around $694. David


The Sips focuser with the built in Paracorr is certainly a nice way to go, no tunable top hassle. The downside is that it pretty limits you to using the Paracorr in only one scope. With the standard Paracorr, it can be used in a number of scopes.. It can also be removed if one want's the widest possible field of view or to view without it at high magnifications.

Jon

#16 TexasRed

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Posted 03 February 2013 - 04:41 AM

Am I the only one with an XT12g (f/4.92) who doesn't see a paracorr as necessary or worth the money?

#17 Jon Isaacs

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Posted 03 February 2013 - 07:13 AM

Am I the only one with an XT12g (f/4.92) who doesn't see a paracorr as necessary or worth the money?


This is the way I look at the Paracorr:

A Paracorr is never necessary. I spent several years with my 12.5 inch F/4.06 without a Paracorr, I just didn't have one, I didn't have eyepieces that were of sufficient quality to benefit. And I didn't have the money. When I was finally able to purchase a Paracorr, it transformed the views, it was the first time I was really satisfied with the scope and appreciated what a fine telescope it was and is.

Since I also had a 10 inch F/5, it was natural to use the Paracorr in that scope as well. While the views in that scope suffered less from coma, correcting the coma took the views to the next level, with reasonably well corrected eyepieces, the field of view were much cleaner off-axis.. And, as I previously pointed at, at high magnifications, viewing the planets and double stars, the sweet spot, the diffraction limited field of view, is much larger.

As to whether it is worth the money, a new one is quite expensive, used ones are around $200. It's a question of expectations and it depends on the eyepieces one is using. There is no doubt that there is visible coma at F/5 and that if one is using eyepieces that are free of off-axis astigmatism, the Paracorr does clean up the field, stars are round most everywhere.

So the questions I have for you are:

- What eyepieces are you using?

- Have you ever tried a Paracorr?

Bottom line:

At F/5, there is coma to be seen and a coma corrector really does take the views significantly closer to perfection. But a coma corrector is not necessary. People put up with various aberrations, field curvature in refractors and SCTs, chromatic aberration in achromatic refractors, they just live with them. Coma is the same way, a very visible aberration, it's one that can be corrected.

I don't recommend that when one buys an F/5 Newtonian they run out and buy a Paracorr, I don't recommend that they run out and buy a set of Naglers either. But... at some point, they both are very nice to use.

Jon Isaacs

#18 GadgetAce

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Posted 03 February 2013 - 08:01 AM

I opted to get a coma corrector (GSO for $120 at AgenaAstro); and I am glad I made the decision to get one for two reasons. Once the telescope (Zhumell 10inch F4.92) is properly collimated, the coma corrector does noticeably improve the view in all my eyepieces towards the edge - not perfect but much better than without it. In addition to the visual benefit, I discovered that the coma corrector solved the problem of my inability to get a focused image in my Nikon D3100 when attempting prime-focus astrophotography. The stock Crayford-type focuser that came with the scope lacks sufficient inward travel; and no combination of focal reducers or tele-extenders will help. The GSO coma corrector has a slight magnifying effect (1.1x), which might have something to do with it. All I know is that now I do not have to buy a new, low-profile focuser to achieve prime-focus astrophotography!

A couple of observations: the GSO coma corrector is heavy and may require you to re-balance your scope. And when seeing is fair to poor, you may find that planetary viewing does not benefit much, if at all, from using the corrector.

#19 northernontario

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Posted 03 February 2013 - 08:44 AM

After several years of tollerating coma in my Dob, I too am considering cleaning up my act and opting for a parracorr.

Coma bothered me last night. First time ever that I found myself getting annoyed. I was viewing the BeeHive cluster in the wee hours.

So at f/4.5, it can be bothersome.

jake


I notice that folks pretty much equate "coma corrector" with "paracor".

The coma correctors from Baader, Highpoint, GSO, are so much cheaper, but I'm not finding much in the way of reviews.

Surely Baader makes a quality product?
The Baader appears to be the only one that has no magnification.

Is anyone having success with any of these more economical alternatives?


Thank you for that bit of knowledge. :bow:

jake

#20 Jon Isaacs

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Posted 03 February 2013 - 09:02 AM

After several years of tollerating coma in my Dob, I too am considering cleaning up my act and opting for a parracorr.

Coma bothered me last night. First time ever that I found myself getting annoyed. I was viewing the BeeHive cluster in the wee hours.

So at f/4.5, it can be bothersome.

jake


I notice that folks pretty much equate "coma corrector" with "paracor".

The coma correctors from Baader, Highpoint, GSO, are so much cheaper, but I'm not finding much in the way of reviews.

Surely Baader makes a quality product?
The Baader appears to be the only one that has no magnification.

Is anyone having success with any of these more economical alternatives?


The Baader is designed for photography, it corrects the coma without changing the effective focal length but it is my understanding that it does add spherical aberration so it is not well suited for higher magnifications. It's a two element coma corrector rather than a 4 element like the Paracorr.

As far as the Paracorr being equated with coma correctors, it has been around far longer than the others and it is designed so that it is easy to use. To achieve the best possible coma correction, the spacing between the focal plane of the eyepiece and the coma corrector is important. With the Baader, this requires changing adapters. With the GSO (I believe they make the others), the spacing is fixed.

The Paracorr has the "tuneable" top which allows the spacing to be easily adjusted when switching between eyepieces. In the field, this works very well and it's easy to dial in the correct spacing when swapping eyepieces. Even easier is the SIPS which a collaboration between TeleVUe and Starlight Instruments, it's a Feathertouch focuser with an integrated Paracorr, once it is adjusted, the spacing between the eyepiece and the corrector is always correct because the Paracorr is fixed.

Paracorrs are not cheap but a collection of eyepieces that are free from astigmatism is considerably more expensive... getting the best possible views from a fast telescope does require an investment in eyepieces. At about $200, a used Paracorr is reasonable investment.

Jon

#21 MikeBOKC

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Posted 03 February 2013 - 11:38 AM

Well I had my maiden outing with a new xx14g last night (f/4.6) and I tested it without the AT coma corrector first to see how the CC altered or otherwise improved views. With it installed coma declined at the edges of the field by about 90 percent. It definitely made view of wide field objects more enjoyable and aesthetically pleasing. I would be somewhat less inclined to invest in the Paracorr which costs about three times as much. I find the AT model, which I imagine is pretty much identical to the GSO one, to be quite adequate for my needs. But I also find on first impression that it is a worthy visual upgrade, not necessary and quite desirable.

#22 cjc

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Posted 03 February 2013 - 01:44 PM

I have no regrets about buying the GSO/Astro-Tech, though mine was labelled Altair Astro. It gives a clean image and does not need a turntable unless your eyepieces require extreme focus excursion. Roger Ceragioli, the optical designer says the spots look good at +/-10mm on the nominal 75mm back focus.

#23 dscarpa

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Posted 03 February 2013 - 05:29 PM

I'll be using a lot of heavy eyepieces barlowed and not in my STS. Having the corrector built into the focuser will be needed so I don't end up with long combos with a fair bit of flex. David

#24 maroubra_boy

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Posted 04 February 2013 - 07:18 PM

A coma corrector ISN'T absolutely necessary. All the posts I've read make it sound like it is madatory in a fast newtonian to use one. How apparent it is depends on a few things though, your preference for AFOV, the focal length of the EPs used, the speed of your scope, and your own perception and viewing habits.

Many people mistaken coma for astigmatism and other aberrations they see through their eyepieces. There's only one tell tale sign of coma and that is the little comet tales that appear radiating out from the centre around the edge of the FOV. Anything else is a lack of correction in the eyepiece being used, and even an optical mismatch between the EP and scope (rarely ever considered). A coma corrector won't deal with astigmatism as this is in the eyepiece (mirror astigmatism aside).

My Newt's are all fast, with f/5 being the slowest and f/4 the fastest. I LOVE wide field viewing. I love my 8" f/4 dob giving me nearly a full 3deg TFOV with my Meade 34mm SWA, oversize exit pupil and all. Is coma present? Yes. Is it significant? No.

Coma appears along the edge of the FOV. In most instances it is not really significant. Also appearing along the edge of the FOV, it is not an area where serious observing is done - the scope is moved to inspect these areas.

I've tried coma correctors in my fast Newtonians. I ended up ditching them as I found that they killed my seeing of fainter stars than the resulting modification of the edge of the FOV was worth.

If you are doing photography I see a coma corrector as indispencible. But for visual I don't see the need as being anywhere as critical. Yes they "work", but I honestly see them as over rated for visual use.

Astigmatism, chromatic aberration, pincushion, field curvature are all much more easily delt with by careful eyepiece selection. After all this has been corrected for, I'd rather deal with a little soft edge from coma than add another set of glass elements for little gain along the edge and loose those precious photons we all crave so much.

BUT where it can become an issue though with visual is using the ultra, ultra wide eyepieces, 100deg +. Here, the increased magification afforded will increase the apparent size of the aberration. Coma in a 21mm 100deg EP will be more significant than in my 34mm 68deg eyepiece by virtue of the increased magnification even though the TFOV is just about identical. Here one's observing habits and prefered AFOV start to play a part. AND as you increase magnification with the same AFOV EPs, coma is reduced. Just remember, you will sacrifice some photons for this. For me, just a blanket necessity for a coma corrector makes no sense. You need to consider these aspects before you make a choice to use one.

If you can, try to borrow one and compare the view with and without before you lay your money down. Not a quick peek-a-boo, but an extended examination with all your viewing habits. You may be surprised.

#25 cjc

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Posted 05 February 2013 - 03:57 AM

A coma corrector ISN'T absolutely necessary...

A coma corrector is necessary if you want a fast Newtoninan to be sharp off axis. Coma degrades the image and like other aberrations, it is something to be minimised.

Just my tuppence.






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