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Equipment Discussions >> Eyepieces

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daveyfitz
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Reged: 01/02/13

Loc: Utah
Coma Corrector Required?
      #5656228 - 02/01/13 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


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MikeBOKC
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Loc: Oklahoma City, OK
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5656250 - 02/01/13 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.

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5656254 - 02/01/13 12:18 PM

Quote:

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


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csrlice12
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Re: Coma Corrector Required? new [Re: daveyfitz]
      #5656256 - 02/01/13 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.....

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stratocaster
sage
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Reged: 10/27/11

Re: Coma Corrector Required? new [Re: csrlice12]
      #5656355 - 02/01/13 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.


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coutleef
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Re: Coma Corrector Required? new [Re: stratocaster]
      #5656364 - 02/01/13 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.


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turtle86
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Reged: 10/09/06

Re: Coma Corrector Required? new [Re: stratocaster]
      #5656394 - 02/01/13 01:41 PM

Quote:

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.


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KaStern
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Reged: 04/18/06

Loc: InTheDark
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5656437 - 02/01/13 02:06 PM

Hi Dave,

you might find that article be helpful:

http://www.astromart.com/articles/article.asp?article_id=50

Cheers, Karsten


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Achernar
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Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5656523 - 02/01/13 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


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dscarpa
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Loc: San Diego Ca.
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5656911 - 02/01/13 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

Edited by dscarpa (02/01/13 07:02 PM)


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Jon Isaacs
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Re: Coma Corrector Required? new [Re: Achernar]
      #5657636 - 02/02/13 06:53 AM

Quote:

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


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dscarpa
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Loc: San Diego Ca.
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5657985 - 02/02/13 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

Edited by dscarpa (02/02/13 12:10 PM)


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northernontario
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Reged: 07/01/09

Loc: Porcupine, Ontario Canada
Re: Coma Corrector Required? new [Re: dscarpa]
      #5658105 - 02/02/13 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


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daveyfitz
member


Reged: 01/02/13

Loc: Utah
Re: Coma Corrector Required? new [Re: northernontario]
      #5659164 - 02/03/13 12:03 AM

Quote:

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?


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Jon Isaacs
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Loc: San Diego and Boulevard, CA
Re: Coma Corrector Required? new [Re: dscarpa]
      #5659165 - 02/03/13 12:04 AM

Quote:

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


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TexasRed
scholastic sledgehammer


Reged: 05/17/11

Loc: East Texas
Re: Coma Corrector Required? new [Re: Jon Isaacs]
      #5659359 - 02/03/13 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?

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Jon Isaacs
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Loc: San Diego and Boulevard, CA
Re: Coma Corrector Required? new [Re: TexasRed]
      #5659437 - 02/03/13 07:13 AM

Quote:

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


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GadgetAce
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Reged: 09/04/10

Loc: Texas
Re: Coma Corrector Required? new [Re: Jon Isaacs]
      #5659488 - 02/03/13 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.


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northernontario
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Reged: 07/01/09

Loc: Porcupine, Ontario Canada
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5659520 - 02/03/13 08:44 AM

Quote:

Quote:

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.

jake


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5659541 - 02/03/13 09:02 AM

Quote:

Quote:

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


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MikeBOKC
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Re: Coma Corrector Required? new [Re: daveyfitz]
      #5659810 - 02/03/13 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.

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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: Coma Corrector Required? new [Re: Jon Isaacs]
      #5660024 - 02/03/13 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.

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dscarpa
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Re: Coma Corrector Required? new [Re: daveyfitz]
      #5660417 - 02/03/13 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

Edited by dscarpa (02/03/13 05:35 PM)


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maroubra_boy
Vendor - Gondwana Telescopes


Reged: 09/08/09

Loc: Sydney, Australia
Re: Coma Corrector Required? new [Re: dscarpa]
      #5662332 - 02/04/13 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.


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: Coma Corrector Required? new [Re: maroubra_boy]
      #5662891 - 02/05/13 03:57 AM

Quote:

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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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Coma Corrector Required? new [Re: maroubra_boy]
      #5662925 - 02/05/13 05:04 AM

Quote:

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.




It also shows itself as reduced planetary detail when one is not centered in the field of view at higher magnifications.

Jon Isaacs


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cadiz - Spain
Re: Coma Corrector Required? new [Re: Jon Isaacs]
      #5662945 - 02/05/13 05:59 AM

I understand that there are people who need to use the Paracorr in their fast telescopes, especially if we talk about a focal below f5, but sincerely, from this focal, I do not consider necessary. In fact, I do not use it on my 16" f5.

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stratocaster
sage
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Reged: 10/27/11

Re: Coma Corrector Required? new [Re: VĂ­ctor MartĂ­nez]
      #5664179 - 02/05/13 07:09 PM

As mentioned previously, the OP will just need to try one out to see if he feels it is necessary for himself. And "necessary" is probably too strong a word for visual use in an f4.9. Let's just slightly rephrase the question as "Is a Coma Corrector (highly) Desirable", or "Is a Coma Corrector Worth It". The OP is only going to know when he tries it out for himself. It was worth it to me even at f5, but clearly it was not worth it to others. There is no global definitive answer to this.

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daveyfitz
member


Reged: 01/02/13

Loc: Utah
Re: Coma Corrector Required? new [Re: MikeBOKC]
      #5667497 - 02/07/13 05:07 PM

Quote:

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.




That is quite a savings over the Parracor, I'm happy to hear that a 4.6 user is content with the GSO type CC.

I would be interested to know what eyepieces you were using to evaluate the coma (or lack thereof).


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: Coma Corrector Required? new [Re: daveyfitz]
      #5668345 - 02/08/13 05:07 AM

Quote:

...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...




This mirrors my experience. This Roger Ceralioli design works well and needs no turntable with a sensible choice of eyepieces. I currently use my scopes with ES82s, ES68s and a Baader Hyperion Aspheric.


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Re: Coma Corrector Required? new [Re: cjc]
      #5668369 - 02/08/13 06:13 AM

Quote:


This mirrors my experience. This Roger Ceralioli design works well and needs no turntable with a sensible choice of eyepieces. I currently use my scopes with ES82s, ES68s and a Baader Hyperion Aspheric.




The original Paracorr did not have the Tunable Top, I've had one for years, it works reasonably well. Those can be fitted with the Tunable Top but instead I got a whole new one, it does work better.

Jon


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Re: Coma Corrector Required? new [Re: maroubra_boy]
      #5668645 - 02/08/13 10:14 AM

Quote:

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.




This has been my own recommendation for the SCT forum for a few years now.

For a given size true field, the shorter the focal length of the eyepeice, the more apparent the coma becomes.

For telescopes with coma or field curvaturre, a Panoptic (or similar high quality 68 degree wide field eyepiece) with the same size true field as a Nagler or Ethos will magnify the abberated blur so much less that it becomes very hard to see.

Some of the reason for why some people say that coma is bad at f/5 while others say it is not may simply be because some people might be using an Ethos or Nagler with a given true field while someone else might have the same scope, but is using a Panoptic with the same size true field.

One says the coma is bad (Ethos user), one says it is OK (Nagler user) and the Panopitic user might say "Coma? I don't see no stinking coma! What are they talking about?"

If the OP wants to use giant AFOV eyepeices, then a coma corrector will start to look better and better.

But here is the problem with that. Once of the primary advantages of a big Newtonian reflector is that it can easily provide a big exit pupil. A big exit pupil makes extended objects appear brighter.

Uaing an Ethos, then putting it behind a Paracorr basically gives a very smaller exit pupil than using a Panoptic with a similarly wide field of view.

And deep sky observing is very much about exit pupil. Lots of people using small scopes routinely see "Difficult" targets like the Whirlpool and Veil nebula because they know that you can see them just as easily in a 4" refractor as you can in a 12" reflector if you use a 6mm Exit Pupiil in both scopes.

My personal recommendation to the OP would be to carefully consider the kinds of eyepecies he wants to use and base the decision on the Paracorr on whether he goes with Panoptics or with ultra-wides types.

And one shold not forget that the "Light Bucket" dob will not show the Orion nebula any brigher than an 80mm APO if it is used with an exit pupil that is smaller than that used in an 80mm refractor.


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Re: Coma Corrector Required? new [Re: MikeBOKC]
      #5668819 - 02/08/13 11:47 AM

Quote:

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.




Though your coma corrector doesn't have a tunable top, you can improve on its coma correction. here's how:
--first, get a few parfocalizing rings for your eyepieces (scopestuff.com or someplace else). Slide them on the eyepiece barrels but don't tighten them down.
--try all your eyepieces in the coma corrector. Pick the one with the focus position at the most inward position of your focuser. Remove the parfocalizing ring from that one.
--re-focus the telescope. When you insert the second eyepiece, instead of focusing using the focuser, pull the eyepiece out of the coma corrector until it is in focus and clamp down on it there. Slide the parfocalizing ring against the coma corrector and tighten it down.
--repeat using the same technique on all your eyepieces. Your eyepiece set is now parfocal and the coma correction is equal in all of them.

But let's say that if you take any individual eyepiece and try sliding it in and out, refocusing after each slide, and you find one position of the eyepiece that results in near-perfect coma correction. Then THAT IS the position around which you should parfocalize your eyepiece set. You may have an eyepiece or two that requires a change in focus from there (because they need to get even closer to the lens than the barrel of the coma corrector allows), and for those you will not get near-perfect correction (just good), but all your other eyepieces will be "optimized" for the coma corrector and be parfocal.

How nice is that?
It's possible an eyepiece with a 2" skirt around a 1.25" barrel may have to be used as a 1.25" eyepiece to get it parfocal with your other eyepieces. If so, use that eyepiece as a 1.25" eyepiece.

But now you have better coma correction for your eyepieces and you've made them parfocal to boot. You don't need to adjust any tunable top to your coma corrector--you used parfocalizing rings to make them properly positioned as you insert them!


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daveyfitz
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Re: Coma Corrector Required? new [Re: Eddgie]
      #5668939 - 02/08/13 12:42 PM

Thanks for that post, Ed.

Please help me with a couple of the concepts:

Quote:

For telescopes with coma or field curvaturre, a Panoptic (or similar high quality 68 degree wide field eyepiece) with the same size true field as a Nagler or Ethos will magnify the abberated blur so much less that it becomes very hard to see.





Is this the same as simply saying that the wider field EPs see farther to the edge, and thus see the outer FOV where aberration is worst?

In other words, given two EPs of similar quality, with the same FL (and magnification), a 68° EP sees the best part of the available FOV. An 82° EP would see that same FOV,
with the same amount of aberration, but would also see an additional ring of FOV, around the FOV the 68° EP saw, where the aberration gradually increases.

Is that the situation, or is something else involved?

Quote:

Once of the primary advantages of a big Newtonian reflector is that it can easily provide a big exit pupil. A big exit pupil makes extended objects appear brighter.




I was not aware of this. That makes the exit pupil an even more important consideration in choosing EPs.

Is the perceived brightness directly proportional to the square of the exit pupil?

Is it just more area of the eye being "lit up", or what?


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Re: Coma Corrector Required? new [Re: daveyfitz]
      #5669016 - 02/08/13 01:20 PM

Quote:


In other words, given two EPs of similar quality, with the same FL (and magnification), a 68° EP sees the best part of the available FOV. An 82° EP would see that same FOV,




Eddgie likes to make this distinction. But, he keeps the field of view constant and varies the focal length so that the the 68 degree eyepiece has the same true field of view but a longer focal length and less magnification. The reduced magnification makes the aberrations less visible.

My own thinking is that the same things that make the aberrations less visible also make small faint objects less visible. Of course I do cheat, use a coma corrector with faster Newtonians and a refractor has 4 elements, two in front, two in back, that provides a flat field of view.

Quote:

I was not aware of this. That makes the exit pupil an even more important consideration in choosing EPs.

Is the perceived brightness directly proportional to the square of the exit pupil?

Is it just more area of the eye being "lit up", or what?




Indeed the brightness of an extended object is proportional to the square of the exit pupil. A 6mm exit pupil is more than twice as bright as a 4mm exit pupil and 4 times as bright as a 3mm exit pupil. Large exit pupils are easier with fast scopes because the exit pupil is the focal length of the eyepiece divided by the focal ratio of the telescope.

Jon


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Re: Coma Corrector Required? new [Re: cjc]
      #5669169 - 02/08/13 02:21 PM Attachment (9 downloads)

Quote:

And deep sky observing is very much about exit pupil. Lots of people using small scopes routinely see "Difficult" targets like the Whirlpool and Veil nebula because they know that you can see them just as easily in a 4" refractor as you can in a 12" reflector if you use a 6mm Exit Pupiil in both scopes.




Hmmm.
If the 4" refractor had a 650mm focal length, you would use a 38mm EP to get a 6mm exit pupil.
That would give you 17X magnification, and with a 68° AFOV EP, a 3.98° FOV.

With, say, a 12" F4.9 Dob, the 6mm exit pupil would result from a 30mm EP, giving 50x magnification, and a 1.36° FOV.

I have tried to simulate the FOV of both telescopes for the Whirlpool view, below.
Note that this image is NOT adjusted for the fact that the Dob would be gathering more than 8 times the amount of light as the refractor.

I don't think you can "see them just as easily", or, see them as well.

I would be ok with "see them".


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Re: Coma Corrector Required? new [Re: daveyfitz]
      #5669189 - 02/08/13 02:33 PM

First, your analysis is correct that an 82 degree eyepiece does indeed see further off axis than a 68 degree AFOV eyepeice of the same focal lenght.

But magnification is what allows you to actually resolve small angular detail. The comatic blur of a star will basically not be resolved as long as it remains smaller than about 2 arc minutes of apparent field. The dark adapted human eye is far less capable of resolving small details than the scotopic eye.

Let's use the example you mention above. I have a Panoptic with a one degree field, and a Nagler that shows a slightly larger true field.

A star at the field stop of the Panoptic would fall inside the field stop of the Nagler by 7 degrees of the apparent field.

Ah, but it would be magnified by a lower amount and this is the key. The role of the eyepeice is to magnify the view, and the Nagler basically makes the comatic blur larger angularly than it is in the Panoptic.

And the smaller it is, the harder it is for the eye to see it as a comatic blur.

So, anywere in the field of the Panoptic, stars will appear slightly sharper simply because the blur is slightly smaller. In terms of angular size on the focal plane, it is identical (it never chances of course), but in terms of the magnification, while I am seeing a point exactly the same distance from the center of the field in the Panoptic, it is being magnified less, so the abberations is simply harder to see. After all, that is what magnification does. It expands the angular size of a detail to the point that the eye can resolve it (about 2 to 3 arc minutes of apparent field for scotopic eye).

Does that make sense?

For the second part of your question, it is more complicated, but you can think about it like that if you like, though it is more about how the energy is concentrated (per linear millimeter) at the focal plane.

In a 3" f/6 telescope, the energy is contained in an area 1/4th the size as it is in a 12" f/6 telescope.

If I use a 10mm eyepeice in both scopes, they both produce an image just as bright but this is because in the smaller scopes, fewer rods are receving more light than in the larger scope. I see the image as just as bright, but smaller.

If I put in an eyepeice with 1/4th the focal lenght in the 3" scope, the energy from the focal plane is now spread out over an area equal in size to the 12" scope, but now the rods are all getting 1/4th of the light that they would receive in the 12" scope. You see the image now at the same mangification as the 12" scope, but because the light is spread over such a much larger area, it appears dimmer to you.


Here is another example. I have Pairs of 24mm 68 AFOV degree eyepeices, 32mm 52 degree AFOV eyepieces, and 40mm 43 degree AFOV eyepieces that I use in my Binoviewer.

All give about the same true field of view (limited by the field stop) but of course all three give different magnifications.

I use the 40s when I want to see Nebula and Galaxies show up with as much brightness as possible.

The 24s give a beautiful wide field, but at kind of small exit pupil. The Orion Nebula fills up the field.

When I put in the 32mm or 40m Plossls, the Orion Nebula still fills the field, but the field is much narrower. But the light is not more concentrated into fewer rod, so the nebula appears smaller, but brighter.

Same scope, same true field, but three different eyepieces that show the Nebula three different ways. One extreme gives a good view of the structure, but it looks dimmer. The other shows the Nebula much brighter. And the 32s are in between.

I went to the 40s in the Binoviewers because binoviwers cut the light into half and each eye is now only getting the light from a telescope that has 50% less light gathering (though binocular summation increases this to the equilivent of a telescope with 70% of the aperture).

This dimming bothered me, so in order to restore some of the brigtness, I knew I could just make the exit pupil larger and turn it back from a C11 to a C14 (brightness wise).

Anyway, it is the exit pupil that determines how bright an object looks and any two scopes using the same exit pupil will produce an image that is about the same brightness, and it is because of the way the incoming light is spread over more or fewer rods in the eye (or pixels in a CCD chip... I can put a 4x barlow in front of the camera on an 80mm ED and get the same image scale as the 12" scope, but I will now have turned my f/6 refractor into an f/24 camera lens!).

Again, I don't know if I have explained it well enough.

Anyway, a bigger exit pupil for a given scope will produce a brighter image, so a longer focal lenght Panoptic will produce a brighter (but smaller) image than a Nagler or Ethos with the same size true field...

If Galaxies and Nebula are important targets for you, then you can trade a little magnification to gain a bit of image brightness.

One caveat... Under light polluted skies, many people prefer to go to a smaller exit pupil for deep sky becuase it has the effect of suppressing sky glow. There is likely some truth to this because I think somone explained that it increases the signal to noise ratio or something.

If I want to see it as bright as possible though, I tend to use my lowest power eyepecies regardless of sky conditions. But that is just me. I feel like it gives the best result.


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Starman1
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Re: Coma Corrector Required? new [Re: Eddgie]
      #5669206 - 02/08/13 02:43 PM

If the goal is simply to detect a faint galaxy, a lower power may suffice.
But if one wishes to see details in the galaxy, or see very small galaxies, magnification is essential.
I can see M51 at all magnifications in my scope (range 59X to 608X), yet I can see the smaller details in the spiral arms and SEE the spiral arms much better at 200-300X even though the galaxy has a lower apparent brightness.
The reason is that details have to subtend a certain angle in the eye for our eye/brain to notice them. Details are easier to see when they are larger.
So there is always a compromise between low power/large exit pupil/high brightness and high power/small exit pupil/lower brightness because of this fact.

There is another thread going right now on apparent field and the visibility of coma that y'all might find interesting:
Coma and magnification


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Re: Coma Corrector Required? new [Re: Starman1]
      #5669461 - 02/08/13 05:23 PM

Yes but the difference in magnification between a 27mm Panoptic and a 22mm Nagler when used in a 12" f/4.9 scope is not going to make that much difference on an extended object, and in the context of the OPs post, the images above really don't even come close to comparing what he would see with a 12" f/5 scope when using a Panoptic with a 1 degree field vs a Nagler with a 1 degree field. The difference in image scale is rather minor. So is the difference in magnification. But the difference may be enough that the coma is not as objectionable or even hard to see when using Panoptics, making him less likely to need a Paracorr.

While I of course used a 3" refractor and a 12 inch reflector in my example to explain image brightness, the OP is not trying to choose between these scopes. He is trying to choose what kind of eyepeices he wants to use in a 12" scope.

I was simply trying to explain why accounts may differe, and the pros and cons of using narrower vs wider field eyepeices types, and the role that may play in his decision to purchase a Paracorr or not.

He is less likely to need it if he uses Panoptics/Delos eyepeices. He gets a bit brigher image, but a bit smaller image scale.

I am personally not at all invested in what he gets and an mot biased in any way. I own both types (Naglers and Panoptics) and while my own preference is to use Pans in scopes with poor off axis performance, this may not matter.

I have had people over the years tell me repeatedly that they only really care what is at the center of the field, and I have learned to stop suggesting that people make one choice over the other because I don't know their tolerance for this or that outcome.

I wish him good luck. Many of these decisions are very difficult, but I always feel that knowing these different compromises helps in the buying decision.


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Re: Coma Corrector Required? new [Re: Eddgie]
      #5669756 - 02/08/13 08:57 PM

Quote:

The difference in image scale is rather minor. So is the difference in magnification. But the difference may be enough that the coma is not as objectionable or even hard to see when using Panoptics, making him less likely to need a Paracorr.




Eddgie:

I own both the 35mm Panoptic and the 31mm Nagler. It's a game you are trying to play, it may work with SCTs but I am not so sure about with a fast Newtonian. There are some differences, coma is linear with radius, field curvature is the square. But the biggest difference is the focal ratio,
SCT's are F/10, the Newtonians under discussion are F/5 or faster.

To my eye, at F/5, the 35mm Panoptic is just not as well corrected as the 31mm Nagler is. This is based on both Newtonians and the NP-101 which is F/5.4. At F/10, one doesn't worry much about eyepiece astigmatism, at F/5, you got to worry. The negative-positive design of the Nagler is just better at fast focal ratios, even with it's wider apparent field of view.

Another issue with the 35mm Panoptic in a fast scope is the size of the exit pupil, in the scope in question, a 12 inch F/4.9, it's 7mm.

Jon


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Re: Coma Corrector Required? new [Re: Jon Isaacs]
      #5670342 - 02/09/13 09:19 AM

i think Jon is right here. Eddgie's comments i found are very sound for SCTs but the coma seen in a f/5 scope with the 35 pan is quite significant. i had a 26t5 and it was better corrected but can not speak for a 31t5.

i may add that the coma with a 35 pan in my newt was a lot worst without a paracorr than the FC of any of the pentax EPs in my SCT ( and the pan). and so many talk against the pentax XW because of FC. it was really much less than what i saw with the panoptic and to my taste a paracorr was needed which was not the case with the 26t5. i realize that there is quite a difference in FL between the 35 pan and 26t5 but Jon seems to have had that experience


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Re: Coma Corrector Required? new [Re: coutleef]
      #5670504 - 02/09/13 10:55 AM

Quote:

i think Jon is right here. Eddgie's comments, I found, are very sound for SCTs but the coma seen in a f/5 scope with the 35 Pan is quite significant. I had a 26T5 and it was better corrected but can not speak for a 31T5.

I may add that the coma with a 35 pan in my newt was a lot worse without a Paracorr than the FC of any of the Pentax EPs in my SCT (and the Pan). and so many talk against the Pentax XW because of FC. It was really much less than what I saw with the Panoptic and to my taste a Paracorr was needed which was not the case with the 26T5. I realize that there is quite a difference in FL between the 35 pan and 26T5 but Jon seems to have had that experience




In this particular case, what I think you were experiencing was what I, too, experienced: the field curvature of the 35 Panoptic made the appearance of coma worse than it really was. After correcting the coma with a Paracorr, I discovered the bloat in the edge-of-field star images due to field curvature.
However, I discovered I could focus half-way from center to edge and accommodate the entire field in the 35 Panoptic, making it a good low-power eyepiece (albeit with a large exit pupil--too large for me). Removing the Paracorr and trying the same thing with the eyepiece and no coma correction wasn't as successful--partially because there was no good star image to focus on that was half-way out, and partially because a 38.7mm field stop still shows a lot of inherent coma in an f/5 scope.

By the way, the only two Pentax XWs that people comment on regarding field curvature are the 20mm and 14mm, and perhaps those are like the 35 Panoptic--simply beyond the accommodation abilities of many observers. My experience with these is like the 35 Panoptic--in a Paracorr the residual field curvature is easily accommodated and the coma is gone.


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Re: Coma Corrector Required? new [Re: Starman1]
      #5670592 - 02/09/13 11:55 AM

thanks Don for the clear explanation

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Re: Coma Corrector Required? new [Re: Starman1]
      #5671198 - 02/09/13 06:44 PM

Quote:

My experience with these is like the 35 Panoptic--in a Paracorr the residual field curvature is easily accommodated and the coma is gone.




I generally judge the correction of an eyepiece in my NP-101 under dark skies. The scope itself is essentially free from off-axis aberrations and at F/5.4 the scope is fast enough to show astigmatism etc if it there.

The 35mm Panoptic is quite nice in the NP-101 but with the 31mm Nagler it's about a perfect as it gets...

In terms of the 35mm Panoptic in a Newtonian, I find that at F/5 with a Paracorr (F/5.75) it is pretty darn good. At F/4 with a Paracorr (F/4.67) things are not so sweet, the edge is just quite as sharp as the 31mm. It is still quite a performer... I take the aberration to be astigmatism but it's been a while since I have carefully evaluated the 35mm Pan under F/5.

My eyes seem to open wide but at 64, they are not very accommodating..

Jon


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Re: Coma Corrector Required? new [Re: daveyfitz]
      #5674084 - 02/11/13 01:37 PM

My daughter and I went to a dark sky site this past weekend. She is a newbie with a capital "N". She is 26 years old and has only recently shown any interest whatsoever in the hobby. This is only the second time she's seriously looked thru my scopes.

The first time we did a session together was in my backyard a few months ago. I introduced her to my various eyepieces, as well as to the paracorr and what it did. We used the paracorr at all times during that first viewing session except for when I showed her the difference when not using it.

This past weekend at our dark sky location there was a televue rep set up almost right next to me. I asked him to look at my paracorr (it's a type I with tunable top purchased used) for possible scratches due to barlow insertion. He said he did not see scratches, but that he noticed there was haze on the lenses (my bad - I only cleaned it with distilled water and probably should have finished up with acetone). So I decided to not use the paracorr in the event it created problems with light scatter or otherwise negatively affected the view.

The first time my Newbie daughter looked thru the eyepiece she asked if I was using the "coma corrector thingy". She noticed the lack of coma correction immediately. This was using a 9mm 100 degree afov eyepiece in an f5 scope.


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Re: Coma Corrector Required? new [Re: stratocaster]
      #5674207 - 02/11/13 03:08 PM

The Paracorr is like an eyepiece and can be/should be, cleaned just like one, too.
Here's a good link:
http://televue.com/engine/TV3b_page.asp?return=Advice&plain=TRUE&id=103


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