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Equipment Discussions >> Eyepieces

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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
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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
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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
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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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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
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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
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Re: Coma Corrector Required? [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
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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
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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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Jon Isaacs
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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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Eddgie
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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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Starman1
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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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Jon Isaacs
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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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daveyfitz
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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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Eddgie
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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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Eddgie
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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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Jon Isaacs
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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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