View Full Version here: : Paracorr

24-08-2006, 11:09 PM
Has anyone had experience with using a Paracorr?:shrug:

I am particularly interested in whether it has a significant negative impact on light transmission and image contrast.

I am thinking about getting an f/4.5 20" truss dob. :eyepop:

25-08-2006, 05:48 AM
I've got the Lumicon Field flatener which is very similar; used it on a f4.5 with no significant problems. Didn't do much for me visually ( 13 Nagler) but certainly appeared to work on photos ( 35mm film). Opinion: unless your using a full frame film/CCD set-up it's a very expensive accessory which may have limited use.

25-08-2006, 09:04 AM

I have used a Paracor for a night comparing a 20" F4 with PCOR to a 20" F5 with and without.

What needs to be understood is that coma is a linear aberation that starts almost at the centre of the field . Whether you own an F5 or F4 etc you are losing faint stars from the view as when the aberation blurs out a faint star it disappears from your view. Check out the star spot radius graghs I've attached for the Paracor. Think of the inverse square of light, expanding a given spot of light to twice the size and you dim it by a factor of 4.

What you can see from the graph is that the radial comatic blur at the edge of an F4.5 field is 50 micron compared to 40 micron for an F5 and that the degradation is linear from centre to edge. Look at the effect th eParacor has in tightening star images across the field down to near the diffraction limit. I was startled by this effect visually and made me realise that an F4 Scope with a Paracor is a better visual scope than an F5 without.

The Net result is that in good seeing conditions, there are many more faint stars to be seen scattered across the field with the Paracaor, as the star images are tighter and brighter. Sure there is the usual 5% light loss , but this is below the human detection limit ( about 10%) and this is more than made up for by the tightening of star images.

The differnce between mirror Coma and eypiece astigmatism needs to be mentioned . The umbrella shaped aberation seen at the edge particularly badly in Chinese low cost low power eyepieces is Astgmatism. This come from the eyepieces in ability to handle the fast light cone. Naglers correct completely for astigmatiam and leave you with pure Coma. With addition of Paracor to Nagler you have pinpoint across the field. It needs to be mentioned too that the Paracor has a 1.15X Barlow effect. An F4 becomes F4.6 and and F4.5 becomes F 5.2.

Edge astgmatism usually about 4 X larger than Coma , so average eyepiece plus coma removing Paracor will only help marginally, as far as the very edge of the field is concerned. Panoptics only correct edge astigmatism partially so still look great with Paracor but not perfect. I did make the discovery recently that the TV Radian 18mm and 14mm have excellent astig and coma correction down to F4. At F4 you can let Jupiter drift into the field and see detail as it first drifts into the field. Amazing. With a rich 4.5mm pupil what a golden combination.

So I guess , you get what you pay for...Nagler plus Paracor is perfection, and Radian is narrower but a very cheap fix. I think at $500 or so the Paracor is well worth the money.

I found these two reviews on the Net which I think confirm my findings: an interesting read anyway.

http://www.excelsis.com/1.0/entry.php?sectionid=28&entryid=5&PHPSESSID=5ac3119c52f72ba2866708e6f 8668

Hope this helps.


25-08-2006, 09:22 AM
Excellent information Mark, thanks!

25-08-2006, 11:56 AM
Thanks for the interesting info Mark.
Looks like a Paracor will be going down on the must get list.

25-08-2006, 01:26 PM
Yes, thanks Mark for that very helpful information. Sounds like, even at f/5, a Paracorr would be a very useful addition to the EP kit. The graphs you have attached are particularly telling.

Don Pensack
25-08-2006, 03:22 PM
"Copy of post I made on Cloudy Nights":

The Paracorr's superiority over other coma correctors is its ability to work with all eyepieces from all companies and still give excellent results. Only one distance from the focal plane of the eyepiece to the coma correcting lens results in the best coma correction, and since eyepieces vary in their placements of this focal plane, you need to be able to adjust that distance for best results. Only the Paracorr is adjustable in that parameter.

Photographically, you only set the distance once and leave it. Hence, a less-expensive coma corrector, like the Baader or the Lumicon, will work just fine.

Technically, a coma corrector is engineered to best correct a particular f/ratio, but there is some flexibilty, with longer f/ratios being better corrected than shorter ones.

Perfect coma correction is when coma is cancelled to the edge of your widest field eyepiece (i.e.the coma is smaller than the size of the Airy disc in that scope).

Which brings up the issue of field width. Coma increases linearly with distance away from the center point of the focal plane. If the eyepiece has a narrow field that does not extend very far from center, then much less coma will be visible than if the eyepiece has a wide field extending quite a ways out.
In particular, eyepieces with large field stops, like a 41 Panoptic, 55mm Plossl, 31 Nagler, 35 Panoptic, etc. will show more coma at the edge of the field than a 10mm eyepiece with a small field stop. True, the comatic image will be magnified in the higher power eyepiece, but ultimately it is the field stop of the eyepiece that counts.

Many people with f/5 scopes do not feel the need for a coma corrector, I believe, because they are using narrow field eyepieces with relatively small field stops. Because the comatic star image at the edge of the field of a 35 Panoptic is 81X as large as a star image in the center of the field. It may not appear that huge because some of it is invisible to a particular eye, but it is measurably there.

All people who put a Paracorr into their scopes see an improvement in star images. How much is that worth? I see people spending hundreds of dollars to get better eyepieces with less inherent astigmatism and better transmission. Why not spend the price of just one premium eyepiece to improve the image from EVERY eyepiece you put in your scope?

When I saw the improvement in my scope's images, I almost cried. True, it extended the focal length 15%, which ticked me off. And it meant my 35 Panoptic went from a 1.4 degree field of view to a 1.22 degree field, but that was a reasonable sacrifice to get star images tightly focused all the way to the edge instead of only the inner 10%.

Coma correction has a place all the way to f/8 if perfect star images are the goal. People spend a lot of money for a premium mirror from someone like Carl Zambuto or Steve Kennedy in order to get better, more tightly focused, star images. It seems a shame to sacrifice that by not using a coma corrector.

As for the visibility of coma, it gets more visible if:
--the sky is darker (dark skies allow the fainter outer edges of the comatic star image to become visible)
--the aperture is larger (same reason as sky darkness)
--the viewer has better dark adaptation or genetically better night vision (more sensitivity to low-light aberrations)
--the field of view is wider
--the edge of field star images are carefully compared to the center-of-field star images
--the eyepiece has inherent astigmatism or field curvature (these both bloat star images--add them to coma, and the edge-of-field images get ugly)
--you expect the edge of field star images to be perfect
--your other scope is a long focal length scope and you can regularly compare.
--the f/ratio gets shorter (shorter f/ratios have more coma and coma starts closer to the center of the field)

I observed all night last night at a site with 11 other telescope users. 7 of them were dobs; all were larger than my 12.5".
Coma corrector use was 6 out of 8 dobs. Of the remaining 2, one wanted a Paracorr but couldn't afford it, and the other one had an f/6 dob (long tube!). The point is, that being exposed to a Paracorr by comparing your scope with and without one, or seeing its effects in other scopes (by viewing regularly at a place where a lot of others view) tends to result in the observer seeing its benefit. I've watched the percentage of Paracorr use steadily climb over the last few years, and, except for the economy dobs where justifying a $300 purchase might be difficult, I think it will eventually become nearly universal for users of f/5 and shorter scopes.

Does it cause light loss? Yes, about 3% on axis, but the star images elsewhere in the field are so much better focused that, overall, fainter stars are visible with the Paracorr.

Does it distort a planet's image as it nears the edge of the field? Less than without one.

One tip to make a Paracorr easier to use: put a number label on each eyepiece that corresponds to the particular setting of the Paracorr for that eyepiece. Number the stops on the adjustable top of the Paracorr with the same labels. You'll never need to refer to a chart or memorize which setting applies to which eyepiece ever again.

Hope all that digression helps.

Don Pensack

25-08-2006, 03:32 PM
Excellent information again, thanks Don.

This thread has really helped my understand of both coma and astigmatism, as well as the benefits of a paracorr.

It's not within the budget for me with a 10" GSO dob, but I would definitely include one in the budget if aperture fever took me over (as it has for Rod ;)).

25-08-2006, 05:32 PM
Don, Thanks a great review ;) I know I was gobsmacked at the scattering of faint stars through the field when I had a good look with a 20" F4 and Paracor: it clearly outshone the F5 in image quality.

As astigmatism is such a dominant aberation at the edge of the field of most ( non-Nagler) eyepieces, I think it has distracted observers from the fact that Coma bloats star images in a linear fashion as soon as you look away from the centre of the field and the Paracor fixes that. I expect that my next large scope , a 24" F3.3 , will have the Paracor `super-glued ' into the focusser :D


25-08-2006, 06:07 PM
Great question and information...someday I will use this information to explain/justify to my wife why I need to buy a Paracorr!

Thanks to all of the contributors for solid information!!!

25-08-2006, 07:36 PM
What's the focal ratio of your truss dob, Scott? Do you experience much/any coma?

25-08-2006, 07:43 PM
Great info! Thanks Mark and Don. Yet another thing to add to my ever growing astro-gear wish list that will keep me broke till I'm 265. :(

25-08-2006, 08:50 PM
Thanks Mark and Don for that excellent insite. Mark, I'd like to see that 24" f3.3:eyepop:someday. With paracorr of course.

25-08-2006, 11:27 PM
It is an F5 (I also have an F6)...I do not find coma in my scope particularly noticable (to me) and it is not bothersome to my eyes but would suggest that I am not the "pickiest" or most experienced observer (still comparatively green in the observing department relative to other posters in this thread) and I think I have a pretty good set of corrected eyepieces (at least IMHO) Naglers, Pentaxs and one Panoptic with a couple TV plossls and a Powermate...I am not saying coma does not exist in my scope, I am saying that my veiwing habits and experience are such that it does not bother me (or hasn't yet but having participated in this thread I'll probably be driven crazy the next time I am out with my scope looking for the coma...talk about being influenced and poisoned by more information).

I have never used a Paracorr in my scope but have often wondered what kind of improvement I might find. I have no doubt that if I looked closely I would see an improvement but have struggled with the justification of buying/owning one to myself without the prior use of one. I suspect/speculate that the benefit would be two fold one in reduction of coma present but also the slight "barlow effect" too...but at a cost that I am not sure I am as yet prepared to pay based on use and experience...

From all I've read (not yet experienced) I'm not concerned by loss of light...I am more concerned with added weight and the likely need to rebalance things...not a real objection I know but it is in my mental decision mix.

I am interested in trying one and most likely will own one someday...not necessarily soon (or necessarily with this scope) but it is on my longer term "wish list".

Having said all of that, if I was buying/building a new big fast dob I'd get one and have it built/counterweighted to carry a Paracorr and the bigger Nagler/Pentax eyepieces standard (leaving the Paracorr in all of the time)...the incremental additional cost is small relative to the cost of the new optical system and my belief is that in scopes faster than F5 it probably pays for itself pretty quickly in noticable improvements for a discriminating eye (and if I spent big bucks on a new scope I'd be much more discriminating)...

As was noted in this thread earlier, if you are going to all of the troube to get a high end (read expensive mirror) you might as well get the equipment to let the thing work at it's peak...I mean, what good is owning a high performance race car if you are going to put cheap tires (tyres) on it and use low octane fuel?

I am looking forward to getting the chance to look through your new scope (and Paracorr) ;)


26-08-2006, 07:23 PM
I borrowed a Parracor for an evening and was very impressed using my 20" f5. My 17mm nagler was greatly improved (though the 13mm made only a little difference). Most surprising to me was a marked increase in contrast that I wasn't expecting - the Homunculus of Eta Carina took a fabulous mottled appearance (confirmed by the others) that had never been so prominent and the coma was beutifully corrected.

One minus point though - the Parracor requires an extra cm of inside travel - I had to shorten my tubes to get it to work!

I didn't rush out and buy one, but it is high on my list.

28-08-2006, 12:22 PM
This is incorrect.
Coma correctors are designed around mirror's focal length, NOT f/ratio. Once design has been set, coma will be corrected for all f-ratios (up to the point - high degree aberrations aren't corrected by a simple corrector so once they become visible, that limits the usefulness. In case of Paracorr this seems to be around f/4 or thereabouts). For people who are familiar with elementary optics this is self evident - every f/4 paraboloid contains f/5, f/7, f/8, f/9 ... ad-infinitum beams already. So if corrector works with well with say f/4, it works with anything slower just as well (in fact better).

This is also incorrect. Again, this is elemetary optics 101.
Let's compare two identical hypotetical eyepieces - 20mm f.l with 20mm field stop, and 40mm one with a 40mm field stop.
Coma is a linear function of off-axis distance (coma blur will be twice as big at 20mm off axis compared with 10mm off axis). But magnification is a linear function too - 20mm eyepiece magnifies twice that of 40mm. In the end, what eye sees right at the field stop is the same angular blur.
The only parameter that counts is apparent field of view - wider the eyepiece is, more coma is shown. 11mm Nagler will show a lot more coma than 55mm Plossl, for example.


PS the example above is only valid if eyepiece's astigmatism does not mask coma; 55mm Plossl will in fact have so much astigmatism in say f/4 mirror that coma will be practically invisible.

Don Pensack
28-08-2006, 02:05 PM
This directly contradicts information on the TeleVue website, information elsewhere on the web regarding coma, and what Al Nagler himself told me directly.
All f/4 instruments have exactly the same angle to their lateral rays, regardless of focal length. So it is not related to focal length, but focal ratio.
Al told me directly he designed the Paracorr for a focal ratio of f/4.5, but that it works better on longer f/ratios. At some length of f/ratio, the Paracorr would be applying reverse coma, but it is above f/8 and no one will be using one above that f/ratio. So it IS f/ratio, not focal length.

Your statement regarding magnification also directly contradicts what you see (but not because you're technically wrong). All my eyepieces have 82 degree fields, yet coma is far more visible in the lower powers. Why? Because the coma in the edge-of-field star image is considerably fainter in the more magnified image. This is why the edge of field stars in a low power 50 degree eyepiece appear to exhibit more coma than a very high power 82 degree field of view. Technically, the comatic star image is the same size, but in practice you can't see it.

28-08-2006, 02:09 PM
Welcome to IIS Bratislav! :)

Seems to make sense. In other words, when you put an aperture stop in front of the mirror, you would expect the same corrector to work at least as well as it does for the entire mirror.

But as you approach the point where the coma blur is comparable with size of the Airy disk, it becomes less obtrusive. Also as the brightness of the image is reduced at high powers (as inverse square of the magnification) I would expect that coma is more difficult to perceive.

28-08-2006, 03:46 PM
I seriuously doubt that Al would tell you anything of sorts. Most likely you simply did not understand the explanation (after all, you don't get the simple fact that f/4 mirror contains all f/ratios from f/4 to infinity).
I have designed several simple (2 element) coma correctors as well as more complex ones (3 element corrector for Schmidt Newtonians, Dall Kirkhams and hyperboloidal astrographs) so when I say something, I'm pretty sure of that. Coma correctors for paraboloids are designed around two parameters - BFL (which is pretty much set in concrete for photovisual use) and focal length.
F-ratio does not figure at all.

As far as second argument, well there is no argument at all. You are wrong, simple as that.
Perhaps you misunderstood astigmatism of a 50 degree eyepiece for coma ?

28-08-2006, 03:56 PM
Er, true. That is why I said 20mm and 40mm, not 2mm and 40mm :)
You aren't going to see diffraction structure in a typical f/4 mirror (which is where Paracorr becomes compulsory) until about 3 to 4mm f.l. eyepieces. For run off the mill deep sky selection oculars (9 to say 30mm) diffraction doesn't factor at all.

Again, brightness doesn't factor in untill you start to see the diffraction structure as extended image. That is pretty much at the same point (well below 5mm eyepieces for f/4 mirror).

Let's have our 20mm and 40mm eyepieces again. Say right at the filed stop they show same size comatic blur - for example 5 arc minutes.
A given extended comatic blur (that subtends 5 arc minutes) will result in same same flux (# photons per retinal area) - so it will be the same brightness, regardless of eyepiece's focal length.

Again optics 101.

28-08-2006, 04:22 PM
I see your point. I have no direct experience of how that looks in practice. I certainly cannot see anything wrong with star images at the edge of a 70 degree FOV of 10mm XW eyepiece at f/5, or even at f/4. Looking at Al Nagler's graphs (above), I should. I remain curious to try a Paracorr.

Let's leave that out of it. Beginning optics courses concern themselves with ideal lenses and mirrors with perfect focus for all colours, and perfectly flat infinite focal planes. Only if real-world optics worked like that...

28-08-2006, 06:08 PM
Depends on many things, mainly on how critical the observer is. Some eyepieces may in fact reduce visible aberrations (some well known examples are Pretoria + paraboloid or Plossl + Jones-Bird). Unfortunately, I don't have any experiences with 10mm XW, but I'll keep an eye on it.
Naglers definitely show coma of an f/4 paraboloid, compared to a much better corrected scope like my homemade f/4 Wright. But difference is indeed rather small compared to say astigmatism of 'ordinary' eyepieces.


PS '101' as in elementary principles, not some ideal cases that don't apply to real life

28-08-2006, 07:53 PM
Bratislav, you're obviously very experienced with optics, and as such your opinions and experience are valued in discussions such as this.

Just please try and stay pleasant, and as Steve said please leave sarcastic or rude remarks out of it.

This discussion has brought up a lot of great points and i'd hate to see it degenerate.

28-08-2006, 09:39 PM
So is a Paracorr a goer for Rod ?:lol:

28-08-2006, 10:06 PM
I think Rod's rods would love it! :D

29-08-2006, 08:48 AM
I agree with Don Pensack....
In my experience as an amateur the coma is related to the fast cone of the f ratio. "faster" systems ie f 4 produce more coma than "slow" systems independent of the focal length. I agree you need to consider the FL for the overall sizing of the optics but coma is f ratio. My understanding is that the Hale 200" at f3.3 was coma limited hence the use of the Wright etc correctors.
My 2c.

29-08-2006, 09:17 AM
Merlin, I think everyone who posted here agrees about faster scopes producing more coma, and the amount of coma being dependent on the f-ratio.

29-08-2006, 09:26 AM
Apologies Mike, I'm a bit thick when it comes to niceties and group hugs.
But rest assured, I won't do it again in your forum.
Cheers !

29-08-2006, 09:42 AM
Aside from all the theory, this was my impression too. Even at F5 , coma blurs the star images bigger than the Airy Disc , quite close to the centre of the field , so contrast and sharpness is improved even near the centre with the Paracor.

This also means that if you are looking at a subtle small object like the Humunculus, you don't have to have it sitting exactly in the field centre to get the best detail. As one of the US reveiwers posted here said, the Paracor is a device that people gradually decide to buy as they have more exposure to it and increase their observing skills.

Its nice to know there is still one bit of kit left that can improve the view in most of your eyepieces, beyond having a good mirror.


Don Pensack
01-09-2006, 06:27 AM
I just spoke at length with David Nagler (Al's son) and he confirmed what Al had told me some time ago:
1.Coma correction is a function of f/ratio only, and focal length has nothing to do with it. The Paracorr applies exactly the same correction to a 4" f/4.5 that it does to a 20" f/4.5. It is the angle of the lateral rays that are the determinant of the coma in the instrument, not the focal length.
2. David also confirmed my second statement that coma becomes less visible in eyepieces with shorter focal lengths (and smaller field stops), even if AFoV is the same, and that the reason is the drop in the brightness of the lateral parts of the comatic star image. I have seen coma in all my eyepieces all the way up to a 5mm, but it is/was nowhere near as intense to my eye as it is/was at lower magnifications. Since coma is linear, the argument that doubling the power and having the field halve in width should show exactly equal coma at the edge of the field SOUNDS right, but my experience says that's not true, and that there's some other factor at work. That factor is the simple fact that the extended star image is 1/4 as bright while the magnification has only doubled. This is the reason many observers (and I mean many) stop using their coma correctors at high powers. The lateral edge of the comatic image is simply too faint to see, so the star image doesn't appear as bloated. A lab instrument or CCD image (compensating for brightness by a longer exposure) would show the star image just as wide, but the eye can't see it.

Sorry for any confusion on the issue. I stand by my earlier comments.

01-09-2006, 10:45 AM
Can you please get Al to confirm this quote of yours ?
"Al told me directly he designed the Paracorr for a focal ratio of f/4.5, but that it works better on longer f/ratios. At some length of f/ratio, the Paracorr would be applying reverse coma, but it is above f/8 and no one will be using one above that f/ratio. "

I'd like to hear it from Al himself, not your paraphasing, please.

Except coma correctors actually correct two things - coma and field curvature. And field curvature is proportional to mirror's focal length. Hence, when uncle Al set to design Paracorr, he picked one focal length (as there will be one and only one f.l. where field is 100% flat), and NOT one f/ratio (as coma correctors will correct for all f/ratios, not only f/4.5).

In simple terms,

1) Paracorr will work best with only one focal length paraboloid (once you move from prescribed f.l. field curvature creeps in)
2) Paracor will work with ALL f/ratio paraboloids (up to the point where high order aberrations including spherochromatism start to bloat images - around f/4 for Paracorr) and
3) there is no one "optimal" f/ratio and "applying reverse coma" only exists in one's head. You can put Paracorr in f/10 Newtonian and coma will be reduced by exactly the same amount as in 4.5 one.

Hence, statement that coma correctors are designed around focal lenght is indeed correct.

The confusion is indeed rather evident.
Your main point of confusion (apart from not understanding that f/4 paraboloid contains all slower beams) is failing to understand that comatic image at the edge of the field of view of a shorter (20mm) eyepiece is smaller by factor of two, so even if it is magnified twice more, its angular size (as seen by eye) is exactly the same as in 40mm one.
Same angular size, same number of photons = same flux reaching the retina == same brightenss.

If you or David Nagler could explain the mechanism to make the same angular size aberrated image appear brighter in longer f.l. eyepiece I'm all ears.

As this is my last post in this forum, if anyone (including Don, David Nagler or Uncle Al himself) wants to continue discussion, please use my bratislav3162 at hotmail dot com address.

01-09-2006, 10:52 AM
That cannot be it. The 1/4-fold brightness reduction due to 2x magnification increase would be perfectly cancelled out by the 4-fold increase in brightness due to linear 1/2 x decrease in coma angular size at the same angular displacement in the apparent FOV. So the star and coma 'should' look the same. There must be something else going on that we have not considered.

Don Pensack
01-09-2006, 11:40 AM
I'm not going to bother to respond to Bratislav, as it is not my job to question what Al and David Nagler have told me directly on the phone and in person. If he doesn't like their answers, he can call them on the phone and argue with them. Suffice it to say, what I understand about coma correctors did not come from only the Naglers.

As for the comatic star image, the size of that image is linear, so Janoskiss is correct that there must be some other factor at work that we haven't considered. It might be an interaction with the exit pupil, or something else. If the star image is half the size, but the magnification is doubled, the star image should be of equal size. Indeed, the night sky will be 1/4 as bright at twice the magnification (4X the area), so contrast with the star images should be larger, making the coma more visible.
And yet it is not.

But, thinking aloud, if the star image at 10mm from center at the focal plane is, say, 1" in diameter, and 2" at 20mm from center, and if the smaller field becomes the edge at twice the magnification, then the 1" star image at 10mm has become 2" by means of magnification. But let's say both star images have equal brightness to start out with (at the focal plane). Then the 2" star image created by magnifying a 1" image will be dimmer than the 2" star image at 1/2 the magnification. If there's something wrong with that logic. let me know.

01-09-2006, 11:54 AM
Tricky thing about star images is that they are points. So you don't have 1" star image or 2" star image. It's effectively a point at all magnifications, at least until diffraction effects become apparent, i.e. when you magnify it enough to see the Airy disk.

Don Pensack
01-09-2006, 03:51 PM
Except a comatic image, which is a LOT larger than the Airy disc. At f/5 it's 9X as wide as the Airy disc in the radial direction at 20mm off axis, which gives it a size (it's even larger at f/4). So it should behave more like a planetary image, where magnification and brightness are concerned.

01-09-2006, 04:29 PM
but it is the size of the coma which does not change when you up the power say 2-fold: at the same point in the AFOV, you are 2x closer to on axis, so coma is 2x smaller, but you're mangifying 2x, so it would stay the same. It's the same star with the same apparent size so you'd expect it would look the same (as long as Airy disk is unresolved). Don, interesting point about background sky brightness, yet things go the other way. These are all things I have not really thought about before all that much and never had a look at what they really mean in practice. Maybe time to have a critical look at stars at different powers with the f/4 Newt. (although I don't think the mirror is very good in that one.)

Don Pensack
02-09-2006, 12:24 AM
That's the issue. Seeing artifacts will impact the evaluation of coma at higher powers.

I can tell you my experience:

My 5 and 7mm Naglers display coma in their outer fields, so I use a Paracorr for every eyepiece. But they exhibited only annoying, and somewhat minor comatic images at the edge.
My 35 Panoptic, on the other hand, exhibited a starfield that looked like I was standing on the deck of the Millenium Falcon going to lightspeed. The star images looked like radial streaks.
After the Paracorr, the star images are perfect to the edge in the 5mm and 7mm, yet still a tiny bit comatic at the edge in the 35mm. [I have 43 years experience looking at star images through hundreds of scopes, so I am compensating for the tiny amount of positive field curvature in the 35mm]
If simple magnification explained the visibility of coma, coma should have been MORE visible in the 5 and 7mm (82 degree fields) than in the 35mm (68 degree field), yet it patently was not even in the same league.

Here's what I think: Star images are not magnified. They appear the same size in all eyepieces until the Airy Disc is visible, and then start appearing larger. The 5mm eyepiece is a 1mm exit pupil in my scope, yet represents my highest magnification (I'm not a planets or double stars observer, so powers above 400X aren't too useful to me, on an undriven scope), but that magnification *just* makes the Airy disc visible.
Accordingly, the only factor having an influence on what coma I see is field stop size in millimeters--a wider field stop will display more coma and magnification has nothing to do with it. Doubling the power doesn't increase the size of the star image because the scope sees it like a point source. It is only the reflection that is comatic. The eyepiece is just a simple magnifier of the focal plane of the telescope. So, doubling the power with an eyepiece of identical apparent field doesn't show the same amount of coma; it shows less because the field stop is smaller.
In other words, you'd have to keep the field stop the same size to see equal amounts of coma at all magnifications.
What do you think? I think that may just be the explanation I was looking for.

02-09-2006, 08:46 AM
The Nagler are designed to handle the fast light cone from an f4 or f5 scope - so of course they will display less coma. That's why you pay so much money for them.

So I am not sure your comparison is valid. I believe you would need to compare different magnifications of the same ocular design to test whether coma is a function of magnification.

I remember one night using x600 on an undriven 20inch f5. The seeing was exceptional and supported this magnification. It is worth keeping a shortish focal length ocular in your kit for such nights.

and it doesn't seem to make a lot of sense that star images are not magnified, then suddenly they are once the Airy disk becomes visible.:screwy:

I suspect both have an effect, along with f ratio and eyepiece design. The combined effect, together with the observers individual variation in visual perception and experience, would be difficult to model. Which is why we have this discussion with Bratislav leaving.

I am sure "Optics 101" says one thing, and the observers eye adds another layer of complexity. We all seem to perceive or react differently to the image presented to us through an ocular. My view is that there is a layer of emotional perception that everyone applies to what they see visually.

That what I think.

02-09-2006, 09:40 AM
Don, I'm not sure I agree, because I'm not sure I understand. :lol: I wish someone could run some objective (no pun intended) experiments with a CCD at different powers. I think that would be interesting.

Now I am no expert. I'm just guessing based on what I've read/heard about EPs. I don't think these EPs fix coma. They just work as well as possible within the limitations imposed by coma of a parabolic mirror. I don't think coma figures into it - they are designed for an ideal coma-free fast scope afaik.

Well maybe not so suddenly, but there are low-to-medium powers where the Airy disk is so small that the eye cannot resolve it: here the star is effectively a point source and magnification has no effect on the apparent surface brightness.

Don Pensack
02-09-2006, 02:48 PM
Coma is produced by the primary mirror, not the eyepiece. Having well-corrected eyepieces reduces astigmatism, lateral chromatic aberration, and a host of other f-ratio-induced aberrations, but coma is not reduced by them. However, reducing the other aberrations does make coma MORE visible because it is not mixed with or confused with other aberrations.

The star image has a definite width, but it is a point as far as our eyes are concerned until the Airy disc becomes visible. At that point, the star image will increase in size with more magnification. But no more resolution can happen once the Airy disc is resolved. that occurs around an exit pupil of 1mm. So, at lower magnifications, the star image stays the same size with magnification change up to that point. This is why fainter stars are visible at high powers--the sky gets darker with increasing magnifications, but the star image itself does not get dimmer until magnifying past the point where the Airy disc has become visible.

Again, eyepiece design will only have an effect on the visibility of coma insofar as other aberrations may swamp the visibility of coma. Field curvature and astigmatism, in particular, can have the effect of reducing the visibility of coma if the comatic star image is distorted in other ways.
I do see that individual perception plays a role in the visibility of coma. Some people see it at f/6, others not even at f/4. Why, I can't say.

02-09-2006, 03:09 PM
I have read of this. The spot diagrams would appear to indicate that this is so. I have also read that coma is not produced by the mirror, rather by the inability of the ocular to handle the fast light cone.

individual variation plays a big factor in what we preceive.

Don Pensack
02-09-2006, 03:44 PM
The aberrations most likely to be produced by an eyepiece's inability to handle the lateral rays of a wide, shallow, light cone of a short f/ratio scope are astigmatism, lateral magnification distortion, and lateral chromatic aberration, not to mention spherical aberration. Coma CAN be produced by a simple lens (say, a one-element eyepiece (not common since the 1600s). Coma, however, is solely an aberration, with today's eyepieces, of the mirror.

I agree that individual variation , such as the genetic abilities to see in the dark, plays a role in the visibility of coma, but the size of the star image at the edge of a 40mm focal plane in an f/5 scope is literally 9X the width of the Airy disc. It's hard to believe that anyone could be unable to see it. I suspect that everyone can SEE it, but not everyone is BOTHERED by what they see. That's a psychological response, not a physical one.

02-09-2006, 04:57 PM
On the subject of perception.. Since I've gotten into astronomy and started fussing over all the possible aberrations in scopes and EPs, I learnt to see chromatic aberration in my own eyes! Streetlights with mercury lamps have a purple halo around them when I look at them with averted vision. I was quite surprised the first time I noticed. Normally the brain would remove/ignore such aberrations.

Don Pensack
03-09-2006, 12:25 AM
I hear you. I demanded better glasses to correct my distance vision from 20/20 to better. Why? Because the images in my telescope where sharp, while my daytime view of distant objects was a little blurry. I wanted the distance to be as sharp as my nearer vision and as sharp as an image through my telescope. I ended up correcting my distance vision to 20/10, and achieved a sharp long-distance vision.:sadeyes:
Telescope viewing definitely warps your perceptions.:screwy:
Now, if I could just get rid of the floaters!:lol:

Don Pensack
04-09-2006, 07:36 AM
I've researched coma more fully. Here are the two best sites I found that explain the concepts:
The best quote is:
"Using this method to specify field position, all telescopes of a given focal ratio have the same amount of off-axis coma, regardless of aperture. For example, a 10” F/4 has the same amount of coma as a 20” F/4 at the edge of a 30-mm diameter fov."
The formula is:
Linear diameter of coma is 3r/(4N)^2 where r=off axis distance in mm, and N=focal ratio. No mention of focal length is made.
Just thought people would like to know.

04-09-2006, 11:33 AM
Interesting inputs from all involved. The physics (geometric optics) demands the coma be the same for optics of the same f ratio. And the paracorr doesn't know how big the mirror is or its focal length, it just deals with the convergence angle of the incoming focused light. So while Bratislav's arguments are thought provoking (not a bad idea) I couldn't see how they would apply. I'll re-read them as the arguments are subtle.

The moral of the story (based on people's reported experience and the various arguments here) seems to be that for any Newtonian of f/8 or less, and especially f/4-f/5, with first class eyepieces, a paracorr is a damn good gadget to have available.

Thanks to all for the enlightening discussion.

One question arises: since astigmatism seems to swamp coma for eyepieces below a certin quality, at what point on the quality curve does a paracorr start to make a visible difference? Anything better than el-cheapo Chinese eyepieces? Something like the Meade 5000 series? Or only for things like Naglers and top Pentaxes? Comments aprpeciated.

2. Floaters: Don, if you find a way to deal with them, please post here!! My eyes are a zoo :(


04-09-2006, 12:01 PM
Don, I think there is a bit of confusion about what Bratislav was trying to get across. He was not arguing that the amount of coma will be different for different focal lengths, but that the optics needed to correct it will need to be designed for a particular focal length, and independent of f-ratio (once you have chosen the fastest f-ratio you want the corrector to still work in).

Thinking about the problem in my simple-minded way (having no experience with designing optics), this seems to be correct, for two reasons.

1) If a corrector is matched to a fast mirror of some focal length, then it also must be a match for all slower mirrors of the same focal length, because the fast mirror can be split into a slow inner mirror and a fast periphery. The corrector must work on both slow inner and fast outer halves of the mirror if it is to work on the mirror as a whole. So you cannot have a corrector designed for fast mirror that will not work just as well on an arbirtrarily slow mirror of the same focal length.

2) If you think about scaling up a fixed f-ratio mirror (increase f.l. but keep f-ratio constant), the amount of coma at some point in the focal plane will increase linearly with size of the mirror. But so will the distance r of that point from the optical axis. Combine this with the fact that for any given mirror, the amount of coma is a linear function of r, it follows that the amount coma will indeed be the same for any given r independent of the focal length, as we all seem to agree. However, when you scale up the optics, the field curvature will change. A scaled up field will have less curvature, i.e., for the same r, you are closer to the focal plane in a long focal length mirror. (e.g., think of circular arc of fixed length as you blow up the circle.) So a small fast mirror will have a more strongly curved field than a large mirror of the same f-ratio.

Because of #1, the coma corrector cannot be specific to any f-ratio.

Because of #2, it makes sense that the corrector must be specific to focal length, because while the amount of coma at a given distance from the optical axis is independent of FL, its distance from to the ideal flat focal plane is not, with field curvature getting stronger as the FL gets shorter.

From this I would conclude that there will be an ideal focal length where Paracorr works best, and there is a range of focal lengths above and below this where it works well. I expect there would probably only be an issue with small mirrors, as the field is practically flat once mirrors get large enough, so a corrector designed for a flat or nearly flat field will work well with arbitrarily large mirrors. However, I would not be surprised if the Paracorr did not work with a 4" f/4, for example. Anyone care to check?

04-09-2006, 01:48 PM
A question: what effect is a paracorr designed to have on field curvature, as distinct from coma?


04-09-2006, 02:40 PM
I don't know if the Paracorr was designed to have any effect on FC (but I have read somewhere that it flattens the field somewhat). Whether it increases, decreases the FC, or leaves in the same, is beside the point within the context of the present discussion.

My intuition tells me that even if a coma corrector leaves the FC unaltered, it can only properly cancel the coma if it "knows" where that coma occurs, which will depend on FC. So the designer must consider the FC of the system the corrector will be used in, even if there is no desire to change the FC. And as FC of a mirror does change with FL, the design will need to be tailored to specific FL.

04-09-2006, 09:16 PM
Interesting point. I'd be interested to see what Don has to say. My earlier post made the implicit (and wrong) assumption that the paracorr scales up with the mirror! (Whoops).


Don Pensack
05-09-2006, 01:55 AM
1. The Paracorr's correction of field curvature is slight, and is related to having the focal length lengthen 15%. this makes field curvature in the telescope less, and since field curvature in the eyepiece and telescope are additive, there is some correction of field curvature.
2.Bratislav's idea that a short f/ratio mirror contains rays of all f/ratios is dependent on the idea that some rays strike and use only a part of the mirror. Unfortunately, and I'll explain what I say in a minute, EVERY RAY HITTING THE MIRROR USES THE ENTIRE MIRROR BECAUSE EVERY RAY STRIKING THE MIRROR IS A WAVE THAT HITS THE WHOLE EARTH. That means that, wherever the star is in the field of view, the f/ratio is the same. What does happen is that the front of the tube and the secondary size do vignette that wave, reducing its brightness from an axial wave, but the wave itself is focused on a point at exactly the same distance from the mirror. If you'd like an illustration, stand 1' from a wall with a bright flashlight. Shine the flashlight on the wall straight ahead. The light on the wall is circular. Without moving the flashlight toward or away from the wall, angle the flashlight 45 degrees to the side. What shape is the light beam hitting the wall? Oval. Now, this is an oversimplification of what occurs on the focal plane of a paraboloidal mirror, but it is somewhat illustrative of why coma is worse farther off axis.
3. When the wave passes through the Paracorr, the outside of the wave is bent by the edge of the lens. Like any light ray passing through a lens, the angle of incidence determines the angle it exits the lens. All f/4 telescopes, regardless of focal length, have the same angle of incidence entering the Paracorr lens. Accordingly, the lens curvature can be designed to fully correct a particular angle of incidence (f/ratio). But what happens when the f/ratio is longer? The rays that hit the lens do so at angles closer to parallel, which makes the angle of exit warped to a greater degree. So, if the goal is to reduce the size of the comatic star image to no larger than the size of the Airy disc, this will happen over a small range of f/ratios. At very long f/ratios, where the rays hit the lens at nearly parallel angles, the warpage of the lateral rays can bend the ray the other way and cause "reverse coma". At what f/ratio this would occur, I don't know, but I wouldn't try a Paracorr in a Schiefspiegler.:)
4. There is a range of f/ratios that would be completely corrected. On the Paracorr, that would appear to be f/5.5 to f/8. That says that Al chose to make the curves gentle to avoid spherical aberration. The 15% increase in focal ratio was necessary to avoid any detectable spherical aberration from the lens for on-axis images. This is also why the other commercially available coma correctors DO exhibit increased spherical aberration, though this is not as important photographically due to the larger size of the star images compared to visual use. It also means that if TeleVue had chosen to fully correct an f/4 telescope, spherical aberration would have had to be controlled some other way, such as additional lenses or aspherical curves, both of which would have made the Paracorr an unaffordable luxury for a chosen few, as well as much heavier.
5.The point to get across, here, is that the linear size of a comatic image will be exactly the same size 15mm off axis in any scope of the same f/ratio, whether having a 25" focal length or a 100" focal length. There is a difference of image scale, though, and it depends on whether you look at coma from an angular or linear distance away from center as to how you describe it. I suspect Bratislav and I were talking about coma from opposite points of view.
But when we look through an eyepiece, we are looking through an eyepiece with a set linear field stop, and the coma present in the image at the edge of the field of any eyepiece is dependent on the linear size of the field, and it is dependent on the f/ratio of the scope.

Don Pensack
05-09-2006, 02:05 AM
Here's some info on floaters:
Note that laser fixes for floaters is not yet considered a standard procedure. But, short of replacing the fluid in the eye, this remains the only current cure.

As for eyepieces, if astigmatism swamps all other aberrations (say, using a Chinese widefield at f/4.5, for example), then eliminating coma will only make a minor improvement. But if the eyepiece is decent (like the new Baader Hyperion or the Orion version), then it can be valuable to add a coma corrector. A few posters on Cloudy Nights have done just that and been pleasantly surprised at which eyepieces were improved and which weren't. It's possible an eyepiece that appears unsalvageable without coma correction may become quite tolerable with it. I think you never know until you try it. But, as you say, the better eyepieces that have little or no inherent astigmatism do seem to be the ones that are made nearly perfect by simply correcting coma.

Don Pensack
05-09-2006, 02:46 AM
I talked with an expert in optical design, and he explained coma to me as a paraboloidal aberration and explained why there was so much confusion. The explanation was really quite simple.
The linear size of a comatic image on the focal plane of any scope is dependent on distance from the center of the field.
How it relates to focal length and focal ratio is as follows:
A 10" f/4 exhibits X coma at the edge of a 40mm field of view
A 20" f/4 exhibits .5X coma at the edge of a 40mm field of view, yet, because the magnification is double, the linear size of the comatic image at the edge of the field of view is exactly the same.
This is why the linear size of a comatic image appears the same at all focal lengths of identical f/ratios at the edge of the same sized field of view, and why a coma corrector can be designed to work with a set f/ratio in mind.
At the same time, the linear size of the comatic star image on the focal plane of the scope, at the same distance from center, will be less with a longer focal length, and it is here that describing coma as lessening with focal length is correct.
A coma corrector that is designed for visual use will take magnification into account, so would be designed to correct for f/ratios.
A coma corrector designed for prime focus photography will be designed for the size of the photographic plane and the focal length of the scope.

Where the confusion lies is that they are one and the same. It depends on your point of view as to which explanation makes the most sense. FOR IDENTICAL FIELDS OF VIEW, THE LINEAR SIZE OF COMA WILL BE IDENTICAL FOR ALL FOCAL LENGTHS OF A PARTICULAR F/RATIO.

So, the linear size of the comatic star image IS made larger by magnification. He had no explanation of why I (and others) see lessened coma at higher powers. I, since (on another forum), have run into some individuals who see the same coma in the field at all magnifications, so it might be a psychological phenomenon on my part.

05-09-2006, 06:05 PM
Don, with the exception of anti-reflection coatings, all telescope optics (mirrors, lenses, eyepieces, barlows, field flatteners, coma correctors etc) are designed to operate in the regime of geometric or ray optics and are based entirely on ray optics principles. Here, wave phenomena such as interference and diffraction can be completely neglected and light can be viewed as consisting of localised rays rather than extended waves that fill all space. Or, equivalently to the ray picture, you can think of light as point particles bouncing off mirrors and bending at air-glass interfaces according to the law of refraction. Ray optics does not contradict wave theory, but it is a special case of it, i.e., the short wavelength limit.

The ray optics approximation is completely valid for telescope optics in normal use, and only starts to break down at the diffraction limit of the scope, i.e., when one uses enough magnification to start seeing diffraction effects (e.g., Airy disk star images). (It does fail to predict some subtleties e.g. diffraction spikes from spider of a Newt but these are irrelevant in the present context.) It is perfectly valid to split the mirror into multiple components (e.g. a slow part and a fast part) and then figure the total light intensity in the focal plane to be the sum of the light intensities from its composite parts. And the statement that a given f-ratio mirror also contains all slower f-ratios is correct also.

Coma is a geometric property of the parabolic mirror. It is entirely a ray optics phenomenon and is corrected by optical devices such as the paracorr within that theoretical framework.

Yes, this has been stated a number of times, and I believe everyone who posted about it in this thread agrees on this point.

06-09-2006, 10:28 AM
Boy oh boy.

It looks like we have had lots of "entertain" from Don so far, but not much of "inform" and "educate" despite vailant efforts of Janoskiss.
I guess I'm to blame as well as so far noone apart from Steve actually understood what I was talking about.
I can't really make it any simpler, I'm afraid if anyone wants to know something about optics I can only suggest a good book like Rutten and Van Venrooij or Conrady.

Let me but in one more time, just to try to clear the confusion.

BTW, I don't really need to talk to an "optical designer friend" - I have designed coma correctors (single element - subaperture menisk, two element - Ross family, and three element units - Wynne/Gascoigne type) for various systems (sphere, paraboloid, hyperboloid, Dall-Kirkhams and Schmidt Newtonians). I've been designing optics for decades, and when I say something, I usually know what I'm talking about.

So let me offer something even simpler to understand. I can show you spot diagarams of two optical systems using the same coma corrector.

Although I have in my posession reverse engineered data for Celestron/Baader and Lumicon units, I don't know the prescription for Paracorr (as noone volunteered a unit to be pulled apart, and I'm not going to shed $$$ just to try to convince someone who doesn't really want to learn anyway).

So, here I will use a well known coma corrector published several times - in May '85 Sky and Telescope (under a peer review of late Bob Cox) and Telescope Optics book (again peer reviewed by R & vV). It is designed for 1200mm focal length (author designed it for 12" f/4 scope).

Let's see what Zemax says about two systems. 100mm f/12 (reverse coma anyone?) and 100mm f/4 (which should be spot on if we are to believe Don).

Vertical bar on the left gives you a reference size (200 microns). Black circle is Airy disc size. I've used monochromatic light not to confuse people even more with polychromatic aberrations. Five spots are spread as to cover field from dead on axis to approx 14mm off axis (very edge of the 1-1/4" size eyepiece and approx. corner of a typical DSLR chip).

A 100mm f/12 scope (attachment 1)

{{{ I'm hopeless, can't get the attachment to be where I want them }}}

Reverse coma ? Hmmmm .... Sure. Want to see f/16 ? F/20 ? It only gets better, trust me.

100mm f/4 scope, focused for best images on a flat focal surface (attachment 2)

Hmmm. But it should be perfect, it is f/4 after all !!!! And those off axis images look like -er- coma ????
Don't let me show you what a 50mm f/4 looks like.

06-09-2006, 12:07 PM
... and to complete the picture here's the 300mm f/4 that corrector was designed for.

I've actually omitted designer's name - apologies to Mr J. Richter.

Don Pensack
07-09-2006, 07:07 AM
I was wrong about reverse coma. The inward bending of the curve in the outer part of the field just indicates complete correction of coma at longer f/ratios, not a backward bending of the curve.
I posted an inquiry about the discrepancy between what I was told and read about coma, and remarks by Bratislav, on another Forum.
Here is a link to that thread (you may have to cut and paste the entire link into your browser address bar):

I think reading the posts will fill you in on where the misunderstanding was, and point out that I believe that what we had here was a "failure to communicate".

This will be my final post on this subject on this Forum. Moderators should be more aware that ungentlemanly behavior on the part of participants should be remonstrated.

07-09-2006, 07:46 AM
Hi Don

Thanks for your contribution on this topic. Much of it is beyond my current level of experience and knowledge, but I have valued the little that I have picked up along the way.

As a reader and contributor to this forum, I know who the gentlefolk are and by your posts, you have shown that you belong to that community on Ice In Space. Although I am always surprised at what clearly appear to be confrontational or combative responses in these threads, I focus on the real data. My respect for the gentlefolk only increases when in the face of such an onslaught, they do not respond to the caustic comments in the same prickly manner.

The intellect can be a powerful tool and when someone has developed a sharp intellect, one hopes that their overall persona has also matured to the extent that this sharp tool is applied not in a cutting manner, but in a generous and helpful manner. Sharp is good; cutting is simply unnecessary and perhaps the sign of an insecure person?

Take care


07-09-2006, 11:03 AM
Most of us are here to learn and work towards a common understanding. I appreciated the positive contributions from all who posted in this thread, and the opportunity to learn a bit about coma. OTOH, I would also like to point out that it should be obvious to anyone with half a brain that the condescending attitude displayed in some posts was totally unwarranted and entirely counterproductive. (see what I did there...? :P)

07-09-2006, 11:06 AM
I am not going to apologise for my nature of commenting things. That is how I am - plain and simple.

I also understand that this forum is a lot more about getting together and sharing love for astronomy - I fully respect that. More on that later.

That said, I really get upset by Don's ability to twist and misquote things (delibeartely or not, I don't really care).
For example, in 'another thread' he now managed to attribute this to my name :
"He stated quite definitely that coma in shorter focal length f/4 scopes was worse than coma in long focal length f/4 scopes".

I never said anything of sorts, as this would be stupid plain and simple. I have no doubts he misquoted Al and David Nagler in a smilar fashion. Gentlefolk or not, this is equivalent to slander.

Now, "unsecure" nonwithstanding, I am going to defend my point of view and my name to the best of my abilities. If that is perceived "sharp" or "cut" so be it. I am dealing with facts - I don't have patience or "maturity" to wrap things in a rosy glow so everyone can be merry and live happily everafter. Science and engineering forums are like that. Put up or shut up.

I do understand that this forum is different and this sort of behaviour is not welcome here - yup, I've got the message - several times. It is entirely my fault for getting into this discussion here in the first place. I really should have known better, and I sincerely regret it. I will do my very best to avoid this in the future.

I'll let you go back to the regular program now.

(I did want to remove my account from IIS, I already asked moderator how to do that. I guess it doesn't matter now, this should expedite things :) )

07-09-2006, 11:16 AM
Thread locked so we can all cool down a bit.

07-09-2006, 11:18 AM
Bratislav, Moderators do not have the authority to be able to do such. On the other hand if we did, I would of gladly done so.