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.
I am particularly interested in whether it has a significant negative impact on light transmission and image contrast.
Rod
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.
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.
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.
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 ).
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
What's the focal ratio of your truss dob, Scott? Do you experience much/any coma?
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)
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.
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.
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).
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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.
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.
Bratislav
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.
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.
... 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).
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.
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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.
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.
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.
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 ?
But as you approach the point where the coma blur is comparable with size of the Airy disk, it becomes less obtrusive.
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.
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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
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.
Paracorr
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