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Thread: Coma correction

  1. #1
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    Default Coma correction



    As I continue my education into Dobs before deciding if that Orion XX14g is really in my future I find I now need to understand comma. So, is there some place I can go for a good primer on the subject and while I'm doing that can folks talk about the need for comma correction on a f/4.9 14" Dob? Mandatory? Should do it ? Just nice to have? And of course if I do need one, which one and why?
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    UO 4mm Ortho, KK 9mm & 12.5MM Orthos, Delite 7mm, Delos 10mm, 14mm, Panoptic 19mm, 24mm, Baader 8-24mm Mark IV Zoom

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    Default Re: Coma correction

    Coma correction is optional around f5. At f4 it's essential. I had an Orion xx14i and I was happy leaving the coma corrector off. That might depend on your eyepieces though. I was using Panoptics and Delos.

    I used the TV Paracorr and the Baader MPCC. The TV offers ease of use and excellent optics as an advantage. The Baader MPCC is cheaper but is more inconvenient to use. I wound up buying a couple of them so I wouldn't have to swap the corrector and rings among eyepieces, spacing is critical.
    Past items italicized. Scopes: Refractors: Orion ST80 (mods for while light solar), SV ED80 f7, Orion and SW 80 and 120 ED doublets, Orion 120 f8.3; Newtonians: AT 8”f4, OC 8”f6.3, Z12 f5, self made 6” f9, Orion XX14i; Catadioptrics: VMC110L, Intes MK66, Celestron 9.25 SCT and 9.25 Edge. EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, TV Nagler, ES100s.

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    UlteriorModem's Avatar
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    Default Re: Coma correction

    Fast newts suffer from coma. No avoiding it.

    Is a corrector 'mandatory' not really depends a lot on the user. Yes it is nice to have but not necesarly mandatory..... For visual
    Scopes: Orion 130mm APO Triplet, Orion 8" f4 astrograph, 10" RC Mount: Celestron CGX-L Guiding: Orion 60mm f4 scope, Starlight Loadstar, PHD2 software - Cams: Planetary: ASI120mc, DSO:Atik 383L+, SGPsoftware Processing : Regisax, Deep Sky Stacker, Star Tools, Pixinsight Assistant: One very large cat
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    Default Re: Coma correction

    ok, thx. Still no plans to ever get into AP .... If I end up getting the scope I'll first see what EPs I end up with since I know I'll want to be swapping out some of the EPs I have now and then can decide after using them a while.

    Fritz: noticed you have the Mark IV zoom. Did you have that when you had the 14" Dob and if so what was your impression?
    Intercepting & temporarily inconveniencing light photons as they travel through the universe since 2017
    What do I have? A M.S. degree in AstroPhysics w/minor in planetary geology & an Orion XX14g time machine which is still in many pieces
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    UO 4mm Ortho, KK 9mm & 12.5MM Orthos, Delite 7mm, Delos 10mm, 14mm, Panoptic 19mm, 24mm, Baader 8-24mm Mark IV Zoom

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    Default Re: Coma correction

    If you do decide to get one for when you use your widefield eyepieces I'd really suggest a Paracorr that is tunable.
    My eyes
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    Default Re: Coma correction

    Quote Originally Posted by Nimitz View Post
    ok, thx. Still no plans to ever get into AP .... If I end up getting the scope I'll first see what EPs I end up with since I know I'll want to be swapping out some of the EPs I have now and then can decide after using them a while.

    Fritz: noticed you have the Mark IV zoom. Did you have that when you had the 14" Dob and if so what was your impression?
    Yeah... But no. They didn't intersect in time. That zoom is recent. Come to think of it haven't had a night out with the Z12 even since they arrived......
    Past items italicized. Scopes: Refractors: Orion ST80 (mods for while light solar), SV ED80 f7, Orion and SW 80 and 120 ED doublets, Orion 120 f8.3; Newtonians: AT 8”f4, OC 8”f6.3, Z12 f5, self made 6” f9, Orion XX14i; Catadioptrics: VMC110L, Intes MK66, Celestron 9.25 SCT and 9.25 Edge. EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, TV Nagler, ES100s.

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    Default Re: Coma correction

    I have a tunable paracorr I for my f4.7 newt (used from astromart), and yes, it does make a difference. If you have decent EPs that have a design amenable for a fast newt (ES82's do very well in this way) then it is by no means a must have (for visual, and for my preferences). What it may do is extend you use of other alternative EP designs, and will certainly improve coma in those EPs that suffer from it.
    Ian
    Scopes Orion XT10i f4.7 "Frosty", old Sears 6305 60mm f15 "Raquel"
    Binoculars Asahi Pentax 7x35 and 7x50, Beck Kassel 8x30 and Celestron 15x70
    Eyepieces: Leitz Periplan GF, ES68s, ES82s, TV Naglers, Meade 4000 super plossl, Nikon CFI, Olympus FK, Paracorr I, Abbe Orthos: Carl Zeiss Jena K7x (35mm), K10x (25mm)
    Asteroid count: 0 Herschel 400: 74 Planets: 5 Novae: 1 S30: 2

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    Default Re: Coma correction

    Quote Originally Posted by kanadalainen View Post
    I have a tunable paracorr I for my f4.7 newt (used from astromart), and yes, it does make a difference. If you have decent EPs that have a design amenable for a fast newt (ES82's do very well in this way) then it is by no means a must have (for visual, and for my preferences). What it may do is extend you use of other alternative EP designs, and will certainly improve coma in those EPs that suffer from it.
    The 70 degree formats like Panoptics need a corrector less. But I find that for them too at f5 they're fine but they need help at f4.
    j.gardavsky and kanadalainen like this.
    Past items italicized. Scopes: Refractors: Orion ST80 (mods for while light solar), SV ED80 f7, Orion and SW 80 and 120 ED doublets, Orion 120 f8.3; Newtonians: AT 8”f4, OC 8”f6.3, Z12 f5, self made 6” f9, Orion XX14i; Catadioptrics: VMC110L, Intes MK66, Celestron 9.25 SCT and 9.25 Edge. EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, TV Nagler, ES100s.

  9. #9
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    Default Re: Coma correction

    I agree with n_F_A. Gotta have one at f/4 and faster, but only needed at f/5 if you want super sharp edges to your panoramic wide views. If you are happy with your Superviews and 15mm Luminos eyepieces in your SCT, you will probably be happy without the corrector. If you prefer the improvement when you substitute Panoptics for the Superviews or a Nagler for the Luminos, then you will probably prefer to use a corrector. Note that most folks who rate correctors as "must have" accessories are using premium eyepieces. This isn't an elitist attitude, it is just the laws of diminishing returns setting in. That last 5% to 10% of performance always has a 20% to 50% price tag.

    As Kanadalainen mentioned, the corrector will help eyepieces with curvature problems, but it won't fix an inherently soft or astigmatic eyepiece. Tunability is a big plus, but you will have to adjust it each time that you change an eyepiece, so if you do a lot of mag changes when you observe, keep that in mind. It is another argument for upgrading to better eyepieces with fewer inherent aberrations so that you are primarily using the corrector only to fix the mirror's coma which is a constant. I would try to get to a star party to borrow a corrector for evaluating the benefits.
    Scopes: 28" f/3.8 self built Dob, 8" Skywatcher f/4 Newt, iOptron 115mm f/8 Newt, Nova 9.5" RC, Celestron 5" f/6 FAST SCT, 6" f/15 Jaegers achro, Celestron Nexstar 102mm achro.
    EP's: WO 40mm SWAN, 20, 9mm, 3.5mm XWA; ES 14mm 100°; Meade 5mm MWA; Gary Russell Konigs and zoom, TMB 2.5mm, 3.2mm, 4mm Planetaries; TV 1.8x 2.5x Barlows; Some older Plossls and Orthos
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    Default Re: Coma correction

    I have several Newts that go from from f/4 to f/5. The only time I find a coma corrector useful is when using my Explore Scientific 30mm 82° eyepiece in the 8" f/4 scope. I don't find coma problemaric in my 17.5" f/4.5 scope when using the same eyepiece, and certainly not at f/5.

    Coma is a phenomenon that ONLY pertains to REFLECTORS, not refractors. Many people say they see "coma" in there fracs, all this shows is they do not know their optical aberrations. People who saythat see "coma" in their refractors are confusing it with field curvature which is a totally different aberration.

    Coma stems from the shape of the optical plane that comes from a concave reflector. This focal plane is also concave. The severity of this bowl-like shape depends on the f/ratio. Scope's focal length also plays a part, but more significant is focal ratio. Coma then comes from the ability for an eyepiece to deal with this shape.

    It doesn't stop here. It is important to also have an eyepiece that is an optical match for a Newt. Refractors produce a convex focal plane, and it is easier to design and fabricate eyepieces for a convex focal plane. Not so for Newts. What an optical mismatch will do with a Newt is introduce astigmatism into the image. This is something a coma corrector will not alter.

    Coma and astigmatism show themselves very differently. With coma, stars along the edge of the field of view have little "comets" coming off them, and these are aligned radially - that is pointing away from the centre of the image.

    Astigmatism sees little "seagulls" coming off the syars along the edge of the field of view and these are aligned co-centric to the image.

    In an optically matched Newt/eyepiece coupling, only coma will be visible. In an optical mismatch, both coma and astigmatism will be.

    Your first step in cleaning up the image is to get eyepieces that are an optical match with your Newt. You may find that the coma aspect actually becomes very tolerable. You may find that you don't require a coma corrector.

    This is my approach with eyepiece selection, as astigmatism is something that can be avoided, I first start with eyepieces that are an optical match. Then it is upto the focal ratio of the scope if I need a cprrector or not, and I entioned at the start of this post where I use one.

    Note also that eyepieces will vary on how they handle coma. EP designers cannot know what focal ratio your Newt is, f/6 or f/3.3 or whatever. So they will design first for an optical match, and only some coma correction. You then need to select a coma corrector that is best suited to the focal ratio of your Newt. And ALL correctors you will neex to adjust the spacing between it and the eyepiece - that is the "tunable" part of some correctors.

    Remember also that the appearance of coma is also dictated by the AFOV of an EP and the magnification.

    There is one other aspect that is often overlooked in eyepiece discussions- the human factor. The eye is an amazing organ, and not only does it vary from person to person, but also the tolerance of different aberrations varies in the mind's eye from person to person. So for two identical eyes in two different people, some astigmatism may be ok for one person but unacceptable for the other. And the eye is also an active element in the scope-eyepiece system, and it can also correct for some aberrations. This cannot be forgotten.

    SCT's and Maks

    These two cassegrains despite being refractors, they actually produce a convex focal plane like a refractor. This is due to the convex shape of the secondary. SCT's will still show a very small amount of coma that is really only problematic for photo applications, not visual. Maks don't show coma as far as I know, due to the different corrector plate design compated to SCT's. But also Maks are next to never used for DSO imaging so any coma they may display never comes into play.

    This post is only about coma. It is not about which eyepiece is better than the other as this is more subjective than objective.

    Alex.

 

 
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