Coma and other issues

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stuartspink86
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Coma and other issues

#1

Post by stuartspink86 »


I have recently purchased the ZWO ASI1600MM Pro camera with Electronic Filter Wheel. I am using the Orion 8" Astrograph which requires me to use a Coma Corrector to produce a great image. I have successfully used my Baader MPCC Mark III with my Nikon DSLR and produced some nice images. I took the plunge to a new camera and have had nothing but issues since. As you look at my picture I took last night of a 5 minute exposure of the Horsehead Nebula in H_Beta you will see a number of issues. I would like confirmation that I have terrible coma and hopefully a little guidance as to the large circle at the bottom of the image and the smaller one near the top. I also attached my train from camera to coma corrector. From ASI's website they say 55mm is the optimal distance but clearly that's not the case. One modification that I had to do is moving my primary mirror a little forward as I could not achieve focus. Now I have plenty of back focus but I believe that moving my primary mirror forward also cause the large halo at the bottom and the smaller one at the top. ZWO has on their website for this camera an example train that I followed to the letter. https://astronomy-imaging-camera.com/wp ... lution.jpg If someone is an expert at where this Coma Corrector should be in the imaging train and can give me some guidance that is greatly appreciated.

Thank you -Stu
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Larry 1969 United States of America
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Re: Coma and other issues

#2

Post by Larry 1969 »


I'm no expert (just a beginner as a matter of fact) but I believe coma only really shows up toward the edges of the image. You have some weird distortion throughout the entire frame...

At the very top just right of center appears to be some type of reflection?

I also think the double refraction spikes indicate something as well.

It will be interesting to hear what the authorities have to say.

Good luck!

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
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Lady Fraktor Slovakia
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Re: Coma and other issues

#3

Post by Lady Fraktor »


Check the alignment of the vanes, they may not be perfectly aligned to the tube. One or more may have a slight twist.
I assume you have checked the focuser alignment to the tube as well as standard collimation?
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
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Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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yobbo89 Australia
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Re: Coma and other issues

#4

Post by yobbo89 »


Is your orion scope f3.9 ? a bit strange that you had to move the mirror to achieve focus even though it's an astrograph .

a few things, That coma corrector does not perform optimally at f4 or lower even though it's listed to work down to f3.5 with "complete pinpoint stars across the field",that is COMPLETELY FALSE... ,which i think most retailers have mixed up the info, you'll find some don't mention the f ratio it corrects down to but mention that you get no vignetting down to f3.5 .. , f4.5 seems to be the sweet spot for this corrector and it works very well, below that f ratio it will struggle to correct even on a smaller aps-c size chip.

It looks like you might need to re collimate
adjust your focus,i can see star diffraction spikes separation,which is a induction of a soft focus
and obviously tracking needs an improvement , this will help you analyzed your star profiles a bit better to see how well the stars are corrected while adjusting the coma corrector .

a handy tool to have is a digital vernier to measure the distance between your camera and the corrector for adjusting spacing.

The reflection is most likely due from the coatings on the filter and the coma corrector and majority of the problem is the extremely bright star Alnitak ,it's a very difficult target that requires a flawless setup and very good processing techniques to remove any remaining reflection,reducing star halos ectt.
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .

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sdbodin United States of America
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Re: Coma and other issues

#5

Post by sdbodin »


Looks like a combination of things, hard to separate, but coma is not one of them. First, as others have said, check collimation, the double diffraction spikes are a possible clue to this. Then squaring of the focuser and coma corrector in the focuser.

What mostly seems to be the problem is guiding and focus, did you autoguide? If not, take a series of shorter exposures on a brighter target, 10 sec and up and try to eliminate the symptoms one at a time.

Going to be a long slog, but we have all gone thru it.
Steve

P.S. why shoot H beta? not much there, is this a visual filter? or a dedicated camera filter?, could be the source of the circle reflections.
Scopes; Meade 16 LX200, AT80LE, plus bunch just sitting around gathering dust
Cameras; Atik 460ex mono, Zwo ASI1600MC-cool, QHY5L-II color and mono
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SkyHiker United States of America
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Re: Coma and other issues

#6

Post by SkyHiker »


Since the coma stretches the stars uniformly from the upper left to the lower right, the optical axis is hugely off - my guess is that collimation should fix most of your problems. Even the spiders have shadows consistent with that direction.
... Henk. :D Telescopes: GSO 12" Astrograph, "Comet Hunter" MN152, ES ED127CF, ES ED80, WO Redcat51, Z12, AT6RC, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S with OnStep, AVX, Tiltall, Cameras: ASI2600MC, ASI2600MM, ASI120 mini, Fuji X-a1, Canon XSi, T6, ELPH 100HS, DIY: OnStep controller, Pi4b/power rig, Afocal adapter, Foldable Dob base, Az/Alt Dob setting circles, Accessories: ZWO 36 mm filter wheel, TV Paracorr 2, Baader MPCC Mk III, ES FF, SSAG, QHY OAG-M, EAF electronic focuser, Plossls, Barlows, Telrad, Laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: KStars-Ekos, DSS, PHD2, Nebulosity, Photo Gallery, Gimp, CHDK, Computers:Pi4b, 2x running KStars/Ekos, Toshiba Satellite 17", Website:Henk's astro images
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