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Coma in an SCT

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#1 revans

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Posted 30 November 2021 - 11:55 AM

I bought a used C6 from a reputable dealer.  It arrived and I was busy working with other scopes so initially just took it out in daylight and looked at the local scenery.  Last night was first light for it.  I had the scope, then an F 6.23 Celestron focal reducer/flattener, then a 2" adapter to the camera's mounting ring and finally a Canon EOS T5.  I focused on Vega through the camera view viewfinder.  I was at about 19x.   I noticed that as I approached focus the star disk was asymmetrical before it finally resolved into a pinpoint on further focusing.  I didn't know what to make of that as my refractors have symmetrical star disks when approaching focus. So I thought of possible collimation error, but I wasn't at high power or using an eyepiece or looking at a CMOS image of the star.

 

I went ahead with the imaging run on M45.  The subs showed excellent tracking through a 3 hour imaging run with no star movement in the field.  The stars were sharp except at the edges (but not everywhere)... sort of in a "V" shape and peripherally.  I used to use a Newtonian and what I saw as abnormal in the SCT images after image calibration was what I would have called some coma.  I didn't know SCTs could get coma.  Are they born with it or is my scope in need of calibration?  I figured you folks would know...  Eventually I'll take the time to do a star test, but I only use this scope occasionally and I don't have time to get deeply into calibrating it just yet. And it doesn't have Bob's Knobs so calibrating it will be unpleasant if it comes to that.


Edited by revans, 30 November 2021 - 12:02 PM.


#2 alphatripleplus

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Posted 30 November 2021 - 12:18 PM

It just sounds like your scope is in need of collimation of the secondary mirror, based on the description of the asymmetry of the out of focus star donut with the secondary shadow in the middle.


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#3 revans

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Posted 30 November 2021 - 12:51 PM

It just sounds like your scope is in need of collimation of the secondary mirror, based on the description of the asymmetry of the out of focus star donut with the secondary shadow in the middle.

Here is more data....  first what the collimation screw area of my C6 looks like.... to me it seems more "pushed in" around 2 o'clock.  Next is my memory of the asymmetry approaching focus on Vega from the outside as I looked through the camera view finder.  Finally the 3 hour image of M45 taken with the scope, showing coma.

 

C6 collimatn
Star Asymmetry

Edited by revans, 30 November 2021 - 01:07 PM.


#4 revans

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Posted 01 December 2021 - 05:19 PM

Looks like the moderator in the ATM forum moved my discussion of this issue there to this forum, probably not realizing that I'd already posted the issue here.  It's a bit redundant... two posts on the same issue in the same forum... sorry about that...




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