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Old 24-08-2008, 01:07 PM
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multiweb (Marc)
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Field Curvature and Coma

I've been trying to improve my images for the past month or so. I purchased a Baader MPCC, a Cateye collimation kit and CCD Inspector. Running my subs through CCD inspector helped me improve my optical alignment and by using the right spacers with the MPCC I managed to flatten my field dramatically. I guess now I'm at the stage where it is "acceptable" but I'm trying to go the whole way. Am I right in assuming that moving the primary towards the secondary up the tube [i.e clipping the primary reflection in the secondary] would improve coma? My reasoning behind this is that the light cone reflected by the primary will be "cropped" by the secondary and not totally reflected so although I'll get less light I'd be using the better part of the primary (no the edges?). Does this sound right? Any toughts? Obviously the scope is a newtonian. 5"/FL650mm F/5. I mostly image at prime focus with a QHY8 [which is a faily large chip] and a 2" nose piece. Thanks for any pointers.
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Old 25-08-2008, 06:39 PM
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Field Curvature and Coma

I posted this originally somewhere else and didn't get any answer so I'm trying here. Maybe it's more relevent to the photo threads.
I've been trying to improve my images for the past month or so. I purchased a Baader MPCC, a Cateye collimation kit and CCD Inspector. Running my subs through CCD inspector helped me improve my optical alignment and by using the right spacers with the MPCC I managed to flatten my field dramatically. I guess now I'm at the stage where it is "acceptable" but I'm trying to go the whole way. Am I right in assuming that moving the primary towards the secondary up the tube [i.e clipping the primary reflection in the secondary] would improve coma? My reasoning behind this is that the light cone reflected by the primary will be "cropped" by the secondary and not totally reflected so although I'll get less light I'd be using the better part of the primary (no the edges?). Does this sound right? Any toughts? Obviously the scope is a newtonian. 5"/FL650mm F/5. I mostly image at prime focus with a QHY8 [which is a faily large chip] and a 2" nose piece. Thanks for any pointers.
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Old 25-08-2008, 11:46 PM
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What you need to find out is what corrections or what spot sizes are given for using the MPCC. Then see if you are within the specs. You may be chasing your tial here if the MPCC isnt able to get more performance out.
Post a picture showing the reults of on and off axis stars.

Theo.
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Old 26-08-2008, 12:37 AM
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The coma is coming from the f5 primary and is part of the design. Moving the secondary etc is no different to cropping the final image. I'd go for the easy way; get the best you can out of the field flatener then crop the edges of the image as required.
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Old 26-08-2008, 07:36 AM
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Moving the primary up will only introduce more vignetting and make no difference to coma produced by your parabolic primary. As you move away from the optical axis there is just less of the mirrors outer zones reaching the focal point.
Gary
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Old 26-08-2008, 08:24 AM
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Quote:
Originally Posted by multiweb View Post
My reasoning behind this is that the light cone reflected by the primary will be "cropped" by the secondary and not totally reflected so although I'll get less light I'd be using the better part of the primary (no the edges?). Does this sound right? Any toughts? Obviously the scope is a newtonian. 5"/FL650mm F/5. I mostly image at prime focus with a QHY8 [which is a faily large chip] and a 2" nose piece. Thanks for any pointers.
You shouldn't be getting any objectionable coma at F5 and an MPCC.

Moving your primary forward will just increase vignetting not reduce the size of the comatic image. Coma is a function of F ratio.

Just put an aperture stop on your scope. Put a 4.1" aperture in front of your scope to make it F6, but experiment with F5.5 also. Remember that as you stop the scope down you'll be decreasing the ratio between the secondary and mirror diameter which will increase vignetting in the field ( which I guess you can fix by 'flat fielding' in image processing ) .
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Old 26-08-2008, 08:39 AM
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Quote:
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Moving the primary up will only introduce more vignetting and make no difference to coma produced by your parabolic primary. As you move away from the optical axis there is just less of the mirrors outer zones reaching the focal point.
Gary
Ouch! Doesn't sound good then... I've already moved it up 5mm and re-collimated last night. I'll do some more testing on the field this week-end.
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Old 26-08-2008, 09:36 AM
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Stop the telescope down a little to reduce coma ( for example 4" to make F6 ). Vignetting will increase slightly.

Coma shouldn't be objectionable at F5 with an MPCC.
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Old 26-08-2008, 10:19 AM
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The two threads have been combine into this one.
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Old 26-08-2008, 12:22 PM
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Thanks for that Andrew. Sorry for the cross posting. I wasn't sure where to start the original thread from when I didn't get many replies from the "tips and techniques" side.

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The two threads have been combine into this one.
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Old 26-08-2008, 12:25 PM
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No problem Marc.

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