dunc66
05-12-2009, 12:41 AM
Hi
I have an ED80 and it focuses stars to a pinpoint through an extension tube and 40mm eyepiece.
I used the same tube and eyepiece on my newly purchased Orion ST80-A guider and the coma is very bad right across the FOV. I originally bought it for guiding but then read articles about tweaking it. Would be nice to have it as a portable wide-field scope. How bad should the coma be?
In my attempts to find the problem I have removed the lenses and focusing tube and reassembled them. I adjusted one focuser grub screw to reduce the slop in the rack and pinion mechanism. While I was at it I shortened a screw in the focuser tube and blackened a few bright areas. I found no problems and everything seated back nicely. There is no way to correct the squareness of the lenses to the tube or the focuser to the tube, easily.
I made a round target out of paper with a compass and fitted it snugly and centrally into the objective lens cap which I fitted to the scope. When I insert my laser collimator (which I have checked), the laser dot hits the centre of the target (roughly, given that the laser dot is oblong!).
When I look down an illuminated Cheshire collimator, I see a centred green ring.
I will now start rotating one objective lens with respect to the other to see if the coma goes away.
What do I do next? Insert shims behind the rear objective lens to try and square things up? If this is the way to go, do I shim on the fan side of the coma or the point side when viewing through the eyepiece?
If I am still unable to fix it. Is there anyone in Brisbane /Ipswich who could help out.
Cheers
I have an ED80 and it focuses stars to a pinpoint through an extension tube and 40mm eyepiece.
I used the same tube and eyepiece on my newly purchased Orion ST80-A guider and the coma is very bad right across the FOV. I originally bought it for guiding but then read articles about tweaking it. Would be nice to have it as a portable wide-field scope. How bad should the coma be?
In my attempts to find the problem I have removed the lenses and focusing tube and reassembled them. I adjusted one focuser grub screw to reduce the slop in the rack and pinion mechanism. While I was at it I shortened a screw in the focuser tube and blackened a few bright areas. I found no problems and everything seated back nicely. There is no way to correct the squareness of the lenses to the tube or the focuser to the tube, easily.
I made a round target out of paper with a compass and fitted it snugly and centrally into the objective lens cap which I fitted to the scope. When I insert my laser collimator (which I have checked), the laser dot hits the centre of the target (roughly, given that the laser dot is oblong!).
When I look down an illuminated Cheshire collimator, I see a centred green ring.
I will now start rotating one objective lens with respect to the other to see if the coma goes away.
What do I do next? Insert shims behind the rear objective lens to try and square things up? If this is the way to go, do I shim on the fan side of the coma or the point side when viewing through the eyepiece?
If I am still unable to fix it. Is there anyone in Brisbane /Ipswich who could help out.
Cheers