Collimating a 9.25" SCT

[5ash]

I know how to collimated this telescope roughly using the defocused image of a star , however I'm thinking of using the telescope for planetary imaging and want to collimated it more precisely. For this I need to examine the airy disks around a focussed star at high magnification. However the atmosphere is rarely still enough to make these observations and I'm considering making an artificial star to do the job. On reading comments on the net about collimating a SCT I find that many have the opinion that if a SCT is pointed horizontally the primary mirror moves forward making accurate collimation impossible and that it must be pointed up at 20-30 degrees to overcome this problem making use of an artificial star very difficult. Would appreciate any comments.
Regards philip:help:

[casstony]

I get good results collimating near horizontal (pointed slightly up) indoors with an artificial star or outdoors early in the morning before sunshine starts making the air unstable. The amount of mirror slop in a particular scope probably affects the accuracy of collimation though approaching final focus with an anti-clockwise turn of the focuser should help.
I use this: http://www.hubbleoptics.com/artificial-stars.html

[5ash]

How far away are you from the Hubble optics device ? My reading suggests 20x the focal length of your telescope ( 47metres).
Regards philip

[casstony]

For collimation the star only needs to be far enough away to achieve focus. Strong spherical aberration results but it doesn't affect alignment of the optics. Slightly to one side of focus you'll see a bullseye with a bright ring which can be centered and made even. I collimated a standard 9.25 at about 60 feet from the star from memory.
My 8" EdgeHD needs about 55ft while I have to do the 11" outside as it needs more distance.

[tim.stephens]

Something I just tried was to use a single ball bearing with a torch to create a cheap artificial star. Works brilliantly for the centre of field.

This was fine but I was getting sick of slewing the telescope to check the corners so I bought a bunch of smaller bearings and glued them onto a board spaced appropriately to fill the FOV of my camera. Have to get a ruler out to measure the distances so they are the same and paint the board backing matt black to avoid unwanted reflections.

Tim

[multiweb]

I have collimated my c11 with an artifical star. It's great to get you ball park but if you're looking for the ultimate collimation then a star test in the area you want to image is the way to go for the final tweak.

[5ash]

Thanks everyone for the info , will take it on board. I am at present building an optical artificial star , not sure how to hyper link this : http://observatory.mvastro.org/library/Star_Test/ArtStar.html
Regards philip