Collimated but with severe spherical abberation?

Hello All,

I am in the process of trying to collimate my 8" dob project and purchased a laser tool to facillitate the process. I was able to get the laser dot to line up correctly, and then took the scope out last night for the first time. I noticed I was getting a soft image, so I went out of focus on a bright star and noticed the disk bending to the right forming a 'C' rather than an 'O' pattern. Below is similar to pattern:

The scope is showing collimated per the laser tool, but I know there must be an adjustment/alignment issue (so I'm confused at this point). Any advice?

Thanks in advance,
Eric

Attached Image:

Eric,

It looks like your secondary is not lined up with the focuser so part of your light cone is being cut off. Try looking down the focuser with nothing in it and see if it looks like what you expect. Try rotating the secondary a bit while you look. Using the laser, I've also looked in the front at the reflection of the seconday to see if the laser was hitting the secondary where I expected it to be (be careful not to get the laser in your eye - hold up a piece of white paper first to see if/where the beam comes out).

When I pull my scopes apart and put back together, I eyeball it all up, use the EP collimation tool, then final check with the laser. The laser doesn't work so well if you are way out.

Ed

If you do not have a Cheshire EP, then take a 35mm Film can (Plastic type) and cut off the bottom and then make a small 1/8" or so hole it the center of the top where the dimple in located. Put this in the focuser and look through it. You should then be able to see the secondary as a round circle and the primary being reflected off it. Adjust the secondary until it is aligned and round in shape.
http://www.fpi-protostar.com/collim.htm
Here is a link to a good site that shows what we are talking about in detail and pictorially.
Floyd

Adjusted the secondary, per the instructions on site and presto the image drastically improved