Mirror image in diagonal

I have a 12.5 inch f5 Cave telescope which arrived at my door in January of 1980. It was probably one of the last Caves out of the plant door. I had the mirror checked by Steve Swayze three years ago and when he tested it he found it to be overcorrected throughout with the exception of the edge which was noticeably undercorrected. He then refigured it for me and it has been safely kept in its shipping crate since then. I do not want to use it until I can build a permanent home for it. I am about 2/3 finished on a roll-off roof observatory. Now with that out of the way here is my question. When I look through the focuser at the mirror image in the secondary, should I see the entire mirror? I ask this because as it was set up from Cave the outer edge of the entire mirror was not in view. I suspect this was done because of the undercorrected edge. Before I go and put new holes in the tube should I see the entire mirror?

Lawrence Regira

Lawrence,
They may have used a smaller secondary to make for a smaller Central Obstruction too. But it is possible that the edge was left out on purpose to correct for the defect.
You can replace the secondary with a slightly larger mirror or you can possibly raise the primary and shim the focuser if it only required a small change.
Yes, you would like to see the primary to the edge or very close. But it would depend on how much is missing as to whether it is a problem or not. If only a small edge is cut off, I don't think you need to do anything. If it is 1/4"-1/2" then you might want to make a change.
I would probably replace the secondary rather than make changes in the tube myself. Cutting on the tube is irreversible where replacing the secondary can be reversed without any modifications to the main scope

I am not an expert in the optics field! But before you start rethinking the Cave, I question your method of looking through the focuser to size the secondary. You did not tell us the minor diameter of your secondary, or the measurements of primary to secondary and secondary to your observation point, so I can't check it.

There are several sources of equations you can use to size the secondary. Any of the books on telescope making discuss it. There is a nice discussion on http://www.catseyecollimation.com/mccluney.html of effects of secondary mirror size on how much of the field will be fully illuminated.

Many of the drawings I have seen show the focal plane of the basic scope without eyepiece coming to focus on the outside of the drawtube (where the eye is). I am not sure that this is really the case. I think the focal plane occurs in front of the eyepiece and the eyepiece picks up the image and reconverges it into the eye. So the focal plane for the primary mirror is down inside the focal tube. This should be possible to check with a piece of translucent material directly focusing on a bright star or planet. By looking at the secondary from farther away than the focal point you may be cutting off the primary mirror's edges.


Gary Klein