Adjustment of the Meade Off-Axis Guider Prism (mirror)

Some time ago, I was out in the desert preparing to photograph some large, dim object, the Rosette Nebula I believe. So I decided to try to use the Meade Focal Reducer. The focal reducer is a lens that increases the field of view and decreases the f/ratio of the telescope from f/10 to f/6.3. So, to the camera, the image looks smaller and brighter.

But when I put the Focal Reducer in, I could see nothing in the off-axis eyepiece. Gave up on it that night.

The next morning I spent some time adjusting things while simply pointed at empty daytime sky. What I learned is that the prism needs to be tilted slightly so that, when you look through the off-axis tube without an eyepiece, you should be able to see the secondary mirror in the center of the field. (What you see if the prism is flat in its holder is one of the walls of the telescope! You don't see out the end of the telescope - you won't be able to see any stars if it is bad enough. They will look like arcs if it is only semi-bad.) If we label the triangular sides of the prism as A, B, and C where C is the hypoteneuse, and where side A is facing the secondary mirror and side B is facing the off-axis eyepiece tube, you will find that the prism needs to be tilted so that the corner, AB, needs to be lifted slightly from the bottom of its holder. By careful successive adjustments, the secondary mirror image can be centered in the field. Then, when the off-axis eyepiece is used, and you focus on something in the distance such as Saguaro cactuses on hillsides in the distance, you will get a beautiful, focused, symmetric full-field view that should be the correct thing for off-axis guiding. (NOTE: I corrected an error in the above. I had previously and erroneously referred to corner BC instead of AB. Sorry! This is hard enough to convey anyway without the added confusion.)

I have been successfully using the Focal Reducer since I made that adjustment. The Pictor 201XT autoguider also has more light than ever to guide by!

The Horsehead Nebula shot was taken using the focal reducer, the Pictor201XT autoguider and the Meade Off-Axis Guider with the prism adjusted to suit my particular telescope.

More detail - Excerpt from MAPUG discussion:

There is no adjustment mechanism built-in. You have to unscrew the off-axis tube, then unscrew a set screw that holds the prism platform in the off-axis guider body. The prism platform can then fall out of the off-axis guider body. The platform contains the prism and a couple of small pieces of paper. There is another set screw to hold the prism in place. The pieces of paper go between the tip of the set screw and the prism to, presumably, prevent damage to the prism and provide friction so that it will stay in place.

To make adjustments, I would adjust the prism, slide it in, check to see whether the secondary mirror was centered, slide it out, make another adjustment, etc., until I got it right. If you are outside, don't drop the prism on rocks or other damaging objects! I used a lot of care. You might spread a blanket on the ground to lower the risk.