APQ 130 and discmountPosted By Tom Peters |
I now have AP/DM-6 tripod adapters in stock.-Tom Peters DiscMounts.
Posts Made By: Tom Peters
June 1, 2004 11:44 AM
Forum:
Zeiss
January 14, 2005 06:52 AM
Forum:
Telescope Making
Aluminum Class. Cassegrain truss projectPosted By Tom Peters |
I built a 16” truss RC and can comment a little. The RC is very critical of primary - secondary spacing. Some designs are more forgiving. You can see the spherical distortion in the star test as you change the spacing; I keep mine with in 1 mm.
Some tips:
Mirror sets are made with the back focus held constant. So to adjust the spacing set the distance from the center of the primary back to the focus plane to the back focus distance. Adjust the spacing to bring focus at infinity. I slide a tape measure up the focus tube to the primary's edge set the back focus then adjust the spacing until I have focus. As you can tell I don't believe in moving the secondary to focus. There was an article in Sky and Telescope a year or so ago that has a chart of the spacing change vs. spherical distortion.
If you must focus by moving the secondary! To get the focus change at the eyepiece, multiply the power of the secondary times the change in spacing. My RC has a secondary power of 2.5 so moving the spacing 1mm will move the focus 2.5mm.
Take a big sheet of paper and draw the tube assembly full scale. One thing you will notice is the size of the secondary is controlled by baffling. If the secondary is to small stray light will pass by the secondary through the hole in the primary and into the eyepiece. This is why most cassegrains have 38% to 40% primary to secondary ratios. The larger the primary the smaller the secondary, unfortunately things don't change much until about 40”. There has been a long time argument as to which is worse stray light or large secondary. You can take a really big sheet of paper and see why the Hubble works so well.
The secondary must be centered with the primary. When I built my secondary support I added a small hole in the center. Before I install the secondary I shoot a laser up the focuser through the hole. Of course this assumes that the primary is centered and the focuser is parallel. Nothings easy!
By using the truss design I was able to keep my tube assembly to 60lbs. Not bad for a 16”!
Good luck. It's a fun project!
Some tips:
Mirror sets are made with the back focus held constant. So to adjust the spacing set the distance from the center of the primary back to the focus plane to the back focus distance. Adjust the spacing to bring focus at infinity. I slide a tape measure up the focus tube to the primary's edge set the back focus then adjust the spacing until I have focus. As you can tell I don't believe in moving the secondary to focus. There was an article in Sky and Telescope a year or so ago that has a chart of the spacing change vs. spherical distortion.
If you must focus by moving the secondary! To get the focus change at the eyepiece, multiply the power of the secondary times the change in spacing. My RC has a secondary power of 2.5 so moving the spacing 1mm will move the focus 2.5mm.
Take a big sheet of paper and draw the tube assembly full scale. One thing you will notice is the size of the secondary is controlled by baffling. If the secondary is to small stray light will pass by the secondary through the hole in the primary and into the eyepiece. This is why most cassegrains have 38% to 40% primary to secondary ratios. The larger the primary the smaller the secondary, unfortunately things don't change much until about 40”. There has been a long time argument as to which is worse stray light or large secondary. You can take a really big sheet of paper and see why the Hubble works so well.
The secondary must be centered with the primary. When I built my secondary support I added a small hole in the center. Before I install the secondary I shoot a laser up the focuser through the hole. Of course this assumes that the primary is centered and the focuser is parallel. Nothings easy!
By using the truss design I was able to keep my tube assembly to 60lbs. Not bad for a 16”!
Good luck. It's a fun project!
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