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Need Help?

Secondary mirror position

Started by fblue, 07/19/2003 10:06AM
Posted 07/19/2003 10:06AM Opening Post
My secondary offset is about 1/10 of an inch, maybe a little less. The formula calls for 1/10 inch so I quess that is OK.
I just wish I could get the shaddow of the secondary to be in the center with the holographic collimator. Just can't seem to get it to collimate and still stay in the center. It collimates fine but the shaddow is always off-center. Not just the offset, but usually a little low as well. I guess I just can't worry about it. I am not sure if I should move the secondary by adjusting the spider bolts to see if I could get it to the center, or just leave it alone.
Any input as to the fix for this problem?
All the articles that I can find on the internet are not that informative about this.
Also, I keep reading that I should see the secondary as a perfect circle when looking through the focuser with a film canister with a hole drilled in the top. I see a perfect circle of the reflection of the primary that is lined up pretty much with one edge of the secondary, but I still see the secondary as a slightly oval shape. The circle of the primary has a crescent of the secondary to the left of it, not too large but still it has one. Is this a problem?
I did not built this scope only just bought it not too long ago and am trying to check out all the optics and get the best collimation possible.
Thanks all.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 07/19/2003 10:46AM #1
Does not sound correct. Can you use the laser as just a dot and not a holograph ? If so, we can help you. The holograph is good for fine work but not so good for the rough stuff. I myself only use the laser to get things started after that I use "like yourself" a small collimating cap. When looking down the focuser you should be able to see the secondary reflection in the center of the primary if not then something is not lined up properly. I already know its a bit hard to descibe in words what you see. I think it best to start over but we need one laser dot.
Posted 07/19/2003 12:10PM #2
Have you already read Nils Olaf Carlin's piece on collimation procedure?
Posted 07/19/2003 12:18PM #3
Here it is
Posted 07/19/2003 01:11PM #4

There's one tricky thing that IMO too little is said about, and that's getting the secondary rotated square to the focuser. The basic instructions would be to adjust the secondary mechanically so that it isn't tilted more in any direction than another (i.e. the axis of the holder is aligned with the axis of the tube). THEN rotate the secondary so that the primary is centered right-to-left in the secondary as you look through a peephole at the focuser. THEN adjust the tilt of the secondary. If you have to make the secondary cock to one side or other of the focuser axis, then the rotation isn't right. Don't every bother with the primary at all until you think you have the secondary right.

From your description it sounds possible to me that the secondary is rotated slightly too far counterclockwise as you look down the tube. If so, you may be able to see that the secondary holder is cocked slightly to the left (the side with the open crescent). Try rotating the secondary just a tad clockwise as you look down the tube and then recollimate from there. I have to warn you, though, that trying to visualize it as I write I may have directions flipped, so if you do that and it makes it worse, then just go back and rotate it the other way. When it's right the secondary will look circular through the focuser and the primary's image will be centered left-to-right in the focuser. Centering it top to bottom has to do with the distance the secondary is from the spider.