Is it normal for collimation to change on a dob slightly as the telescope orientation changes? I have an 11" Starmaster ELT. I can get the collimation spot-on with the laser, but as I move the truss assembly in altitude from zenith to the horizon, I can't keep the red dot within the center-spot ring on the primary without tweaking the secondary some more. I tried tightening the spider assembly screws to make sure nothing is sagging, but this doesn't help. Suggestions?
Changing collimation
Started by Ed Ashton, 11/07/2005 05:57PM
Posted 11/07/2005 05:57PM
Opening Post
Posted 11/07/2005 06:35PM
#1
Ed
As I understand it most all truss dobs will do this to some degree. That's why most folks recommend collimating at the position you are going to use the most. However if you are collimating at about a 45* and the dot moves outside the ring while moving to zenith or the horizon then that would be too much IMO. I have a SM11" f5.4 model and I collimate at about 60* and when I move to zenith or down to 45* the dot will move off center a bit (less than the width of the laser) and may need a touch up if I want the best I can get on any particular target.
Hope this helps
Buck
As I understand it most all truss dobs will do this to some degree. That's why most folks recommend collimating at the position you are going to use the most. However if you are collimating at about a 45* and the dot moves outside the ring while moving to zenith or the horizon then that would be too much IMO. I have a SM11" f5.4 model and I collimate at about 60* and when I move to zenith or down to 45* the dot will move off center a bit (less than the width of the laser) and may need a touch up if I want the best I can get on any particular target.
Hope this helps
Buck
Posted 11/07/2005 06:48PM
#2
Hi Ed,
Almost every truss and solid tube Dob I have checked have some degree of this movement. Some is from spider sag, truss sag, or just plain flexing in the structure. Even mirror flop can be present. But I have a 14.5" Starmaster and it certainly does have it. I too would say to collimate at about 60° and see if that does not help. You rarely use a scope at the horizon, there is just too much atmosphere to get a good view. Also Zenith is one of the least used positions due to the difficulty is movement with a Dob in this region of the sky. So you want to concentrate on getting the alignment best at the normal viewing angles.
Do I think this should be normal.No, I think that it is possible to build without the flexing. But the cost would be a heavy scope with large dimensions and most do not want that.
Almost every truss and solid tube Dob I have checked have some degree of this movement. Some is from spider sag, truss sag, or just plain flexing in the structure. Even mirror flop can be present. But I have a 14.5" Starmaster and it certainly does have it. I too would say to collimate at about 60° and see if that does not help. You rarely use a scope at the horizon, there is just too much atmosphere to get a good view. Also Zenith is one of the least used positions due to the difficulty is movement with a Dob in this region of the sky. So you want to concentrate on getting the alignment best at the normal viewing angles.
Do I think this should be normal.No, I think that it is possible to build without the flexing. But the cost would be a heavy scope with large dimensions and most do not want that.
[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue
Amateur Imager
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