:S Hello out there. I just purchased a used 4" ED refractor and am having troubles with collimnation. I'm using the startest technique with the sun on a ball bearing around 100 feet from the scope. Initially, I noticed some very outstretched difraction patterns (rays) at 9 and 7 o'clock extending from the focused image. Upon defocusing, it was clear that colimnation was the issue as the center spot was nudged to one side and the diffraction rings were noticably brighter in the same area. So far a releif! I then began adjusting the mirror cell. After considerable adjustment, to the point where the bottom of the mirror cell was almost half a centimeter from the tube flange, and the top of the cell was touching, the brightness in the diffraction rings became balance and the bright spot was centered. But, now there is a serious amount of astigmatism showing. The scope will only focus to a cross and opens into slits perpendicuallary oposite to each other on either near side of focus. Astigmatism was not abserved when the collimnation appeared bad, but after tilting the lens cell for best diffraction uniformity it is evident. Do I have an astigmatism issue, or am I not collimnating correctly?
Refractor collimnation
Started by jonbanger, 09/25/2006 04:32PM
Posted 09/25/2006 04:32PM
Opening Post
Posted 09/25/2006 04:50PM
#1
I think you mean lens cell. In any case, tilting the lens cell will probably not work in collimating a doublet refractor. Most doublets are corrected for coma, so if coma is present, it is not due to a tilt in the lens assembly. Coma is usually caused by a tilt in the spacer between the cell. This is not something that can be compensated by tilting the entire assembly. The other possibility is that the spacer is not tilted, but the two elements have shifted laterally with respect to each other. Then usually a small amount of lateral color is also present.
Trying to fix a tilted spacer is not a job for a beginner. You will need an autocollimation setup to do the job right.
If it is a lateral displacement of the elements, one can try a simple thing first before disassembling the cell. Sometimes a temporary fix can be accomplished if the front lens retainer is loosened, and the lens cell is gently rapped with a piece of wood. This will loosen a stuck lens element and may cause the two elements to realign themselves against the inner walls of the lens cell. Do it with the scope aiming approximately 45 degree (point it at Polaris, and use that star to see whether it has realigned itself). When you are done, try not to tighten the lens retainer against the glass. Leave just a tiny smidgen of gap so that the elements are not being squeezed together and can move around inside the cell. The lens should rattle a tiny bit if shaken.
It could be that some dirt has fallen down the inside of the lens cell and has displaced one of the elements to one side. In that case you will need to disassemble the lens and clean the inside of the cell.
Rolando
Trying to fix a tilted spacer is not a job for a beginner. You will need an autocollimation setup to do the job right.
If it is a lateral displacement of the elements, one can try a simple thing first before disassembling the cell. Sometimes a temporary fix can be accomplished if the front lens retainer is loosened, and the lens cell is gently rapped with a piece of wood. This will loosen a stuck lens element and may cause the two elements to realign themselves against the inner walls of the lens cell. Do it with the scope aiming approximately 45 degree (point it at Polaris, and use that star to see whether it has realigned itself). When you are done, try not to tighten the lens retainer against the glass. Leave just a tiny smidgen of gap so that the elements are not being squeezed together and can move around inside the cell. The lens should rattle a tiny bit if shaken.
It could be that some dirt has fallen down the inside of the lens cell and has displaced one of the elements to one side. In that case you will need to disassemble the lens and clean the inside of the cell.
Rolando
Funding Member
