Laser Pole finder

WARNING: A green laser is very bright, and might impair vision of people, especially when it is dark. Since it is pointing upwards in this case, it should not be used in vicinity of air traffic.

The original Vixen Polaris mount has a serious shortcoming when it gets to polar finders. At higher latitudes it is impossible to use such device, because the mount base actually blocks the eyepiece. The idea that came up is to insert a green laser pointer into the RA axis tube, that points at the North Celestial Pole (NCP). It may seem less accurate, but in fact the telescope itself can be used to enhance the accuracy by magnifying the laser beam endpoint.

The tube forming the RA axis has a diameter of roughly 19mm, so a 15mm copper waterpipe will fit well enough. A rubber ring is used to lock it into place at the outgoing side. The three setscrews on the lower side can be used to adjust the laser beam direction until it is parallel to the RA axis.

 

The whole thing is constructed from 15cm of 15mm copper tubing, a green laser module, an old Nokia wall-wart and a current source based on an LM317. The current is set to approximately 250mA, which results in a nice and bright laser beam.
The current source and laser module are stuffed into the tube, using a plastic inner tube to fit them snugly; especially the laser should be rigidly fixed. The tube is slid through the RA axis, and then the rubber ring is rolled on. Pulling back the assembly into the RA tubepartly rolls back the ring, but it should end-up about halfway to give sufficient room for adjustment.

 

Next, a process of rough alignment must be performed. To do so, the RA axis is pointed at a white-board, and the RA position is rotated from left to right. Rough alignment is done with the three setscrews until the deviation circle on the whiteboard is only small.

 

At various RA positions the location of the spot is marked on the whiteboard. The dots should all be on a circle, and the RA axis actually points at the center of this circle. This is marked and then final adjustments ar made with the setscrews. Larger distance between mount an board will improve accuracy.

Final step is to try it under the night sky:



It works like a charm! It appears that the calibration has done its' job, since rotating around the RA axis hardly shifts the target. The misalignment between OTA and RA axis proves to be a lot larger, when looking at the beam through the eyepiece at 100x!