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These are the draft instructions. They worked for me and I cannot think of anything particular I would change. I used TSX's jog command to move the star about and their feature of being able to download and save subframes, which speeded up things a lot. I stacked 15 images and with reasonable seeing, the readings were quite stable. I only managed one iteration before the clouds came over. I'm hoping for another clear night to complete a second iteration and then image a star cluster as the acid test. GoldFocus Plus Off-Axis Cassegrain Collimation Analysis.pdf

Well, after quite a few frustrating sessions trying to use the GoldFocus mask readout as an alternative assessor in an out-of-focus star test, I'm pleased to announce I was able to get the GoldFocus mask to work very well indeed. Jeff worked on an updated instruction manual for dual mirror adjustments. It was not something you would have figured out by random intuition but looking at the procedure, it does make sense. In an hour I was able to collimate both mirrors on a 10"RC with high accuracy. That was also helped with a night of good seeing and changing my locking grub-screws to pointy tipped ones. The original ones were cork-screwing the main mirror by a few microns (laterally) and causing the air to go blue. The great thing is that it achieved this using numerical analysis and one iteration between secondary and primary movements. I could have gone for a second iteration but clouds stopped play. Out of focus stars were even symmetrical donuts but I'm sure that a second iteration would have nailed the last few percent.

Mike - It is sort of like three Bahtinov masks looking at the variation of focus by angle. When all the light focuses at one point, the three focus positions agree, i.e. collimated. For any coma, astigmatism or other abberation, the focus is smeared and the readings for the different orientations of the mask differ. It is virtually impossible to distinguish between small primary or secondary errors with one reading, just like star testing. The focuser part is really useful, since it can directly tell you (after calibration) just how far to move the focuser with one reading. The collimation is intriguing but more elusive at the moment, hence the post.

Yes Chris - I have one of those too - part of a big evaluation of all the major techniques for collimating a RCT. It is running to 25 pages already. I made a discovery early on with my Altair Astro Truss RC and that was to do all primary adjustments with the OTA vertical. If you don't the mirror sometimes shifts when you loosen an adjuster. Currently a simple HG laser to line up the focuser/secondary and the hall of mirrors on the primary get me very close. I found an excellent reference by Deep Sky Instruments on a particular form of star testing that is very sensitive too. According to the website, the focus mask technique should be able to improve on that, especially in medium seeing conditions. The science is compelling but so far, ahead of my practice. Collimation Procedure

I'm trying to collimate my 10" Truss RCT from Altair using the GoldFocus mask. I'm not having much success and suspect I am doing something wrong. I can get 'good' collimation with classical outside focus star testing but when I use the GoldFocus mask, to try and improve upon it, for some reason the adjustments make things worse. Originally I was using the mask in the same way as the DeepSky Instruments use star testing - adjusting the primary mirror to set a central star and the secondary to balance the appearance of peripheral stars. After getting some advice, I switched over, with little effect. I would be interesting to see if any SGL members have used this clever device to set both mirrors, and how they went about it. regards Chris