starhoo

2019.09.26 - Collimating 14" AstroTech RC Update Some months ago after some success using the Howie Glatter laser collimator, I still wasn't seeing good collimation. I decided to focus only on the secondary mirror, and making collimation adjustments only there. What I found was that the collimation screws (which I had replaced with Bob's Knobs) were not long enough to reach the back surface of the secondary mirror housing. Both the original screws and Bob's Knobs version were the same length (about 1.75"). The spacing between where the screws pass thru the collimation holes on the housing to where they contact the backside of the housing is about 1.5" - so there is little extra length in the screws for adjustments requiring more than I guess it was designed for. I ended up taking some washers and placing them between on the backside of the secondary housing and where the screws make contact. These shortened the length needed for the screws to make contact. It worked. Since this change I have not had signficant collimation issues.

I purchased an Astro-Tech 14" RC a few months ago. Suffice to say I am still trying to figure out how to collimate this scope. There are three collimation options: the primary mirror, the secondary mirror, and the focuser. When I first got this scope I looked thru it visually at the moon and it was very ice. I defocused to see the collimation rings that they looked like evenly concentric circles. Being used to an SCT (this replaced a Meade LX850 which I regret selling now). With an SCT you've got the coarse focus knob that makes it pretty easy to fit any imaging train onto the OTA. With the RC, since it is a fixed mirror (and that's one reason I wanted one), you have to attach the correct extensions to the focuser housing in order to then have your Optec TCF or Startlight Instruments temperature compensating focuser have the ability to reach focus... This took several trials but I got it working. The next thing was to take some images and see how the whole rig performs. So far - not very good. The stars were ovals, revealing some collimation issue. But where? Primary mirror, secondary, focuser? I decided that since the primary and secondary seemed ok before I attached 9 lbs. of imaging and focusing hardware that I should start with the focuser. The focuser has three 3mm hex screws and two smaller hex screws beside the 3mm ones. I have found ZERO documentation about guidelines for adjusting these screws despite sending requests to Astro-Tech twice. Anyone know? The only choice with no reference is trial and error. So I was able thru much unintuitive adjusting to get the stars to appear circular, but this caused a good deal of coma in the image (see attached). Also, although it is a brand new mirror and I was imaging on a clear moonless night, there are many odd circular reflections in the image that I'm not even sure flats could remove. I've been trying to figure out what they are - are they surface defects on the mirror(s)? Are they reflections off the baffles? (Oh, why did I sell that Meade LX840 14"!!!)... There's much more to this saga, and I am not hopeful that anyone out there will even read this post, but if you do, please, I welcome your thoughts, insights, help!