Quote:
Originally Posted by ericwbenson
Hi Peter,
How do you know the camera and laser are centered on the rotator? On off-center chip (wrt the rotator) will make a stars rotate around the rotator axis, not the sensor 'axis'.
I went through this whole exercise on my system and it was pretty involved. In the end I had to shim the focuser and shim the camera separately (in different planes). It's not perfect but being remote I am hesitant to mess with it since I might 'fix it'! The closer you get to perfect the better the seeing has to be to see what you are doing.
So the order to iterate is
-collimate
-shim cam to rotator so that 180 deg rotation does nothing except move star around due reason above
-shim focuser to make aberrations even in corners
Repeat with finer collimation and small changes to shimming until happy or exasperated! And in your case you can skip the collimation step...
I found CCD inspector fairly useless for this task, the values would bounce around on consecutive readings when not doing anything to the alignment. What was really useful was Platesolve2 which analyzes 5 or more frames across focus and calculates the average focus hyperbola in the nine sectors of the chip, then displays the difference of the estimated best focus of the nine sectors relative to the center. Very robust measurement and somewhat immune from seeing variations. From these values I could calculate an approximate tilt plane and shim accordingly.
Remember it doesn't matter if the components are individually crooked, what matter is to get the CCD orthogonal to the rotator axis, then the camera+ rotator unit orthogonal to the optical axis. The camera not being centered on the rotator axis is annoying but not deal breaking.
HTH,
EB
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I don't know! And I have no adjustments possible to fix lateral displacement. I can only adjust tilt. The biggest question in my mind is how to know whether I should adjust for tilt, or more precisely, how much tilt? I pretty much assume most of what I see is tilt due to the quirky nature of fitting the rotator adapter. But if there is a collimation issue and I try to "fix" it by tilting I will make things worse. Though, worse than what I'm seeing is hard to imagine!
I was shown a technique with CCDI where you take 10 short exposures and CCDI averages these together to give an indication of curvature. CCDI calls it collimation but is it? Or, is it more an indicator of tilt? I think this is based on FWHM across the chip. I found results pretty repeatable using this technique. I will check out Platesolve2 and I appreciate this recommendation!
I'm attaching a test jpg I out together with the last time the rotator was on the TEC180. It's pretty clear what is happening on rotation! I'm also attaching a photo that shows what happens to a centered star on a full rotation. These are with two different cameras. The star trails camera has a KAF16200CCD and the CCDI test was done with a KAF8300 chip camera after the camera was shimmed at 270 degrees. Clearly shimming the camera was not the correct order to do things in. I should have shimmed the rotator fitting first. But, still I'm confused as to how I would do that and "know" whether remaining tilt is due to the camera or rotator adapter?
re the star trails test, I subsequently discovered a screw adapter that was moving which explains the non-smooth rotation. The yellow arrow indicates the start position of a centered star.
I still guess that the majority of my tilt is coming from the rotator adapter and that I can shim quite a lot of it away. The 5 arc-sec error means that every slew must involve a plate solve and it obviates my arc-sec pointing model (last night 6.5 arc-sec RMS).
Eric, I appreciate you help quite a lot!