Conlin Hill Observatory -- 42 7' N, 71 54' W



Techniques - Star Elongation Analysis  



The issue:
For a long time I've experienced issues with elongated stars in my images. Sometimes there's little to no star elongation in an image, and other times, the images are ruined by it.
Here's an example, this image is a single 1 minute exposure, taken through the LX200 with an F6.3 focal reducer. The scope was being guided by an ST8 imaging through an ETX90 riding piggyback, and PEC was running.

Possible Causes:

I believe the horizontal arrows above reflect guiding/tracking errors, where the stars appear to be moving from right to left across the image. However, the diagonal elongations in the upper corners I believe are due to collimation problems. I've heard numerous theories from very knowledgeable people about what causes these type of problems. The explanations tend to fall into 3 main categories, with possible explanations for each one.
  • Tracking problems
    • Here's a movie I made from 8x1 minute shots like the one above. I need to re-run this, unguided and with shorter exposures.
 
  • Inadequate polar alignment
    • Need to check again. Last time I checked was in Summer and it was dead on.
  • Poor or no PEC training
    • I recently erased and retrained the PEC. I need to measure it to see what it's like now
    • I've read that PEC can cause guiding problems, I may erase it again and retry the test with no PEC.
  • Scope out of balance
    • The scope was well balanced in 3 dimensions
  • Guiding problems
    • Flexure of the guidescope
      • The guidescope is not well secured. I'm using a Losmandy dovetail camera mount with a single screw type connection to the rear of the guidscope, need to replace with solid 3 point rings.
    • Over aggressive corrections
      • I've only used the defaults for agressiveness, so I could try making adjustments and trying to measure the effects. As noted above, I've heard that PEC can cause the guider to work harder so PEC may be causing over agressive guiding.
    • Mirror flop
      • I use a mirror lockdown bolt and I'm pretty certain my primary isn't moving around.
  • Collimation
    • Secondary collimation
      • Per the image above, I believe that the elongation in the top left and right hand corners are due to collimation problems. I have been working on collimation but more needs to be done to get it right. I tried collimating while using the focal reducer and got poor results. I collimated again at F10 with better results.
    • Focal reducer spacing or alignment
      • I'm using the Agos focal reducer adapter with distances between scope, focal reducer and camera measured out carefully.
    • Flexure of the camera
      • The Agos focal reducer adapter elimates this problem.
Next Steps:
  • Working with short exposures, continue to correct collimation problems
  • Verify polar alignment
  • Verify PEC. Take a large number of short exposures. Periodic error can be measured by plotting movement across all the frames. Also, the individual frames can be examined for elongation which is caused by collimation problems (since the short exposure time eliminates mount problems).
  • Get 3 point rings to lock the guide scope down better.
  • Verify guiding by taking a large number of guided shots with PEC turned on and off to see which works better.
  • Verify the limits of guiding by taking guided shots at various exposure lenghts to see if there's a maximum cutoff.
  • Verify the effects of guiding agressiveness settings
UPDATE: 2004-01-18
Collimation: I continued to work collimation issues with some success, but still not perfect. Here are some out of focus images that I was using to collimate with. The first is how it started out and the second is what it looked like when I was done. The third image shows what the in focus results look like. There is some improvement although elongation is evident in the corners and edges. It took 2 hours making small adjustments to the collimation knobs just to reach this modest level of improvement. A nice clear moonless night went by and I had to endure 10 degree temperatures so it's disappointing not to have achieved more progress.

Polar Alignment: I verified my polar alignment. It was good, although I did make a very small RA adjustment. Over 5 minutes a bisected star at over 200x remains bisected.

PEC: I took 60x10second images after tweeking polar alignment and collimation. I found my periodic error to be almost exactly what it was when I measured it 1 year ago, averaging around 2 arc seconds between 10 second frames. This still needs work. The camera wasn't 100% orthogonal to the scope and I noticed some dec drift so the polar alignment may need a little more work as well. I used the ST8i for this test but would get better results using the Sac7. Need to run this again.

Guide scope attachment: I have purchased a very good set of rings from Losmandy that should solidify the guide scope attachment. Haven't tried it yet.

UPDATE: 2004-02-24
Since the last update I received the Losmandy rings, they are very solid and work extremely well. I have also purchased a visual back for the ETX90 that I hope will keep any camera movement or flexure to a minimum, still need to try it out.

I worked for some time on the collimation and have it tweaked pretty well at this point. This was done as described above, with the camera. There's no doubt it could be tighter. I would love a program that could analyze images and suggest which collimation screws to turn but as far as I can tell, none exists.

I erased and retrained PEC very carefully by hand. I did a learn and 3 updates, all very carefully with my reticle eyepiece. The mount is tracking as well or better than it ever has.


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