Yesterday, I took the time to disassemble the cooled 350D from the main OTA and here is what I did:
- Inspected the T-Ring to make sure that the central portion of the T-Ring was square with the outer ring (i.e. the central portion is maintained in the outer ring by 3 small screws). A few weeks ago, it became loose and I had to tighten the screws.
- Made sure that all the threads were screwed correctly (i.e. T-Ring, field-flattener and light pollution filter);
- Re-seated the field-flattener in the focuser and made sure it was square with the focuser and that the focuser thumbscrews were tight but not too much.
- Made sure that the focuser tension knob was tight enough so the drawtube would not to slip but not too much so it would introduce tilt.
Then, last night, after a fair amount of rain during the early evening, the sky cleared for a few hours so I decided to to take a few test shots to verify if those adjustments resolved my elongated stars problem. Here is one 4 minutes guided exposure (you can click on it to zoom in)…
As you can see, there is still some elongation (in the range of 0.12 to 0.28 flatness using the Maxim DL informaiton window). I then, took 15 unguided 10 seconds exposures not far from the zenith and analyzed the results using CCD Inspector. Here are the results.
Conclusion, This is better that earlier this week, however, I still have some elongation and the curvature map is far from ideal.
Next step, I will take some test exposures with my un-modified DSLR (i.e. Canon 450D) to try to determine if the problem was introduced when I disassembled my 350D.
More to come.