Elongated Stars – Differential Flexure Confirmed

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Elongated Stars – Differential Flexure Confirmed

| Troubleshooting | October 23, 2012

October 20th I imaged LBN640 taking 45 X 4 minutes sub-exposures at ISO 800.

Using DeepSkyStacker and Maxim DL, I did some analysis on the sub-eposures produced during the session. Here is what I found:

  1. Over the course of the 3 hours, the entire imaged drifted 38 pixels;
  2. Since my image scale is 0.812 arcseconds/pixel, the total drift was 30.9 arcseconds;
  3. This translates into a 0.17 arcseconds of drift per minute;
  4. All the stars drifted parallel from each other;
  5. The drift was oriented in the RA axis of the image;
  6. My guiding was relatively good in both axis (i.e. 0.4 pixel RMS);

Here is a video showing the drift.
LBN640 Drift Effect


Because the entire imaged drifted although the guide star was maintained relatively fixed by PHD Guiding, I can only conclude that my system suffers from differential flexure.

Possible cause of flexure:

  1. The modified 350D DSLR being 2 pounds heavier with the cooling system (total 3.5 pounds quite extended from the focuser) might be pulling down on the focuser too hard.
  2. The Orion Starshoot Autoguider installed on the ED80T CF, although installed at the end of an extension tube, is focused at 70mm on the drawtube scale. This seems to create noticeable flexure in the focuser drawtube.

Next Steps..TBD

About the author

You can call me Frank. I've been an amateur astronomer since 1988. In 2004, I purchased my first astrophotography equipment. In 2008, I built my own observatory. I cannot count the number of hours I have spent building, learning and trying. It as been a humbling experience that I hope will continue for a lifetime.

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