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Still suffering from elongated stars due to differential flexure

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Still suffering from elongated stars due to differential flexure

| Troubleshooting | November 16, 2012


This week, the weather has been very good…it’s the fourth clear night in a row. Futhermore, I am attending a CCD conference on Saturday…what more could you ask to satisfy an astrophotographer?

On October 25th, I contacted Moonlite Telescope Accessories about the broken bearing on my focuser, Ron Newman replied on the 27th telling me to ship the focuser to him and explaining that this should not have happened. My focuser was still under warranty and he would take care of the repairs (i.e. change all the bearings, replace the draw-tube, test everything, shipping, etc.).

I shipped the focuser the next day and it got to Moonlite on November 2nd. Upon receiving the focuser, Ron contacted me to offer a motor upgrade. My motor was the original stepper motor with the mini controller. Ron was offering me the new slip-clutch version at a very good price. He informed me that the new version had two main benefits (i.e. the ability to unlock the clutch and use the focuser manually and an improved sturdier motor bracket).  I accepted his offer and sent my payment. I got the focuser back on November 12th. The motor is definitely attached more securely to the focuser and the slip-clutch option is really useful (I use it to reset the focuser to zero manually).

The Moonlite after sale service is as good as the focusers they produced. Thanks to Ron and his team.

On november 14th, with a working focuser, I did some tests to confirm wether or not, the bad bearing was the culprit for the elongated stars created by differential flexure. The quick answer…no…I still have elongated stars!

So on the 15th, I conducted 2 more tests:

1. I rotated the camera 90 degrees and took a guided 4 minutes image. I verified if the elongation orientation changed on the images and it did (i.e. the star elongation was now horizontal instead of vertical).

2. To verify if the differential flexure is being introduced by the Hotech field flattener connecting the camera to the focuser, I replaced it with a standard 2″ T-Ring to focuser adpater and took another guided image. The stars were still elongated.

It leaves two possible sources of differential flexure:

a) The imaging camera is just two heavy(i.e. 3.5 pounds) and create some differential flexure in the imaging train. As the mount is tracking higher and higher toward the zenith, gravity diminishes and the flex gets reduced and thus creates the elongation.

b) The guiding scope, its focuser and the guiding camera are flexing relative to the imaging train which creates the elongation.

If you have any comments or advice to help me resolve this issue, please feel free to provide them.

I will continue to investigate the cause of this differential flexure. However, I feel I am spending a little too much time on the technical aspect of the hobby and not enough time doing imaging. I will try to balance it out a bit.

Thanks.

PS, I am currently working on a paper that will try to determine if a german equatorial mount performance can be improved by disassembling, cleaning, polishing, lubricating and adjusting its inner mechanical parts. More on that soon.

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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