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The Unknown Cause of a Desired Behaviour

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The Unknown Cause of a Desired Behaviour

| Imaging Sessions, Troubleshooting | March 18, 2014


Making beautiful astronomical images of deep sky objects like nebulas and galaxies is not easy. One of the challenge lies in being able to ensure the telescope is precisely following the movement of the sky caused by earth’s rotation.  First, one need a telescope mount that is of sufficient precision. Secondly, the mount needs to be properly aligned with the celestial sphere. Once done, it can fairly synchronize the movement of the telescope/camera with the movement of the sky.

This tracking is unfortunately not always sufficiently precise to ensure that the subject does not appear blurred in the image. In order to keep the subject perfectly centered in the image, a method called auto-guiding can be used. The principle is simple: Use a second camera to monitor the position of a star, if the star moves from its initial position, tell the mount to nudge a little to compensate and, using software, repeat every few seconds. Sounds easy enough…does it?

Those of you who’ve been following this blog for a while know that auto-guiding can be plagued with all sort of issues and problems. Finding the root cause of those problems can be very challenging. Sometimes, one might think problem A is caused by issue B; Fix B and the problem goes away….huh no!  Wish life was always this simple!

Over the last couple of years, numerous attempts were made to get rid of various technical issues in order to be able to auto-guide properly. Issues like focuser sag, too much weight, improper balance, differential flexure, backlash, friction, stiction, etc. Many imaging sessions ended up being cancelled or jeopardized due to these issues. Last November one of these sessions got cancelled. Here is the story of the events.

November

November 3rd: The sky was clear and the target was NGC1491. An emission nebula in the constellation of Perseus.
Specialized Narrow band filters were to be used (i.e. Hydrogen Alpha, Oxygen III and Sulfur II) for shooting the nebula.
The session what “scripted” in CCD Commander for a complete automated acquisition.
There were numerous issues during acquisition:

  • Selected star for focusing was too bright (entered wrong focus star coordinate)
  • Unable to find a guide star through the off-axis guider (camera settings were tweaked)
  • CCD Commander stopped responding around 1h am for unknown reasons
  • Auto-guiding was not that great and showing signs of star elongation.

With eyes full of tears, the observatory was manually closed and the equipment turned off around 1h30 am.
The session was a FAILURE.

December, January, February

Winter, in the region, was really bad for astrophotography. We were afflicted with very cold, cloudy and windy conditions. Not a lot of imaging was made. The lack of determination and of braveness to face these conditions could also be invoked.

Theses cold winter nights were spent working on a face lift for Clearskypix. The Resources and Techniques sections are still under development but feel free to browse around the website and send your feedback.

March

March weather was not so great either. Snow and cold conditions continued.

March 3rd: Clear sky, -18 Co, average transparency and seeing, an imaging session was planned.

Again, more issues:

  • Human error: Narrow band filters were replaced by LRGB filters. The filter wheel face-plate was unintentionally re-installed in reverse! Took 1h to figure out why the camera was not “seeing” anything and then re-installed the face-plate on the right side!
  • Guiding camera driver stopped responding. PC had to be rebooted.
  • Unable to find guide star through off-axis guider

The session was another FAILURE.

March 6th: With the intent to resolve the “not finding a guide star” problem, the off-axis guider was removed and the original guide-scope was re-installed, everything realigned, balanced and configured exactly as it was originally.

Then, suddenly, for the first time in years, auto-guiding was surprisingly good.

No more star elongation in the RA (right ascension) axis during auto-guiding! Why, why, why-why and why? No clue! The mount was now behaving as expected but the cause of the star elongation was gone.

A miracle? A coincidence? Simple luck? An omitted technical detail? Some time was spent trying to figure out but the cause couldn’t be found. A decision was made: Why waste time looking for the root cause of a problem that is no longer there? Let’s start imaging.

March 7th: The mount’s polar alignment (i.e. aligning with the celestial sphere) needed some adjustment. A precise polar alignment was performed using PEMPro. Once complete, the alignment was close to 1 arc-minute in both azimuth and altitude. Auto-guiding was re-tested and was now better than ever.

March 8th: 2.5h of total exposure of M108. A barred spiral galaxy in the constellation Ursa Major.

March 9th:  Bias and darks were captured at -40o Celsius to create master calibration frames.

March 11th: Image processing of M108. Tried a new LRGB processing workflow and obtained nice result.

March 23rd: The sky was clear and the target was the Sunflower galaxy (M63). The session what “scripted” in CCD Commander for a complete automated acquisition of 4 hours. Image acquisition started at 9h30.

Then, something unexpected and unfortunate happened: More auto-guiding problems!  But wait…this time the issue was now with the other mount axis (i.e. the declination).  The “Astronomical Gods” were amusing themselves at a poor imager’s expense. The session was cancelled around 10h30.

April

At the beginning of April, a few nights were spent troubleshooting the declination problem. The star would drift and drift and drift but it looked like the corrections were not being sent to the mount. After a few minutes of guiding, the star would jump up for over 20 pixels, ruining the image.  The theory suggested that there was too much static friction or “stiction” in the axis gears. Untightening of the gears should resolve the issue.

As opposed to the problems experienced on the RA axis, after only two nights of troubleshooting, the root cause was found. It ended up being the opposite of what the theory had suggested. That is why it is so important to  confirm theories with experiments! There was too much mechanical backlash or “slack” in the declination gears.   Tightening everything got the mount to auto-guide correctly. Here is the proof…

Autoguiding results 9 avril 2014

Now… what do the “Astronomical Gods” have in store for the future?  Prayers and sacrifices might be an option!

One thing is sure, to put up with years of technical torments, one need to have either an infinite amount of patience and resilience or, be a little crazy. I like to think of myself falling in both categories.

Clear Sky

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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