Astrophotography at the Veluwe Logbook, results, lessons learned

New polar alignment method

Not really new, ofcourse. There are a few specialised commercial and public domain packages that use the same principle. Check out PolarAlignMax and Tpoint.
I got inspired by reading about Astrometry.net. They do "blind solving" of astrophoto's. That is pretty cool. Read about it here http://cosmo.nyu.edu/hogg/research/2006/09/28/astrometry_google.pdf

My method in short:

1. Aim the mount's polar axis at Polaris. As accurate as possible, but spend no more than 1 minute.
2. Put a camera on the mount with a medium telephoto lens (I use 135mm)
3. Take 3 or more photo's. Between the photo's: sweep the mount in RA in considerable amount, but do not move in DEC! Write down the time of each image
4. Do a blind solve of all images. I use Elbrus (offline) or Astrometry.net (online)
5. I write DEC and RA and time measurements in my Excel spreadsheet.
6. The spreadsheet  uses Excel's  "Solver" to calculate  the Azimuth and height misalignment of the polar axis.
7. By using Azimuth and height finetune knobs I then can change the direction of the polar axis to correct for the misalignment.
8. Repeat the above steps starting from step 3 for another iteration and higher accuracy.

I will try to use this method to obtain a polar alignment error of less than 2' within 20 min.

I will report about my results here!

Edit sep 2 2010: I tried to determine the accuracy of my method. I did 6 series of 5 measurements without changing Azimuth and Height and thus found 6 separate polar alignment error (in degrees)  for Height and Azimuth.

So it follows that my method has an error of less than 0,15 degrees, about 8 arcmin. I think I can improve on this using larger sweeps on the sky within a serie and by using a telescope instead of a 135mm telephoto lense. The accuracy might improve too if I do the sweeps with the RA-motor instead of of manually after decoupling the motor.

But first I need to get Elbrus working with images taken with my 550mm telescope. They have a smaller pixel scale and Elbrus cannot plate solve then, yet.

Quality of my Min-EQ mount

Today I tried to do a good polar alignment of my Min-EQ travel mount and measure it's periodic error using the Drift Explorer of K3CCDtools. With some luck (I think) I succeeded in a pretty good polar alignment resulting in a DEC drift of less than 1"/min, during a measurement of 80 min. There is a slight periodic drift in DEC. This cannot be explained from a mechanical viewpoint and therefore I think it is caused by non-perfect angle measurement of K3CCDtools resulting in "dripping" a small fraction of the true periodic error in RA into the DEC drift.

The RA-drift was much higher ofcourse, caused by the periode error of the mount, a mechanical issue. The graph shows the RA drift during the same 80 min measurement. It is a combination of:

1. a somewhat irregular periodic drift with a period of 861sec and an amplitude of about 160" top-top. This has a mechanical cause: one turn of the wormshaft.
2. a lineair drift of about 4"/min. This can be caused by a polar alignment error. The star used for measurement was in the south-west. Only on the south meridian the drift caused by polar alignment error results in true DEC drift and no RA drift. It can have a mechanical cause either but it requires a much longer measure ment of at least 4-6 hours to make sure.

Then I tried manual guiding during a 10 min drift measurement to find out what accuracy is possible. I looked at the drift explorer screen and corrected RA drifts  with the handcontroller. The conclusion is that I can keep the RA variation well within 20-30" during 10 min without too much concentration. Long exposures with a 200-300mm lens as long as concentration allows can be used without noticable guide errors. In addition a good polar alignment is required because DEC drift cannot be corrected manually with this mount. DEC drift should be under 4"/min to start with to allow 5min+ exposures with a 200-300mm focal length.

See this link for a very instructive program created by Niels Noordhoek to simulate star drift caused by Periodic Error and Polar Alignment Error

SQM-L

Today, my Sky Quality Meter arrived (SQM-L). The Sky Quality is a measure of light pollution. The SQM measures the Sky background Brightness in Mags/Arcsec2. Values are between 16 (heavy light pollution, moon light) to about 22 (very dark).

The SBB  is strongly related to the limiting magnitude (the brightness of the faintest visible star) and to the maximum exposure time of astrophoto's.

See this page for a graphical relation betwee SBB and Limiting Magnitude:  http://members.ziggo.nl/jhm.vangastel/Astronomy/visibility/PredictionTool.htm.

From now on I will write down my SQM-L value (SBB) with all my astrophoto's on this site.