Astrophotography at the Veluwe Logbook, results, lessons learned

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