Fast and accurate polar alignment
After setting up my equipment in my garden I always start with a polar alignment procedure. I aim for a moderate polar aligment (with a DEC drift < 10" /min) when planning for autoguiding sessions with my telescope with 550mm focal length. And I aim for a better polar alignment (DEC drift < 3"/min) when doing non-autoguided session (either short-exposure with my telescope or longer exposure with telelenses). The polar alignment procedure takes 5-30min depending on the accuracy I need. High accuracy polar alignment , with a DEC drift of <1"/min, is surely possible but it may take considerable time or requires a little luck. Even when autoguiding, good polar alignment accuracy is usefull as it will lead to less frequent and smaller DEC corrections with no risk over overshooting or oscillations.
Moreover, accurate polar alignment during autoguiding will eliminate field rotation and therefore will increase guiding accuracy and will allow longer exposures and longer focal lengths.
So I came up with this idea to reach excellent polar aligment within 5 minutes: I glued a laser diode to the hour axis of my Vixen GP mount. This laser beam is about horizontal when the mount is polar aligned. The position of the tripod is fixed with a little hole in my terrase that hold one leg of my tripod. <add some photo's>
Then I do the best possible polar aligment possible. I have spend two evenings using the well-known drift method to reach near perfect polar aligment. I think I have obtained a DEC drift less than 0.25" /min. The Polar alignment error is less than 1'. This is much better than what is possible with a polar axis finder scope. is Now I mark the laser dot on a wall 6 meters away.
In subsequent sessions all I have to do is put the tripod in its fixed position using the hole for the single leg and then adjust the Azimuth and Height adjustment knobs of the mount to aim the laser to the marked location.
I will report the accuracy I obtain using this method.
Almost 3 hours integration time and guiding problems
Yesterday I made my longest exposure ever: 34 subframes of 5 minuten, making a total of 2h50 at ISO 800.
Unfortunately I had to discard half of them during stacking because 17 had unacceptable drift due to guiding errors. Adding these frames to the stack would have made the end result worse.
In fact during the night I switched from 10 min subframes to 5 min subframes because I saw guiding errors in most of the 10min frames. So I need to improve my guiding quality. This can be done by doing better polar alignment, using a longer focal length for my guide scope of by tweaking the parameters of PHD. But I expect the best improvement by using a PCMCIA-USB adapter on my Thinkpad T30. This will increase the USB speed from USB-1 to USB-2. At the moment, with USB-1 speed, PHD looses the guiding signal from the DSI guide camera for a few seconds while a frame is downloaded from the DSLR camera. When the guide signal is back, PHD overshoots and lacks accuracy during 10-sec-3min after the frame download. This sometimes leads to a guiding error that is just visible in the next subexposure.
Here is the end result of my long exposure record: http://www.astro.pwng.nl/item/leo-triplet
BTW 1: apart from the guiding problem, when looking at the histogram values, 10-12min@ISO800 seems to be about the max for the combination of this telescope (f/6.9), camera (modded 20D), and my local skyglow conditions (SQM-L 20.5).
BTW 2: My new 12V adapter for the Canon 20D worked great. It worked from 21h-3h without an accu reload.