Sorry for the slightly off-topic discussion, but I thought it might be useful...
Quote:
Originally Posted by LewisM
Peter, I found the other day that if I did a 5 min exposure using a 2 second refresh, I got star trails (small, but noticeable). If I dropped the refresh to 0.2s, a 5 min sub showed no star trails.
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If you get star trails with 2 second guide captures at relatively short focal lengths (900 mm on your ED100), then I suspect one of three common issues:
1. Polar alignment isn't accurate enough. Have look in the extreme corners of your 5/10 min images. Are the stars equally round as in the corners? Poor PA with good guiding will give you elongated corner stars (field rotation) while the centre area will still look great. Another way is to do a "difference" mask with the first and last sub from a session (e.g. 2 hours apart) and see whether the corner stars have rotated.
2. Mount is unbalanced. If the mount is unbalanced, you can end up with lots of oscillations every time it moves. Very short guide exposures would try to fix the oscillations. This may work great for wide fields (< 1000 mm) but isn't effective with longer focal lengths. Many people find that balancing the EQ6 so that it's slightly east-heavy reduces slop-related oscillations.
3. Mount needs tuning. The gears on your mount might not be moving very smoothly. My first EQ6 (bought new) was terrible in this regard - I could actually see it periodically getting "caught" on the PHD guiding graph (and hear it on the mount too). My second EQ6 - hyper-tuned by another IIS member - is much, much better and runs far more quietly than my first one did.
I've attached a couple of examples taken last weekend with my Canon 5DmkII attached to a GSO RC8 (1600 mm focal length, ~0.8 arc sec/pixel). The first is an unguided 60 second framing exposure at ISO 25600 (H2) - notice round stars with slight elongation due to periodic error of the mount. The second is a 360 sec light frame taken at ISO 3200 and guided with an Orion 50 mm mini guide scope (only 162 mm focal length) with 2 sec exposures - round stars
