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An alignment accident causes better auto-guiding - what is happening here?
The other night just before I put my scope into hibernation I clunked into the counterbalance in the dark fairly hard. I figured I might have thrown off alignment so I slewed to a guide star, re-adjusted aim and re-synced. Problem sorted - no!
So next evening I did a full 6 star alignment and check polar alignment under the Celestron Utility -> Display alignment -> Polar alignment. It read over 3 degrees off! Big opps - It's normally about 9 minutes of arc. So I grabbed the base of the mount and checked for looseness of fit - yep I'd left the East/West adjestment knobs loose and it had rotated anti-clockwise. So I just rotated it back until it hit the screws then locked it in. When I re-did a six star alignment (much more goto accuracy this time) it still read -1 degree of polar alignment (so I probably rotated it off too far).
But here's where things get surprisingly wierd. How long can I do a long focal length exposure with auto-guiding from a 80mm scope without stars looking blobby or streched? In the past my limit was about 240 secs as a hard max. Since upgrading PHD from version 1.4 to 1.5 last week I've done 300 - 400 second duration shots with almost no sign of bad alignment. The upgrade puts in buffers to the guide routine to stop over corrects or oscilliations. I suspect my mount had bad backlash on +ve RA and -ve DEC and this lead to oscilliation on my early shots.
Well since moving from 9 arc minutes off the celestial pole to 60 arc minutes my autoguiding has allowed me to do 11 minute shots with no evidence of stars looking wrong! This is really counter-intuititive to me. I guess it means the bias or error in my shots is always significant, and its always in a consistent direction, so it's easier for the autoguider to be better tuned out of the box.
On the surface I'd always assume being close to the celestial South pole would be less strain on your auto-guider, but maybe the reality is you have to very carefully adjust the guiders settings (pulse length, aggressiveness etc) to your precision of alignment and focal length. Maybe my knock just got to a sweet spot between PHD's current settings and my level of alignment.
When it was at 3 degrees out the autoguider couldn't prevent long star trails on my 6 minute shots. At 4 minutes without guiding the trails are about 4 mm. With auto-guiding given 1 degree mis-alignment with the pole, on a 11 minutes there isn't a bump more than say 0.5mm on any star visible.
What are folks thoughts and observations on what I experienced. I'd be very interested to hear!
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