Electronic Polar Alignment Scope

[AndrewJ]

Gday Ken

it's just that I just like symmetry

Then you should buy a fork mount instead of a GEM

Andrew

[kens (Ken)]

So tonight I tried aligning using the camera mounted just like a guidescope. At first I had some trouble getting it aligned with the RA axis but eventually got the hang of it. The trick was to start with small rotations to make a small arc while the collimation point was off screen then as it adjusts in you can use larger arcs. I used ICMeasure to record three points on the arc zooming right in to get pixel level accuracy. The I drew a circle connecting the three points. Over that I overlaid three concentric circles of 114, 92 and 75 pixels diameter corresponding to the distances of BQ Oct, HD99685 and TYC-9518-405-1 from the pole.
I ahd to cheat a bit and platesolve to get near the pole. I had just plonked the tripod down on the ground and was over 5 deg out as it turns out and too far to home in without some help.
Once I could see the polar asterism I adjusted alt and az to place the stars on their correct circles.
Attached images are the zoomed in screen shot of the final result and the actual image with overlays. At this focal length each pixel is 11.1 arcseconds so in theory I am within 1 arminute of the pole.. Unfortunately I'm not set up right now to test that with a drift alignment

[bojan]

Originally Posted by kens

So tonight I tried aligning using the camera mounted just like a guidescope. At first I had some trouble getting it aligned with the RA axis but eventually got the hang of it. The trick was to start with small rotations to make a small arc while the collimation point was off screen then as it adjusts in you can use larger arcs. I used ICMeasure to record three points on the arc zooming right in to get pixel level accuracy. The I drew a circle connecting the three points. Over that I overlaid three concentric circles of 114, 92 and 75 pixels diameter corresponding to the distances of BQ Oct, HD99685 and TYC-9518-405-1 from the pole.
I ahd to cheat a bit and platesolve to get near the pole. I had just plonked the tripod down on the ground and was over 5 deg out as it turns out and too far to home in without some help.
Once I could see the polar asterism I adjusted alt and az to place the stars on their correct circles.
Attached images are the zoomed in screen shot of the final result and the actual image with overlays. At this focal length each pixel is 11.1 arcseconds so in theory I am within 1 arminute of the pole.. Unfortunately I'm not set up right now to test that with a drift alignment

Most likely drift alignment (and the method you described is actually drift alignment procedure, only applied to the stars near pole) will show the same result..
On both my transportable mounts I have precision bubble level and magnetic compass attached. They are both "null-ed" at the stage your mount is at the moment. by just using them I am always near the pole, which is OK for short focal lengths (less than 300mm) and exposures up to a minute.

[kens (Ken)]

Tonight I used my regular guide scope following the same technique as before. Then once I finished the polar alignment I did a PHD2 drift to see how good it was. Net result was around 4" dec drift over 15 minutes. This corresponds to less than 2 arc minutes PA error and I suspect some of the drift was in fact flexure plus I was hampered by some light cloud cover. A good result seeing as I didn't take any special care with the alignment. And it was nice to have a confirmation of the accuracy of the method.

[AndrewJ]

Gday Ken

Just for fun, i saw Altronics are doing a special on vari focus CS lenses.
Just picked up a 2.8-12 unit for $32
http://www.altronics.com.au/p/s8955-...ual-iris-lens/

Best thing is being a CS lens, it screws directly onto my DBK41 with no adapters reqd.
It has a built in focusser and also can go from wide view to telephoto mode.
Haven't tried it under the stars yet, but it will be interesting to see what it can pick up in wide mode vs narrow mode, as you could start in wide mode to ensure stars stay on the sensor, then zoom in as you get closer??
Now, when will it be clear in Melb again???

Andrew