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Old 15-11-2013, 09:46 AM
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pmrid (Peter)
Ageing badly.

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Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,665
Quote:
Originally Posted by alocky View Post
I've been wondering the same thing myself. There's two challenges, one is to be able to resolve drift into RA and dec vectors. The other is apply a sufficiently small adjustment.
I use a geared head and the vixen polar scope, but hang the camera off the front of the polarie during the process via its strap, as there is a surprising amount of flex in the RA drive.
I'm getting good 120s subs at 180mm focal length, I might try longer and see how good it is.
Cheers,
Andrew.
I just had a closer look - the rear cell that removes to admit to Vixen Polar scope device has a strange thread that defies my entire junk drawer full of "bits" . But because it is threaded, means that it would be a simple matter to cut a suitable length of a finder scope - about 100mm I'm guessing, and lathe up a collar to connect that to the back of the Polarie. I have already modified an old finder by adding a 1 1/4 inch removable eyepiece holder so attaching a QHY5 or similar to that would be a doddle.

At the front, there will need to be another collar turned up to allow the 50mm ID of a finder scope to sit over the 44mm OD of the RA plate - adding a few locating screws to hold it in place - so there is a relatively easy pathway to attaching (and removing) a guidescope to a Polarie. An afternoon on the lathe would do the job.

Assuming all that could be done and a guidescope could then be atached along the RA axis of the Polarie, the next question would be how to make best use of it.

If you just leave the power to the Polarie off and allow the camera to run for say 10 minutes you'd get some star trails which would be a coarse guide but because of low-elevation atmospheric distortion, you couldn't use that to ID the SCP because you'd be out by whatever that error was on the night.

Going to the east and placing yourself on a star and allowing a 10-minute exposure with the RA drive operating would give you an elongated star that drifted either north or south to give you an altitude error and the same near the zenith would give you your azimuth error.

So you could tell which way the image was drifting, I'd do a 10 minute exposure but put a bit of black card across the front after 60 seconds and hold it there for about the same - so that you'd still get a single image but with a distinct break marking the starting point.

Peter
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