Photo method polar alignment.

[ollypenrice]

I gave this a try on Friday night but cannot find the original web page, nor can I find Sara's original post on the matter! (Apart from that this is going to be a great post!!!)

So, scope points South at Declination 0 degrees. I used the handset to put it there. I sent it to Antares then input 0 degress Dec and slewed. Next I used the handset RA button to slew to due south.

I set the capture software to give 130 seconds, finger on the West button at Guide Rate, poised. Watching the exposure countdown I let it expose for 10 seconds then pressed West for the next 60 seconds, then East for the last 60 seconds.

This gave a load of elongated Vee shaped star trails with the starting point clear on all of them from the blob created by the first 10 secs. You need the blob to know which was the 'outbound' trail.

I gave the Azimuth knob a tweak and repeated the experiment. The angle of the Vee was wider, so wrong way. Tried again making small tweaks in Azimuth till, quite quickly, the Vees were not Vees but pixel perfect lines, the one on top of the other.

Brilliant method.

There's a but....

On trying the same thing in the East I found the sensitivity to altitude adjustment near-ludicrous! A Gnat's Crotchet of a turn on the Knob and the vees swapped sides. Also I never got straight lines but lines whith a curious diverging bend to them. Maybe I was not pointing at the right bit of sky. However, I did my best and the resulting PA was pretty good over a three hour run.

I need to suss out how to get the Alt adjustment method sorted but first signs suggest that this is a demon method.

I'd have spent longer but the Ha for the Elelphant Trunk was calling.

Olly

[swag72]

Look at this thread, post 6 - Is this the guide you were referring to?

Glad that you got it to work. Were you suprised at the amount of alignment was needed? I was surprised at the amount of alignment that I didn't need!! I have the NSEW now sorted on my handset, but not sure whether the camera was rotated correctly. Would that make a difference?

[ollypenrice]

Thanks Sara, yup, that's the one. I can never find anything. Never could! Sorry about that.

Olly

[George]

Polar Alignment using a CCD Camera

Think this may be the article your looking for, the method is often referred to as DARV (Drift Alignment by Robert Vice)

Works a treat and is very accurate

[malc-c]

The only "fault" I think I've found with this is that it relies on everything being 90 degrees to each adjustment. So at zero degree declination with the scope pointing south there are generally clear views and lots of targets, but at 0 degree west or east most of us will have obstructed views. If you then increased the declination so you acquired a target the fact that it's not at that 0 degree IMO may have an affect on the adjustment of the altitude ?? - Or would it ??

[ollypenrice]

I think it would and perhaps did in my case...

Olly

[swag72]

OK, so here's a question. I can see at 0 degrees DEC to the south, but to the East I am looking up a hill and to the west I have some buildings. How can I work around that?

[malc-c]

I think this is a limitation with any form of drift alignment. Unless you have clear east / west horizons, and can hit a target near them then the geometry for alignment will be slightly off... or that's how I interpret it

[swag72]

So, no one can do any formm of drift alignment (tradional method or DARV) if they can not see at 0 degrees DEC on their east or west horizon?

There must be loads of people in that position - There must be some way around it.

[ollypenrice]

So, no one can do any formm of drift alignment (tradional method or DARV) if they can not see at 0 degrees DEC on their east or west horizon?

There must be loads of people in that position - There must be some way around it.

Not sure that there is. You might have to do a classic drift if you can't see 0 Dec in the East. You can still use the camera provided it's orthogonal.

How to find N/S and E/W on the chip? Take a 30 second sub and half way through give the mount a tiny gentle push N for a couple of seconds. You will get a star with a protruding tail. The tail points N. Repeat for E/W. Andy's Shot Glass website has a nice alternative but on some mounts you can't just turn off the motor as he suggests without going through a palaver to restart.

Then just loop short subs for a star on the camera reticle and watch the drift.

Olly

[rfdesigner]

I have always drift aligned.. and the problem with the E/W alignment is that you must NOT do it too close to the horizon. If you do you get the earths atmosphere bending the light and the stars appear to rise at too steep an angle, so you miscorrect for everything else. With the Azimumth alignment the stars are not rising or setting so you don't make that error.

What I've found is that it is critically important to get the Azimuth alignment done perfectly first. If that is even a little bit out then the E/W alignment will be substantially incorrect.

Personally I always try to use E/W stars that are at least 25 degrees above the horizon, then it all seems to work ok... but then I'm not doing hour long exposures yet.. when I am then I'll know if it's all good enough.

All IMHO and I'm probably wrong etc.

Derek

[Merlin66]

In the southern hemisphere we had little choice but to use the drift method....

I agree, do the zero dec on the meridian first then swing to the clearest east/west horizon and work with whatever you can find......