Drift alignment question

[Joel Shepherd]

Before the clouds declared victory tonight, I was trying to drift align (in hopes of capturing M94). I aimed south and got a pretty decent RA alignment. Then I returned the scope to its starting position (aiming north), and the next step was to find a star to the east.

I usually look for a star in the direction that I consider east, but tonight I started thinking that maybe "east" means "turn the scope in RA-only towards the east". And at that point the clouds shut me down.

Anyway: is that the idea when adjusting declination? Align in RA, return to the start position, turn the scope in RA to the east and then align in DEC? Or is being some way of celestial east okay?

Hope this makes sense; sorry if it's silly -- Joel.

[Merlin66]

The two drift alignments are semi independent....

When you swing the scope over to the East it doesn't have to be exactly the same dec. setting you used on the Meridian drift.....somewhere close to zero Dec (the equator) is good enough.

[kens]

Both drifts are done close to declination zero. One near the meridian to adjust your azimuth and another near the horizon to adjust altitude.

I start by setting my declination to zero then slew in RA only to near the meridian (or a bit east of it) and look for any nearby star in the finderscope.

When I'm done adjusting azimuth I slew in RA only back to the eastern horizon and look for another suitable guide star there in the finderscope. 

[bogdog]

here is a link explaining (all credit to others)

http://www.cloudynights.com/page/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

[Joel Shepherd]

Does "setting declination to zero" mean literally zero: parallel to the ground? I thought ideally declination and latitude were supposed to be more or less the same when the mount is properly aligned.

Thanks -- Joel.

[Merlin66]

It means zero reading on the declination scale....this is when the telescope points to the celestial equator.

[Joel Shepherd]

Thinking this over, I've realized part of my confusion. The fixed angle of the mount -- that needs to be aligned with the earth's axis -- is adjustable in altitude and azimuth: it moves in a fundamentally different way from how the telescope itself moves. I hadn't really considered that before. So my terminology (aligning the mount in RA, etc.) was incorrect, and I incorrectly interpreted "setting declination" to zero.

So, with my head screwed on a bit straighter (?), it sounds like the guidelines are to find stars close to declination zero (the telescope, not the mount) to align from -- one near the meridian and one to the east (or west), and more or less at 90 degrees to each other. How exactly the telescope slews from one to another isn't much of a concern. Please holler if I'm going further off track.

Thanks -- Joel.

[Stargazer33]

There is a useful video on Andy Raiford's site here. Unfortunately petesastrophotography site is no longer there. It had a very nice drift alignment simulator that you could use to practice drift aligning.

[kens]

Sounds like you are on the right track now Joel. How are you doing the alignment? Are you using the computer on the scope, PHD2, DARV or totally manual?

I use PHD2 as my scope doesn't have a computer (but is pulse guideable) and it's quicker than the manual methods.

[Thommy]

What if the view to the celestial equator is blocked towards east (or west)? Just go to higher declination well knowing that the accuracy will be lower?  (sorry for stealing your topic, Joel)

/Thommy

[Davey-T]

What if the view to the celestial equator is blocked towards east (or west)? Just go to higher declination well knowing that the accuracy will be lower?  (sorry for stealing your topic, Joel)

/Thommy

Yes as the stars don't conveniently sit on the zero line you always have to be some way off just takes longer to drift.

Also be aware that the North and South is celestial not magnetic.

Dave

[Joel Shepherd]

I'm aligning manually: CG-4 mount, motor on the RA axis, illuminated reticle, no view of Polaris and obstructed horizons in all directions. :-)   So the results -- while steadily improving -- haven't been great. Mostly trying to determine if that's a factor of where I'm aiming (which it sounds like it's not so much) or just limitations of my observing site and patience.

Thanks -- Joel.

PS: I do have an Advanced VX mount arriving Thursday which will probably simplify things. That said, I like the deeper understanding that comes from learning to do these things manually.

[Davey-T]

If you center a star in your illuminated reticle then slew back and forth in RA and rotate the reticle so the star follows the center line when slewing then tighten it in that position if it drifts sideways from the centre line adjust to bring it back to the line until it drifts the other way then gradually get it to stop drifting for ten minutes.

Dave

[msh1]

here is petes drift simulator http://web.archive.org/web/20090416115204/http://petesastrophotography.com/ its worth playing with 

[Joel Shepherd]

Wow: nice find. Thanks for sharing the link!

[Atreta]

I was meditating (taking a nap) and i thought one thing, don't know if anyone has tried this before.

Place a camera in the scope and align it near where polaris or sigma octans is(easier for polaris i think), and  take lets say a 5 minutes exposure and check it. if one of the stars doesn't trail that is polaris. so you check it's position in the frame, make the corrections to center it   and use some software like APT or BYEOS to zoom in and center again as much as it allows and again try a long exosure picture.

would this be worth doing?

Frank

[Davey-T]

You don't want Polaris in the centre for imaging, good enough for visual though.

Dave

[kens]

You'd want the camera fitted to the polar scope. It's hard to ensure perfect alignment of the OTA with the polar axis of the mount.