Dec guiding shows strange drifting.

[Garbz (Chris)]

Hi All,

During Astrofest I was having a hell of a time with my EQ6 to the point of where I simply gave up using the C8 and stuck with ED80 and wider scopes. Some of what I was seeing can be logically explained by the gears hanging up, and my mount is now in many pieces because of that.

What I can't explain and had a few people scratching their heads is that during drift aligning I noted that my Dec axis was wavering. It almost looks like periodic error which is absurd as the dec axis shouldn't move at all. By all accounts I should be seeing a linear drift one direction or the other.

The screenshot below shows the results during a drift alignment session. During this time Dec guiding is disabled so you expect a linear drift in some direction.

I've ruled out seeing, 3 second exposures and the long time span should take care of that. I checked and triple checked that everything was tight although there is backlash in the mount. The period shown in the graph is about 8minutes long.

Any ideas what is causing this?

[lazjen (Chris)]

I can safely say that I have no real idea why, but perhaps a couple of questions might help eliminate options:

How good was the balance of the scope, especially when all cables were attached? If you alter the balance does it change this behaviour?

You mention Dec guiding being disabled while drift aligning (of course), so RA was still being guided? Do you get the same behaviour with no guiding at all and drift aligning?

[Rod771 (Rod)]

I'm relatively new to guiding in both OAG and guide scope so I could off the mark, however my questions are:

Were you guiding the C8 with an OAG? If so could it have been tube currents??

Were there a lot of people walking around your scope? Or was it windy?

And as Chris has mentioned, how is you dec balance, is it slightly biased toward the Camera end?

[Garbz (Chris)]

Originally Posted by lazjen

How good was the balance of the scope, especially when all cables were attached? If you alter the balance does it change this behaviour?

Scope was slightly unbalanced both in North for Dec and east for RA to keep the gears engaged at all times, to eliminate backlash. Cables were tied with a loop and someone did suggest it at Astrofest so we went to great lengths to set them up so they weren't touching anything or putting any strain in any particular direction.

Originally Posted by lazjen

You mention Dec guiding being disabled while drift aligning (of course), so RA was still being guided? Do you get the same behaviour with no guiding at all and drift aligning?

Yes RA was being guided. It didn't occur to me to disable guiding altogether. Though from the graph you can see that while the first half of the curve appears to correlate the drift with RA corrections, the second half does the opposite. I.e. when correcting east the scope drifted north at first, but at about the 6 minute mark when correcting east the scope is drifting south.

Originally Posted by Rod771

Were you guiding the C8 with an OAG? If so could it have been tube currents??

No 50mm finder guider. No telescope in use. Just drift aligning at this point. The scope was still cooling down.

Originally Posted by Rod771

Were there a lot of people walking around your scope? Or was it windy?

No wind but someone did setup near me so we actually considered this as well. I tried star-jumping like a drunk lunatic doing exercise to dubstep which caused a bit of comment on the imaging field but it didn't even cause a blip on the scope. The ground there was surprisingly firm.

Originally Posted by Rod771

And as Chris has mentioned, how is you dec balance, is it slightly biased toward the Camera end?

Yep, and more than enough to overcome any cable strain. This is actually a problem with my setup, I can't balance it the other way currently because my dovetail is too short. Fortunately since I spend most of the time imaging towards the south it's not too much of an issue.