Periodic error or drift alignment issue?

[traveller (Bo)]

Finally managed to get things going again after a break. I set up my ED80 on EQ6, running EQMOD and Stellarium.
A couple of issues and questions for more experienced members:

1. The target in scope is slightly off centre from Stellarium. By the time I centre it on the finderscope, it has moved "off target" on Stellarium. How do I go about re-centering it back? I tried pressing the "Align Accept" button on my pad controller on EQMOD, but it didn't do anything

2. I managed to get drift align working on a couple of stars. One in Orion low in the west and Canopus overhead. I can get 60-90 secs via Drift in Backyard EOS. I can get 3-4 good 60 sec sub exposures, but then it starts drifting again. Similarly for 90 sec subs, I can do one good one then the next 2-3 will have trails Is this drift alignment error or periodic error?

Thanks for your help in advance!
Bo

[alistairsam]

hi Bo,

suggest you try the following.

1. verify your start / park position. try and get RA as close to 90deg or vertical and dec as parallel to your eq6 body as possible. some use a bubble level or inclinometer to do this.
2. watch for drift in Dec only. ignore drift in RA. Choose a star at the intersection of the meridian and equator that's at zero dec. observe drift in dec, and adjust your azimuth till drift is negligible over atleast 2 or 3 minutes.
3. choose a star below 25deg in alt/az in the west or east, and again watch for drift in dec. adjust the elevation to get the drift negligible.

once done, check the sync points in eqmod. make sure they're zero.
slew to a bright star, manually centre with your game controller or buttons in eqmod, sync (usually ctrl+3). you should see the reticule/scope indicator in stellarium move to the star. repeat for atleast 2 or 3 more that are as far apart as possible and across the meridian.
once done, save your sync points in eqmod and tick the two boxes to load/save on park/startup.
this way as long as you park and don't loosen the clutches, your pointing will be fine and should land targets on the centre.

for imaging runs, choose a relatively bright star near your target, slew and sync on that, and then goto your target. that should keep building your sync model and improve your pointing accuracy.

if you've done most of this and its still bad, its more likely backlash than PE. can you feel play in any of the axes?

Cheers
Alistair

[alistairsam]

to answer your first question, ignore how far off the pointer is in stellarium, just get the star in the centre in your finder and then in your camera. once centered, it should stay there as you'll be tracking.
you need to hit ctrl+3 in stellarium to sync and add a sync point. you can verify the keys by looking at the stellariumscope window where it shows "listening".
http://stargazerslounge.com/uploads/...1363726447.jpg
the "key" in this window corresponds to the key sequence. ctrl+1 for slew, ctrl+3 for sync and ctrl+5 for cancel.
so once you center in the camera, make sure the star is selected in stellarium and press ctrl+3.
the scope indicator will move to the star and both will coincide.

[traveller (Bo)]

Thanks Alistair
1. Ctrl+3 for synch, check, should have remembered that
2. Mount set up, I did everything the best I could using bubble level (the bubble level in my mount is not 100% accurate), compass and my iphone (compass, inclinometer). I will re-do the drift around the 3 min mark and see if anything happens. I have not saved the synch points, so that may be an issue.
Have not checked mount backlash, so will do that. I did have to tighten the counterweight bar as it was loose and wobbled a bit.
I will post some images soon, managed to get some M42 and more data on Eta Carina.
Bo

[Astro_Bot]

Don't use the bubble level on the mount - get a good one from a hardware store or just use your digital inclinometer (or get a more accurate one).

[alistairsam]

hi,
you mentioned you checked for drift with two stars, one in the west and one overhead??
you need to watch drift on a star at zero dec and intersecting the meridian, not overhead. thought I'd clarify that.
and the star doesn't need to be bright. its very unlikely to get a bright star at below 25deg east/west and zerodec/meridian. so just use your guidecam in your main scope and loop 1second exposures.
the new version of phd (phdv2) has a module for this.
http://openphdguiding.org/man/Tools.htm#Drift_Align


Cheers
Alistair

[traveller (Bo)]

Thanks Alastair
Yes, need to choose my alignment stars with more care.
Astro-Bot, I got a good hand held level as the bubble on my mount is just a nice feature and woefully inadequate!
Bo