Drift issue - Should this concern me?

[swag72]

I asked in a previous thread about the difference between drift and flexure. At the time, I put a number of subs through DSS and there seemed to be little problem with drift. Well, now I have lots of it and I don't know if it's a problem or not!

I recently changed my finder / guider setup to a dedicated Altair Astro finder / guider package. My reason for this was that with the old guider I couldn't get the guiding camera pointing in the same place at the imaging scope as the bracket I used was not adjustable.

Fast forward to now and my guider is pointing in exactly the same place as the imaging scope. But I now seem to have masses of drift. The DSS figures are shown below, a total of 62 images in 5 minute subs.

So, how will this manifest itself? And what does it show? The guider is on the same saddle as before, nothing on the scope or mount has changed apart from this mini guider package. I would really welcome some thoughts and comments in this as I do find this type of thing very daunting and difficult to comprehend.

Alternatively, if the subs don't show a problem, should I just ignore it and look upon it as natural dithering?

[Stephen]

I have no idea, I'm just chucking this out there ...

Given the angle of rotation is pretty constant, and the delta in each direction is pretty constant ... does this suggest your polar alignment is out? (Assuming one set of figures is calculated as an offset from the previous one.)

[dph1nm]

Assuming one set of figures is calculated as an offset from the previous one.

I think the figures are absolute offsets from the frame it chooses as the reference frame.

NigelM

[swag72]

As per the instructions, the reference frame was the first of the night and what you are seeing in the above is the last 10 frames.

[Shibby]

Hi Sara,

The dY does look a little concerning. I also use a finder guider with a short focal length and, looking at my last session, the dY didn't really change over the course of an hour (it did vary by up to about 0.5 from sub to sub - 10min subs).

I'm assuming the offsets are measured in pixels??

[Shibby]

You say the bracket is now adjustable - maybe you have cable pull? Have you secured it to the OTA with some tape? A trick I learned from Uranium235!

[swag72]

No cable pull - I've not used tape as until now, the other guider sat on the saddle bracket with no movement at all. In fact a good tug of this one in the saddle and it's not moving.

[themos]

Sara, can you copy the entire DSS list and paste it into a Google Doc to share?

[swag72]

Just need to get the offsets again - Leave it with me!

Just uploading an excel spreadsheet to my Dropbox with all the info in it.

[swag72]

Here's the dropbox link - http://dl.dropbox.com/u/41385123/OSC%20DSS%20settings.xls

Really interested to see what you make of it.

[themos]

Ok, I see steady increasing rotation. That tells me it's probably polar alignment error that is producing this. The dx and dy are induced by your guider trying to counteract the polar alignment error induced field rotation.

[swag72]

Cheers Themos, looks like I'll check the PA next time I'm out. Odd though as I did drift align a while back and seemed to get everything working well!!

Could my previous guider have masked that a little with it pointing away from the target?

[swag72]

Just had a look through the polar scope and Polaris is where it should be for GMT. Now living out here in Spain, I am on CET time, that being UTC+1. All of my system (computer, software, timings etc are all based on GMT - Even my obs clock!) - So with the clocks going forward do I have to alter where polaris is in my polar scope? I thought I could just keep everything to GMT, knowing that for instance it's 2145 out here and so that relates to a time of 1945 for the polar scope reticle.

Something is clearly wrong with polar alignment, it can only be this? Can someone explain please?!!

[themos]

Now you've confused ME! If you're using EQMOD , don't you use EQMOD's Polaris Angle widget? I would set the correct time first, though. I don't see why you use a time zone at a different place.

[swag72]

Yes I did use EQMOD's PA thingie - Hence why I was surprised when it showed as being so far out!

[themos]

Well, then, if you have set the wrong computer time, EQMOD will show you where Polaris would be but NOT RIGHT NOW.

[swag72]

I think I may have sorted it, but with these time zones I have managed to confuse myself. I'll keep you posted with some subs!