Drift alignment or flexure?

[swag72]

I think I know what my guiding issue is but does this sound right to you ............

.......... Ever since moving my guider to the saddle on the left of the focuser (I've got 2) it's got worse. I think that the guider is pointing WAY off from the scope. That is why when I get my finder guider package, where the finder will be aligned to the scope, and so the guider will too, my issues will probably improve.

I know I could just move the guider back to the right hand saddle, but then I can't use my finder as my filter wheel is at a funny angle!

My scope is at 900mm.

[themos]

It would be nice if you could get a number for how far the guider center is from the imaging center. But remember, a large angle for that would show up as image rotation and I'm not convinced you have that problem.

[swag72]

Determining how far from the target my guidestar is could be interesting!!

So, I'll wait and see what the all in one finder guider comes up with and see if it's better with that! I do hope so as I was able to routinely get 20 min subs with no guiding issues. So I know my setup can do it, just not at the moment!!

[JacobvonChorus]

I once managed to take an image at a pristine dark location with my dslr consisting of two 30 minute exposures at ISO 800. I polar aligned with a polar scope and imaged at 500mm. Guiding was done through a separate scope at 800mm using the SSAG. The image was good enough to get published in a magazine: http://celestialwondersphotography.com/ImagePages/M45Page.php

Jacob von Chorus

[sulaco]

Interesting, I wasn't aware that there was a need for he finder guider to be aligned as I free misalign it to find a better target star or phd.

I have noticed that the last few times I've been out I'm getting occasional dips in my Dec axis on the graph, I otherwise manage 300s subs

[Russe]

Sara, you should drift align anyway.

But separate guidescopes nearly always suffer from flexure to one degree or another.

There is a method you can determine the cause of the eggy stars, detailed here:

http://deepskystacker.wikispaces.com/Measuring+differential+flexure

Great link! Thanks for putting this up. Will have a look at my data again. Is 0.5px/min a reasonable estimate for DF then?

[fwm891]

Wiki DSS link no longer working

[rickwayne]

Re sub length: IIRC Sara is doing her (absolutely gorgeous, I go to your site every time my Envy Level drops and I need a refresh) imaging from very dark skies, so her narrowband subs will be much longer than we wee mortals'. ?