Drift during guiding !

[RobertM]

I noticed that I had drift to the east during the 300s M81 subs that I captured.  It equates to about 32 pixels over the 90 mins that the exposures were taken i.e. about 1.5 pixels during each sub.  It isn't noticable in the larger stars but the smaller stars are oblong as a result.

Has anyone any experience of this - I would imagine that it's not normal or expected.

[RobertM]

Forgot to mention:

Guiding was 80mm F6.3, QHY5+PHD,          5.2um pixel size @500mm f/l yealding 2.1 arcsec/pixel
Main scope 235mm F6.3, SXV-H9+MaximDL  6.4um pixel size @1480 f/l yealding 0.89 arcsec/pixel

Thanks
Robert

[Mike]

What are the aggression settings?

[RobertM]

They would be the default settings:

RA Aggressiveness : 100
RA Hysteresis : 10
Min Motion : 0.25 pixels

[MarkS]

You said  "drift to the East" - what does that equate to in temrs of  R.A. and Dec?

The reason I ask is that I have experienced 3 causes of star trails during my (limited) experence of guiding:
1) I accidently switched off Dec tracking so it was guiding in R.A. only (therefore trailing in Dec)
2) There was flexure in combination of scope and guide scope - as the main scope changed orientation over 2 hours or so, gravity increasingly acted on the guidescope to tilt it away from the axis of the main scope.
3) Wind caused the whole mount to vibrate - but this doesn't manifest itself as a gradual change in star in the image position over 2 hours.

[RobertM]

Drift for one of the stars relative to the image was:
from RA 9h 56m 3.7s, Dec 69deg. 6' 22"
to    RA 9h 56m 9.1s, Dec 69deg. 6' 26"

So thats about 5.4s total over 90 mins, Dec drift appears to be negligable.

[MarkS]

Flexure is my bet ...

[RobertM]

Mark, I think you're right.  Looking around it seems it be in the same order of magnitude that others are experiencing.

Great ! Now comes the problem of what's causing it and how to fix it

The easiest to check is your point 2.  The guide scope is held on a single point through two screws - this is an obvious weak point and I think I'll have to consider a change to heavy duty guide rings.

[MarkS]

Sounds like you've found the problem and it's very similar to the one I had.  My "guidescope" (a 300mm telephoto lens) was sitting on the piggyback camera bracket on my Celestron - just held in place with 2 screws.  So I invested in a proper rail and guiderings (still piggybacked).  This made a massive difference.

[RobertM]

What rail and guide rings did you get ?

[MarkS]

ADM
http://www.rothervalleyoptics.co.uk/detail.php?id=1228

[RobertM]

Thanks, will investigate...