Does guiding mean less stack alignment required?

[PunkJay]

Howdy all,

I went out for my first guided imaging session last night, and despite expecting the worst (i.e. nothing working) it all went quite well!

18 x 7 minute exposures with ease - excellent!

The only minor issue I uncovered was that I had assumed that as the scope was being guided, all of the subs should be aligned with each other. Therefore I thought that when I loaded the subs into my stacking utility (Keith's), that they would all be more or less aligned. To my surprise they all needed a fair degree of nudging to get them into alignment!

I cannot for the life of me figure this out, as each sub was captured within seconds of the previous and I was not touching the setup at all, so if the image was shifting that much then it should have shown as star trails on the subs?!

Any suggestions on what could be causing this? I'm only asking as I was hoping to not have to spend 30 minutes nudging subs into alignment every morning :-)

cs,

- Jason

[Freddie]

Flexure between imaging and guide scope. Use DSS (free) and it will align automatically saving you all that time.

[PunkJay]

But the stars are not drifting during the sub - and the time between subs is less than 10 seconds. If there was that much flex between the imaging and guiding scope, then surely you would see trails during the sub exposure?

[ollypenrice]

No, it isn't flexure. It's very simply down to polar misalignment. You would need an incredibly high order polar alignment to avoid this and it wouldn't necessarily be an advantage anyway. Corrections in Dec are best if they all go the same way (ie to correct the polar misalignment). Also you get a natural 'dither' by being misaligned; ie the same pixels don't sample exactly the same morsel of sky on every sub.

You need to use a stacking software which will rotate and re-align between subs. DSS has no trouble doing this. Most softwares nowadays do a good job of co-registering images slightly shifted with respect to one another. In the most sophisticated forms of autoguiding a slight 'dither' between frames is built in. Then Drizzle stacking algorithms from the Hubble team can derive sub pixel resolution from the stack. Amazing. A free lunch!

Olly