Is drift more noticeable in different areas of the sky?

[Cimitar]

Hi,

Question from a viewer - is drift more noticeable in different parts of the sky?

The reason I ask is that I was out taking images 2 nights ago. I ran a 10min sub (my first ever!) on the Sculptor Galaxy and Helix Nebula, both of which worked perfectly. The time was around 1.30am and both DSO's were near the meridian (slightly to the West).

However when I went to the Horsehead and NGC 1365, both of which were in the East and away from the Meridian, neither of these worked and I got quite a bit of drift. PHD had a great deal of difficulty keeping up (I ended up with eggy stars or lots of L shapes on my stars). There was no wind during the night, it was dead still.

PS. The L shapes also lead me to a follow-up question - when the L shape occurs, why doesn't it happen to all stars in the image?

Thanks in advance!
Evan

[niharika]

Yep drift is different in different parts of the sky.

[Somnium]

if your polar alignment is out you will get a lot of field rotation when pointing close to the celestial pole, when you are pointing to the east or west the drift will be present if the altitude is out of alignment and when you are imaging near the meridian drift will happen if the azimuth is out.

the L shapes should happen across the field, maybe it didnt show up very well on stars that were not so bright ?

if you dial in your polar alignment it should sort these issues out.

[ZeroID]

L Shape I think can also be caused by optical misalignment. Is your mirror shifting slightly when you switch targets ? May explain why some stars are still looking ok but others are more affecting by the direction of shift.

[Cimitar]

Perfect, thanks Aidan! That'll do it. I think my Az was close, but the Alt alignment must have been out by a fair bit. Not bad considering I'd spent all night trying to get alignmaster to work (EPIC FAIL) and ended up doing a simple one-star alignment instead.

Obviously when I was lower in the eastern sky the drift was much greater. Thanks the TIPS as well RE: drift alignment. After reading your comment I finally understand how drift alignment works! (silly I know :rolleyes:) Hopefully I can try for a proper drift alignment procedure on the weekend.

PS. You're right, the L shapes were on the dimmer stars, I just had to ramp up the light curve to see them :thumbsup:

[Cimitar]

Thanks Brent, it's quite possible. I didn't have the mirror lock on so the image may have shifted somewhat as well.

[Cimitar]

Hi Aidan, as a quick follow-up question - I've generally struggled to get a decent image of the Tarantula Nebula, usually because of drift.

Is it still hard to do (drift-wise) even with excellent polar alignment?

[Somnium]

Hi Evan, i didn't ask this question originally but do you have an EQ mount or an alt az mount ?

the Terantula Nebula is quite close to the SCP, meaning that you are going to get field rotation with an alt az mount. for an EQ mount, if you have it polar aligned well (and you are auto guiding) then you shouldn't get any drift, just corrections due to periodic error

polar alignment is only needed for EQ mounts

[Cimitar]

Thanks Aidan, yep - I have a wedge for my LX200 so it's EQ mounted.

I've previously tried to image the Tarantula, however it didn't turn out very well, hence my question. I think my polar alignment must have been out at the time.