Stars pointing in one direction

[smr]

I'm trying to work out why sometimes, tonight for instance quite a lot, the Stars in my subs are pointing towards the left. If you look at the picture they seem to be slightly elongated and 'pulled' left. I've seen this on other nights. 

There is a bit of a breeze tonight, if that has anything to do with it, but they are always to the left it seems so I'm not sure if that's a factor.

I'm balanced properly.

Interestingly, a night or so ago where there was absolutely no breeze the Stars looked nice and round, if it was wind though, why would they always appear as if they're slightly pointing to the left and a bit elongated, this is the centre of the image, you can see the HH in the top right.

 

[CloudMagnet]

If all the stars in the image are pointing in the same direction, then I would first blame the wind. If you had coma then you would see the stars pointing in different directions, towards the center of the image so can rule that out.

You could also check against your guiding performance, wind usually shows as a sudden spike, like a dither. So if you can see big spikes in the guide graph then most likely wind. Reason why it pushes the stars in one single direction is wind is not random, in a single night, the wind will blow from an particular direction (west for example) so it will naturally push the scope in one direction as well.

[smr]

22 minutes ago, CloudMagnet said:

If all the stars in the image are pointing in the same direction, then I would first blame the wind. If you had coma then you would see the stars pointing in different directions, towards the center of the image so can rule that out.

You could also check against your guiding performance, wind usually shows as a sudden spike, like a dither. So if you can see big spikes in the guide graph then most likely wind. Reason why it pushes the stars in one single direction is wind is not random, in a single night, the wind will blow from an particular direction (west for example) so it will naturally push the scope in one direction as well.

Thanks for the reply. Yeah it could be wind, I'm not sure. Not an expert at understanding guiding.

I've had a look at the subs again, this last one is worse - so you can see what I mean, it's particularly bad tonight, usually I get some subs like this but mostly OK.

 

Here's a screenshot of the guiding...

[CloudMagnet]

I dont think it would be scope optics if you said on other nights your stars were round, what image scale are you shooting at? If your guide performance in arc seconds per pixel is higher than your image scale, then you will have issues with stars.

I would also look at the SNR of your guidestar, around 20 is quite low, a good way of increasing this is to make sure your guide camera is operating at 16bit rather than 8bit- you can change this on the"connect equipment" page on PHD2, i think its a tiny box next to the connect icon for your camera. You can also turn on 2x2 noise reduction on the camera tab in the advanced settings. You may need to change your gain settings but it should help improve SNR. in turn, that can help your guiding.

Another option could be camera tilt, making sure the camera is fully fixed and is level with your focus tube might play a part as well.

[smr]

Here's a gif I made of the first 10 subs from the imaging session before it, this was on the 17th, where to me at least I thought the stars looked ok. So if they looked ok on this night, it would rule out flexure etc.?

Edited January 21, 2022 by smr

[smr]

3 hours ago, CloudMagnet said:

I dont think it would be scope optics if you said on other nights your stars were round, what image scale are you shooting at? If your guide performance in arc seconds per pixel is higher than your image scale, then you will have issues with stars.

I would also look at the SNR of your guidestar, around 20 is quite low, a good way of increasing this is to make sure your guide camera is operating at 16bit rather than 8bit- you can change this on the"connect equipment" page on PHD2, i think its a tiny box next to the connect icon for your camera. You can also turn on 2x2 noise reduction on the camera tab in the advanced settings. You may need to change your gain settings but it should help improve SNR. in turn, that can help your guiding.

Another option could be camera tilt, making sure the camera is fully fixed and is level with your focus tube might play a part as well.

I'm not sure what imaging scale I'm shooting at, how would I know that? I didn't know about the 16bit thing, will have a look at that, thanks.

[900SL]

It would help if you posted details of equipment and exposure

[The Lazy Astronomer]

3 hours ago, smr said:

I'm not sure what imaging scale I'm shooting at, how would I know that? I didn't know about the 16bit thing, will have a look at that, thanks.

Imaging scale ("/px) = pixel size (μm) / scope focal length (mm) * 206.3

[michael8554]

In your PHD2 screengrab there are the guide error figures:

RA = 1.32 arcsecs, Dec = 2.10 arcsecs.

So Dec error is about 60% more than the RA error = elongated stars.

"the Stars in my subs are pointing towards the left"

Is that elongation in Dec, I have no way of knowing ?

But looking at the guide graph there appear to be roughly equal amounts of north and south deviations in Dec.

But without a PHD2 GuideLog it's all conjecture.

Michael

[rickwayne]

It's pretty  simple to do a plate-solve on your image (Platesolve2 or ASTAP will work and are free), to find out where the RA and DEC axes are. If the elongation is goes along RA, probably periodic error. If  DEC, might be polar misalignment that guiding isn't fixing.