Star shape/advice

[Hicks]

The other night I gave a new off axis guider a go. Attached is a 30s single quickly stretched image (m27_32_stretched and raw fits file) of M27. Equipment was a LX90 8" using an f6.3 reducer and SXVR-H9.

A few of the stars look a little egg like to me. I think my drift alignment was off as the guider had to correct in Dec quite a bit rather than just a tiny amount now and then in RA. However, for those with more experience in imaging, does that look like the most likely explanation?

My focal reducer spacing is not yet quite right, 90mm rather than the ideal 85mm giving a f6.27 rather than f6.3. Could that explain it or is it close enough?

Any other advice anyone can offer based on the image? I'm assuming focus can be improve a little more although given the seeing conditions, 2x2 binning may have been a wiser choice for the night.

Also attached is a stacked and quickly processed image 30x30s. Not a lot of data I know and as the black area at the top shows, there's some drift in the image sequence.

m27_30s.fits

M27 30s single frame, stretched image:

 

[PhotoGav]

I see what you mean about the star shapes, but thankfully they are not too far off acceptable. What mount are you using? In my experience, the seeing can influence star shapes greatly through the guiding being pants. From what you say, with the RA being much steadier than the DEC, that would suggest an elongated pattern of guidestar plots in your guide application (PHD2?), which I always find translates to the shape of the stars in the image, which in this case are slightly elongated. I think that you would find much better star shapes when the guiding is more stable thanks to improved seeing.

There is not much sign, if any, of coma, which is what would be present if the focal reducer spacing was having a detrimental effect.

[Hicks]

16 hours ago, PhotoGav said:

I see what you mean about the star shapes, but thankfully they are not too far off acceptable. What mount are you using?

It's an LX90 8" on the standard fork mount/tripod with an equatorial wedge.

In my experience, the seeing can influence star shapes greatly through the guiding being pants. From what you say, with the RA being much steadier than the DEC, that would suggest an elongated pattern of guidestar plots in your guide application (PHD2?),

There is not much sign, if any, of coma, which is what would be present if the focal reducer spacing was having a detrimental effect.

I'm using Linux, guiding was done via Ekos from within KStars. The dec graph showed quite a lot of variation compared to RA. I think my polar alignment was still quite out and will focus more on that next time. In addition my mount isn't too responsive to fine dec motion so that likely made dec guiding harder. There's a few options on the handbox I can try tweaking to improve that.

Thanks for the feedback.