How good is good guiding?

25 Mar, 2018 13:35
Hi All!
Since I am relatively new to Astro-Photography and looking at some of the wonderful shots here on AB I asked myself what good guiding really is. After conquering the initial setup steps in PHD I thought that my guiding was fairly good, but my pictures show significantly larger stars than other really good pictures. Of course the exposure time plays a role, still, when taking 60 seconds exposures I expected more pinpoint stars (being fully aware that stars with mag less than 8 will be bigger since overexposed). So I started to blame my guiding, which might be wrong, maybe it is the resolution of the camera. To eliminate the guiding from this chain of thought it would be nice to hear from you what guiding accuracy you have.
Mine is around an average of 1-2 arc seconds over a span of 5 minutes (PHD gives you a nice statistic), is that too much? Is the goal to be below 1 arc second? Would be great if you can pass some of your results so that I can compare.
25 Mar, 2018 14:37
I don't think guiding has anything to do with the size of stars, only the shape of them after long exposure.

I think the pinpoint stars come down to the optics and type of filters used.

I have no idea what my guiding accuracy is, I just look to see if the stars are round.

25 Mar, 2018 15:58
Can you tell us about your equipment?  Focal length, aperture, camera with pixel size, and mount.  I can tell you what I get from a guiding error standpoint but may not be relevant to your situation.  Also, do you know what your seeing is in arc seconds?

Edited 25 Mar, 2018 15:59
25 Mar, 2018 16:15

I saw that you were using basically the same optics as me.  I'm getting between 0.5-0.8" RMS over subs. (windy nights in the 1.40's) I have it mounted on a EQ-6R Pro

Also from some of your images, the stars are round in one corner, but oblong in another.  Maybe the sensor isn't parallel with the imaging train. (ex. M1 the stars look round in the bottom right but oblong mostly elsewhere), also the same for several others.

Focusing could also be a culprit.  I recommend a motorized focuser for that wonderful refractor you have.  It really does it justice getting pinpoints with this apo.  If you go the motorized focuser route, kstars/ekos works very well to use an autofocus routine and monitor HFR (Half flux radius) of a select star in the frame. That program works very well controlling autoguiding/autofocus/platesolving/planetarium/camera sequence control. (example of it all working in a stream i did not long ago
25 Mar, 2018 17:09
I ran the numbers and at f/7 your imaging resolution is 0.71 arc seconds per pixel, because that camera has tiny 2.4 micron pixels. With a 0.8x reducer, it is at 0.88 arc seconds per pixel. That is going to be a tough image scale to use with the AVX… unless you were to use the camera on a smaller APO like a WO GT71 or SV70 = 1.47" per pixel.
25 Mar, 2018 17:54
Thank you all so much for those answers, gives me a perspective! The oblong stars at the edges is definitely a problem, Alan, you might be wright that the sensor is not exactly parallel with the train, or even the telescope itself, I have to check this out again. The telescope arrived with the focuser not mounted correctly, maybe I did not do a 100% job to get it back in place. Robert, your numbers are a good explanation, thank you for calculating this out for me! One of my other suspects is the light pollution that I am taking the pictures in, Bortle scale 6 to 7. And i noticed when I try to reduce the back ground noise in the pictures by cutting of the light at the low end it increased the star sizes (wonder why?). In a couple of weeks I will do a short vacation in a less light polluted area, that at least should give me an answer to this. But as always, the challenge is part of the whole, without it everybody would make great picture and then no one would care as much as we do right now.
Again, thanks to all who answered, very much appreciated!
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