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Examination of FWHM vs Guide RMS Error


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I was looking again at the data I gathered on Saturday night of M51. For the first couple of hours I gathered 300s images, and then moved to 600s when it seemed that the guiding was good. Not being very happy with the results of stacking all of the data, I tried to determine if the 300s or the 600s data was better. So I had a look at it in the PixInsight Subframe Selector. There was a trend over the evening for the FHWM to increase (from 4.8 to 7.7). Initially I thought, maybe it was the change from 300 to 600 second subs that caused the increase, but when I graphed them, the increase looked to be linear, there was no step change when I changed exposure length, just a gradual increase.

So then I went and pulled out the PHD guiding log for the evening. As PHD Log Viewer kindly shows the dither points I could (very laboriously) select each individual 300s and 600s sub frame and have a look at the Root Mean Squared guiding error for the whole frame as well as the peak RA and DEC error. Then I added these to the graph. Everything is in Arc Seconds so that the scales match up.


The orange lines at the top of the graph are the FWHM for the 300 and 600 second frames, and the three lines at the bottom at the RA Peak, Dec Peak and RMS guide errors. Certainly the guiding RMS error went up from about 0.8" to about 1.1" as I moved from 300s to 600s, but it was interesting that the peak error increased so much more. I wonder if this was more to do with M51 becoming almost overhead as the evening went on? Interestingly the eccentricity went down as the FWHM went up. I assume that this is to do with the seeing smoothing everything into a round shape, or the guiding error being evenly distributed leading to more of a circle.

So now I am asking: Is 600s pushing it, should I have stayed with 300s? However, the trend was already increasing, I wonder if I would still have had 7 arc second FWHM even on 300s images if I had stuck with them? Was this a change due to the seeing, or in the altitude of the target? did the focus slip as the telescope ended up pointed straight up and the 1.5kg of camera dragged it down slightly (I did tighten the focus lock thumbscrew so I hope not)? Are there just too many variables to ever find out what was wrong?

Using an online calculator the Airy disc for my scope should be 8.5 microns in diameter. The camera has 4.5 micron pixels, so with perfect seeing I should get stars that are 2 pixels wide (I think) which is about 3.5 arc seconds. I have no idea how to calculate what a good FWHM is from that information ;(

Is there any way of calculating what a good FWHM is for a given scope / camera combination? I can find lots of "that looks pretty good", or "I get x" discussions online, but nothing mathematical to show what should be theoretically possible.

I wish there was a way of embedding the guiding RMS and peak error into the individual subs with SGPro, doing this by hand is labour intensive and error prone.


Edited by frugal
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