FYI I was looking at an imaging run tonight, and saw a lot of those SNR dropouts you were talking about. FWIW, I traced it down to having a StellarSolver profile for guiding with MinSize set to 1.5. I guess my guide star must have been near an HFR of 1.5. I moved Min Size much lower to 1.05 and my dropouts went away.
This is part of the StellarSolver configuration. There's configuration like this in several place, Focus, Guide, Align--whereever star detection is run. For guiding, you look in the Guide Options menu, the Guide tab, and you'll see a "SEP Profile" line as the 2nd line in "Other Settings".
Rob has provided several choices for starting. There is "1- Guide Default", "2- AllStars", "3-SmallSizedStars", "3-MidSizedStars", "3-BigSizedStars",
You can choose one of them. Then, given your choice, you can further edit these starting points by clicking on the pencil to the right of the chosen profile.
That brings up a menu where you can modify all the parameters (shown below)
You can see MinSize is the 2nd row from the bottom on the left side. There are mouse-over help messages and some links on that menu.
Thanks for this! Will try when I get another clear night.
One thought... whenever you change the minimum, isn't there a possibility of EKOS selecting a star that is near the new minimum and the problem recurring? So, things might be good for this session... but the next night the star select is closer to the new minimum and the problem is repeated?
Also, what is the disadvantage of setting the minimum to zero?
Mounts: Sky-Watcher EQ6-R Pro, Meade LX85, Celestron NexStar Evolution Alt/Az
OTAs: Celestron 8" Edge HD w/Celestron Focus Motor, Meade 80mm APO Triplet Refractor w/ZWO EAF
Cameras: ASI533MC Pro, ASI183MC Pro, ASI224MC, ASI120MC-S, ZWO ASI290MM
Raspberry Pi 4 with Stellarmate OS, MacBook Pro
The algorithm that selects guide stars isn't perfect, for sure. It tries to find stars that are reasonably bright, but not totally blown out. It tries to find stars not really close to other stars. It tries to find stars not right next to the image border. Here it is: invent.kde.org/education/kstars/-/blob/m.../guidestars.cpp#L139
If you lowered the StellarSolver min size parameter to 0, I'd guess the scheme would probably start detecting some bad pixels or noise as stars, and perhaps sometimes those stars would be added to the mix and could affect guiding. Real stars that are that small are also probably not very good guide stars, as they probably are harder to detect consistently. However, if all it detected were small stars, then for sure it should be using them, instead of sticking to an arbitrary cutoff of MinSize=1.5HFR.
Like everything else, it can be better and I occasionally get back to it. Probably the key thing needed to improve it is some idea of what stars look like for your particular camera and scope and exposure time. I suppose I could get it to take several "practice exposures", evaluate detected stars with a variety of settings, find ones that seem best, and then adjust settings accordingly. Some day I may get to that
For now, it's best if you adjust your StellarSolver settings so that it does a good job of detecting stars for your setup. The hope was that the default StellarSolver settings would do a decent job, and it probably has, but if you see signs of issues (like these dropped guide stars) then if you know how, make adjustments to the star detection parameters. Obviously, be conservative with your changes, try your best to understand what the parameters are, and write down what your old parameters were so you can revert back to them.
Also, BTW, Rob provided a binary with his StellarSolver called StellarSolverTester. It allows you to load fits files and run star detection and/or plate solving on the fits file, and to play with the parameters you use to get the best detections.
So, you don't need to wait for another clear night. You can do this during the daytime if you have any stored images. It may be that you don't have any saved guide images, but next time you can, take a few guide images (e.g. in the capture module with the right exposure/gain/camera), save them and use StellarSolverTester to improve your detection parameters.
excellent discussion, thanks a lot! I've been experiencing the same SNR drops and as you suggested it might indeed have to do with min size in Stellarsolver settings. My guiding was not affected (at least I didn't notice) but when I saw it happen I worked around it by increasing the exposure time of the guide cam. The size of the guide stars increased thus the algorithm didn't "lose" the guide star anymore (or, at least, not as often).