I would very much like some advice on what SGP settings to use for imaging with a Celestron 11" Edge. In my particular case I am using the Celestron 0,7x focal reducer, a QSI 683wsg-8 camera (KAF-8300 chip), and I have installed the Moonlite focuser designed for this scope. I am interested in how you have configured auto focus under this or a similar circumstance. For example:
What change in temperature would you recommend between auto focusing? I have set 2 degC. Any reason to make it more or less?
What step size (in motor clicks) would you recommend? I had this initially way too high and my focus was thrown off completely during an auto focus session. I probably have it now set too conservatively at “10”.
Are there any other settings you may recommend that would deviate from the SGP default settings?
I have seen others provide some very nice looking auto focus curves, however mine tend to be quite jagged and chaotic. I wonder if the nebulosity and number of stars parameters need to be adjusted:
What nebulosity setting would you recommend?
How many stars?
Finally, is there perhaps some guidance how to tweak many of these settings before committing to a night’s sequence run?
Thank you for your attention to this, and I look forward to your suggestions and comments.
I have a Celestron EdgeHD 14 so am in a similar boat. I’ll take a shot at answering your questions:
I think 2 degrees is probably too much. I have noticed that 1.5 degrees is about the maximum temp change I can tolerate without leaving the critical focus zone.
I can’t answer this without knowing what focuser you’re using. Even then, it will vary quite a bit from scope to scope. Here is what I recommend - start with the camera in very good focus using a Bahtinov mask if you have one. Start SGP and go to Frame and Focus. You want to take a shot that is the same binning and duration that you will use for autofocus. I use 2x2 binning and 10 seconds. This is probably a good place for you to start. Take that shot, and then turn HFR on. Look at what stars are selected. You will probably want to leave the number of stars slider all the way to the right unless you are clearly picking up a lot of hot pixels. Adjust the nebulosity rejection slider to make sure that you aren’t selecting things like galaxy cores, etc. Once you have these set, leave them alone. Then start moving focus (it doesn’t really matter which direction), taking shots as you go, and keeping an eye on the overall HFR. When your overall HFR gets to about twice what you started, that is about as far out of focus as you want to go (don’t go to three times the HFR as is recommended for refractors). Note how much you moved focus. You use this number to determine your approximate step size. If you are using 7 points, then divide this number by 3, if you are using 9 points, divide by 4 to get your step size. So, for example, if your initial HFR was 2.21, and it took you 250 steps to get to 4.45, you would use about 80 steps for 7 points, and about 60 steps for 9 points. This will give you a good starting point. You may still have to do some tweaking, like determining the number of points that works best for you. This is a good exercise for a full moon night.
You also want to disable “Smart Focus”. It works well for small refractors, but not so well for large SCTs.
You’ll also want to determine your backlash - if any. I have a Moonlite CHL and it has essentially zero backlash. If you are focusing with a motor on the focus knob, you will have a lot. The easiest way that I know to do this is to put a Bahtinov mask on the scope and point at a moderately bright star. Bring the scope into good focus. With the mask still on the scope, move the focuser a relatively large amount, and then move it back the same amount. If the focus is still perfect when you move it back, you have no backlash. Do this for both focus directions. If you do have backlash, you can determine the amount by seeing how far you need to move the focuser to bring the image back to focus after moving out (or in) and back. Add about 50% to that number and enter it into backlash correction.
Finally, you should probably just get in the habit of tweaking AF prior to every night’s run, particularly if the targets are different. You will primarily need to adjust nebulosity rejection for the particular object you are imaging. Once you have the rest of the settings working, you can pretty much leave them alone.
Thanks for your thorough and helpful response. The steps you laid out definitely sound like best practices and offer a good and relatively quick calibration of the auto focus settings. I am also using the Moonlite CHL focuser. This evening I will definitely make these measurements and adjustments as you suggest.
Hi Tim,
Wonderful outline of the optimum process for setting up your auto-focus. So nice I have copied it into a text file for easy reference. I am ordering an Edge 11 so I will be following it.
Thanks. Jerry
Last night I went through the procedures that Tim suggested. There were a couple of deviations I had to make, however. First, after determining what I think is the best setting for nebulosity rejection for my main target of interest, I discovered that I did not want to allow auto focus to adjust so far out that HFR was 2x that of the minimum. Getting to this value was so far out that stars were big doughnuts and the algorithm got confused. I settled on a more conservative number, something on the order of 1.5x, but once I did this auto focus performed very well. I went through a couple of auto focus tests before committing to the night’s sequence. I haven’t gone through the logs and images yet to make sure it stayed in focus the whole night through, but from the times I was able to watch it worked well.
Another thing I discovered is that, at least with my equipment, I needed to stop guiding during auto focus. This made perfect sense (after the fact) in that as the scope is brought out of focus, I lose the guide star. Consider that the guide star with OAG is way out at the edge of the FOV, so it’s going to get real distorted when out of focus. SGP was hanging as it was waiting for the guiding to resume, and the auto focus procedure was halted. Turning off guiding during auto focus was the way to go.
I did note that there was no clean way for me to stop SGP while it was hung up waiting for PHD2 to resume guiding (which it never was going to do under the circumstances). I had to hit CTRL-ALT-DEL and stop the various processes, which is messy. The usual Pause Sequence or some other type buttons were unavailable during this time. The Stop Auto Focus button didn’t work for me.
I have another ongoing SGP issue that has affected me the past couple of nights, but since it doesn’t have to do with the auto focus I will submit another topic.
Thanks again, Tim, and I’m sure this thread will be helpful to others, not just with my particular scope and system.