Hi Folks been off of here for a long while after a 2500 mile move to a much more well lit part of the country (Seattle Area).
Question(s)
Can I expect the autofocus routine to work in less than stellar (pun intended) seeing conditions? The best I can get shooting out of my garage is 3.67 HFR.
Is there any point of dinking around with FWHM and will help? (side question is what is the real difference and benefit of one over the other?)
I did a search through the forum archives and this post is the closest I found to what I think my problem is, so here I go:
I’ve gotten AutoFocus to work once or twice with my gear, so I know it’s possible, but it’s not reliable. My theory is SGP is “chasing the seeing” and losing. My gear: permanently mounted MI-250, CFF 14" f/15 classical Cassegrain (5250mm focal length, yes, very long), QHY 268M camera, Optec ThirdLynx focuser. Currently, autofocus exposures are set up in SGP to be binned 2x2 and I’m using the HFD algorithm. I’ve tried 7, 9, and 13 data points.
What I observe is: when I’m lucky, I’ll get three or four data points which trend smoothly downward toward optimal focus, then the HFD value will jump (I’ve seen it jump from 3.xx to 10.xx between 2s exposures) and I’ll get an “outlier” which messes up my saddle. If I’m unlucky, instead of even the beginning of a nice saddle, I’ll get a saw-tooth shape with no pattern whatsoever.
I’ve had some success performing the AutoFocus routine manually. I’ll take three or four exposures and make a mental average of the HFD value. Then I’ll move my focuser a pre-determined number of steps and take three or four more exposures, average, determine if I moved in the right direction and continue with the process. I looked in the AutoFocus settings to see if there was a way to ask AutoFocus to take multiple exposures at each focuser position (to “average out” the seeing), but didn’t see a way to do it.
If that’s not a feature of SGP, I’d like to request it.
If your camera has the ability to bin 4x4 I’d use that. It gives you the smallest stars, and the widest scale. Also try setting your exposure up to 5 or 6 seconds. With a really long focal length the longer exposure time will help to average out the star wiggle from the atmosphere. If you are using a narrow filter a bit more exposure time doesn’t hurt. I use 7 data points with 350 steps on my Optec focuser on my Meade LX600. This puts my HFR at about 5 at the outer edge. I also make sure when ever bin 4x4 is selected for the focus that the camera is also set to high gain.
AutoFocus via long focal length (or just star-poor regions) is a known weakness for SGPro. In addition to account for variations in seeing, SGPro also needs to have a better understanding of when the law of averages (with outlier rejection) will actually be detrimental to the overall result (i.e. whole image HFR). I am not 100% sure what will make this better, but ideas are welcome. In the past, I have considered automatically switching to monitoring of the median star (not on the edge), but I have no data or anything to show if this method would be better.
AF7JQ, thank you for your suggestion. I already bin 2x2 for autofocusing, but haven’t tried 3x3. My camera won’t bin 4x4. I haven’t tried exposures quite that long for autofocus, but I’ve tried as long as 8s for guiding. That didn’t work.
Ken, when I asked a friend of mine about autofocus this weekend, he suggested using multiple exposures for each data point so the software could average the HFD or FWHM. I suppose my friend doesn’t use SGP for his autofocusing; as I said, I looked for a way to take multiple exposures per data point but didn’t see that feature. I wonder if that would work? Since seeing is often a short-period transient, three or four exposures might be enough to get a decent average. The algorithm could even reject outliers- that would be easy with my imaging train since I’ve seen my HFD jump a bunch as I said. If the algorithm got one step more sophisticated, it could remember the averages from one autofocus run to the next (within the same “autofocus event”) and refine the criteria for a “good star” versus a “seeing compromised star image”.
I’d be happy to beta test anything you come up with. My observatory is remote, so beta results would not be immediate.
When you see a massive change in HFR, can you see what the whole-image HFR is picking up when it does so? Nebulous objects that are more present in some frames than others? Just more stars in some than others?
Being remote always makes things more difficult…
Something else to consider with big scopes is the focus change caused by temperature. If the temperature is quite a bit different from the last imaging session then a auto focus routine might not work because the new focus is outside the curve. On my scope, once you get beyond about a HFR of 6 or 7 the star doughnuts start getting large enough that SGP starts seeing bright points within the doughnut as stars and and tries to focus them. My fix for that is to run frame and focus and use the course adjust button in the focus module until the HFR gets down to something more reasonable…2 or 3, then the auto focus routine works fine.
Reducing your step size yields no better results? It might create a sitation where it doesn’t push out to HFR 6 or 7, but at the same time not produce enough of a delta to provide any kind of meaningful results.
Good Morning Ken,
My scope is a LX600 12" F/8 (F/5 with focal reducer). Because I don’t have mirror flop to deal with my focuser is mounted on the scopes main focus knob rather than in the optical path so I am focusing by moving the mirror. My results may be different from a F/10 scope focusing in the optical path. My normal settings are 350 step size with 7 data points. If I use 9 data points, the right side of the hyperbola gets flattened out because the star doughnuts get large enough that SGP no longer sees them as stars. Usually, anything beyond HFR of 5 is unreliable. If I reduce the step size to 300, thinking it might be more accurate, the bottom of the hyperbola flattens out. This causes the optimum focus point to become more nebulous…repeated focus runs return different numbers. Also the smaller step size means the HFR jump between data points is smaller, and atmospherics can cause a data point to be outside the curve. Overall, your focus routine allows me to get repeatable results every time once I have accounted for changes in temperature from one session to the next.
Ken, I don’t know how to look for the HFR for the whole image. I’ve been looking at HFD numbers for individual stars.
A question about the GUI: last night I was looking at frame-and-focus images, HFD specifically. I right-clicked the image then selected Show HFD. But when the next frame and focus image displayed, the HFD data disappeared. Then I had to right-click the image, deselect Show HFD, right-click again and select Show HFD to get the data back. Is that normal behavior?
Possibly. It depends on your settings. Because HFR is a computationally expensive operation, it is not calculated every frame unless the Image History options is enabled. You can enable or disable this feature in the Image History module:
I’ve just starting using the SGP autofocus and had similar issues with “noise” at the bottom of the focus curve and a flat bottom rather than the V or U shape expected. I noticed that SGP was finding lots of stars at the defocussed ends of the curve, but very few when close to focus (sometimes just one or two). Those one or two were also quite bright stars (therefore with high HFRs) and therefore prevented the curve from going any lower.
At first I thought this might be due to having the Minimum star size set too high (so as they come in to focus and get very small they get rejected). I changed it from the default of 6 down to 2 but didn’t see any improvement. Eventually I found that changing the Image Analysis Precision (found in Tools>Options) from “Auto” to “Normal” completely solved the problem. SGP now found lots of stars at all focus points and my HFR values at focus went right down to ~1.4 where they previously had bottomed out around 2.5. The curves were also superb smooth shapes with “quality” figures (whatever they represent) of 99 or 100%.
I’m not sure if this helps anyone else but it certainly worked for me !
The “quality” of your autofocus represents how closely the data represent a (I’ll guess) parabola (or some other smooth curve). If the data represent a smooth curve like a parabola, the algorithm can create a curve fit with high confidence and then use calculus to find the minimum of that curve, which represents your best focuser position.
From the Help File under Detect Stars Settings: Apart from the “auto” options, the general rule of thumb is that the more precision you use, the more stars you will find, BUT, this will come at the cost of speed.
So if the default Auto didn’t work well in your case, perhaps try other “manual” settings and see if you can get a good compromise between detecting enough stars and keeping speed high enough for you.
I had a break through last night. I tweaked collimation, improving it a bit. After I got everything put back together, I manually focused and got much tighter star images than I had previously. After seeing that I decided to try AutoFocus again. My first attempt failed, I discovered due to bad settings. I revised my settings and it worked. It worked for the rest of the night.
