SGP + longer focal lengths


#1

I am just curious of how many users of SGP are imaging at longer focal lengths (>1500mm). Just wondering if I need to back down on my FL (1680mm via c925 w/ reducer) to have been success with autofocus?


#2

I can’t imagine why focus would be much of a problem, just fewer stars to focus on and maybe if you were in a relatively star poor area you might need to increase the exposure time for focus but it should do fine. I have only used it up to 1000 mm but noticed no difference between that and my other scope at 530 mm.


#3

This has been a tough one for me with my setup for some reason. I have even moved over to focus-friendly fields with a 6-7mag star and plenty of other fainter stars in it. The autofocus routine just has issues with my setup. If I switch over to FocusMax and MaximDL, I get very good focus quickly. Even running the V-curve in FocusMax produces a decent V/U-shaped curve. I only get that with SGP once in a while with the same setup. I have adjusted autofocus parameters many, many times and have not found a good, consistent configuration for me. Current parameters include: Lum filter, 8sec exposure, 2x2 bin, dark frames. I have worked with the step size and sampling points numbers using the ideas in the help file. The only thing that seems “off” is when doing the autofocus, most stars tend to have the red box around them, suggesting SGP is ignore them.

I was using an external crawford focuser and switched back to using the main SCT knob to move the primary. Same results, FocusMax can do it every time (focus confirmed with a Bahtinov Mask). Maybe others using a SCT and SGP can chime in?


#4

I often use my AT8RC at its native focal length of 1625mm and haven’t had a problem since I increased my exposure time to at least 10 seconds. I also had to use twice my calculated step size. However, I use a DSLR - D5100.


#5

Thanks for the info Tim.

I had some clear skies last night and did a test with SGP and FocusMax. I did a vcurve run in FM and wrote down the step size and number of steps (40 / 36). It made a nice u-shape curve and the it matched my previous vcurve runs. Then ran the autofocus routine and got a nice well focused star.

So I next went to SGP and used the same step size and number steps on the same FOV. The results were way different, I did not get a curve at all, just the a lot of bouncing around on a sort of horizontal line. I would expect at least for the two systems to behave roughly the same with same step size and sample points. So now I am scratching my head. I really, really would love for focusing to work with my setup in SGP. But my current plan now is to pause imaging when I need focus, disconnect the camera in SGP, run FocusMax (acquirestar), reconnect SGP, and continue the imaging run.

I am running the latest version of SGP (2.3.13.2473) btw.


#6

Hi there,

First off, ignore, the colors of the boxes, they indicate size and not usefulness. If it has a box, SGPro used it in the HFR calculation. Can you provide a log of a failed AF run? We can help inspect some other params that way (on dropbox is fine, please don’t post logs on the forum).

Also… we will probably add this to the help file, but we have starting fiddling with other methods to help users get AF setup properly. Try, instead to use 9 data points, LUM, 2x2… then start big with the step size… maybe 3 times what you think it needs to be. This should be the only param you need to tweak.

Run, AF manually. If, for the first AF frame, stars are so far out of focus, none are selected or SGPro has resorted to hot pixel selection, dial your steps down a bit. Keep doing this until you get a decent number of stars read. Once you have achieved this, let SGPro run through all 9 data points. Eventually, when you find your “low” HFR for that fov, tune your step size such that the first AF frame HFR is 2-3x times larger than the ideal HFR (please don’t use HFR calculations from different software products).

Ken


#7

Hi Ken,

Thanks for all that information and suggestions on getting this tuned. I will look to see if I can find a failed AF run in the logs. If I cannot, I will use the steps you outlined. If there is any more issues that I think you need the log file for, I will get them to you. Sorry for being a squeaky wheel with this.

Pete


#8

Also you might want to play with the Nebula Rejection. In the summer I can’t get my STL-11000 down to -20C and it’s fairly noise for shorter frames. Bumping the Nebula Rejection up helps a good amount.

If you continue to have issues turn on Auto Focus Packs and send us a link to an auto focus run that you’ve uploaded to Dropbox (it will be quite large!) this will at least enable us to see what you’re seeing and potentially adjust the AF algorithm or make suggestions on settings.

Thanks,
Jared


#9

Ken, Jared…thanks for the tips and ideas on this. It was clear last night and I implemented your suggestions was able to get successful AF runs while pointing at Abell 39. I had to tweak the Nebula Reject to get it to go, but after that, it worked very well. I need to tweak my step size and exposure a bit because the last AF frame was tending to lock onto hot pixels, but I still achieved good focus. HFR was around 1.8, which seemed pretty good.

Too bad I did not go out to the observatory before starting the sequence, my scope was on the wrong side of the pier and I only made it 12/30 frames before I hit the safety on my CGE-Pro while I was sleeping. Guess I need to install a webcam too now.


#10

I won’t slew my mount without looking through my security cam! I have a Foscam fi9821W v2. Prior to that I have a Foscam fi8910w.

Both of these allow you to turn the IR on and off which is important for astro photography!

Here’s a shot inside my obs:


www.mainsequencesoftware.com