Focus issues (auto)

Sequence Generator
1

I had used focus with excellent results. I just plugged in some course and fine numbers and it just did it’s job perfectly.

So I change from a 071 OSC camera to a ASI1600MMC. and I add a filter wheel. I now go to auto focus and it’s all over the place. No nice “U” shape or “V” shape I get points up and down and zig zagging.

It’s like it’s confused. It will pick a starting point, go down and get better and down better… then may JUMP UP on the grid a huge jump. As if it stepped to far or something in the train just we WAY out of focus at that point? Any ideas?

2

I would start by doubling (at least… maybe more) your step size and increasing exposure time to 10 sec. You don’t need to keep it at 10 sec, but it’s good to eliminate that as a problem while troubleshooting.

3

The course and fine numbers on the focuser control panel have no bearing on auto focus. You need to use the settings in the Auto Focus tab.

Step size is pretty dependent on the size of your pixels as well as seeing.

SGP isn’t confused, it’s just displaying the information it has available. Garbage in, garbage out. It sounds like you need to either take longer exposures or increase the step size in the Auto Focus Settings panel.

Thanks,
Jared

4

I also have this problem. Waiting for a clear night to experiment with different step size.

5

Longer step size worked, but my “U” is lop sided (is that ok?) seemed to be. Though it’s lop sided because auto focus seems to want to start on one side of what it might think is prime? So a not to miss prime focus? And it starts to far off… so HFP of 7-8 first exposure. (yes mine always 10 for lum). Then it takes 5 more exposures to get down to HFR of 2. Maybe that’s because it’s forced to step to small? (75 currently). HOWEVER What I worry about is if my steps are TO BIG - then it will not hit prime focus because it will be forced to step past it?

Like say its in decent focus but is 50 off and the step size is 100. (It won’t ever reach the 50?)
Seeing had to be decent, I was getting HFR of under 2 with most filters.
I’ll up it to 100 and see if that reduces the exposures to get from first focus frame to near focus. My “U” is much more like a italicized… J

Does that matter? I’ll try even larger steps.

6

Step size is going to depend greatly on your equipment, but I can tell you that in my case a step size of 140 gives the most “V” shaped and closest result to what I get with a B-mask, so don’t hesitate to try step sizes beyond 100. Also, perfect focus is calculated based on what I presume is a regression analysis of the set of the points on each side of the “v”, and the intersect of the two slopes, not on the lowest point measured. As long as you have a curve something close to a “V” or “U” and about 2-3 fold difference between the lowest and highest FWHF, the AF calculation should be pretty close.

That said, I usually try to a “V” or “U” more symmetrical than the lopsided “J” you are getting, though in my case the right “leg” always seems to be a bit longer. If it’s really lopsided then a lower step size may be needed. Also, you are getting ~4x difference between lowest and highest FWHM, suggesting a smaller step size. 2-3 is usually sufficient.

7

Absolutely. This is pretty common. Both kinds of lopsided actually, the one where the left and right have differing slopes and the one where the focus point has shifted left. Go as high as you can, but beware of wings on the edge of focus… means that your outside frames are not being measured properly.

8

Step size now 100 and focus working very well. Nice “U” though I’m using to many points I think (9) Will try 7 next time.

I’d like to speed things up I have no idea what binning is? should I turn on hardware binning and try some binning during focus?

I put in focus differences in all filters. Last night things went VERY WELL with filter changes.

9

Hardware binning is only available to CCD cameras, Software binning is available to DSLR images and should be used only when the computer is having difficulty computing whole image HFR in a reasonable period of time.

10

Thanks I’ll go to that link. I enabled hardware binning once and I noticed the exposure/crop difference. I turned it back off as I don’t want to mess up to much at once. But things are coming together again so time to explore. I have CMOS so I suspect you mean only on CCD/CMOS type cameras. I was thinking I could do hardware binning simply because I have no idea how to “setup” the 4 binning options in SGP. Heading to your link, Thanks Ken.

11

yes… old habits. 2x2 is a very popular option for AF because it is a great balance between download time and resolution. It can speed up AF significantly.