Hi guys, I am new to SGP, and have just shifted from using MaximDL. I made the move due to the autofocus feature SGP offers. So far I like the software!
As my title suggests, I am not getting the results I expected with the autofocus feature. I have followed the online tutorial, and re-read it a few times with no luck.
I ma using a USB_Focus stepper motor on the stock 4" focuser on my TAK TOA-150. I have measured the step size to be about 3 microns per step.
Sorry for the long explanation below!
I’ll explain what I’m doing, and what my results were, and hopefully someone can point me in the right direction.
- Frame and focus with my Bahtinov mask, and my focuser position is at 614, at good focus.
2.in the autofocus settings, I have used a sample sizes of 7 and 9, and a step size of between 50-80 which works well. I’m using Bin 2x2 as recommended, and an exposure time of 4 seconds.
3.Click “Run” in the focus control window.
4.The focuser moves out to its expected position, and starts taking frames, then moves in by the required amount. 90% of the time I end up with a good graph, with the green lines, however I ALWAYS get the warning message saying that the HFR is out by 0.8 or so.
5.Check focus with the mask at the new computed position, and focus is clearly out.
Now after running this quite a few times, and watching what goes on I have noticed a few things:
I watch the graph as the sequence runs, it starts with a high HFR, then drops to begin forming a “V” as expected. When the graph is showing the HFR at position 614 (Excellent focus) I expect it to start to rise again, completing the “V”. However it ALWAYS overshoots this value, and falls further, ALWAYS by approximately 100 steps. The graph keeps plotting as the focuser travels in, and I end up with a nice “V” curve, with green lines, and a best fit position of say 490 or 514, 495, or 528. So I pop the mask back on, and as expected, the star is out of focus. Move back to 614, and its in focus.
I hope this part makes sense below!
Now this is the part I really don’t understand. As the graph is plotting its curve, I am watching the HFR values in the little yellow boxes in the image. When the graph begins to overshoot MY expected good position of 614, the HFR values indeed do decrees, making it appear that its working, That is; at position 500, the HFR is 0.5, compared to 1.0, at 614.
When I check this manually it doesn’t add up. If I compare HFR values of the same star, at position 500 and 614 manually using the frame and focus tool, position 614 always shows a lower value.