I have Starizona Microtouch primary mirror focuser on an Edge HD 1100. I am imaging with a modified Canon 6D full frame DSLR using the 0.7x reducer. I have always done manual focus, but I would like to give the SGP auto focus a try. I am hoping someone with a similar setup could provide some guidance on a good place to start with some of the settings. In particular I am interested in the starting point for the following settings:
Step size - Is 100 a good step size to start with for the microtouch focuser?
Focuser Backlash Compensation - what backlash compensation is recommended with the microtouch focuser on a SCT? I have read elsewhere I may need a backlash comp of 300, but thought I would ask here if that sounds correct. Also, am I correct to assume the compensation direction should be âInâ?
What exposure time and ISO should I use with the Canon 6D for focus frames?
Is there a general guideline for Minimum star diameter when using a DSLR?
What other considerations should I consider as I start working through this setup?
I focus an sct with robofocus on the primary - so the mechanics are similar but the exact step size stuff wonât be.
I use 9 steps and 2 second exposures with sct and ccd - and I have backlash compensation set to about a full turn clockwise of the primary focuser knob. I make sure the focus curve is taken from high numbers to low (right to left) while turning the focuser knob counter clockwise - and backlash compensation will kick in any time I need to go toward high numbers.
I hope the above makes sense because it applies to any system for focusing with the primary focuser knob.
The 9 steps provide a parabolic plot and not a sharp âVâ so that I can see the shape and width of the bottom of the curve. Opinions differ on that - but I like to see the true shape of the curve near focus.
Taking out a full turn of backlash may take some time - and you may be able to get away with less, like half a turn.
I would use an ISO that is pretty high but doesnât saturate the stars.
With a dslr you may be completely limited by the download time of the frames - so maybe 7 steps would be better.
I would use about 2" as the minimum star size - if you are sure your in-focus stars are not smaller than that. Use the pixel size in arc-seconds to find out how many pixels that is. You donât want to lose stars when out of focus because they are too big - or lose stars near focus because they are too small.
Here are my settings using an Edge 9.25 with no reducer, Feather/Micro Touch and a modded 6D. Second pic is the nice v - curve that results. 300 for backlash compensation is good, and you are correct that the direction is in. These settings give me excellent, repeatable results. Let me know if I can help if you have any issues, since my setup is very similar. Note that you no longer need to disable smart focus with the newest focus routine in SGP.
To initially set it up and find your step size, start by getting excellent focus using a Bahtinov mask or other method, then try to run the auto focus. With my setup, I find that the tops of the v curve, where the focus is the worst, give me around 5.5 HFR when the seeing is such that the best focus is around 2.0. If the v curve is not steep enough to get you in this general area, try to increase the step size. If your step size is such that the tops of your v curve are much above about 5.5 or 6 HFR (assuming that the seeing is such that best focus that night is around 2.0 HFR), try reducing the step size until you are in that area. In general, you are shooting to have the HFR at the tops of the v curve around 2.5 to 3 times what it is at perfect focus. So just adjust your step size until you achieve this. You can have the worst focus positions of the v curve showing small donuts and it should still find focus unless the donuts are really big.
Once you have the right step size, unless there has been a really drastic temperature change from the last time it was focused you will not likely need the Bahtinov mask to start the night. You can just run the auto focus routine.
And I promise once you have this set up properly, you will actually enjoy watching it go through its routine as it brings you to perfect focus every time. I use temperature compensation, set at -6 steps per degree C. This lets me image for hours without having to stop and do an auto focus run while still maintaining perfect focus.
Frank/Dean thanks for the advice. Dean I really appreciate you sharing your settings and you right because we share similar setups hopefully the settings will be close. If we ever get clear skies again here in Houston I will give the process you guys described above a try and let you know how it goes.
I finally got a clear night when I could try out the autofocus in SGP and I just wanted to let you know your settings work brilliantly. I played around with them adjusting the step size and backlash comp up and down a bit, but in the end I found your settings work the best. Thanks again for sharing. Here is a screen shot of the results
I have one more question about the auto focus routine in SGP. As you can see from the graph in my previous post I am only getting about 1.5x difference between the bottom of my curve and the top. I keep reading that ideally you should try and get 2 - 3x difference. Is the fact that I am only getting 1.5x difference a reason for concern?
Currently my backlash comp is set to 300 and the step size is set to 120. Should i consider increasing both? If yes, by how much? I hate to mess with it too much as I am getting such a nice V shaped graph right now (it took me a lot of trial and error get get my graph to look this good). My seeing was crap the other night when I was trying to get this to work so a HFR value of 6 was about the best I could get while focusing on Polaris.
Fantastic! Very happy that my settings helped. I found that when I started using SGPâs auto focus with my Edge that I couldnât get near 2.5 - 3x the HFR when out of focus, compared to at focus. Step sizes that big would put me in donut land, and at that point, the auto focus routine became unreliable. With the latest improvements to the auto focus, I found that I was able to increase the step size and get closer to that 2.5 - 3x ratio without any problems. The improvements to the auto focus routine seem to make a big difference in donut land. It is easy enough to increase the step size for a few auto focus runs to see how well it works for you. If it doesnât show any improvement, then you can always go back to the settings that you have now that are working fine for you. Your setting of 300 for backlash compensation should be fine.
I also highly recommend dialling in the temperature compensation with the SGP auto focus. With that properly calibrated, you can go hours with perfect focus without having to stop and do an auto focus run.
I have a similar setup to Andrew except I am using Hyperstar. Can anyone tell me their settings in this situation? Also, Where do you set the backlash compensation in SGP?
