Auto Focus Fails

I too am having issues getting Auto Focus to complete successfully. For some reason, the routine cannot seem to find the proper positions and when it graphs the focus attempts, it always appears as a straight line rather than the nice “V” curve. I am using the Celestron Focus Motor, and SGP seems to run it just fine. Here are my SGP settings:


Here are the error messages I am getting:

Cannot Focus
Cannot Focus 2

Any suggestions for new settings would be very much appreciated … thanks,


It is likely that your step size is way to low. Increase your data points to 9 and then increase step size until the HFR of the first AF frame is 2.5 to 3 times the value of the in-focus HFR.

Start at 130 for the step size and adjust it up or down by increments of 10 to get the V just right. I use 5 points instead of 7 for the RASA. Also, use 6-10 second exposure unless you have a very tight Narrowband filter.


Hi my in focus hfr value 1.18 then I move the focuser till hfr value 4.5 then deduct the focus position value with in focus position value then I multiply with 2 and then divided by data point the value igot 225 ( I did this as per youtube tutorial).i enter this value in step size…first time I got v curve but next auto focusing time it says cannot find focus point and failed to do autofocus…please let me know what I’m doing wrong…
My equipments
Esprit 100ed apo
Sesto senso focuser
Zwo asi071 mc pro
My settings for auto focus
Exposure time 10
Bin 2x2
Data point 9
Step size 225
Auto close delay 10
Minimum star diameter 6
Crop autofocus 20% checked.

It took me a while to get autofocus to work properly with all of my filters. I have a Celestron 9.25 Edge HD OTA with FL = 2350mm. Key learnings were:

  1. Step size must be just right, especially for my SCT. I spent several hours increasing my step size until the HFR went to about 3X the minimum value. There is a limit with SCT’s as the central obstruction creates large “donuts” that causes HFR calculation problems. I ended up with 9 steps, step size 700, min star size 4 pixels.

  2. Exposure times must be increased significantly for narrowband filters. I set my ASI1600 camera gain to 300 and 2x2 binning. Lum exposure was 7 sec, R,G,B 10 sec, narrowband 25 sec.

  3. Point to an area with a sufficient number of stars. Some targets, like galaxies, may have very few stars with sufficient brightness. This is especially true for narrowband filters. I used an open star cluster for focusing.

  4. Don’t point the scope too low in the sky, especially for narrowband. I was imaging M1 and my sequence changed the filter from Oiii to Sii. Transparency was reduced as I pointed further West toward the Oregon coast. The focus curve flattened and HFR was increased from about 2.5 to over 5.

Mark W

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