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Geordie mc

SGP autofocus with Celestron Edge 800

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Hi. Just starting to use autofocus with the Celestron motor focus and SGP. Can anyone give me some starting points for step size, backlash etc. I’m getting there but wasting a lot of imaging time just trying to get this to work accurately. I’m currently trying a step size of 75 and a backlash of 50. Any tips most appreciated.

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I use SG Pro with a MKIT20 controller and Microtouch Focuser, which are arguably the most sensitive (certainly most expensive). It took me ages to answer the same question. I followed the step size calculation guidance in  the SG Pro tutorial and never got a deep enough V curve. However, it has a range between 0 and 65,000 steps. Therefore, a step size of (say) 30 isn't going to make a fat lot of difference! The step size that eventually worked for me was 11 points at 150 step size.

I don't know what range of sensitivity is offered by the Celestron focusser.  But let's say it was 0 to 30,000, half that of mine. That might suggest a step size of 75. But I would start by doing the calculation in the on-line tutorial. If you don't get a deep V, just keep incrementing the step size until you do. I might have got lucky by this crude trial and error methodology, but autofocus has boosted my observing experience no end. 

 

 

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Thanks for the info. The Celestron has a range of about 44,000 steps so I may need to up the step size a little. The right side of the `v ‘ curve doesn’t look bad but the left is very steep. Think I need a much bigger backlash compensation too. Good to know I’m on the right lines. I have a Hyperstar for the Celestron which I hope to use when galaxy season ends and I’m hoping autofocus will give me much sharper images. I’ll try upping the frame count to 11 too when I’m getting a more even V. Thanks again.

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3 hours ago, Geordie mc said:

Thanks for the info. The Celestron has a range of about 44,000 steps so I may need to up the step size a little. The right side of the `v ‘ curve doesn’t look bad but the left is very steep. Think I need a much bigger backlash compensation too. Good to know I’m on the right lines. I have a Hyperstar for the Celestron which I hope to use when galaxy season ends and I’m hoping autofocus will give me much sharper images. I’ll try upping the frame count to 11 too when I’m getting a more even V. Thanks again.

From the outset, I have been using SG Pro autofocus with Hyperstar. The extra photon gathering power probably means I don't  need to be too fussy about the exposure lengths I use for autofocus. If you remain at f/6.3 or f/10 you might need slightly longer focus exposures to capture the required HFRs.

A range of 44,000 steps might suggest you need something of the order of 44,000/65000 × 150 = 101 steps based on a crude conversion of what seems to work for me. What threw me completely was the SG Pro tutorial diagrams showing a step size of merely 22. I was getting a flat line! At that! 

The other thought, try 11 curve points not 9. After a bit of trial and error with larger step size, that seemed to help me find the ideal curve. I suspect that now I know my approximate start focus setting I could cut back to 9 points to speed the process up and reduce step size. But the key is to get progressively close. 

 

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Must admit that I hadn’t looked at anything before my first attempts and my step size was  at 20! I watched a video on YouTube where the guy was using 175 steps. That’s when I realised I’d better up the number. I’m hoping that as we move into warmer weather I can tolerate being out long enough to get things sorted with settings for Hyperstar and when using the reducer. I never shoot at F10 using that just for lunar or planetary stuff where you can see much better when you’ve got the focus right. I was so cold last night I was shaking so making adjustments was tricky. I got some subs done of M81 in LRGB but they aren’t as sharp as I’d like. Now I need to learn Pixinsight which I’ve just got. I’ve watched some videos and got a book so that’ll keep me busy on cloudy nights. 
Really appreciate the input. Thanks again. Clear skies.

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Unless you are starting from zero (racked in\out) every session, the actual number of steps that your focuser takes to go from 0% to 100% is irrelevant.

When a focus routine is run, the software will attempt to go back and forth, through the lowest average HFR value of the image, repeating the process till the best value is found.

As my system is usually set-up ready to go (I wish), outside of SGPro, I remotely drive the focuser, to obtain a reasonable focus, I then set the step number of the focuser to a value somewhere to the middle of its range, which then allows SGPro plenty of 'wiggle' room to get the focus the best it can, fyi, I set mine to 9 data points & a step size of 50.

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Step size varies from system to system and asking for recommendations won't help unless someone has exactly the same setup.

Step size is pretty easy to calculate. Get good focus with a mask, or by getting the HFR the lowest you can, note the focuser position. Step out until the HFR is around 3.5 - 4 times your in focus figure. Note your focus position, subtract your in focus position, double this and divide by nine. This should give you a good starting point for the step size.

Backlash is even easier, you don't need an exact figure you just need enough to make sure you take up all the backlash, more won't hamper your autofocus, it'll just take fractionally longer. If you get a nice downward curve for the first few steps you have enough in your backlash setting, if the first two or three steps look odd you need more or you have the wrong direction set for backlash.

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