Stepper motor focuser and Maxim, best focus is not in focus!

[blinky]

So, I got 10 minutes before it clouded over tonight so went out for a 1st try at autofocusing with my new stepper motor focuser..... After 1st discovering I  needed to reverse the motors as in was out and out was in I was all set!

So in MaximDL, I set my microns/step to 3 and F Ratio to 4 and clicked start, the first attempt was terrible so I adjusted the HFD down a bit to 4 and tried again, this was a bit better and initially I thought it was fine but when I put my mask over the scope and took another image I could see it was slightly out.  The clouds have rolled in now so I thought I would post and see if anybody had any ideas?  Backlash.... Dont know, I dont see any but that does not mean there is none - The motor is directly coupled to the focuser, I could add say 10 steps and see if it makes a difference?.  HFD, not sure what this should be, I noted the best HFD measured on the 1st run and set it to a couple higher than that as it suggested....  Maybe it was just bad luck, the scope seems out of collimation, the brightest star was not dead centred and there was some wind and bits of cloud...  The only other thing is when reading it suggests to BIN the image but that option was greyed out on the exposure setting tab of focus - my camers is a QHY8 OSC so that was why it suggests to use Binning.

Anyway, any ideas would be most welcome, the weather is not looking good for a while so it might be ages before I get another attempt!  I could also try Focusmax as I have an old version of that but would rather stick with Maxim as its one less application running on the machine etc

[Davey-T]

How did you arrive at your step size ?

You need to set some backlash, maybe 20 as it's not critical as long as you've got some.

The Maxim focuser and Focus Max 3 are pretty much the same.

Dave

[blinky]

step size - The stepper has 6330 steps/revolution (and  I confirmed this by making a mark on the mounting bracket and telling it to move by that many steps, the motor returned to the mark), and as its directly coupled I moved it by that many steps, measured the distance my focuser moved with a vernier gauge and divided the number in mm by 6330 steps to get roughly 3 microns.

I went out a few minutes ago to try and figure out the backlash (although the stepper is directly coupled it has a gear box, planetary I presume?) and I think its somewhere between 20 and 30, so I put in 25 but it's still cloudy so I could not try it again!

[Davey-T]

Never tried using an OSC so don't know how / if that affects things regarding autofocus, if you bin it certainly confuses plate solving in Maxim.

Backlash 25 should be fine.

The HFD figures during autofocus should alter  by about 1.5 during steps in the autofocus routine you can alter the step size to achieve this.

Dave

[LightBucket]

10 minutes ago, blinky said:

step size - The stepper has 6330 steps/revolution (and  I confirmed this by making a mark on the mounting bracket and telling it to move by that many steps, the motor returned to the mark), and as its directly coupled I moved it by that many steps, measured the distance my focuser moved with a vernier gauge and divided the number in mm by 6330 steps to get roughly 3 microns.

I went out a few minutes ago to try and figure out the backlash (although the stepper is directly coupled it has a gear box, planetary I presume?) and I think its somewhere between 20 and 30, so I put in 25 but it's still cloudy so I could not try it again!

That’s the correct way to work out step size, or at least it’s the way I do it, mine works out at exactly 1 step = 1 micron... 12mm the drawtube moves in one rev of the focuser, divided by 8400 steps per rev my focus motor moves.. 12/8400 = 0.001 = 1 micron..

its also useful to know the critical focus zone of your imaging scope, mine is approx 100 steps, so my scope is in focus anywhere within a certain 100 step zone of my focuser..

find it out here, it’s at the bottom of the page

http://www.wilmslowastro.com/software/formulae.htm

 

 

[blinky]

Critical focus zone of mine is 35 microns I think (calculated a few days ago so cant remember exactly) so that give around 10 steps 

[LightBucket]

What version of Maxim do you use..?

[blinky]

Not sure, it's an old version I got years ago and it was a case of it ain't broke so I just used it and never upgraded to newer versions

[blinky]

So I went and got V6 of Maxim and tried again tonight - same issue as last night, nearly in focus but not quite..... I also downloaded SGPro trial and had a go with that and it was the same!  In Maxim I tried every setting I could tweaking it but it never made much difference until I changed the focus direction, rather than in, I changed to out and managed to get it to focus perfectly twice before the inevitable clouds rolled in.  So it seems for some reason I need to have my focuser set to in rather than out???

BTW, foun SGPro quite confusing, I guess its just what you are used to....

[blinky]

So went out tonight again and could not get Maxim to reliably focus - then tried FocusMax and it just works!  Really pleased....