FocusMax with a DC Motor Part 2
Star Date: 26th Feb 2011
I finally got some free time and clear skies to try and finish getting FocusMax set up and working with the Orion Accufocuser on my ED80. From last time I had done (what I thought) was the initial calibration and was waiting to get the V-Curves running.
Well, after some playing around and rethinking, I swapped the start and end positions round, so now ZERO is fully racked in and the movement of the focuser now increments the position in positive values. It now makes a bit more sense!
OK, so that done, I refound the approx focus position and noted that value. I used that as the starting point for the V-Curves run. I had actually previously done a run of V-Curves but the numbers didn't look "good". The PID values were between 60-120 and I had asssumed that these were not good. It wasn't until SteveL asked if I had actually tried focusing with the profile, that I realised that I hadn't!!
I posted on the Yahoo Group to see if I had missed anything, and after some helpful tips on things to check from Joe Mize, I went about redoing the profile - clearing out the previous measurements and starting with an empty profile
When I had run the V-Curves previously, the Centre Point seemed to continuously move in an outward direction - which I knew to be wrong as I knew where the approximate focus position was. My conclusion was then that the focuser moving but recording the position as a negative value confused matters!
This time, although the PID values were big (and to be honest, I still don't know what they mean!) the Centre Point was pretty stable - it moved a few points in either direction which told me that it was doing what it was supposed to do! The PID values were closer to 200 this time round and I think this means that the difference in the left and right slopes (i.e. inside focus and outside focus) was quite big.
After about 8 iterations of the V-Curve process, I thought it was time to see how the focusing would run - if at all! I jogged the focuser a couple of hundred points in one direction and hit the 'Focus' button. FocusMax then went through it's routine of determining which side of focus it was on, and calculating the best focus position. When it had finished, I did an exposure in Maxim and measure the FWHM value (as I would normally do) and it was bang on!!
I then jogged the focuser in the opposite direction and hit 'Focus' again - and after a few seconds, it calculated the focus position, and again I checked it in Maxim and again it was bang on! Whoo-hoo!!
So, having proved that the system works, I now need to set about refining the settings and getting the numbers down. Joe tells me that the values in the slopes should be way closer than they are. Back to the drawing board it is then!!
Once I get this sorted, I will rewrite the articles with top-to-bottom steps... if nothing else, to get it clear in my own head for when I go do the 12" scope!!
