I changed my flattener as I was having reflection issues and this changed my foacal length. I changed all the specs so they are right in setting.
Last night, after not using the scope for a while, I could not get a good focus no matter where I was pointing or what I tried, including backlash, step size, data points, exposure time and min star size.
I know there is a software update from SGP in the mix although it seemed to work fine a few weeks ago.
I get a dog ear on the outside of focus, a sharp drop into a curve from there, the bottom of the curve generally flattens out and then the inside of focus is fine, like what you would expect, except the green lines are in the wrong spot due to the outside of focus curve shape.
I’ve tried to start in focus but it doesn’t seem to matter where I start or what I do it is always the same.
I am using a WO FLT132 with a Feathertouch 3.5" focuser driven by a Sharpo Sky Pro motor and controller.
Any advice appreciated, please ask if you want more info.
Which camera and what image scale are you imaging at? In my home observatory I use the same scope and a kaf8300 ccd with the original P68 Flattener and also AFRIV reducer without issue at 1.2”/px and 1.47”/px respectively.
I will look up settings and post up later (maybe tomorrow) as I am leaving for work shortly.
What were sky conditions like while you were experiencing these problems? I have copious focus troubles with my EdgeHD 8” and have been wondering whether poor seeing affects focus V-Curves? I think that I have fewer problems when the sky is clearer.
In my experience this is caused by backlash. You can usually address it by manually moving your focuser out prior to starting autofocus (to take up any backlash slop) or by having a sufficiently large backlash compensation step size set.
I am using a KAF8300 based chip as well. Image scale is 1.2. Steps are 25, 9 data points, bin 2, 8 sec exp, backlash 50 in, star size 10 and Lum filter with offsets. This has always worked well in the past.
Hi PhotoGav,
I have to say that sky conditions were quite good, I can usually tell from the guiding. Usually at this location if seeing is crap, so is my guiding.
Hi Steve,
I am going up to the obs tonight (remote location to me) and will go over all my settings including the motor driver although I always run backlash adjust in the software anyway. I did try a ridiculous 100 steps for backlash the other night.
+1 I am using exact same setup right now with the P88 flattener and KAF8300 and have extremely good U curves. I can share settings too when I get home.
The focus motor driver has a backlash setting for its gearbox, so for the hell of it I doubled that from 25 to 50 steps. I set the Backlash in SGP to triple I would normally set it at to give me some way of seeing if there is a difference, so 150 steps (I have measured backlash and 30 steps should have been enough). I’m still getting a wonky line on outside of focus but it is much better.
Although I tried to focus as best as possible using the star button at the top of the image display to give me figures, I noticed that the first run was very one sided and used a lot of points on inside of focus to determine the focus point. From there on I was getting fairly consistent and good results with even curves with reasonable v shape. I then added 5 steps to the step size and that seemed to improve it a lot.
So I’m guessing backlash as Steve suggested, although I have measured backlash and it shouldn’t need that much.