Focuser step size < 1


#1

I’m a longtime MaximDL user but it seems that Main Sequence listens to users and is relatively quick at implementing bug fixes/user requests/etc, unlike Diffraction Limited. Tonight when/if it gets dark I’ll be trying out SGP but I’m wondering if I’m going to have an issue with the focusing routine. As per Starizona’s website, the step size for my EdgeHD9.25/stepper (retrofit not crayford) is 0.26 microns per step–not a typo. I guess I’ll just have to enter 1 as the value but I wonder how much of an effect this will have. I’m using a hyperstar and have had great success with FocusMax so the potential is there. Anyway since my entered step size will be approximately 4 times greater than actual should I anticipate focusing problems?


#2

It’s possible you may have AF troubles with super fast scopes. Instead of
asking our users to calculate step size mathematically, we find it is more
tolerable to do this empirically. When your scope in is the CFZ, your
average HFR for the target will have a particular HFR… say 0.7 or so. In
terms of AF settings you should use no less than 7 data points and a step
size that yields 2-3x times your CFZ HFR value. This setup should yield
generally acceptable results so long as your field has a decent sample set
of stars (> 50).


#3

Chiming in because I might be the one missing something here :slight_smile: I use a Edge11 with Starizona Microtouch, and image w/ a DSLR. I didn’t think SGP knows anything about the number of microns per step. It is simply asking you how many steps to move the focuser between images in auto focus. I believe it’s up to you to determine how many steps to take. (Although I do remember seeing a calculator somewhere, perhaps on the SGP help site.)

In my OneNote Astro folder, where I keep all my notes on numbers like this since I can’t remember them off hand, I have the critical focus zone to be about 9 microns or about 35 steps. So 35 would be the right step size.

A couple of things: I found that I actually need to go higher so either my math or specs or something is off, and with a DSLR, I found it pretty critical to go with a low ISO (100/200) when doing autofocus. Lastly, with my Microtouch, I had to reverse the motor direction (I did it via the microtouch software, but SGP also supports this) because in and out were reversed. You want “in” to move the mirror against gravity closer to the secondary, and by default it was the other way around.


#4

Thanks for the replies. I guess I should have continued reading the ‘step size’ section in the help file. My brain must have turned off after the section about calculating the focuser step size and missed that you then use this value (in my case somewhere around 0.26 microns/step) as a divisor with the CFZ being the dividend. Sounds easy enough!

Last night was a mess–I used a faulty cable to connect to my focuser so while it buzzed a bit and sounded like it was moving but instead it wasn’t. I quite literally sat there for over two hours trying to focus before I clued in that something must be terribly wrong. Oh well. I work tomorrow so I’m not sure I will be able to try this again tonight.


#5

I used a Hyperstar lens last year with SGPro too. I found that most issues
could be fixed by extending the exposure time. Because the CFZ is so short
at f/2 you want to have the 2 slopes left and right as accurate as possible.

The other thing is to use temperature compensation - otherwise you end up
with a new focusing run over and over again.

Better luck next time!

 MarkS

www.mainsequencesoftware.com