Moonlite Autofocusing

[umasscrew39]

I am not sure if this is an Astroart or Moonlite question. I setup my Moonlite focuser with the ASCOM driver and did an initial focus on Dubhe using the scope focuser with a Bahtinov mask per the Moonlite instructions. I then set the Moonlite draw tube at midpoint of its total steps which I calculated to be about 4700. So, Dubhe was in focus at about 2350 steps. I took a few 30s images and then opened the Focus Window and clicked the Autofocus tab. I placed a box around the star and pressed START and it ran through a series of calculations before telling me the best V was P03850. When it finished, I took another 30s image and Dubhe was a donut shape, out of focus. When I manually set the steps back to 2350 steps and took another image, the star was in perfect focus. So, I am obviously doing something wrong. Any suggestions?

[Dunmunro]

I am not sure if this is an Astroart or Moonlite question. I setup my Moonlite focuser with the ASCOM driver and did an initial focus on Dubhe using the scope focuser with a Bahtinov mask per the Moonlite instructions. I then set the Moonlite draw tube at midpoint of its total steps which I calculated to be about 4700. So, Dubhe was in focus at about 2350 steps. I took a few 30s images and then opened the Focus Window and clicked the Autofocus tab. I placed a box around the star and pressed START and it ran through a series of calculations before telling me the best V was P03850. When it finished, I took another 30s image and Dubhe was a donut shape, out of focus. When I manually set the steps back to 2350 steps and took another image, the star was in perfect focus. So, I am obviously doing something wrong. Any suggestions?

IIRC, your focuser should be set to absolute rather than relative and you would set the start position as 2350.
I had something similar happen when I first began to use AA7's AF routine. I think the problem was that I had the autofocus steps per focus image set incorrectly so that AA7 ran way past any possible correct focus value, and ran out of range during the AF routine. In your setup, the focuser can move about 15mm or ~15000 microns and so each step is about 3.3 microns. You would probably want the AF routine to move about 30 steps per image and for the default 7 image positions that would mean a total range of 210 steps or ~700 microns.

[umasscrew39]

Thanks, Duncan.

I'll give it a go again tonight if the sky cooperates.

On another note, I ordered the new Celestron ASCOM driven autofocuser which is due in April. If it works as advertised, it would be a much better option for me as it allows me to use again my hyperstar lens for f/2 which gives a very nice wide FOV, in addition to using the .7x reducer and prime at f/10. The main difference is that the Celestron autofocuser works off of the scope's focuser instead of locking it down to focus with the Moonlite dew tube moving. Of course, the disadvantage is that mirror shift and flop can become more prevalent if doing long exposures.

[fabdev]

Hello,
usually it's not possible to autofocus on a bright star as Dubhe. It will give a lot of saturated pixels and/or blooming, unless you use an extremely short exposure time (example: 0.01 seconds) which most deep sky cameras cannot handle well.

If you need to autofocus 3-4 times per night, I suggest to start using autofocus on the same field of the object, so using stars of 9-10 mag and a focus exposure time of 0.5 - 2.0 seconds.
For this kind of fine focusing, a lower "Step" is recommended.

Clear skies,
Fabio.

[umasscrew39]

Thank you both for the tips. I think I got it working after making some of the suggested adjustments but I want to try on multiple nights to make sure it wasn't just dumb luck.