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Focusing issues

imaging CMOS
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#1 dchamb


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Posted 12 June 2019 - 06:55 PM

First I want to thank this group for being here and looking forward to some helpful advice!

Here is my equipment setup: Astro-Tech 14" RC, Paramount ME II mount, Moonlite Nitecrawler focuser and ZWO ASI1600MC-Pro OSC camera. I also have a ZWO ASI1600MM-Pro with a ZWO 8 position filter wheel.

My frustration is in focus. Initially I use a Bahtinov mask on a bright star and get nice symmetrical lines. My Nitecrawler focus index ends up around 10000-11000. I remove the mask and I should be pretty close, right? So I start imaging and notice faint background stars are appearing as doughnuts. I use @focus3 in TheSkyX Pro and it makes a few small adjustments. Still not looking good. The diffraction spikes on brighter stars show faint doubling which is another giveaway that focus is off. My Nitecrawler is in the 9000-10000 neighborhood.

Now I begin to pick out those fainter stars that looked like doughnuts and start dialing the Nitecrawler in so they are smaller, brighter and no doughnuts. My Nitecrawler index is around 6200 now! Imaging starts to look pretty good.

I know temperature and sky conditions can play a role in focusing, but that seems like a big difference in focus position. Could it be that the Bahtinov mask isn't that effective on an RC scope? Should I leave my focuser at the last position before each new session, assuming I haven't changed the optical train of course.

Looking for any insights!



#2 kathyastro


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Posted 12 June 2019 - 07:51 PM

Temperature certainly makes a big difference to focusing.  I need to refocus if the temperature changes by 1 degree C during the session.  I really should do it every 1/2 degree.


I leave the focuser where it is at the end of each session.  Obviously I need to refocus at the start of the next session, but at least it starts out close, and I can usually predict which way I need to adjust to get a new focus, based on the temperature.

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#3 rgsalinger



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Posted 12 June 2019 - 08:37 PM

It sounds to me as if you are not starting your automated focus run near best focus focus. Personally I have trouble using a mask and haven't bothered with one for years. If you use a mask and have donuts as F8, then either there is something wrong with your mask or you aren't using in properly. In either case, it's a waste of time as the focal point only moves a bit with temperature with an F8 scope.


When I first set up my systems I just get the focus close (no donuts) and then allow the auto focusing routine to do its thing. Even if you set up de novo each night, keep the focus where it was the last time and you should be at a reasonable starting point. Each night after that I run the routine and pay attention to what it's doing. That's because a really wide temperature swing from my last focus at 4AM to right now can mean that the initial run never achieves best focus. If that happens, adjust the focus based on the data you got and try it again.


I also find @focus3 to be more or less worthless. Use @focus2 and see if that doesn't work more reliably. Even better buy a copy of FocusMax and get the best focus possible. It has a rough learning curve but it's the most reliable method I've found with my big scopes. With my F8 scope and it's carbon fiber tube I refocus every two hours. I can see, using FocusMax, that it's about 100 microns for a 3 percent change in HFD. So, every two hours is probably overkill but it just takes about 3 minutes so I don't care.



#4 dchamb


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Posted 14 June 2019 - 06:42 PM

I setup FocusMax 4 but don't have it operational yet and the skies have gone cloudy again. Last night I started out with my last focus setting of 6200 and the dimmer stars were donuts. I went all the way to 11000 before the donuts went away, and got down to 8600 to capture the Owl Nebula. After that my FWHM values started going bad and by the end of the evening I was back around 6200.

Do you think the primary mirror fans are playing a role here? Should I start earlier by letting the fans run longer before trying to focus?

#5 Monkeybird747


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Posted 16 June 2019 - 04:43 PM

Have you properly characterized your focuser in the @focus 3 settings, and disabled backlash compensation? I would pick a bright star in the center of frame and create a very tight manual subframe around it and give it a go that way (@focus3). You don't really need the bahtinov mask given the tech that you have in your setup. Like others mentioned, leave it in last position after your session, and then get it as close as you can before running autofocus. I use @focus3 with a small subframe and trial and error to get rough focus, then run focus twice. Don't forget you're dealing with .2667 micron steps. If your scope is not fully acclimated you are going to chase your tail until it is. I haven't used @focus2, but my understanding is central obstruction scopes work better with single-star focus algorithms, when compared to multi-star full frame ones. Maybe give it a shot as Ross suggested.


The Robostar focus routine (single star) in Voyager by Starkeeper is excellent, as is it's Local Field focus (star field).

#6 CharlesW


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Posted 17 June 2019 - 09:56 AM

Characterize your system in @F3. Select your L filter, bin 2x2, 10 second exposures. NB filters will need 14 seconds. 

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