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Focus drift or?

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#1 Maxtrixbass

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Posted 15 July 2020 - 01:04 PM

I have noticed recently that I get about an hour of good focus, a second of usable focus, and beyond that enough drift that I start to see donuts in dimmer small stars.

 

My setup: 205mm/1220mmfl newtonian F/6, Losmandy autoguided mount, QHY 163m mono camera, Baader MPCC coma corrector, 30s subs

My previous setup where I didn't notice this was the above except modded dslr, 2-3 minute subs

The seeing has also been better lately (usually terrible 2-3 a/s, now 1.4 a/s)

 

The scope goes out at 7PM, once the sun is behind the house,  imaging starts at 10:30. Temps are 70s high, 50s low. Humidity low. I checked the focuser and it hadn't sagged.

 

Is it:

 

1. Your shorter subs, highre rez camera, and better seeing has exposed something that was always there. Congratulations

 

2. One to two hours??!?  I have to refocus every (fill in short time frame here).

 

3. Its a quirk of (netwonians, telescopes, imaging trains, pretty much all astrophotography)

 

4. Something else

 

If focus normally drifts over the night, does the focuser end up going slight inward or outward?

 

If autofusing is the answer, is there an option that won't break the bank and or isn't too heavy (I'm pushing the mount as is)? I use SGP if there is something that interfaces with that...I'm open to DIY and am pretty handy.


Edited by Maxtrixbass, 15 July 2020 - 01:10 PM.


#2 kisstek

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Posted 15 July 2020 - 01:35 PM

I'd guess #4: temperature change causing the focus shift. I hadn't notice that in my setup until recently when my focusing, collimation, ... all got better. Then I started noticing donuts about 2 hours in. Short of some kind of autofocuser, I don't have a solution for you.


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#3 Ryou

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Posted 15 July 2020 - 01:42 PM

Probably mostly a combo of all of your options tbh.

 

1. You've probably had similar issues all along if you don't go re-focus during the night.

2. 1-2 hour is a pretty good amount of focus time depending on your conditions, I believe that some people set auto-focus routines to run every hour or so.

3. The donut out of focus is definitely quirk of newts/sct

4. There is ALWAYS something else

 

 

To be more specific what you're probably dealing with is the TEMPERATURE causing the focus to change. Chances are your focuser itself is staying in one position (though obviously do check this), however the actual optics (glass/mirrors/etc) and tube assembly is slightly shrinking/expanding over time as the outside temp drops. This is just the laws of physics, and even though you are probably only talking about a couple MM (if that) of difference... well this is a place where 1mm can make a huge focus difference.

 

You basically will need to either go out every hour or two and re-focus manually or get an auto-focuser of some kind.

 

Although I have no first hand experience yet with it, I decided to go with Rigelsys for their autofocus based on recommendations from here. They seem to be good quality, modular, responsive to inquiries, probably average weight, and also not super expensive. Price is on par with the ZWO offering however hopefully higher quality.


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#4 bignerdguy

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Posted 15 July 2020 - 01:49 PM

I saw the same thing with major changes in temperature, especially when it got REALLY cold (i'm from Texas so Really cold here is a relative term wink.gif ).  I had a nights imaging session where the temps dropped down into the 20's (Fahrenheit) and when it did the images were  WELL out of focus and would not refocus.  I guess the extreme cold warped one of the lenses or mirror too much or accurate focus.  In your case the change may not be so extreme but even a small shift in temps can cause big changes in the image focus. 

 

If you try to refocus, does this fix the issue?  if so it is definitely temp related. 

 

Otherwise might also be fog building up if the temps are cool enough at night.  Do you let your equipment cool down before you use them?  It can take at least 30 minutes to an hour for equipment to equalize depending on what you are using.  If you start imaging right after you setup and haven't given it time to equalize this could also be a factor.  This may not be an issue in your case, just covering all the bases so to speak...


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#5 Maxtrixbass

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Posted 16 July 2020 - 12:06 AM

Thanks for the responses. Good info and the Rigelsys looks promising.

 

Just realized tonight as well: As I was setting up my routine is to collimnate, focus, then wait for it to get dark enough to start the camera. Will probably help to move the "wait" part before the focus. I could be loosing an hour in the wait time.




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