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Old 13-01-2014, 11:02 PM
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Focus shift with temperature change

Hi All,

What does everyone do about focus shift at this time of year. Today was low-mid 30's here in Melbourne. It's nice and clear tonight, big moon, but OK for some narrowband imaging. The ASA N12 take quite a while to cool down, I have just achieved stable focus at about 2330, which leaves little time for imaging before the sun rises again tomorrow morning.

Other than putting a cooler in my observatory what can I do? I know that Bert has some fancy temperature control stuff on his fast scope, but I'm hoping for something simpler.

I have been focusing between subs, but the focus shifts during 15 or 30 minute exposures enough that I get sub-optimal star shapes.

Cheers
Stuart
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Old 13-01-2014, 11:49 PM
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Hi All,

What does everyone do about focus shift at this time of year. Today was low-mid 30's here in Melbourne. It's nice and clear tonight, big moon, but OK for some narrowband imaging. The ASA N12 take quite a while to cool down, I have just achieved stable focus at about 2330, which leaves little time for imaging before the sun rises again tomorrow morning.

Other than putting a cooler in my observatory what can I do? I know that Bert has some fancy temperature control stuff on his fast scope, but I'm hoping for something simpler.

I have been focusing between subs, but the focus shifts during 15 or 30 minute exposures enough that I get sub-optimal star shapes.

Cheers
Stuart
In summer I have been opening the observatory dome about 30 -60min before the sun sets which gives me 2-3hrs of cooling before imaging starts and I run the AG12's three mirror cooling fans (which suck the air out of the tube rather than blow on the back of the mirror) from dome open to camera shut down, all night.

Mike
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Old 14-01-2014, 07:07 AM
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Hi Mike,

I run the fans, but the setting sun in the west would hit the scope until just before sundown (when it drops below my neighbours roof line). I don't like having the scope out in full sun.

Cheers
Stu
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Old 14-01-2014, 09:03 AM
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multiweb (Marc)
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I have been using the SharpSky pro kit a little now and logged in the temperature with my C11/hyperstar over 3 nights. Focus is pretty critical. I found that I needed to go out 20 steps per degree. The body is CF. I refocused manually for the training period. Focus shift is maximum at the beginning of the night and early morning. But it's usually safe during the night as temp doesn't really shift that much.

Those 3 nights temp started at 24c and dropped to 20c over a period of 3h, then stayed there for most of the night.
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Old 14-01-2014, 09:24 AM
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strongmanmike (Michael)
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Hi Mike,

I run the fans, but the setting sun in the west would hit the scope until just before sundown (when it drops below my neighbours roof line). I don't like having the scope out in full sun.

Cheers
Stu
Yes, I guess I have the dome which keeps the direct sun off the tube...mind you a setting sun is much less intense than a higher one so perhaps if you can just shade the OTA during that time...?
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Old 20-01-2014, 10:19 AM
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I have CCDAP do a focus run every 30 minutes, that means at the end of each sub generally, I do a preliminary cool down of the scopes once civil ends and that gives me about an hour of cooling before the first focus run. It does not need to be focus after about 3 hours but the software only allows for focusing on a regular basis or at the change of a filter. Keeping the scope cooler might help you with focus shift, but in the end it has got to stop moving enough to get some subs in. I thought you had a carbon tube on the 12"Stuart? I don't understand why you are getting shifts like that?
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Old 20-01-2014, 07:48 PM
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Hi Paul,

Unfortunately, until someone writes an X2 driver for the ASA focuser I'm stuck doing manual focus. I'm not sure that I trust @focus anyway.

The tube of the ASA is indeed CF, but I think that the focus shift is from the mirror cooling down, or the Wynn corrector, or the large lumps of aluminium in the focuser. The CFZ for the scope is about 30 microns, I normally focus in 10 micron increments, after initial cool down (i.e. sunset) to stable focus is normally an hour or so and the focus will shift by between 30 and 50 microns. On hot days and cool nights I don't reach stable focus for about 2 hours.

I think I've had a bit of mis-collimation as well, which has had be struggling to get (and keep) round stars. Hopefully that's now fixed.

Cheers
Stuart
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Old 21-01-2014, 11:00 AM
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It might be worth considering getting an Atlas focusor for your system and then you can use Focusmax to focus the scope and give you really reliable results. In my experience Focusmax never fails to deliver the sharpest frames. I found that even trying manually to focus was good; it is not as good as HFD focusing. If it were me I would really consider the Atlas.
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Old 21-01-2014, 01:48 PM
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sure is a problem Stuart - a real pain. In the region of f4, there is next to no tolerance. CF helps, but as you point out, the Al stuff does it's own thing anyway and you still need to refocus at the slightest temp change.

FWIW, I need to refocus at about every 0.2 degrees with my current steel tube f4 - I have just bought a CF scope, but back of the envelope calcs show that I will still need to refocus about every 0.5 degrees.

Do ASA provide any temp sensing for their focuser? - would have thought that it should be relatively straightforward to get a high end focuser like that to track with changing temps, which is what you will need if the focus is drifting within subs. If they don't, there are a few stepper focuser drive systems out there that incorporate temp sensors - doubt there is anything magic about ASA stepper motors, so it should be fairly straightforward to use an alternative interface box and get one that can operate with your chosen software.

The other thing that is useful is a low read noise camera, which allows short subs - only a consideration if you are in the market for a camera though.

Could an aircon in the obs be an approach to get your system ready for imaging before midnight?

Last edited by Shiraz; 21-01-2014 at 02:30 PM.
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Old 21-01-2014, 04:35 PM
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Originally Posted by Paul Haese View Post
It might be worth considering getting an Atlas focusor for your system and then you can use Focusmax to focus the scope and give you really reliable results. In my experience Focusmax never fails to deliver the sharpest frames. I found that even trying manually to focus was good; it is not as good as HFD focusing. If it were me I would really consider the Atlas.
Hi Paul,
The ASA focuser should run in focusmax, but again, Windows program. I was really hoping to ditch Windows altogether, but the ASA has to run it, so I might end up transitioning back to run TSX under Windows. I'm a bit of a Mac zealot, so this hurts, sometimes I'd rather just go out and focus myself...

The ASA focuser software is a bit flaky, but it should work with Focusmax, they say it will, if anyone out there knows about it I'd be grateful to hear about their experience.

Even then this won't help if the focus shifts in the 30 mins I'm taking an exposure.

Cheers
Stuart
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Old 21-01-2014, 04:39 PM
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Originally Posted by Shiraz View Post
sure is a problem Stuart - a real pain. In the region of f4, there is next to no tolerance. CF helps, but as you point out, the Al stuff does it's own thing anyway and you still need to refocus at the slightest temp change.

FWIW, I need to refocus at about every 0.2 degrees with my current steel tube f4 - I have just bought a CF scope, but back of the envelope calcs show that I will still need to refocus about every 0.5 degrees.

Do ASA provide any temp sensing for their focuser? - would have thought that it should be relatively straightforward to get a high end focuser like that to track with changing temps, which is what you will need if the focus is drifting within subs. If they don't, there are a few stepper focuser drive systems out there that incorporate temp sensors - doubt there is anything magic about ASA stepper motors, so it should be fairly straightforward to use an alternative interface box and get one that can operate with your chosen software.

The other thing that is useful is a low read noise camera, which allows short subs - only a consideration if you are in the market for a camera though.

Could an aircon in the obs be an approach to get your system ready for imaging before midnight?
Hi Ray,

Thanks for the input.

I do believe that the ASA has temperature compensation built in, looks like I'll have to do a calibration run. It's just that with so few good nights to image on, sometimes it feels like a waste to spend one calibrating the focuser. I suppose the next hot day/clear night combo might be ideal, as I won't get any decent subs before 2300 anyway.

I like long subs for NB imaging, the longer the better. Limited to 30 mins as that's all my dark library is good for. Too much can go wrong with longer than 30 min subs, unless you're driving the HST.

Cheers
Stuart
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Old 21-01-2014, 05:56 PM
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I use Sequence Generator Pro to refocus my Moonlight focuser every degree drop of temperature and on each filter change.
Works perfectly and I never have to worry about focus shift aha in. Using the sharpsky Pro and mount hub devices on different scopes.
I don't bother with temp compensation as such as I find it's more advantageous to refocus on one degree C temp drop.
Allan
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Old 21-01-2014, 06:00 PM
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sure is a problem Stuart - a real pain.

FWIW, I need to refocus at about every 0.2 degrees with my current steel tube f4 - I have just bought a CF scope, but back of the envelope calcs show that I will still need to refocus about every 0.5 degrees.
Could an aircon in the obs be an approach to get your system ready for imaging before midnight?
Sequence generator will allow re-focusing at a 0.2 or 0.5 degree temp drop and will also allow use of TemperHum.
Allan
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