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Mirror cooling?

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#1 Speedy1985

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Posted 24 June 2021 - 10:43 AM

Last night I went out for a quick session as the conditions were supposed to be better than they have been recently. I typically will get everything set up in the yard long before dusk and then collimate. That was very good, only requiring a small tweak of the primary. The fan on the primary had run for at least 75 minutes. Early on, the views were great as I was able to go right up to 300x on M13 and could also split the double double in Lyra. As the night progressed, I noticed that I was having a more difficult time getting pinpoint focus. The temp when I started was about 62-64 but over the 90 minutes or so that I observed, it dropped into the lower 50’s. Is it possible that the views deteriorated due to the mirror temp not keeping up with the quick temp change? I never noticed any dew on the scope, so I’m pretty sure it wasn’t that. 



#2 wrvond

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Posted 24 June 2021 - 10:49 AM

Yes.

Ten or more degrees in an hour to an hour and a half is a bit much for a mirror this size to keep up with. Did you keep your fan on? Additionally, a single computer case type fan is really a bit small to be truly effective.


Edited by wrvond, 24 June 2021 - 10:51 AM.


#3 Speedy1985

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Posted 24 June 2021 - 10:57 AM

Yes.

Ten or more degrees in an hour to an hour and a half is a bit much for a mirror this size to keep up with. Did you keep your fan on? Additionally, a single computer case type fan is really a bit small to be truly effective.

I always keep the fan running when I observe. And the scope is stored in my non climate controlled garage, so the initial temp is very close to ambient right away.


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#4 havasman

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Posted 24 June 2021 - 11:09 AM

I always keep the fan running when I observe. And the scope is stored in my non climate controlled garage, so the initial temp is very close to ambient right away.

That's very good procedure there.

 

Some causes of change over a session are external, and common. Upper atmospheric conditions can have big effects on higher magnification views during a session and there's little remedy available when they degrade as a night goes on. Watch what happens over a number of sessions and see if there's a trend. For example, if you are able to get and maintain good 300x views the next time you go out there's little evidence your optics have a problem and more indicators that the air moving overhead may have contributed to your problems last session.

 

I monitor this site before a session to watch for forecast changes to upper flows so that I'm less surprised and worried if the seeing goes  -  https://www.aviation...er.gov/windtemp


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#5 Speedy1985

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Posted 24 June 2021 - 11:14 AM

That's very good procedure there.

 

Some causes of change over a session are external, and common. Upper atmospheric conditions can have big effects on higher magnification views during a session and there's little remedy available when they degrade as a night goes on. Watch what happens over a number of sessions and see if there's a trend. For example, if you are able to get and maintain good 300x views the next time you go out there's little evidence your optics have a problem and more indicators that the air moving overhead may have contributed to your problems last session.

 

I monitor this site before a session to watch for forecast changes to upper flows so that I'm less surprised and worried if the seeing goes  -  https://www.aviation...er.gov/windtemp

Thank you! I’ll have to pay close attention in future sessions. I haven’t noticed the views deteriorate in prior ones, but I don’t recall the temperature changing as drastically in that amount of time either. 


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#6 Jon Isaacs

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Posted 24 June 2021 - 12:15 PM

Thank you! I’ll have to pay close attention in future sessions. I haven’t noticed the views deteriorate in prior ones, but I don’t recall the temperature changing as drastically in that amount of time either. 

 

Every night is different.  Atmospheric stability is associated with weather patterns so a transparent night where the temperature drops quickly after sunset may also be associated with a less steady atmosphere.  In many parts of the country, hazy, lazy nights are associated with steady seeing.

 

Jon


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#7 spereira

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Posted 24 June 2021 - 12:16 PM

Moving to Reflectors.

 

smp



#8 Speedy1985

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Posted 24 June 2021 - 12:29 PM

Every night is different.  Atmospheric stability is associated with weather patterns so a transparent night where the temperature drops quickly after sunset may also be associated with a less steady atmosphere.  In many parts of the country, hazy, lazy nights are associated with steady seeing.

 

Jon

Jon, I figured it would be better than most of my other sessions where Astrospheric listed the transparency and seeing as no better than average/average. Last night was listed as above average/average. To me, it seemed pretty accurate because some of the brighter stars I use to align were visible much sooner than they had been previously. 



#9 Tom Stock

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Posted 24 June 2021 - 02:30 PM

I always keep the fan running when I observe. And the scope is stored in my non climate controlled garage, so the initial temp is very close to ambient right away.

You would think this but garages are insulated and will still usually be a little warmer or cooler than outside temp.  


Edited by Tom Stock, 24 June 2021 - 02:31 PM.

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#10 Speedy1985

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Posted 24 June 2021 - 02:33 PM

You would think this but garages are insulated and will still usually be a little warmer or cooler than outside temp.  

This is true, but being outside for more than an hour with the fan running should overcome any small temp difference there I'd think. 



#11 Bean614

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Posted 24 June 2021 - 08:32 PM

I'm  in Western Massachusetts,  and though not right "next door" to your NJ location, you're just a hundred or so miles from me, as the crow flies.  A cold front came through my location yesterday mid morning, and continued through your location early yesterday evening.  Here, at Sunset, it was 70F, and a hour later still 67F.  In spite of the nearly full Moon, seeing was steady, transparency excellent.   Then, it all went to heck in a handbasket!  That cold air mass finally sank like a rock, and stars started twinkling and jumping around.  Seeing (Pickering Scale) went from 8/9 to a 3 in a flash.  And I was only using an 8-inch f/6 Dob, with 2 fans on, set up on dirt.  By 5am this morning, it was 42F here.  Pretty tough for a mirror to keep up with that, even an 8-inch with fans.

   You'll  start to notice these things as your experience piles up.  You'll even start paying attention to weather reports, especially those with maps showing  Fronts and Interactive Radar.  You'll have some "Ah-Ha!"  moments, even a few epiphanies,  and then you'll relax and be patient,  knowing it's not your scope, and be happy to take what Mother Nature gives you and make the best of it.


Edited by Bean614, 24 June 2021 - 09:26 PM.

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#12 havasman

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Posted 24 June 2021 - 08:47 PM

goodjob.gif



#13 bokemon

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Posted 25 June 2021 - 03:59 PM

I have had seeing "go bad" through no fault of the mirror.

I imagine that you can tell whether it is due to the mirror or atmosphere by looking at a highly out of focus star image.  


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#14 Asbytec

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Posted 25 June 2021 - 05:16 PM

 

I imagine that you can tell whether it is due to the mirror or atmosphere by looking at a highly out of focus star image.  

Yes. Generally if you defocus on a star, atmospheric seeing is usually very fast moving disturbance. Atmospheric seeing looks a lot like a fast running stream of air blowing across the defocused star. If you focus outward, you can actually "focus" on the upper atmosphere and the seeing becomes apparent. Thermal activity in the tube, including the primary mirror, are more slow moving distortions. Sometimes you can see bright "caustic" lines (like those seen at the bottom of a pool on a sunny day) snaking across the defocused star image, dark areas that appear to slowly move about randomly, and even some spikes protruding from the edge of the circular defocused pattern. If the mirror is warm, you will likely see slow moving thermal activity caused by the boundary layer near the surface of the warm primary. You can blow that off with a fan as the mirror cools. Also the defocused image should also show some over correction as the center of the primary remains warm and expanded while the edge tends to cool a little faster and contract a little. 



#15 Keith Rivich

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Posted 25 June 2021 - 06:37 PM

My guess is there was a big drop in temperature higher up in the atmosphere leading to your bad seeing.

 

Not your mirror temperature.

 

 As Norme states defects due to a warm mirror (tube currents) are easy to identify using an out of focus star. 


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#16 Speedy1985

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Posted 25 June 2021 - 07:03 PM

Thank you everyone, definitely some points to consider should it happen again. The upper atmosphere temp change is likely the culprit considering the temp drop at ground level. I also baffled the rear of the OTA now to help direct more air flow on and around the mirror, just to try and help alleviate any potential issues there. 



#17 tloebl

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Posted 27 June 2021 - 07:47 PM

This is a handy site for a quick look. I usually don't even bother to set up if the Jet Stream is overhead which  happens more often than I would like in eastern Massachusetts.

 

https://www.wundergr...wind/jet-stream




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