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Nasty Tube Currents

Started by fugardi, 07/23/2006 06:43AM
Posted 07/23/2006 06:43AM Opening Post
Last Weds, I attempted to finely tune my collimation using my 50um artifical star 100feet away, again (for the 5th time). Even after 2 hours of scope cooldown with the observatory door open & dew shield on, I am still suffering from nasty tube currents (see attached). Looking at my last Jupiter image taken before moving to my dome showed much better detail than I am achieving now. Previously, I could see the red spot visually no problem (of course in 2004 it was higher in the sky). Since my local seeing improves with a dew shield and having the observatory door open, I have to conclude that my 3+ tons of concrete foundation/base are retaining heat and slowly radiating it up to the scope affecting the tube currents. Next time out, I will use a floor fan to pull the air across the scope and floor out the door. Using this setup 2 hours earlier and maybe leaving the fan on should help. All this recent talk of air conditioners is making me think that besides observing comfort, the AC may be reducing tube currents as well. There's a lot to be said about not having sweat drip onto your 8.8mm eyepiece when observing Jupiter...

I think the roll off roofs have a big advantage here with tube currents since the entire area is open to the ambient. Does anyone else suffer from these currents and what do you do? I may have over-engineered the concrete and killed my own seeing...

Regards,
Steve

Attached Image:

fugardi's attachment for post 32590
Posted 07/23/2006 07:47AM | Edited 07/23/2006 08:01AM #1
Hi Steve

Solve temp issues in the structure and you will solve temp issues in the instrumentation.

Passive ventilation just by openning the slit both sucks and blows...in a sense.

Being in the greenhouse business, I can cool greenhouses of warm air by passive ventilation and openning roof vents..analagous to you dome slit. Only difference is that I have vents the entire length of the structure; you're stuck with a tiny narrow openning compared to the area of the dome surface. Big picture this is an inefficient method of cooling so it needs some mechanical help. I utilize fans to forcefully exhaust warm air and introduce cooler ambient air which cools the interior air but also the infrastructure and instrumentation.

Warm air rises...I suggest blow a fan "up" out of the slit to forcefully exhaust warm, trapped air. In the doorway, place a fan to blow in ambient cooler air to more rapidly replace the warm air and cool the infrastructure and instrumentation. I'm using "Essential Home" 20 inch fans purchased from K-mart, they can be assembled for use on the floor, on a stand or hanging; first to aid in air circulation in my greenhouses at work, and now for air movement in my observatory.

I'm coming to the conclusion (as has been suggested by others) that the key, like in a greenhouse, is to avoid the buildup of heat inside the observatory in the first place...and the issue of temperature management is multifacetted. White reflective exteriors, insulating factors both interior and exterior (to trap cooler existing air "in" and keep radiated heat "out") and cooling devices (we use evaporative coolers in greenhouses) such as AC units with internal or external thermostats to take the edge off the extremes to maintain as much as an ambient temp as possible.


8" f/12 D&G classic refractor
Astro-Physics 1200 DA equatorial mount
BlueStar Observatory (roll off) by Backyard Observatories of Lodi, Ohio
Previous telescopes A-Z...Astro-Physics to Zeiss
Posted 07/23/2006 11:10AM #2
What kind of scope? SCT?

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