Re: TASS Cameras and rain.

["Shawn Dvorak" <sdvorak@bright.net>]

----------
> From: Chris Albertson <chrisja@jps.net>
> To: tass@wwa.com
> Subject: TASS Cameras and rain.
> Date: Tuesday, May 05, 1998 11:37
> 
> I can never stop thinking of potential problems.  Here's one:
>   
> I had the idea that
> I'd set up the Mk III camera I have on a tripod with no housing or
> permenent setup.  Just put a garbage bag over the whole thing when not
> in use. (get it operational now, improve the setup later) Well the other 
> night it was clear in Redondo Beach I could
> have set up the camera that way.  But, later that night it rained.  I
> was
> woken up by rain hitting the glass skylights.  Had the camera been set
> up it would have gotten soaked.  So I guess I'll finsh the enclosure
> and with luck it will rain proof the camera.
> 
> What about the Mk IV?  What is the plan with reguard to rain?
> 
> First let's concider the easy case whare the Mk IV is operated at
> someones home.  If it is raining or looks like it may them I assume
> you don't turn the system on that night.  But what about the above
> senario?  It looks clear, you turn the system on, then while you
> are sleeping an unexpected storm blows in.  The system gets soaked.
> 
> Now the harder case:  A remotely operated scope.  You need to know
> the weather at the remote location before yu open the roof.
> 
> Now what does water do to a Mk IV?  Maybe nothing.  If water gets
> into the optical tube assembly it will not come out on it's own.
> What about the moters and control electronics?
> 
> We can't simply say "be carful" or "watch the TV Weather Chanel".
> It is the nature of an automated survey instrument that it be
> operated without the direct supervision of it's owner
> -- 
>    --Chris Albertson             home: chrisja@jps.net        
>      Redondo Beach, California   work: chris@topdog.logicon.com
>

The best way would be to automatically detect clouds (and therefore the
chance of rain).  And, attempting to do photometry with any appreciable
clouds present is a hopeless task, so it's better to shut down for the
night anyway.  I know that I personally would not want to get up every hour
or two to peek out the window, and that's certainly not the direction that
TASS wants to go.

So how do you detect clouds?  I've seen a description somewhere of a simple
cloud sensor that uses the fact that during a clear night the sky is much
colder than on a cloudy one.  Thermoelectric sensors are pointed towards
the sky and the ground and the resulting differential signal gives an
indication of the sky's temperature, and therefore its clearness.

Another method would be to watch the signal from the Mark III/IV itself. 
Obviously clouds are going to be much brighter than the nighttime sky, at
least from a suburban area, and this effect would probably be even more
pronounced in the I band.  This only tells you that there is a cloud in the
direction you are looking, but a prolonged spell of elevated background
signals would indicate lots of clouds - time to shut down!

A rain sensor could be added to give additional warning - something like an
array of exposed wires with alternating polarities that would conduct a
current when rain drops completed the circuit between two wires.  Push a
little current to keep it warmer than the dew point and I think it would be
reasonably sensitive to even a light rain.  The camera might still get a
little wet, especially in the event of a sudden downpour, but it would be
better than a total drenching

Ideally this would all happen automatically, with a motorized lid closing
over the camera when it detects adverse conditions.  Maybe something like a
small roll-off shelter with motorized roof or maybe just a flip-open box if
the camera is compact enough.  Worse case would be a buzzer in the
observer's bedroom - probably not too popular with ones' spouse.

Shawn