Clear Sky Detector Sees Clouds

[Tom Droege <tdroege2@earthlink.net>]

As threatened, I have built a Clear Sky Detector and attached it to the 
Mark IV.  This detector consists of a TEC attached to a plate which is "C" 
clamped to the frame of the telescope.  The TEC sits at the bottom of a 
foam/foil sandwich well.   The attempt is to make a directional radiation 
device.  The TEC radiates to free space in a cone of about 10 degrees. It 
points where the Mark IV points.  The signal from the detector is put into 
one of the Mark IV data acquisition channels.

Last night was pretty clear here in Batavia.  The Clear Sky Detector 
measured between +0.8 and +1.4 volts all night long.  There was a 
consistent change depending on where in altitude the telescope was 
pointing.  This does not yet make much sense as I expected the sky to look 
clearer the higher in elevation the telescope was pointing.  It did not 
follow the expected pattern.   I don't know the real sky conditions, so it 
could be correct.

The software puts the measurement in the .fits header under the key word 
CLARITY.  I will be able to work this into the pipeline to try to match 
what the Clear Sky detector sees vs the quality of the data.

This morning after the sun came up it was still clear.  The Clear Sky 
Detector read +0.8 volts.  Now some clouds have moved in.  There are some 
clear spots in the clouds.  The Clear Sky Detector is now reading between 
-0.3 and -0.8 depending on the cloud cover at the moment.

It would appear that the detector is sufficiently sensitive for the 
intended purpose.  Both drift and noise seem quite low.  My plan is to use 
this as one more measure of data quality.  The hope is that this will be a 
good tool to pick out the exceptionally good data for calibration of the 
all sky photometry.

The Clear Sky detector does not care if the sun is up or not.  It is 
measuring the radiation loss to space, and unless the sun comes into view 
it will not change much.  I emphasize that this idea is not new with 
me.  My guess is that we could find modern scientific references back to 
1850 or so of such measurements, if the Greeks didn't mention it BC.

The design is reasonably directional.  I estimate a beam width of 10 
degrees compared to the Mark IV detection beam of 4 degrees.  I may change 
the design to provide a narrower beam to try to match the Mark IV.  I have 
some great stuff to try.  It is a bubble wrap foil sandwich.

For those with enough electronic experience that want to duplicate this, I 
am using a relatively low noise (cheap about $1 at Digi-Key) OP Amp, the 
Burr Brown OPA134.  It is in the normal gain configuration.  The input 
resistor is 100 ohms and the feedback resistor is 100,000 ohms.  Thus Gain 
= 1000 if the TEC is low enough in impedance.  I suspect it is a few ohms 
or less, but I did not measure it.  The TEC I am using has 127 
junctions.  The TEC is mounted to the frame of the telescope with the side 
where the two leads are mounted glued to the telescope frame.  The other 
side faces space.  Just bare, nothing is attached to the space facing 
side.  The Red lead is connected to common, and the Black lead is connected 
to the summing junction through the 100 ohm resistor.  This should get you 
the same polarity that I see.  Clear sky gets a more positive signal.  I 
have bypassed the OP Amp +/-15 volt power supplies to ground through 
tantalum capacitors to try to keep the whole mess from oscillating.  There 
is a 0.1 microfarad capacitor across the 100,000 ohm feedback 
resistor.  OK, this should be enough for most of you that are familiar with 
basic electronics to duplicate the circuit.  Later I might try to describe 
it in enough detail that one unskilled can duplicate it.  I would not have 
picked the OPA134.  I just happen to have it in my bin.  I would pick an OP 
Amp with lower guaranteed offset.  A few microvolt unit.  The 134 lists 500 
microvolt, though the one I happen to have picked is more like 200 
microvolts.

Tom Droege