Re: [ATM] Dark Sky Meter

["Jim Miller" <jim@jtmiller.com>]

perhaps i'll start by listing my goals and current thinking on an approach.

goals
- should be replicable by others; all circuits and software to be published
- should give an absolute value to allow reporting of sky brightness
- should be under $100 (preferrably much less) to build
- should be capable of reporting down to 21mag/sqarcsec
- should be battery operated
- should not require a computer to generate results

approach
- silicon photodiode with lens to give 4°FOV (matches telrad) and suitable 
collection area
- charge integrating preamp and comparator under microprocessor (PIC) 
control
- beeper (morse code?) readout with optional phone jack to connect to 
computer
- aim device and press start button to capture data, device beeps result 
when done

so far i've been looking at the perkin-elmer vtb1013b which has a visual 
response curve and using it with a fet input integrator. this combination 
appears to be capable of sub-picoamp (actually ~20fA) dark currents if the 
photodiode is operated at zero bias. comparator and pic are pretty easy by 
comparison. initial leakage can be minimized by good construction and 
guardring layouts but leakage resulting from longterm contamination will 
need to be addressed.

the diode response is approximately 1uA/100fc. my first challenge is 
determining whether this combination is actually sensitive enough.

calibration target would be vega at zenith on a "clear" night.

may require computer to establish calibration. pic has internal nonvolatile 
memory to store calibration once established.

my reason for suggesting a yahoo group was that this might be too 
electronically oriented for some here. but i'll continue until i hear 
protests.

all comments and suggestions welcome!

jtm 

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