[SQM] Directional Accuracy of SQM-L

Glenn & Gail Muller gmullers at primus.ca
Thu May 14 13:56:53 UTC 2009

 Hi Tony,
 That's an interesting thought. While Anthony would, no doubt be
better able to answer the question about aligning the instrument
during assembly, I think several units need to be tested, together, to
see if there is A) a standard offset, or B) slight differences from
unit to unit.
 As you say, a proper lab test would be more conclusive one way or
the other.
 Glenn Muller
 On Thu 14/05/09 12:57 PM , Tony Flanders
tflanders at skyandtelescope.com sent:
	I’m curious if anybody else has explored the question of whether
the center of the field measured by an SQM-L lies along the long axis
of the instrument. I have owned one SQM-L for some time and recently
purchased a second unit to loan out on a monthly basis (see
[1]). I have concluded that with both units, the optical axis is about
8 degrees “above” the long axis of the body. (Above meaning toward
the button.) 
	If true, this has serious implications for anybody trying to do a
full-sky brightness map. In a light-polluted environment, the sky 38
degrees above the horizon is typically about 1/4 magnitude darker than
at 30 degrees above the horizon. 
	Here’s how I figured this out. Immediately after buying the second
SQM-L, I naturally decided to compare the two units, which I did both
holding both back-to-back with the buttons facing out, and pressing
both buttons simultaneously. I got systematic differences between the
two units, which I initially attributed to instrumental variation. But
when pointing the two units closer to the horizon, I soon realized
that the top unit always read much daker than the bottom unit,
regardless of whether the new or old one was on top. The obvious
explanation is that the optical axes of the instruments are pointing
in different directions, so that the top unit is measuring a darker
part of the sky than the bottom unit. 
	Last night I tested this more systematically by holding the units
against a photo tripod pointing due north and 45 degrees above the
horizon. I turned each unit four ways: button up, button right, button
down, button left. Unfortunately, I had to fudge the button-down
position in order to access the button, so those readings are less
reliable than the others. Nonetheless, I’m pretty confident in the
results.For both units, the button-left and button-right measurements
were essentially identical, the button-up reading was about 0.15
magnitude darker than those, and the button-down position was about
0.15 magnitude brighter. Other readings indicate that the
light-pollution gradient at this point was about 0.02 magnitude per
degree, suggesting that the optical axis is roughly 8 degree
buttonward of the long axis of the body. 
	Obviously, this could be done much more accurately in a lab. 
	    - Tony Flanders  

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