[SQM] Directional Accuracy of SQM-L

Tony Flanders tflanders at skyandtelescope.com
Thu May 14 12:57:14 UTC 2009

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 http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3075428/page/0/view/collapsed/sb/5/o/all/fpart/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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