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No, the SQM-LE is not weatherproof. For permanent mounting outside, it should be mounted in a weatherproof housing.
We sell such a housing here.
For people using the unit only during telescope observations, the meter can be stowed away with the telescope.Here are some sources for plastic domes:
Yes, the heat generated inside the SQM-LE by the internal web server is high enough get rid of dew. When used with this housing, no dew was ever seen inside the unit. In fact, drops of rain on the glass cover evaporated after a few hours.
It may be important to allow moisture to escape, that is why we have an air hole on the bottom of our housing.
As far as frost goes, the unit is too warm to allow that. If the unit is un-powered then moisture will likely build up. It is probably best to always keep the unit powered.
A PoE system can be used to transmit "Power Over the Ethernet" cable to the SQM-LE. A PoE system consists of two units:
The PoE Injector would be located near the router, and the PoE splitter is located near the SQM-LE.
The PoE soution is very handy if you do not have an electrical outlet near the SQM-LE. You can just run the Ethernet cable to the PoE Splitter then short wires come from the splitter to the SQM-LE.There are two models that we have tested successfully:
We are in the process of developing a webpage tool for correcting the SQM reading for the Milky Way as seen from a given longitude, latitude, SQM reading, and date/time. We are basing this on Schlosser & Hovest (A&AS, 128, 417, 1998) "Collection of Major Surface Photometries of the Milky Way". We need to integrate over MW surface brightness (involving two filters), the responsivity of the SQM with angle, and extinction with zenith angle for each map cell. This is straightforward but time-consuming to verify and so it is not yet available.See Surface Photometries of the Milky Way (Schlosser+ 1997) for more information
The SQM's readings are assuming 'best transparency'.
You can get an updated definition of the transparency in your area from Attila Danko's Clear Sky Clock. Also, frequently local weather stations can provide "visibility" and "relative humidity" numbers which could potentially be used as surrogates for actual transparency measurements (which aren't possible with a handheld meter).
Magnitudes are a measurement of an objects brightness, for example a star that is 6th magnitude is brighter than a star that is 11th magnitude.
The term arcsecond comes from an arc being divided up into seconds. There are 360 degrees in an circle, and each degree is divided into 60 minutes, and each minute is divided into 60 seconds. A square arc second has an angular area of one second by one second.
The term magnitudes per square arc second means that the brightness in magnitudes is spread out over an square arcsecond of the sky. If the SQM provides a reading of 20.00, that would be like saying that a light of a 20th magnitude star brightness was spread over one square arcsecond of the sky.
Quite often astronomers will refer to a sky being a "6th magnitude sky", in that case you can see 6th magnitude stars and nothing dimmer like 11th magnitude stars. The term "6th magnitude skies" is very subjective to a persons ability to see in the night, for example I might say "6th magnitude skies" but a young child with better night vision might say "7th magnitude skies". You can use this nifty calculator created by SQM user K. Fisher to do that conversion, or this chart.
The "magnitudes per square arcsecond" numbers are commonly used in astronomy to measure sky brightness, below is a link to such a comparison. See the third table in section 10 for a good chart showing how these numbers in magnitudes per square arcsecond relate to natural situations:www.stjarnhimlen.se/comp/radfaq.html
Each magnitude lower (numerically) means just over 2.5 times as much more light is coming from a given patch of sky. A change of 5 mags/sq arcsec means the sky is 100x brighter.
Also, a reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80.
The value produced by the sensor in the SQM is affected by temperature. There is a temperature sensor in the SQM that compensates for this effect. However, when the SQM is first powered up, the light sensor is colder than when the power has been on for a few seconds. Depending on the ambient temperature this will result in the first reading being slightly higher than subsequent readings.
For the most accurate results, it is best to take many readings and disregard the very first reading.
This issue is due to the transient response of the TSL237 which briefly changes its light-to-frequency characteristic when it is warmed by applied power. Ironically, the normally very sensible practice of leaving it out at the environmental temperature probably makes it more significant.