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Calibrating Sky Quality Sensors

SQM Cloudsensor Unihedron Bol

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#1 nakbrooks

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Posted 26 September 2014 - 10:41 AM

I have both Unihedron SQM/LE and Boltwood CloudSensorII sky quality sensors but so far haven't found an easy way of correlating them against each other or calibrating them, so I don't know how accurate their absolute readings are. Any advice would be welcome.

 

Nigel


Edited by nakbrooks, 26 September 2014 - 10:41 AM.

Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
http://www.facebook.com/stratisobservatory

#2 nakbrooks

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Posted 02 October 2014 - 08:59 AM

Just a follow up to this. I've plotted data from both instruments over a period. It is not practical to correlate their readings directly because obviously they are doing different things: SQM is essentially a photometer and is purely measuring "darkness", whereas CloudSensorII is detecting cloud cover (by comparing sky temperature to ambient temperature).

In principle there should be some correlation (as cloud cover will tend to magnify light pollution because it will reflect street lighting, etc) but there are confounding variables - notably the Moon will affect SQM but will not affect CloudSensorII.

Really they are complementary tools. CloudSensorII is useful in determining whether it is worth attempting to image and its rain detector will enable the roof to be closed (I'm assuming remote automated operation here) whereas SQM places some metric on sky quality which is useful in assessing images taken.

I'm going to make an attempt at calibrating SQM in October, weather permitting, as the Moon will be both New and below the horizon at my site in the middle of the month (and if I wait until after 1.00am the church and street lights in the village are off). It will be interesting to see whether SQMs reported naked eye limiting magnitude actually corresponds to what I can see for myself in the sky.
Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
http://www.facebook.com/stratisobservatory

#3 afesan

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Posted 17 October 2014 - 07:00 PM

You will can value also how the pass of Milky Way over zenith also affects SQM readings :-)... Mine is working here from past couple of years ( LE version) ...and works OK. As you stated.. Cloudwatcher and SQM instruments have different purposes... But SQM helps me to establish if my images can be of quality and to determine point when I usually begin to image : I never image until SQM readings are over 21.1 .. Evidentey this is a local measure imposed to myself...

You will enjoy it and will find your own utilities..

Antonio


Edited by afesan, 17 October 2014 - 07:01 PM.

Best regards:
Antonio
Spag-Monfrague Remote Observatory
http://afesan.es




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