nakbrooks Posted March 20, 2015 Report Share Posted March 20, 2015 Best seeing so far this year last night at Stratis Observatory, at least according to my SQM/LE sensor. Unfortunately I wasn't there to confirm and I still need to check calibration. At least I've got the live update to the web site working (uses PHP, JavaScript, JQuery and Google Charts to display sensor info). Link to comment Share on other sites More sharing options...
pesa Posted April 3, 2015 Report Share Posted April 3, 2015 Nigel, very dark indeed! can you give a rough estimate of seeing in terms of possible resolution might have bin? perhaps 1.5" ? What is a normal value, 2" (fwhm) ? At my obs I might get 2 twice a year! 3 is more usual. All numbers are rough esitmates but not far off I think. Woluld be interestng to hear what other folks get....Any takers? pelle Link to comment Share on other sites More sharing options...
nakbrooks Posted April 3, 2015 Author Report Share Posted April 3, 2015 That's an interesting question. I can measure limiting magnitude and cloud cover objectively using the SQM and CSII sensors, but seeing is more difficult to measure objectively, particularly remotely. I've not tried it, but probably monitoring double stars of known separation would be a reasonable way of getting repeatable results. Nigel Link to comment Share on other sites More sharing options...
GeorgeCarey Posted April 4, 2015 Report Share Posted April 4, 2015 How does that 6.62 equate to SQM? My best ever SQM was 20.11 Link to comment Share on other sites More sharing options...
nakbrooks Posted April 4, 2015 Author Report Share Posted April 4, 2015 (edited) An MPSAS of 20.11 equates to a NELM of 5.6 (at least according to the algorithm used by Knightware in SQM Reader Pro 2). A NELM of 6.62 equates to an MPSAS of about 22.05. MPSAS is the native metric generated by the Unihedron SQM sensor, and is a luminosity measure (in magnitudes per square arc sec). NELM (naked eye limiting magnitude) is computed from MPSAS. I prefer to use NELM rather than MPSAS as I find it more meaningful (although I do record and graph both). I have attempted to independently check the accuracy of the reported NELM data by eyeballing the sky near the zenith and seeing what are the lowest mag stars I can see, but I haven't had time to do this thoroughly, and in any event my "naked eye" is probably not typical (I am 63 and wear glasses). Also of course, the SQM sensor can only measure darkness, it can't assess other aspects of seeing (such as transparency). Edited April 4, 2015 by nakbrooks Link to comment Share on other sites More sharing options...
nakbrooks Posted April 4, 2015 Author Report Share Posted April 4, 2015 Attached are the formulae in the Unihedron SQM user manual for converting between MPSAS and NELM. Not sure if it is these that are used in SQM Reader Pro 2 but I assume they probably are. Link to comment Share on other sites More sharing options...
GeorgeCarey Posted April 4, 2015 Report Share Posted April 4, 2015 Thanks for those equations. I worked through and got 20.11 is NELM 5.57, as you said. An SQM of 22.05 I can only dream of being 12 miles from Birmingham, UK. George Link to comment Share on other sites More sharing options...
nakbrooks Posted April 4, 2015 Author Report Share Posted April 4, 2015 Sounds about right George. My system records data every 1 minute but I take the mean over 5 minute periods for upload to the web site to reduce data volumes so this does give some rounding. Link to comment Share on other sites More sharing options...
MarkS Posted April 5, 2015 Report Share Posted April 5, 2015 I read somewhere in Unihedron's literature that SQM of 22 is the maximum possible, but I don't know why this should be so. Can anyone shed light on this? Best reading I have obtained (at Mt Macedon 80km from Melbourne) is 21.55 - whereas at my home in suburban Melbourne typical 'dark' sky is 18.6. Mark Link to comment Share on other sites More sharing options...
nakbrooks Posted April 5, 2015 Author Report Share Posted April 5, 2015 You're right that Unihedron do say that an MPSAS reading over 22.0 is "unlikely". I need to try and find a way of calibrating my meter as, although it does get very dark here, the maximum readings I am getting are s bit implausible. Not sure how to calibrate though (other than buying several other astro photometers and taking a mean - which would be expensive). Link to comment Share on other sites More sharing options...
lukepower Posted April 5, 2015 Report Share Posted April 5, 2015 Hm, good question... I Put one of the logging SQM meters for half a year up high here in the Dolomites (around 2400m), and constantly got to 21.7-21.8 on moonless nights. Considering that this place is only about 5km from my observatory, it is constantly better In theory Unihedron has calibrated each unit before shipping... Link to comment Share on other sites More sharing options...
MarkS Posted April 5, 2015 Report Share Posted April 5, 2015 Nigel, According to my calculator(!) and the equation you presented, your NELM of 6.62 gives SQM of 21.99, not 22.05 - so maybe you don't need to chase accuracy, calibration etc. Mark Link to comment Share on other sites More sharing options...
GeorgeCarey Posted April 6, 2015 Report Share Posted April 6, 2015 (edited) I put Nigels equations in a spread sheet. These are the values around 6.62: 22.16 6.696 22.15 6.691 22.14 6.687 22.13 6.682 22.12 6.678 22.11 6.674 22.10 6.669 22.09 6.665 22.08 6.660 22.07 6.656 22.06 6.652 22.05 6.647 22.04 6.643 22.03 6.638 22.02 6.634 22.01 6.629 22.00 6.625 21.99 6.620 21.98 6.616 21.97 6.611 21.96 6.607 21.95 6.602 21.94 6.597 21.93 6.593 21.92 6.588 21.91 6.584 21.90 6.579 21.89 6.574 21.88 6.570 21.87 6.565 21.86 6.560 Edited April 6, 2015 by GeorgeCarey Link to comment Share on other sites More sharing options...
nakbrooks Posted April 6, 2015 Author Report Share Posted April 6, 2015 The variance identified by Mark is about 0.3% and probably due to rounding errors in the SQM Reader Pro program. I would be perfectly happy with a repeatable variance of around 0.5 mag, but can't find an easy way of calibrating my SQM/LE as there is no way of getting a "true" reading without investing in lab-quality equipment. I'll probably just have to trust the manufacturers calibration. One of the documents referenced on their website was an independent review of 9 meters, including the SQM/LE. It determined that it was impossible to identify which was the most accurate and they had a spread of around 14%, but they were all very close in terms of the relative variances in luminosity. Link to comment Share on other sites More sharing options...
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