Jump to content


Photo

Best seeing so far (at least in theory)

NELM SQM

  • Please log in to reply
13 replies to this topic

#1 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 20 March 2015 - 09:49 AM

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).

Attached Files


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

#2 pesa

pesa

    Member

  • Beta Tester
  • 81 posts
  • LocationEnköping Sweden

Posted 03 April 2015 - 01:14 PM

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

#3 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 03 April 2015 - 09:39 PM

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

#4 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 343 posts

Posted 04 April 2015 - 09:51 AM

How does that 6.62 equate to SQM?

My best ever SQM was 20.11

 

 

 


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#5 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 04 April 2015 - 10:20 AM

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 by nakbrooks, 04 April 2015 - 10:27 AM.

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

#6 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 04 April 2015 - 10:34 AM

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.

 

Attached Files


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

#7 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 343 posts

Posted 04 April 2015 - 10:44 AM

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


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#8 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 04 April 2015 - 10:53 AM

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.


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

#9 MarkS

MarkS

    Member

  • Beta Tester
  • 400 posts
  • LocationMelbourne, Australia

Posted 05 April 2015 - 11:16 AM

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



#10 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 05 April 2015 - 12:01 PM

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

#11 lukepower

lukepower

    Member

  • Beta Tester
  • 611 posts
  • LocationDolomites, Italy

Posted 05 April 2015 - 06:16 PM

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...


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#12 MarkS

MarkS

    Member

  • Beta Tester
  • 400 posts
  • LocationMelbourne, Australia

Posted 06 April 2015 - 12:56 AM

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



#13 GeorgeCarey

GeorgeCarey

    Member

  • Beta Tester
  • 343 posts

Posted 06 April 2015 - 11:44 AM

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 by GeorgeCarey, 06 April 2015 - 11:45 AM.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#14 nakbrooks

nakbrooks

    Member

  • Beta Tester
  • 268 posts
  • LocationWiltshire (England) & Hautes-Pyrenees (France)

Posted 06 April 2015 - 10:09 PM

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




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users