Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_android_vs_ios_winners.thumb.jpg.803608cf7eedd5cfb31eedc3e3f357e9.jpg

scarp15

Sky Quality Meter

Recommended Posts

I would like this topic to raise the profile and awareness for using a Sky Quality Meter aimed at dark sky seeking, DSO observers.

A sky quality meter, is a hand held device that reads in magnitude per square arc second, night sky brightness. A popular model, and one that I use myself, is a Unihedron SQM-L. This device has an angular sensitivity response of 42 degrees, which is used to take readings at zenith and can also be used to aim at other angles towards an intended target. Observers using this device will take a frequency of readings throughout an observing period, when transparency is good and will log the lowest, highest and average reading. A SQM is also handy to make comparisons if you travel to varied sites.

A few years ago, when I made the decision that travel to explore varied dark sky locations was to be my primary aim as a visual astronomy, I purchased a Unihedron Sky Quality Meter from FLO. The first time I took it out, I had been at the time a member of a regional astro society and attended their dark sky site. I was so pre-occupied that it never occurred to me, that is until I'd packed up, to take a reading with my new meter. Following this I made a conscious effort to ensure that taking readings was to be integral to a dark sky session, yet even then it was still almost an after thought late on in the session. However this was to change as gradually I began to place value and priority upon taking a frequency of readings intermittently during the course of the night. It steadily occurred to me that much of my time at a dark sky location was actually occupied just looking up, becoming increasing dark adapted and taking SQM readings.

Each of my dark sky trips has to be within an hours drive, perhaps a little more, from home, as factoring in the return journey is essential. There are many locations to explore and I have discovered some quite special places gained within this time frame. For each though, there are still light domes, to get away from this scourge will involve traveling further, deeper. On a recent backpacking, wild camp trip in the Cheviots, Northumberland, I took along my trusty, compact SQM. Unfortunately, whilst there was a great sunset, transparency deteriorated somewhat and it was not required (though great views were had of Jupiter, Venus Moon). My plan for the autumn is to go, further and deeper, wild camping and taking with me my Sky Quality Meter. I do not have aperture fever, what I do have is a strong desire to experience gaining high quality dark sky places and take along what ever optical equipment I am able to carry.

Observing in the UK is challenging as the night sky fades and diminishes due to excessive lighting and constant pressures for development. The Green Belt is severely under threat, villages are being transformed into small Towns. Valuing our pockets of sky quality is crucial and measuring, recording on here in reports helpful. One final point, there are on-line map resources that convey magnitude readings for any location. This might be useful as a very general guide, but is as far as I am concerned highly inaccurate and ought not be quoted and included in reports, or other forum accounts. For example I like to go stargazing in the Breamish Valley in Northumberland. The map reading for this location is 21.84 mag, my own readings have never improved - so far, at just over 21.5, it may be slightly darker deeper within the valley but the map reading is still an exaggeration.   

It will be interesting to learn if you use a SQM or intend to use one.

 

    

 

 

P1080598.JPG

  • Like 8

Share this post


Link to post
Share on other sites

Interesting post! I used to see these meters and think what is the point? The sky is what it is, measuring it doesn’t make it any better!

I can’t recall why exactly I bought one, but I now use it regularly to assess sites that I visit. It is often hard to tell which is better without comparing the same object visibility, so the SQM-L is very useful to be able to build up a picture of where the best sites are. I think the best I have reached is mag 21.3 at Bignor in the South Downs National Park which is around an hour 15 from me. My normal home readings are around 19, and our local observing site 19.3 I think, so not that good!

A very handy tool!

  • Like 4

Share this post


Link to post
Share on other sites

Great thread Iain, these meters are extremely valuable as you point out as they a give fast, reliable method of proving sky darkness without the subjectivity of NELM. I am very interested in your findings on the deeper camping trips :thumbsup:

  • Like 2

Share this post


Link to post
Share on other sites

Getting out to a dark sky location in the UK can become a real sense of occasion. Not least because of the probable infrequency and quite committed effort involved and more specifically if fortunate enough to be able to enjoy lengthy periods of good transparency. To collect data to validate the sky darkness can be relevant information to include in an account for if a particular enhanced observation, such as for example, the Crescent Nebula is achieved. The SQM model option provides a wider angular reading, that will detect any light dome impacting the vicinity and will give a variable reading compared to the SQM-L model. If located in a more enclosed environment, such as close hillsides, it will though give a more distorted and inaccurate reading. For UK skies, it is the dark patches at or within a few degrees to zenith that are most intriguing to gain a reading, the narrower SQM-L serves this purpose. 

  • Like 3

Share this post


Link to post
Share on other sites

Excellent post, Iain. It’s amazing how much skies can charge over the course of a night. Having something that can give an accurate reading as opposed to a subjective estimate but very helpful. I would think the biggest benefit would be in target choice. I’ve certainly lost lots of time by seeking out targets that weren’t possible under the conditions at the time. I’ve looked at purchasing a meter a few times but ultimately have more pressing purchases to make first. It’s certainly on the list though!

  • Like 3

Share this post


Link to post
Share on other sites

A very interesting post, thank you.

I too have wondered about these meters as I am probably needing to find observing sites away from home due to spot lights issues on two sides, I once had a darkish site at home, I no longer do, but that’s typical for many.

The accurate assessment of the sky using such a meter sounds very practical and reliable.

But as Neil says above, many other items have a bigger call on funds at present.

 

  • Like 2

Share this post


Link to post
Share on other sites

@scarp15, Iain, do you know what, if any impact transparency has on the results?

Share this post


Link to post
Share on other sites

Hi Stu, the last time I got to observe at the Breamish Valley and took some final readings, the meter response time to calibrate each reading was prolonged, registering between Mag 21.49 to 21.53. In comparison, on my recent wild camp trip and with average, poor transparency (thin haze, not thick cloud), I used the meter for one brief attempt of readings, the time it took to calibrate was very quick, almost immediate, I think approx mag 19. If the Milky Way was high, the narrow cone of the SQM-L will produce a lower reading, and equally if there is moisture in the air, picking up light. Of course it should not be used at a dark site in thick cloud as this will give an inaccurate / misleading high reading.  

  • Like 2

Share this post


Link to post
Share on other sites

Thanks Iain a very helpful report. I organise outreach events for my Astronomical Society and it would be useful to measure the sky from the various locations. Last season we used the grounds of a National Trust venue and it was dark and one of our members took a reading (I will find it shortly) which proved the quality of the site.

I am currently in discussion over another venture which is a short distance from Lucksall which is used for the SGL star parties.

Perhaps I should purchase a meter for the Society and my own use at home.

  • Like 2

Share this post


Link to post
Share on other sites
5 hours ago, Stu said:

@scarp15, Iain, do you know what, if any impact transparency has on the results?

I hope I can add a comment on this as well Stu?

Under VG to excellent transparency there is little effect on the SQM-L readings and as transparency worsens or actual clouds appear false dark readings can occur. Common sense kicks in on this though.

There is an interesting phenomena with respect to light domes and scattered clouds- if trans is avg to poor and there is an individual cloud over a light dome, light is scattered all over the place giving a lighter reading than normally would occur... this phenomena also makes normally invisible light domes appear to the eye as well IME. This is of particular significance if choosing a spot near the edge of an actual light dome or when in a "pocket" of darkness.

The SQM's really do help pick spots, not only for the darkness levels but for optimum viewing with varying conditions.

  • Like 6
  • Thanks 1

Share this post


Link to post
Share on other sites
24 minutes ago, jetstream said:

I hope I can add a comment on this as well Stu?

Of course! Thanks Gerry :) 

  • Like 2

Share this post


Link to post
Share on other sites
1 hour ago, jetstream said:

I hope I can add a comment on this as well Stu?

Under VG to excellent transparency there is little effect on the SQM-L readings and as transparency worsens or actual clouds appear false dark readings can occur. Common sense kicks in on this though.

There is an interesting phenomena with respect to light domes and scattered clouds- if trans is avg to poor and there is an individual cloud over a light dome, light is scattered all over the place giving a lighter reading than normally would occur... this phenomena also makes normally invisible light domes appear to the eye as well IME. This is of particular significance if choosing a spot near the edge of an actual light dome or when in a "pocket" of darkness.

The SQM's really do help pick spots, not only for the darkness levels but for optimum viewing with varying conditions.

As a user of an SQM in a professional capacity I have found exactly the same. And another thing, there can be a slight variance in 2 SQMs! I have use of at least 2 SQM-Ls (one of them being my own) and a few SQM-LU-DLs, too.

The 2 SQM-Ls produce a consistent difference of .21-.23 MPSAS....it's not a precise art but it provides useful ways of determining how dark a site is, if used properly.

Other things to consider: low battery power can produce false, high readings.

Disregard the first reading due to initial sensor temperature being out of whack with ambient - several readings should be taken and draw out the average.

A fabulously but cloudy dark sky with little to no light pollution can produce readings in excess of 22 MPSAS...22.55 about six weeks ago!

Another phenomenon discovered with a lot of SQM users is that readings tend to improve after midnight...

  • Like 5
  • Thanks 1

Share this post


Link to post
Share on other sites

very interesting post Iain for those seeking a good spot for a nights session or maybe checking one campsite with another.

Unfortunately for the mob, we are kinda stuck where we are as finding another seclude spot in Elan would only end up with us being moved on.

Would be nice to find out the proper reading of where we go tho....

  • Like 2

Share this post


Link to post
Share on other sites

I look at the visibility of a few nebulae to assess the transparency. It can be alarming to watch stars just vanish at a dark site.. same SQM reading, just nothing to look at.  If you use the SQM indoors in a cupboard wrapped up so it can’t see anything it can fail to give a reading.... but you have to wait a while for it to give up. The more SQM readings of different good sites the better, there used to be a website/map to log them,, but it seems to have vanished.

After midnight darkening could well be then streetligths that have curfew times.. good to see a benefit.

PEterW

Share this post


Link to post
Share on other sites
13 minutes ago, PeterW said:

I look at the visibility of a few nebulae to assess the transparency. It can be alarming to watch stars just vanish at a dark site.. same SQM reading, just nothing to look at.  If you use the SQM indoors in a cupboard wrapped up so it can’t see anything it can fail to give a reading.... but you have to wait a while for it to give up. The more SQM readings of different good sites the better, there used to be a website/map to log them,, but it seems to have vanished.

After midnight darkening could well be then streetligths that have curfew times.. good to see a benefit.

PEterW

I think I remember that site, it was a user created one. Didn't know it vanished.

I use this:

https://www.globeatnight.org/maps.php

2018 map: https://www.globeatnight.org/map/

  • Like 1

Share this post


Link to post
Share on other sites

Fantastic report. So well written. Clear and very interesting. £129 for a SQM from FLO at the moment I see. Hmmm.....

  • Like 1

Share this post


Link to post
Share on other sites

Very interesting. At £129 these things aren’t cheap. There are some cheap phone apps available purporting to do the same thing; has anyone assessed how accurate they are? 

It would be very interesting to be able to compare skies more objectively. And use other people’s data to find sites that are good’uns mor easily.

  • Like 3

Share this post


Link to post
Share on other sites
30 minutes ago, FenlandPaul said:

Very interesting. At £129 these things aren’t cheap. There are some cheap phone apps available purporting to do the same thing; has anyone assessed how accurate they are? 

It would be very interesting to be able to compare skies more objectively. And use other people’s data to find sites that are good’uns mor easily.

Globe at night accepts the 'Loss of the Night' app for mobile devices which must mean the apps is okay to use!

 

Certainly saves some dosh! :icon_mrgreen:

  • Like 2

Share this post


Link to post
Share on other sites

Clubs should have one, people who go on observing trips should have one, people interested in mapping or tracking dark skies should have one. Like quite a few bits of Astro kit, not everyone needs to own their own, we are happy to share.

 

PEterW

  • Like 4

Share this post


Link to post
Share on other sites
3 hours ago, FenlandPaul said:

There are some cheap phone apps available purporting to do the same thing; has anyone assessed how accurate they are? 

yeah..rubbish..tried it at Elan, Isle of Skye and the Peak District...said the peaks was the darkest sky..?

  • Thanks 1
  • Haha 2

Share this post


Link to post
Share on other sites

What would be good if someone that is attending the SGL star party in October takes a measurement at Lucksall. Assuming its clear ever night ?

  • Like 1

Share this post


Link to post
Share on other sites
1 hour ago, Mark at Beaufort said:

What would be good if someone that is attending the SGL star party in October takes a measurement at Lucksall. Assuming its clear ever night ?

Good idea and there could be a 'guess the MPSAS' competition... :icon_mrgreen:

  • Like 1

Share this post


Link to post
Share on other sites
22 minutes ago, Beulah said:

Good idea and there could be a 'guess the MPSAS' competition... :icon_mrgreen:

Nice idea Sam - I will link @Grant into this as we are discussing topics for the star party

Share this post


Link to post
Share on other sites

Mob are in Skye in October....I might get a meter for it

  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.