Sky Quality Meter.

[ollypenrice]

Although I've seen these regularly in the hands of our guests, I've never had my own... until now that is, because two kind guests, John and Viv, kindly sent me one as a present yesterday. I was thrilled to bits because these devices are so useful, espeically to the imager. Many members will know of them, but for those who don't they look like this;

You simply hold them in this position, pointing to the zenith, and press the button. A sensor on the top measures magnitudes per square arcsecond. If this sounds a bit arcane, don't worry. It appears as a number on the digital scale. The scale is logarithmic so is highly compressed by the time you are in dark sky territory. A value of 21 is roughly where good sites begin and the best reading I've ever heard of was 22.1 in the Australian outback. Sky and Telescope suggest that 22 is generally considered to be the level found under a totally dark site on a moonless night. They discuss the use of the meter here; http://www.skyandtelescope.com/astronomy-resources/rate-your-skyglow/

Last night, with twilight still clearly around, we started at 21.1 and were at 21.6 by midnight. The best anyone has recorded here is 21.9. If I ever see 22 I'll throw a party! (In the morning...)

One use of the meter is to scout around for the darkest site in your area.

For me its usefulness arises from the fact that, when imaging at the PC screen, it is impossible to assess the current sky quality until your eyes are dark adapted - and that takes ten minutes as a bare minimum. With the meter you can check the sky reliably and immediately and so decide whether the clouds have scuppered you or not.

The manufacturer's website is here.  http://www.unihedron.com/projects/darksky/

Very highly recommended.

Olly

[michael.h.f.wilkinson]

Never seen one of  these, although I have heard of them. Interesting tool for scouting out dark areas or assessing how good the sky really is. I have often set up the scope in what looked like dark conditions, but once the eye adapted fully were not that good (not hopeless, but not as good as hoped for).

[Beulah]

Excellent products - I have one myself and have obtained the darkest reading of 21.6 MPSAS at Zenith, January 2015, dead of winter, in a remote location in Wales so far. That's the 'best' reading from many dozens of readings over six months.

21.9 in your area is amazing - 22.0 is very probable.  

They aren't necessary as part of the astronomy hobby but can be useful when scouting out dark sites.

[Martin Meredith]

I agree Olly. I've been using one since last autumn and find it really useful to guide what might be worth going for on any particular session. I take several readings during the night to monitor changing conditions too.  The best I've obtained in the garden is 20.5 here in a village with a few street lamps and a well-illuminated (but largely congregation-free) church. I intend to use it to find better sites nearby but just haven't got round to it yet. Once I have enough readings I'll plot a graph of faintest object recorded vs SQM.

While on the subject of meters another useful device is a combined digital thermometer/hygrometer (about 5 quid on ebay). Humidity levels change very rapidly where I am and reach nearly 100% surprisingly often.

Martin 

[PhotoGav]

What really gets me with your picture, Olly, is that beautiful wall in the background... I can feel it's warmth emanating from my screen. Looking out of my window, it's another damp, 'colder than average at this time of year', English day.

The SQM looks interesting too, but what is the lowest it can register? My skies might manage double digits on a really good night!

[Knight of Clear Skies]

I've had readings in the low-to-mid 21s when out imaging at Caradon Observatory in Cornwall, with a high of 21.6 IIRC. There was one report of a reading in the 22s there but that was a one-off - I can't find a link now but I've read that the meters tend to overestimate once they hit the high 21s.

[ollypenrice]

What really gets me with your picture, Olly, is that beautiful wall in the background... I can feel it's warmth emanating from my screen. Looking out of my window, it's another damp, 'colder than average at this time of year', English day.

The SQM looks interesting too, but what is the lowest it can register? My skies might manage double digits on a really good night!

It's funny, I thought of the wall as I took the picture. It is very un-English, as you say. Lime mortar and the warm tones of the stones, which have no moss or other growth on them. Our walls are home to little rock lizards and we have redstarts nesting under the edge of the roof tiles at the moment.

Inside the house under natural light it reads 7 so it will read in poor observing conditions (which I think would describe our stting room...  )

Olly

[Ags]

Want one!

[Vox45]

It is on my wishlist

Will you be adding your measurement to the SQM project database ?

[ollypenrice]

It is on my wishlist

Will you be adding your measurement to the SQM project database ?

I should, I think. Thanks for the link.

Olly

[Lowjiber]

I use one for a slightly different purpose... visual observing.

In my light-polluted backyard, I have varying degrees of darkness depending on the amount of light glow from the Las Vegas strip, the Moon, and other variables.  It may seem wierd, but it is really handy when even attemptng dim fuzzies at various angles in the sky.  I need (want?) to know how dark a particular area is in the sky.

The company makes a version called a "Sky Quality Meter with Lens" (SQM-L) that measures only a 40^o cone around the direction (up/down/left/right) it is pointed.

The readings are the same, but sometimes I find it easier to "get my head around" the somewhat simplier Naked Eye Limiting Magnitude (NELM) numbers.  Below is a conversion chart that helps.

SQM readings aren't for everyone, but those of us who use 'em sure do like the results... just another "tool".

Clear, Dark Skies

[ollypenrice]

I reckon I could see the Las Vegas light dome from Dante's Peak above Death Valley so you have your work cut out lving near it!

Olly

[Dave_D]

i'd just end up depressed using one of those...

[Corpze]

Olly; Approx. how long does one reading take at say 20mag/as2? I made my own SQM a couple of years ago and always wanna now if the time it takes to measure is the same.

For me it is maybe 20-30sec at 20 mag/as2. for 21 or even 22 i suppose the reading would take 30-45 sec?

/ Daniel

[ollypenrice]

It seems to be almost instananeous. Maybe a couple of seconds.

Olly

[swag72]

I like my SQM gadget - It's interesting to just sit there and measure the sky reading

[Beulah]

It seems to be almost instananeous. Maybe a couple of seconds.

Olly

A reading of 23 MPSAS is achieved in 50 seconds. Of course, in a darkened room on a cloudy night.

[scarp15]

Very simple to use and good to take multiple readings through a session. I like to use varied dark sites and have locations that I have so far not been able to use, this devise therefore is very handy. Sometimes I only use it towards the end of a session, as I have not quite got into the habit yet and and likely being to preoccupied. A good reference for any report. The only location I have not used it is at home, no point in even knowing. I have typically had readings of 21.3 alone Hadrians Wall. Look forward to using it at Kielder. 

Here is a review expanding a little on Olly's as published through Cloudy Nights

Unihedron Sky Quality Meter - Review

[perfrej]

Well, both Corpze and I know what sensor is in those things (hint: it is a dollar or two) so this is a prime target for DIY. Corpze built one, and I have built one, but the software published needs som polishing as the processor takes a total hike if you try to measure in daylight. Eventually I will put a permanent one in the obsy, Olly, and then the readings will be readily available (and logged) for us all. Maybe I should speed this project up now that you have a "real one" to calibrate against...

/per

[steppenwolf]

I use  an iPhone App. for this purpose ( see my review here ) and although I have no doubt that it isn't as accurate as Olly's hardware solution, for relative readings, I reckon it is a pretty good guide.

[perfrej]

Downloaded... Looks OK. Will test at Olly's tonight, weather permitting.

I just wish people would stop using red for apps and computers. For anyone wiht astigmatism it is a nightmare, and for me, red ruins my dark adaption completely. I prefer dim with normal colours...

/per

[Corpze]

It seems to be almost instananeous. Maybe a couple of seconds.

Olly

Oh, that fast! Is it the same time for every magnitude?

And as Per sais, great DIY-project... even though i haven´t used mine that much

/Daniel

[PhotoGav]

I use  an iPhone App. for this purpose ( see my review here ) and although I have no doubt that it isn't as accurate as Olly's hardware solution, for relative readings, I reckon it is a pretty good guide.

Excellent Steve. Downloaded (only 79p... If only everything in astroimaging were this cheap!) and tested... My home office has an SQM of 16.81. I look forward to pointing my phone at the heavens tonight.

[ollypenrice]

From what I've seen the SQMs which have been here have been remarkably consistent but, as Steve says, for my purposes and, I guess, his, it's being internamlly consistent that matters most.

In a break from the norm the whole of this month's dark time has been with visual observers. The Meter has behaved exactly in accordance with my night adapted impressions since it arrived so I'm more than happy with it.

I haven't had a significantly bad sky on which to test that side of things, though. A pleasant enough problem as problems go. I trust Per won't be imporitng too many clouds tonight! You can't rely on customs to weed them out these days...

Olly

[beamer3.6m]

Is there an android version of this app?