SQM meter

[Hemi]

This arrived yesterday,

Any experience or tips for use?

Cheers

[MrB]

Tips:
1: It's a relatively expensive bit of kit so don't go cheap on the battery. Some Alkalines eventually leak. Maybe you'll be lucky, maybe you won't. Battery Russian roulette. Use lithium (buy once cry once)
2: Aim at a relatively 'empty' patch of sky. Bright stars will skew the reading.
3: Measure a few times and ignore the outliers, sometimes you get an oddball reading.
4: Submit reliable readings to www.lightpollutionmap.info
5: Have fun

[Hemi]

Thanks Simon,

It came with a battery, I put it in without considering that but given it will likely get very sporadic use I’ll replace it with a lithium one.

Ps: probably a very silly question, but do clouds matter in assessing sky quality for light pollution?

Cheers

[iborg]

Hi

My guess, I think it depends on what you are after. Strictly speaking, yes, because it is only without clouds (and their reflected light) that you can get a measure of the quality of the sky.

However, if you are more interested in the quality of the seeing at the time of measurement, then no, because they are affecting the sky at the time you are viewing, or photographing.

I'll be interested to read the responses of those that actually know the real answer.

A suggestion, if you are only using it occasionally, don't worry about the type of battery, but, always remove it when you have done your measurement. Any leakage won't damage the meter.

Philip

[glend]

I have owned my SQM for maybe 10 years, and have used it on all my dark site trips, and it still gets used from my backyard observatory to track light pollution levels over the years, and discriminate between good and great nights. It also surprisingly correlates well with the reading on the global light pollution map section for Australia.

As far as aiming goes, I am always pointing it straight up, for the sky should be darkest at the zenith ( the atmosphere is thinnest straight up), and most of my imaging is done at elevations above 60 degrees.

There are some seasonal factors that affect readings, for example the Milky Way overhead on a perfect night will yield a lower SQM reading than a night when it is rotated out of view ( assuming your somewhere that is actually reasonably dark enough to see the Milky Way in all its glory).

Regarding cloud, obviously cloud is bad, even cloud in the distance can reflect back ground light. Also there are times you may think there is no cloud but high misty cirrus maybe present, and affect your reading. This is why it is important to keep a record of your reading by site, so you immeadiately know based on the history, what your dealing with.

As far as the Seeing debate goes, a SQM reading is just one indicator, and the value displayed can not be taken alone. Often upper winds, thermal inversions, etc can affect imaging significantly. An experienced user, with a site history log, can make assumptions about upper atmo Seeing, but there is no substitute to actually looking with your scope to confirm Seeing.

I also remove the battery when it is stored, and there are many posts on forums, like Cloudy Nights, where they discuss batteries affecting readings. If you ask your question on Cloudy Nights you will get heaps of responses, and opinions (not all based on experience).

If for nothing else, it will allow you to benchmark all your dark site trips, and occasionally settle arguments about what site is darker. Have fun.

[MrB]

Yes clouds will affect the reading.
Lithium battery was maybe $20 or something about 10ish years ago(theyre about half that now) and I've never replaced it. So maybe $2 a year so far... without the hassle of remembering to remove/reinsert it.

[Hemi]

Thanks everyone,

Very helpful. It was bought for the sole purpose of having fun :)

Cheers

[ReidG]

A rough idea of atmospheric conditions can be obtained from
https://www.windy.com/-Satellite-satellite?satellite,-27.468,153.033,5
The cloud cover can be shown using three different techniques.
Choosing the Infra Red option I find gives a fair idea for those nights when there is a lot of upper air moisture but not true cloud


It would be interesting to see how that compares with what the meter shows.

[Hemi]

First play with it…
Moon set 15m ago, 30m after astronomical twilight ended, patchy thick clouds. getting 19.21 averagely. So my part of the world has gone from Bortle 5 in 2015 to Bortle 6 now.

[strongmanmike]

Hi Hemi

One thing...

If this is a brand new unit, there should be no problem, if it is a second had unit however, just make sure that the little LED looking sensor on the top of the unit is a clear glassy transparent blue (bottom unit in attached photo), if it looks at all milky or uniform light blue (top unit in attached photo), ice has formed within in the lens and it is likely out of calibration and it will return incorrect, higher (ie darker) readings for your night sky than reality. The only way to recalibrate then is a trip back to Canada for many $'s :sadeyes:

To get an accurate reading of your true sky darkness, try pointing your unit away from the Milky Way and towards a field at or near the zenith and no, don't measure the sky darkness if it is cloudy, can't see the point..?

It is quite interesting to see variations over time and at different times of the year and solar cycles (sky glow) etc at your location, or take it to new locations and compare.

I am extremely lucky, on average Moonless nights at my location I get consistent readings (https://pbase.com/strongmanmike2002/image/172228789/original) around 21.90, on the those crystal clear moonless nights, when all factors are in favour, ie. no sky glow, a front has recently passed through, low dust, low humidity etc I get readings that hit 22.00 and even just beyond and the sky looks absolutely incredible :)

Have fun :thumbsup:

Mike

[Hemi]

Thanks for the additional tips Mike. Ive been following your Eagle View thread from the beginning and I’m envious of that location and those skies.

I think my SQM is not destined to display those numbers…

[strongmanmike]

He he, well maybe not at your regular/home location..?.. but worth chucking in your bag if you go out bush to dark skies and it's good to confirm those "Hmm?...the sky doesn't look quite right tonight" :confuse3: nights, to confirm you are not going mad or seeing things :lol: :thumbsup:

Mike

[Don Pensack]

Some things of note:
The SQM and SQM-L may give different readings.
See:
http://unihedron.com/projects/darksky/images/fovcompare.jpg

Clouds will interfere with readings, as will trees or buildings, so even with the SQM-L, be well away from buildings and trees.


What is it?

It is a hand-held sky brightness measuring meter that gives a reading of the night sky's brightness in magnitudes per square arc-second, the most common measurement figures used by professional astronomers.

Why would I want one when I can just look at the sky and estimate the naked-eye limiting magnitude using star counts in selected areas?

Well, the problem is your vision and mine will vary. We may determine different limiting magnitudes because of visual acuity, yet the night sky has a true, objective, brightness that is independent of our abilities to estimate it. The Unihedron SQM doesn't get tired or give different results if you forget your glasses.

Other than measure the sky brightness, what can I do with it?

Here are some ideas:

Measure different remote sites to determine which is better.
Check new sites to compare to your usual site.
Check from night to night and hour to hour to steer your observing session. If you discover your site is darker after midnight, wait until then to push your scope to its limits. If the night is poor, spend it observing the brighter, more easily seen, objects and leave the faintest DSOs for better nights.
Compare your readings over time with the Bortle Scale to see how it compares to your site and to put actual numbers on the scale. I've determined that, at my site, mag.21.3 corresponds to about the Bortle Class 3.
Send your data to the International DarkSky Association. With enough data, they can target the worst offenders to make the biggest difference in sky brightness.
You can accurately gauge how bad your home is, compared to your favorite dark site. If the difference is small, push yourself to observe at home in between outings to the dark site. If, like me (3.5 to 3.9 magnitudes brighter skies at home), your home is really poor, start a lunar observation program at home that you wouldn't waste your time on when you travel to that dark site.
Keep a log of your site over the years. This will document the encroachment of lights, and may also display a Solar Cycle variation in readings.
Eliminate the subjective evaluations of night sky brightness and make your notes more useful. If the brightness is noted in your observation notes, this may tell you what's visible in certain darkness of skies.
Compare your data with the Light Pollution maps on sites such as the ClearDarkSky Clocks site. You may be able to put a number with the color that is better defined than the estimates they used, and you may be able to see a result from altitude changes.
With the SQM-L, you can measure your site in different directions of the compass. This can tell you if you need to find a site a little farther in one direction, or even as little as tell you where you need to set up light shielding.
Save time in observing by getting a sky brightness measurement in only a few seconds, rather than waste precious observing time counting stars.

How does it work, and how can I best use it?

The SQM measures brightness in an 80 degree wide cone. If pointed at the zenith, it will measure the average sky brightness down to +50 degrees off the horizon in every direction. This is its strength and its weakness. It truly gives you an idea of how bright your overall sky is. You cannot aim the unit lower than +40 degree altitude and not have the horizon cut off the light cone, giving a false reading. But, if you have more light pollution in one direction than another, the inclusion of the brighter section can penalize the section of the sky that is darker (see the link above).

You can measure the brightness of a smaller section of sky with the SQM-L.

How accurate is it?

An ISTIL International report by Cinzano shows that it is very accurate. If you wish to convert to V magnitude (for certain magnitude comparisons), you have to subtract 0.17 magnitude from your reading (21.0 becomes 20.83), +/- 0.07 magnitudes. In Moonlit or light polluted skies, you'd subtract 0.11 magnitudes to yield the V magnitude (21.0 becomes 20.89) +/- 0.14 magnitudes.

More importantly, to convert to a point-source brightness, you need to add 0.3 magnitudes to the reading (21.0 becomes 21.3). This is the most important characteristic to remember when figuring out the stellar NELM at your site.

Careful to not let nearby trees or buildings cut off some of the field or the reading will not be accurate.

Are there problems with the figures you get?

To a certain degree. A truly dark site with clouds (no light nearby to reflect from the clouds) will read darker than a site without clouds. The Milky Way overhead will add 0.1 to 0.2 magnitudes (read brighter, or a lesser number) to the brightness compared to a period on the same night when the Milky Way has rotated out of the way. Duh. Anyone could have told you that, because we all know the Milky Way makes the sky brighter at a dark site. This is probably a reason to note the magnitude measured on an hourly basis when using the meter, and noting where the Milky Way was in the sky.

Also, brightness does not equal transparency. I've measured nights of relative darkness that had more horizon extinction and muted Milky Way appearance than other nights of the same measured brightness. It's obvious that we need both darkness AND transparency (or Clarity, as I call it) in order to have superior views. But, in normal, slightly light-polluted sites, the presence of water vapor in the sky causes a general sky brightening and a diminution of transparency at the same time. In such sites, the measured brightness will more directly correspond to what you can see through a scope.

Any final comments?

I have found the SQM to be easy to use and its readings to correlate with my personal evaluations of the darkness of my observing site. The more of us who have and use one, the more information we will have about the nature and growth of lighting that surrounds us. Having objective measurement figures for a host of sites all over the world will allow us to more effectively target the sources of light pollution and to effectively avoid the worst of it when we travel to darker observing sites.

Uses:

Find out how good the night or site REALLY is.
Compare the sky brightness at different sites quantitatively.
Document the evolution of light pollution in your area.
Set planetarium dome illumination to mimic the skies people are likely to experience elsewhere in the city.
Monitor sky brightness through the night, night-to-night, and year-to-year.
Determine which nights show the greatest promise for finding the 'faintest fuzzies'!
Calibrate the effect of sky brightness on subjective measures such as the Bortle Scale.
Investigate how sky brightness correlates with the solar cycle and month-to-month sunspot activity.
Help provide local ground truth for future sky brightness prediction with the Clear Sky Clock.
CCD users can make a correlation between the SQM reading and when the background reaches some ADC level.

[xthestreams]

With regards to clouds I’ll add my own experiences.

Yup clouds in a light polluted area will result in a lower mag reading due to reflected light as mentioned.

However after moving to a dark site (Siding Spring area) I found nights where I was in excess of mag 24 (IIRC sometimes as high as 25), for the obvious reason that the clouds have eliminated the last source of LP, the stars themselves.

I’m now in search of a site with no stars so I can get even better SNR….

[Hemi]

:lol:

Underground maybe?

[RB]

:lol:

[xthestreams]

Near perfect darkness, now I can finally shoot some amazing images. Wait. Oops… ;-)