I have a handheld SQM-L for personal use, the "L"ens is said to result in a slightly more focused measuring cone, making measurements more meaningful in real-life circumstances. Measurements are quite repeatable so I guess it's reasonably accurate. Zenith is where you usually measure, yes, but you don't have to if the area you're interested in is not at zenith. At the club, we have the computer controlled version of the SQM-L (I've forgotten the exact name), which makes time series and even car-mounted surveys etc a lot easier. The SQMs are able to be calibrated, instructions are on the Unihedron website.
Does anyone have one of these? Are they accurate? Which is better, the wide field SQM or the narrow field SQM-L model..?
PS. errr?..don't suppose anyone has one I could borrow to test..?
Mike
Hi Mike,
Sorry I don't have an SQM meter. I've always been interested in getting one to compare home versus away sites.
Instead there is a somewhat cumbersome method one can use with a camera imaging the zenith. I get results that are reasonably close to published SQM figures using it and it is a good way to compare two sites. I was going to write it up in another thread some time ago.
Mike (or JA for that matter), I don't actually use my handheld one as often these days, mainly because there's a few of these floating around in our group, and the club's device is there to do serious surveys. You could have it for a very reasonable price, let me know whatcha think...probable better that posting it back & forth across the Ditch
I have an SQM, been using it for years, and have some historical data I gathered at dark sites like Bretti and Coolah, as well as tracking changes in light pollution in my backyard observatory. I am not sure if the narrow lenses version actually gives you any better results, other than measuring away from a light dome if your afflicted by that. Any light dome is likely to impact your imaging anyway.
In terms of your request about accuracy, ...compared to what? What other tool is available? Even the global light pollution map lists SQM readings in its menu.
The SQM is a very useful measuring tool, for long term tracking of changes to your environment. And thus gives you a benchmark that you can use to compare your site to others. I was not at all surprised that Bretti had the darkest sky that my SQM measured, but the difference between Bretti (which lacks a light dome) and Chaffey Dam (located 40kms from Tamworth, and has a light dome) was significant, even though Chaffey is regarded as dark. At Chaffey the narrow lenses SQM might not notice the light dome, depending how you aim it, but the wider (and it is really only slightly wider standard lense) does notice it. So i would argue for the wider SQM, but always point it straight up. Consistency in aiming gives you consistency in data logging.
I've had the SQM-L for about 5 years now. For me it's great just for personal use/info, even when measuring from the same location over a period of years (to see how LP's gotten worse in my area). Whilst I have shared once onto lightpollution.info when I first had my SQM-L, I realised later I just made my (or my neighbourhood's) location public .
Most amateur astronomers probably prefer to keep their dark site private, I've not shared since (esp when I don't own the property).
Yep, I bought mine over ten years ago. An SQM-L - I don't think they had many options in those days. Really just to get a reading, particularly comparing darkish locations when you wanted some backup to what your eyeballs thought.
Point it to the bit of sky that matters to you. It will certainly tell you when the Milky way is at the zenith - a noticeably lower reading.
I recall some discussion years ago on IIS about keeping a well-charged battery in it.
I have two SQM-L meters.
I bought one in 2010 and the other in 2020.
They certainly are a useful tool as trying to gauge how dark a sky is by eye alone is pretty subjective.
Like all instruments, there is no exact reading.
Generally, you won't get exactly the same number each time for repeated meter readings. If the surroundings are brighter in one direction, there will be some affect on the reading even if pointing directly up. You can test this by holding it at chest height in front of you and turning 90 degrees each time. I usually average these readings.
The newer meter consistently gives a lower value than the older one, anywhere from about 0.05 to 0.12 mags/sq arcsec difference. I guess that's close enough to the typical precision of 0.10 mags/sq arcsec given in the documentation. I prefer to use the older one because it gives me a darker sky reading!
So whether, someone's sky is darker at 21.8 or 21.9 mags/sq arcsec with two separate instruments is a moot point.
I have one of the early ones, which I got in 2007 or 2008. The newer SQM-L is obviously a slight improvement, but the old one still does the job and is pretty accurate. It certainly serves the purpose. I have found that the time of year, which affects milky way position has an effect on the readings. From the exact same dark sky location (Warrumbungles) I usually get readings of 21.4/21.5 in early autumn with, Crux, Carina, Centaurus etc overhead compared to 21.7/21.8 in October / November when the Milky Way is less prominent.
The important thing is to use the same device keeping changeable parameters as close to the same as possible, you can then compare relative readings and the absolute number itself is less important.
So with the narrower sensing L model, is it a good idea to point it away from the Milky Way in general..or just accept that the readings will vary overhead throughout the year?
I guess it is all relative in the end, if the measuring method is consistent then readings can be compared and will mean something.
Readings would vary from one press to another . So advisable to just take a few readings and average it out. Spread it out over a couple of years, and I see my backyard readings change - then again I live in the burbs close to the city so naturally light pollution's increased over the years.
So with the narrower sensing L model, is it a good idea to point it away from the Milky Way in general..or just accept that the readings will vary overhead throughout the year?
Yes definitely Mike. Point it away from the Milky Way and preferably to get the most relatable readings point it at the zenith when the milky way is nowhere near the zenith. It all relates to the concept of the SQM meter being to measure the underlying or background Sky brightness, away from the moon and other sources like the Milky Way and other large sources or city light domes. That is then the best way to compare Site A with Site B.
If for instance Site A was a REALLY DARK site, but an SQM measurement was taken with the moon up, the milky way core at zenith, a few clouds scattering the moonlight, etc.. then it might not appear that dark, from an SQM reading point of view, compared with perhaps a Site B a modestly dark site, but with an SQM reading taken at the zenith, on a clear night, with no moon and with the Milky Way at the horizon.
It's all about trying to measure the background sky brightness under similar conditions so that different sites or even nights can be compared.
Only one thing to consider is if near a city with a VERY large light dome, the zenith may not always be the darkest patch of sky. In that case I suppose I'd measure off to the side a little. This is sometimes seen when a city borders the sea or in some rural settings near a large city.
Many factors can affect a reading. Haze or cloud will produce more light pollution. Early evening readings tend to be a bit worse than late night or early morning ones when more lights go off in the suburbs. I have an oval nearby with pretty fierce flood lights.
The Milky Way at zenith can make a big difference. Agree with John on the difference. Pointing away from the Milky Way is better but the light pollution reading is worse at lower elevations. I would take the readings when the Milky Way is lowest in the sky.
With several measurements over a period of time, you should get some consistency. At my place, it was about 20.2 10 years ago but seems to be heading to 20.1 or worse due to light pollution. Hence, the need to head out further from Sydney to get darker sky.
[B]preferably to get the most relatable readings point it at the zenith when the milky way is nowhere near the zenith.
Best
JA
Well I sort of agree with that, but not quite.
If you're at a true dark site with no background lights, or any light domes, pointing it at the zenith shouldn't matter, if the Milky Way is not a factor. I usually only observe under these conditions and if I am not observing under these conditions I don't waste my time with the SQM, because I know what I have is far from ideal.
I always try to point the SQM at a darker patch of sky, obviously above the horizon, so trees, clouds and land structure are eliminated from the equation. This way you can usually get reasonably accurate readings, notwithstanding some Milky Way influence.
As Mike mentioned in his reply, the important thing is to try and be as consistent as you can in what you do.
I usually only use the SQM as a bit of a backup to my mental estimation of sky conditions. I have a number of targets towards the limit of naked eye visibility that I use to ascertain conditions, depending on season. Eg. If I can see M33 naked eye low in the north in October, I know its pretty decent and don't need an SQM to tell me that.
If you're at a true dark site with no background lights, or any light domes, pointing it at the zenith shouldn't matter, if the Milky Way is not a factor.
Yes of course, with the added caveat that the sky is clear of cloud which can diffusely reflect any moonlight and that there's not too much moon (better none if you seek a true background measurement).
The "requirement" to measure at the zenith really depends on what you want the SQM data for. Possibly:
1. If you want it to compare different sites (dark and otherwise) across the world, Australia whatever, using resources such as lightpollutionmapinfo, then it's important to measure at the zenith, since that is how all the data is presented there (pic below). Of course not measuring at the zenith will effect PERFECT dark sites less than urban or suburban locations readings.
2. If you want it to help with imaging to for instance achieve a similar signal/noise ratio in comparison with another location then one might even aim the SQM at the target sky area.
3. If you want it to measure local changes at a fixed location (say home) then provided you always point it appropriately away from light sources and possibly the same area then sure.
The zenith also provides an easy reference point for most and is often the darkest part of the sky, especially in light polluted areas.
When the absolute sky quality of a given site becomes less important than the relative sky quality (for example, compared to other sites measured withg the same device, or other times at the same site), the argument becomes stronger to just point it at the SCP. Admittedly this doesn't work so well when there's some anomaly south of the observing site, or when there is a huge latitude range between sites, but for intra-location comparisons it's great. Consider the attached chart, produced during the total lunar eclipse earlier this year by my observing buddy Paul in a display of unbelievable diligence (imagine a timer going off to annoy you every 5 minutes as you're trying to observe the most spectacular lunar eclipse you've seen in years) with the SQM-L. The site is a Bortle 1 but the graph bottoms out at 21.5, nowhere near that. No matter. What it does show says quite a lot about the uselfulness of these devices.
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. So advisable to just take a few readings and average it out. Spread it out over a couple of years, and I see my backyard readings change - then again I live in the burbs close to the city so naturally light pollution's increased over the years.
