Bob Hill gives us his impressions and test results of our new Unihedron SQM

The Obnoxious Observer

Bob Hill

SQM and other stuff

About a year ago at our regularly scheduled club meeting, the subject of the Unihedron Sky Quality Meter somehow came up. For a couple of years now, I have been seeing reports of this device circulating around some of the observing sites that I haunt on the web, with various reports touting and decrying the usefulness of this tool. The folks who live under lousy light polluted skies said that they did not work, because it did nothing to improve their skies, and the people who are blessed with nice dark skies claiming that it proved that the skies were dark.

Well, the upshot of all this is that everyone at the meeting ganged up on me, and I ended up ordering one each SQM Mark 1. The ordering process was simple, just point the browser to www.unihedron.com, read through the glowing advertising copy, do the PayPal or credit card dance to the tune of $119.99 plus 6.00 s/h, and one small package was en-route from Grimsby Ontario to Amarillo. After a brief stop at the border for interrogation, (hey, the Homeland Security types have to do something to justify the largest plutocracy in the nation’s history)  it showed up in my mailbox seven days later.

First off, the name is a slight misnomer. It does nothing about measuring sky quality, but for measuring the brightness of the night sky, it does that very well. I guess that calling it a Sky Brightness Meter would not sell as well. It uses a sensor about the size of the old TI 192, around a tenth of an inch square, to integrate the brightness of whatever you point it at. Just hold it overhead, press the red button, wait for the ticking to stop, and read the digital readout. It gives the sky brightness in magnitudes per square arc-second, and the ambient temperature in both Celsius and Fahrenheit. I have found that the first reading can be thrown away, and generally use an average of three readings thereafter. It reads a cone of 80° centered on where it is pointed, so pointing is somewhat important.

So what are skies like around here? First night I had it, I went out back to my driveway and read 18.31, with some thin high clouds. I can generally see stars down to around 4.5 – 4.6 from that location, if my neighbors motion sensing lights don’t blast me from his garage 15 feet away. I then drove out to Jim’s place and measured 20.03, again with the clouds in part of the sky. At his place, you can see stars in the 5.8 – 6.3 range on a dark night.

The next night was clear, so I then drove around the city measuring some of the places we have set up scopes in the past. My driveway had improved to 18.70, showing that even thin clouds reflecting the Amarillo Nebula is harmful to your observing. Nick’s observatory read 18.44, same as the Discovery Center that enjoys the light shared by the hospitals. Wildcat Bluff weighed in at 19.45, a full magnitude darker. That is why we moved public observing from the Discovery to Wildcat several years ago.

Through the next several days, I tracked readings from my yard for the current lunar cycle. They ranged from 18.73 with a 4.4 day old moon, down to 15.79 with a 14.8 day old moon, a decrease of 3 magnitudes in sky darkness. So I guess it is not a fear of werewolves that makes us avoid going out on a full moon, it is because we cannot see a dang thing!

Over the course of the last year, I have taken measurements at various locations that we tend to haul our scopes at the drop of a hat. Last January at the Claude site I took readings over the course of a 3.3 day old moonset starting one hour before moonset. The readings increased from 21.22 to 21.60 at moonset, decreasing to 21.40 a half an hour later with the onset of some thin high clouds. This decrease was due to the way we see clouds spread the Amarillo nebula light over a larger portion of the sky.

Clayton Lake last spring came in at 21.62, about the same as Claude. So why do we travel to the lake? The same reason we travel to OkieTex, about 1500’ less atmospheric muck to decrease the transparency we peer through. The darkest location I have measured is at the alternate site, the roadside park on the south side of Palo Duro Canyon. Readings there ranged from 21.69 to 21.80 on one exceptional night. As I recall, the side lobes of M76 that are normally only seen on a deep image, were plainly visible that night. I have only glimpsed them faintly on a very few other occasions.

OkieTex this year ranged from 21.55 to 21.74, with most nights in the 21.55 to 21.61 range. At OkieTex, Peter Lipscomb from Santa Fe had brought the improved SQM-L, so we had an opportunity to compare the two generations of the instrument. The SQM-L uses a lens to decrease the sky area sampled to a 20° radius cone, to try to get a more accurate reading using a smaller chunk of the sky. Our readings were not that much different, averaging about .05 magnitude different, with my meter usually giving the darker reading. I am not sure if that is worth the $25 price increase.

To summarize,  this is another tool we can use to try to quantify the sky under which we observe. It does have some limitations, it tells you nothing about either the seeing or transparency. All it can tell you is how the sky brightness at your location compares to the brightness of other areas. It could be a useful tool for tracking light pollution, and I tend to feel that that would be it’s more proper function, because a reasonably skilled observer can usually tell what the sky conditions are going to be that night while setting up their scope. I can usually tell just from aligning the Telrad on Polaris.

So take the readings you see from other parts of the country with a grain of salt. If they are not accompanied by other pertinent information, i.e. seeing, transparency, altitude etc., it is just a number bragging about how dark it is. I will leave you with an example to illustrate.

The last night at OkieTex, we brought it out for one final reading. It was pointed at the sky, activated, and 15 seconds later (it normally only takes 5-6 seconds under a normal dark location) the reading popped up on the display. 22.59. This night met all the requirements for a Bortal class 1 sky, moonless, so dark that you could not see your hand etc. It was also totally overcast. Clear skies, y’all.