Sky Quality Meter

[Allan]

I was originally going to pick up a SQM-L to measure the brightness of my observing sites, but the more I think about it, I feel that this is the wrong meter to buy. The lens permits sampling of only around 20 degrees of sky in the SQM-L. So pointed at zenith , it is possible the meter will show a dark site and a less than dark site as being reasonably similiar. Multiple readings around the sky will be required with the SQM-L to indicate the true brightness of the sky. This IMO would make the SQM-L a pain to use.

So I am thinking now that the standard SQM is the better option as it measures around 80 degrees of sky, so provides a truer indication of the overall brightness of an observing site, not just a point in the sky. Only one reading would be required with the meter pointed at zenith, and this will give a more accurate indication from site to site as to which is darker.

I would be interested to hear from any SQM users, and if your experience backs up my argument. Cheers

[glend]

Allan I have a SQM-L and wrestled with that question when I considered my purchase. I resolved it in favour of the SQM-L because my observing site at home on Lake Mac has stray light sources, like a pesky street light on a hill , that I can shield the scope from but would potentially influence an SQM wide field reading even if only through humidity scatter. My reasoning is that a lot of my observation is quadrant related and the L allows discrimination as to where the best viewing is, for example is Orion better tonight than last night. I can always do a few other readings, which literally only take a few seconds, if I want a wide reading - and this allows you to examine the sky in 20 degree cones to potentially identify the effects of your local light pollution sources. This ability to discriminate, or map your local sky in cones is a big advantage when your deciding where to point the scope. If I take the SQM-L away to somewhere like Bretti then functionally there would be little difference with the standard SQM as the whole sky is dark. When I spoke to the Unihedron guy, via email, he recommened the L for suburban users. Hope that helps. Btw my best readings here have been above 21 at zenith, which is considered dark.

Glen

[BPO]

It also depends on your purpose. I have two SQM-LE aimed at the area of the SCP. These mainly measure changes in brightness/darkness caused by sky glow and other natural factors. This probably wouldn't be useful, or even possible, from a site afflicted with significant artificial-light pollution, so usage of these devices is going to vary.

[jamespierce]

I have the L version, and in retrospect would prefer the standard version... Point it at the zenith and have a consistent reading, milky way, light pollution etc all considered... The variation with the L is quite high just based on where the milky way is in the sky.

If you want an L, I'd happily sell mine and buy the standard version instead !

J.

[Allan]

It's a very interesting debate. Old posts on Cloudy Nights could never sort the argument either. Although Don Pensack was a very strong supporter of the standard SQM and I tend to agree with his points.

James, I think our views are aligned on this one and you may have helped make my choice.

Glen, I want a way to rank sites in their order of darkness as a way of determining the best places to observe. If you point an "L" at the zenith at Lostock and Bretti you are going to get the same reading. Someone who has never been to those sites will conclude they are equally as dark. But the light dome in the SE from Newcastle affects the darkness at Lostock. So a standard SQM will reveal the difference between the 2 sites. I guess this is what I want an SQM to do for me. Your argument about using the "L" version in suburbia is a good one. But the SQM will only read down to about 40 or 50 degrees above the horizon, so won't directly sample ground lights. Anyway, thanks for your input.

[pjphilli]

Hi

I have been interested in getting a LQM for my site in Thornleigh Sydney
which is moderately light polluted on most nights and much worse say
on dewey nights. As far and narrow and wide views are concerned would
it be possible to make a cone mask to reduce the "field of view" of the
wide SQM to look at various sections of the sky when required?

Cheers Peter

[Allan]

I read a piece of information from someone who believed it was possible. However it involved using it without the cone initially, getting readings, putting the cone on, getting more readings, and then adding some sort of compensation factor. I just shook my head and thought, that's not really practical. So it got back to deciding between the wide and narrow versions.

So I've decided to pick up the standard SQM in LA tomorrow.

[Fizics]

Hi Allan, have you heard of Dark Sky Meter for the iphone 5? It is an sqm that works well on the iphone 5 but not so great on the iphone 4.

[Allan]

Hi John. Yes I was reading about that one. I can't remember now, but there was something about it that I didn't like. I still have an iPhone 4 so that might have been the issue. Anyway doesn't matter, I have decided on the SQM now. Cheers

[BPO]

I suspect you'll find the meter will detect the mask as an obstruction and return seriously incorrect readings.

See the darkest readings on Unihedron's records database:

Unihedron database of readings (http://www.unihedron.com/projects/darksky/database/?statistics=true).

At least the top seven are not true and accurate readings, mostly as a result of obstructions such as cloud or trees. (That's per Anthony Tekatch of Unihedron.)

The highest readings attainable at the darkest of sites are no more than up around the very low 22s (eg, 22.00 to 22.10, and even the latter is suspect) according to Anthony and other experts.

If you fit a mask to your SQM and get very high readings it'll be the result of the meter seeing it as an obstruction.