Hi Alex,
Quote:
Originally Posted by mental4astro
Thanks everyone for the replies.
I'm still struggling to see how comparing SQM readings is valid though. A sky quality meter can only gauge "how dark". It cannot tell you anything about those other factors that go into making one site better than another, even when they share the same SQM value. It can't, even by your own thinking Les. Instead, it can only give a false sense of security if one hasn't undertaken an appropriate and thorough site selection process. ...
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With respect, I disagree. The SQM and SQML-L reading will be affected one way or another by three of the four criteria I listed -- Artificial LP, Natural LP and to some extent, altitude.
Sure it won't be able to read how much air you have to look through nor how "dirty" it is, but in my experience with using it, you won't get a good reading at a crap site, nor will you get a crap reading at a genuinely good site.
I do agree that it probably should not be used as the sole criterion in assessing a site and thee are other factors that play in, but it is at least objective (provided the readings are faithfully recorded) whereas any other method is going to be mostly subjective and subject to the frailties of memory.
On a side note: It is one of the things that disappoints me about our particular discipline within the wider astronomical community is that it relies very frequently on the honesty and objectivity of the observer. Sure, you can on many occasions have another confirm what you see, or claim to see in the eyepiece, but many times, if not most that isn't available and you have to try very hard to be objective (ie don't inadvertently mislead yourself) (averted imagination) and not to tell out-and-out "porkies". I've read (to be honest it is mostly "read' people infrequently make outlandish claims "in-person" -- face to face) hundreds of patently silly claims about what can and cannot be seen/done in this, that and the other aperture. When I was a younger man I used to often argue the toss on these ones but as the years have rolled by I'm more inclined to sprout a "knowing grin" and pass by claims of that type.
... and this is the point of the above diatribe: More often than not on the point of whether something can or cannot be seen/done, it falls to claims about how good the observing site is or the conditions on the night were. But these factors, again, are easy to either "pump-up" or simply lie about. ie I could see magnitude 7.4 stars at zenith, the Milky Way cast shadows, the Gegenschein could be seen at only 20 degrees altitude etc etc. The SQM if properly and faithfully used introduces a significant degree of objectively to assessing the conditions. As I said, not the be-all and end-all and does (again) require the claimant to be truthful about the reading. But it doesn't lie about what it "sees".
Please
anyone reading do not see what I have written above in a general sense, as an accusation against you or a person you know of gilding the lily or lying about what you do or don't observe. It is a general observation about the nature of our observing discipline and the reporting of what is or isn't seen.
Best,
L.