How good is your site?

[acey]

I came across the following formula for rating the quality of an observing site, the "observing site index" or OSI

OSI = (T+W+A+S+V)/C

where:

T= limiting magnitude

W=weather (percentage of clear nights)

A=altitude (in thousands of feet)

S=seeing (1 to 10, 1 worst)

V= visibility (percentage of unobstructed sky)

C=convenience (0 to 100)

I found it in Stephen O'Meara's book "The Messier Obbjects", and he attributes it to Tim Hunter of the IDA (International Dark-Sky Association).

Problem is, this formula is clearly wrong. If you've got a very convenient site (you live in your own observatory), then C = 100 and that makes the OSI small. But if your site is very inconvenient (you've got to drive miles to it), then C is close to zero and your OSI is enormous. Maybe we should multiply instead of divide? I think so.

But I can't find any reference to this formula at the IDA website or anywhere else. Also intrigued that there is one dimensional quantity in the formula (altitude), meaning you get an OSi measured in feet. Hmmm.

In any case, I've heard that altitude isn't all it's cracked up to be, because oxygen shortage reduces visual sensitivity (and makes you faint).

Maybe a better formula is

OSI = TxSxVxCxW.

My site has mag 6, average seeing (let's give it a 5), good all-round visibility (call it 85%, or 0.85), moderately convenient because it's only a 20-minute drive (C=50). Weather's hard to judge - seems like I only get to observe one or two nights per month but that's only when you factor in the moon, social life, need to get up in the morning, light summer nights, etc. What the heck, let's stick with that - a measly 5%, or 0.05

Then my OSI = 63.75.

Any advance on that?

Andrew

[Kaptain Klevtsov]

My site has mag 3, average seeing (let's give it a 5), visibility (call it 45%, or 0.45), convenient because it's only a 2 minute lid off (C=100). Weather's hard to judge - seems like I only get to observe one or two nights per month but that's only when you factor in the moon, social life, need to get up in the morning, light summer nights, etc. What the heck, let's stick with that - a measly 5%, or 0.05

Then my OSI = 33.75.

Kaptain Klevtsov

[Andrew*]

Mag 4.5, above average seeing: 6, visibility 0.65, covenience 80, Weather's hard to judge - seems like I only get to observe one or two nights per month but that's only when you factor in the moon, social life, need to get up in the morning, light summer nights, etc. What the heck, let's stick with that - a measly 5%, or 0.05

which makes my place a lovely 70.2!

Andrew

[S2]

There's a review of O'Meara's book here, by Brian Tung.

He covers the equation as well:

Right off, you can see there are problems, because a low convenience will increase the rating rather than decrease it. Furthermore, O'Meara rates his own site as follows:

OSI = (8.0+0.75+4000+8+0.9)/90 = 45

When I first saw this expression, it struck me as rather odd that altitude was so dominating, until I recognized that O'Meara's expression gave his altitude as 4 million feet. That works out to 800 miles or so, which would give you outstanding transparency if the hypoxia didn't hit you so rapidly.

Up at the club site - Mag 6, seeing: 5, visibility 0.85, covenience 50, I'll go with the consensus of 0.05 for weather (it can't be much different from Aberdeen) - giving me 63.75.

[acey]

We're probably all being too hard on ourselves regarding weather (I think Herschel reckoned one night in three on average was good for observing), but it's pretty much fixed for all UK sites so it's the other terms that matter. Thinking again about the formula, though, I wonder how important sky visibility is. Rosse's 72-inch Leviathan could only move up and down, as I recall (or maybe be moved just a little in azimuth), but that still gave him a view of the complete southern sky if he waited for everything to come by. That would leave transparency, seeing and convenience as the clinchers (personally I don't even worry too much about seeing for deep sky observation). So maybe the only formula we need is

OSI = T x S x C.

I do feel, though, that half the convenience would be well worth it if I could double the magnitude, even though the formula would give the same OSI. Maybe further modification is still needed....

[The Warthog]

It works better if you turn the convenience scale upside down, to give a low number to nearby sites, such as your back garden, and a high number to sites you have to drive to, then my back yard, with 3.5 mag, 200ft altitude, say 50% clear, average seeing of 6, visibility of about 65%, yields an OSI of 1.365, while Cherry Springs with numbers of 6, 2000, 60%, 8, and visibility of about 90%, but convenience of 100 (300 km away,) yields and OSI of .5184. I don't know if this is very meaningful, though, as I can't choose to go off to Cherry Springs on a whim, but I can go out into my back yard any time I want. If I were to go to a local campground, with figures of 4.5, 500ft, 50%, 6, 80%, and convenience of 30 (have to pack the car, etc.) then I get a OSI of 0.18, although the site is clearly preferable to my back yard, and much closer than Cherry Springs.

Seems to me it works better without the convenience factor at all, or some objective way of measuring convenience.

[S2]

I think you ought to leave visibility in.

My back yard is pretty dark, Mag 5 on a good night, and 100 for Convenience - but I can only see the top 30 degrees of sky or so. That's very limiting.

I agree with The Warthog that convenience has too high a weighting, though.