The thing with the observers fstOffset

Some of you might have seen this field in the observers dialog: fstOffset (given in mag)

fstOffset

It’s time to explain this a little.

You might already know that there’s a relation between the fst (faintest star seen with the naked/unaided eye) and the background brightness of the sky (which can be measured i.e. with a Sky Quality Meter.)
The below given picture illustrates this relationship:

Picture taken from: http://www.unihedron.com/

(Please note that fst is sometimes related to as NELM (Naked Eye Limiting Magnitude)

So if you step out under the nightly sky and you estimate a fst value of e.g. 5.0 mag, this curve tells you that the brightness of the sky is about 19.4 magnitudes per square arcsecond. Fine. But now, everyone looks to the skies with his/her own eyes which adds a little personal “fuzziness” to the estimated fst value. This is what the fstOffset value is intended to represent. It tries to nail down the numbers of that very personal “fuzziness”. Or the difference between your estimation and an impartial fst value (whatever this is…).

Fine. So how do you get to the fstOffset value?
- First you need something to measure the nightly sky brightness (e.g. a Sky Quality Meter). (Maybe you can borrow one from some other observer)
- Secondly you estimate the fst value as you always do
- Once you got your fst value, take the Sky Quality Meter and let it measure the brightness of the nightly sky.

Now comes in some math… Calculate the fst value which the Sky Quality Meter measured. How? Use this:

fst_Calculcated = 5 * (1.586 -log(10^((21.568 -BS ) / 5) +1))

where BS is the brightness of the nightly sky (given in magnitudes per square arcseconds) as measured with the Sky Quality Meter.
The fstOffset can now be calculated as:

fstOffset = fst_Calculate – fst (the one you estimated)

Repeat the above steps several nights to get a better and better approximation of the fstOffset value.

Fine.  And what does OM with all those numbers?
The final version of 0.920 will calculate the brightness of the night sky (if not given by the user) and in case you’ve entered a fst value for your observation. (The same holds true if you only set the “Brightness of night sky” value  on an observation, but here the fst value get calculated for sure)
The more precise you enter a fstOffset to your OM observer, the more precise OM can calculate the brightness of the night sky.

Calculated brightness of night sky value (indicated by ~)