Members’ Pages – Bortle scale – suggestion

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  • #575137
    Alex Pratt
    Participant

    Increasing numbers of members are adding the Bortle level of their sky quality to their image descriptions, usually appending it to their Location text. I am impressed with the quality of work being done under skies as bright as Bortle 8.

    Could ‘Bortle’ – with a picklist – be added as an optional field by the web team?

    Many thanks,

    Alex (4 miles outside Leeds – midwinter Bortle 7, midsummer Bortle 8)

    #585124
    Paul Leyland
    Participant

    I support this request and would add another, closely related one.

    The Bortle scale, as well as being a simple number, is also a measure of sky brightness in terms of equivalent stellar magnitude per square arcsecond. I can never remember that Bortle 4 is, for example, 20.49–21.69 and invariably have to look it up at https://en.wikipedia.org/wiki/Bortle_scale

    If the Bortle scale is added, could it be something along the lines of “Bortle 4 = 20.49-21.69 mag/(sq-as)”?

    Thanks,

    Paul

    #585126
    Alex Pratt
    Participant

    Hi Paul, My interest was to see the range of sky conditions affecting the members, so a simple ‘Bortle 4’ tag suffices for me. Not everyone will have (access to) a SQM, so they can assess their site by using online references to the Bortle Scale and either use the partly subjective descriptions, e.g. “Bortle 4- NELM 6.1-6.5 and M33 is a difficult averted vision object, only visible when high in the sky” criteria or via their SQM reading, as you say. Clear – and dark – skies, Alex.

    #585127
    Paul Leyland
    Participant

    I don’t have a SQM either, but I still find the numerical value useful, and for essentially the same reason as you like the NLM.

    Both give an indication of when the brightness of a stellar object is comparable to the brightness of the sky when observed with a particular optical train and detector. My eyeball is 100 times (5 mags) less sensitive than my CCD and has a resolution 30 times poorer (circa 1arcmin compared with 2 arcsecond seeing limit), so one resolution element of my eye collects 900 times as many photons as in one of the CCD, another 7.4 magnitudes.

    Plugging in the numbers, a NLM of 6.3 corresponds to 6.3+5+7.4 =18.7 sky limit for my scope.
    The corresponding instrumental estimate is 21.2 – 2.5 * log (2 * 2* π) magnitudes for the sky, which evaluates to 18.5.
    That is a remarkably satisfactory agreement!

    #585128
    Paul Leyland
    Participant

    Note that I. like many imagers, routinely image objects which are fainter than the sky. Long integration times and background removal software are much more feasible with a CCD than a retina!

    #585130
    Nick James
    Participant

    There’s at least one thing in the descriptive Bortle list that I think sounds very wrong. Bortle 7 (the suburban/urban transition) is probably where I am in Chelmsford most nights but the description says “when it is full moon in a dark location the sky appears like this, but with the difference that the sky appears blue”. This must be rubbish surely? My image processing pipeline produces an estimate of the sky brightness for each image. On a good, dark, transparent night from here I get around 19.2 mag/arcsec^2 near the zenith. A couple of nights ago at full moon I got 16.5 mag/arcsec^2 at the zenith. That sky brightness would be the same even in a normally very dark place since it dominates scattered artificial light so to suggest that my “dark” skies are the same as full moon skies in the countryside seems a bit daft. 

    #585131
    Paul Leyland
    Participant

    It is a long time since I observed in Bortle 7 skies so I can’t really remember what the sky looked like. What I can say is that in La Palma to me the night sky is very definitely blue for 2 or 3 days either side of full moon and I can’t see anywhere near as many stars with the NE as I can two weeks earlier or later.

    Colour perception might be a personal response. I’ve unusually light-sensitive eyes as a consequence of a bad attack of measles when I was a small kid.

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