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Stellarium
Displaying the value of the luminance of the sky
Feature request:
Dear all,
It would be extremely useful for users interested in visual issues to know the value of the luminance (brightness) of the sky at the alt-az points the celestial objects are located. This could perhaps appear as an additional data when clicking on an object (at the end of the list with its magnitude, coordinates, et al)
As far as I could see in the user's manual, this luminance is already calculated by Stellarium, in order to determine the visibility of an object against the sky background (natural or light polluted). According to the manual "The skybright class The class governs physical sky brightness values based on Bradley Schaefer’s VISLIMIT.BAS basic source code (Schaefer, 1998). The basic sources are available on the Sky and Telescope web site"
Displaying the sky luminance in the SI units of cd/m^2 (easy conversion from Lamberts) would really be a very welcome addition. Please feel free to contact me for further info: salva.bara at usc.gal
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Thank you @salvabara for suggesting this feature.
This has been on the unwritten wishlist for years. I just am unsure if the value is fully OK. Some LP expert told me the Bortle scale is not really good enough, and I prefer not to show numerical values that are bogus if all the computation was implemented for was visual effects.
Hi Georg, Thank you very much for your response. I rather meant the sky luminance computed according to Schaefer's VISLIMIT.BAS algorithm as implemented in Stellarium, not the Bortle scale nor the NELM value. This luminance (in nanoLamberts, nL) seems to be stored in the variable BL of the code mentioned in the Users guide (*). This is a robust estimate of the visual brightness of the natural sky due to atmospheric effects (daytime, twilight and moonlit skies). Knowing its value (preferably in the SI unit candela per square meter, easy conversion from nL) would indeed be very useful for users involved in LP and/or visual science research and education. Schaefer's model of twilight and night sky luminance is widely used nowadays in the research community.
(*) https://skyandtelescope.org/wp-content/uploads/vislimit.bas
If the value is available without further addition (did not look at that part for years...), we could always add it e.g. to the mouse pointer coordinates plugin or indeed the object infostring.
I now remember my earlier concerns. The code as from S&T needs some fixes, given the super simple implementation with a fixed calendar, 11.00 years solar cycle etc.
for the users that are interested:
https://github.com/Stellarium/stellarium/blob/06a3181bc7cd87e574bdffa70ef33628133d8551/src/core/modules/Skybright.cpp#L101-L152
I support Salva Bara's request. Schaefer's VISLIMIT.BAS code is a de facto standard that is useful for researchers looking for alternative solutions or for comparison with direct measurement of sky luminance or for developing mechanistic models to account for visibility of celestial bodies. Sky luminance values are already being calculated and used to decide if selected objects are visible (and therefore displayed) or invisible (and therefore not displayed). So the request is to simply report those sky luminance values to interested users.
Adding this feature as an option of the mouse pointer coordinates plugin might be the less invasive choice, avoiding to overcharge the already long object infostring. And it would avoid potential misinterpretations of this luminance as a property of the objects themselves.
I agree with Salva’s assessment. Whatever works for the programmers works for me! Anticipated future improvements of Schaefer’s code should not delay the initial deployment of this useful feature.
Larry Thibos, Professor Emeritus Indiana University School of Optometry email: [email protected]mailto:[email protected]
On Dec 15, 2020, at 5:30 PM, salvabara <[email protected]mailto:[email protected]> wrote:
Adding this feature as an option of the mouse pointer coordinates plugin might be the less invasive choice, avoiding to overcharge the already long object infostring. And it would avoid potential misinterpretations of this luminance as a property of the object itself.
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It should. The code as from S&T still needs some fixes. First make numbers meaningful, then show them.
Agreed. Thanks for your help!
Larry Thibos, Professor Emeritus Indiana University School of Optometry email: [email protected]mailto:[email protected]
On Dec 15, 2020, at 7:40 PM, Georg Zotti <[email protected]mailto:[email protected]> wrote:
It should. The code as from S&T still needs some fixes. First make numbers meaningful, then show them.
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Georg - if you choose to use the mouse cursor for selecting sky location, please include azimuth & altitude along with luminance in the display. Thanks again.
Larry Thibos, Professor Emeritus Indiana University School of Optometry email: [email protected]mailto:[email protected]
On Dec 15, 2020, at 7:40 PM, Georg Zotti <[email protected]mailto:[email protected]> wrote:
It should. The code as from S&T still needs some fixes. First make numbers meaningful, then show them.
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What is the status of this request? I am still very anxious to have this feature available for my research on astronomical photometry. Thanks!
Me too. Still no time for it. We should provide numbers only when the code is really correct and has not been adopted for aesthetic reasons. Currently I cannot guarantee this.
Georg - I may be able to help build confidence in Stellarium’s calculation of sky luminance if I could just get numerical readout of the values used for graphical display.
Over the past few years I have collected a lot of readings of sky luminance for which the time and date have been recorded. This was done in conjunction with first sightings of stars and planets. I have been impressed by the ability of Stellarium to predict the visibility of these celestial objects, which is typically accurate to within ± 5 minutes of my own first-sightings. I suspect that this small error may be due to a discrepancy between the model’s prediction of sky luminance and my measured readings. But to test that hypothesis, I need numerical values of computed sky luminance near the stars and planets being observed.
Would it be possible to get an experimental version of Stellarium that would provide numerical values of sky luminance at the cursor location? It doesn’t have to be the current version. Even a “beta” version from a year or two ago would be fine for my needs.
Thanks for your help, Larry Thibos, Professor Vision Science Department, Indiana University
On Oct 4, 2021, at 11:55 AM, Georg Zotti @.@.>> wrote:
Me too. Still no time for it. We should provide numbers only when the code is really correct and has not been adopted for aesthetic reasons. Currently I cannot guarantee this.
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This is a good task for the community to participate in the contribution into Stellarium. Who wants to help us?
I did some work on this on prof. Thibos's behalf. I included it in the PointerCoordinates plugin so that the sky luminance is shown for the current mouse cursor position. https://github.com/matt-jaskulski/stellarium/commit/f668c4d8d13cd433ef11a65fb62deb1082853379
