SharpCap Forums

Hello,
in order to not to get much frustrated by 1 month of constant and uniform cloudy sky (night and day) I decided to invest in studying a bit.
In particular I am interested in sub-exposures.
tools.sharpcap.co.uk is a great tool, and I wanted to reverse engineer it to put the logic in a spreadsheet (that I can use offline).
What I found by examining the javascript embedded in the web page is the formula attached to this topic as an image ("," is actualy decimal point "." in the equation, px is the pixel size in micron, skyMag is the value reported by Lightpollutionmap.info's SQM, f_sky is the "flux", electrons per pixel per second).
If I didn't make any mistake (apparently I didn't as the equation provides results matching those ones calculated by tools.sharpcap.co.uk), I would like to ask some questions (to some expert or even the sharpcap's creator Dr. Robin Glover):

1. Where the 3.837 factor comes from ? and why 550 instead of the full 300 (nm) visible bandwidth spectrum ?
2. The skyMag (SQM) is usually calculated at Zenith... but it is not unusual to try to photograph a DSO which is low on the horizon (e.g. in this period spring galaxies are too small for my gear, then 1 month ago - the last clear night - I tried the out-of-season Rosette and Heart); any hint on how to get a good estimation of the skyMag in the case of an object not much high in the sky ?
3. I would like to extrapolate a similar equation for the photons' flux converted into e- (e- per pixel per second) in the case of a faint deep sky object (I would say something like: S_dso=(3.837*BW/550)*(QE/100)*(px/F_ratio)^2*(10^(-0.4*Mag_dso)), where Mag_dso is the value that I can get from astro catalogs, Stellarium, etc.)

Thanks a lot for any hint and wish you more clear skies than here

Alvise

Hi,

I can't quite locate the notes that I put together when working out that equation, but this is the source I used for the calculation of the photon flux based on magnitude :

https://web.archive.org/web/20180817124 ... /mags.html

(Note that there is a worked example for V band photons from mag 23.90 star about 3/4 of the way down - this is a good starting point).

A sky brightness measurement gives you mag/arcsec^2. Apply the formula from the page above and you get photons/s/m^2/arcsec^2. The diameter of the scope gives you your area (m^2) and the focal length/pixel size gives you the number of pixels per arcsec^2. This should all be enough to give photons/pix/s (and the focal length and diameter combine into f-ratio). Hopefully the 3.837 (or something close to it) drops out too

I suspect that estimates for objects away from the zenith can be done, but it will depend on your local light pollution situation (for instance maybe you are on the edge of a town - bright on one side of the sky, darker on the other). I've not looked into this in any detail.

For DSO estimates, you need to have figures for the surface brightness of the object (Mag/arcsec^2) rather than the integrated total magnitude of the object (which is the normally quoted magnitude figure). Some object databases may have this info, but of course for many objects it will vary massively depending on where in the object you are.

cheers,

Robin

Thank you very much Robin !

Alvise