I hadn't really planned this feature for 4.2, but I started thinking about light pollution measurement, Bortle numbers, etc, and realised that this could all be calculated based on captured images, sensor analysis data and three more things...
* A 'Quantum Efficiency' or QE value for the imaging sensor and
* The F-ratio of the telescope being used and
* The optical efficiency of the telescope system (what percentage of light makes it from the entry of the telescope to the imaging sensor)
Fortunately these figures are fairly easy to find out, or estimate. For instance the QE for modern sensors is probably about 80% (in the most sensitive green part of the spectrum anyway). Camera manufacturers often give a QE graph showing how it varies with wavelength which you can consult.
The f-ratio is just the focal length of your telescope divided by the aperture. Don't forget to include the effects of any barlow lenses/reducers/teleextenders in the calculation. It's probably a good idea to use plate solving to measure the effective focal length of your imaging system, since it doesn't always exactly match the published values!
Finally, the optical efficiency for modern telescopes is usually pretty high (95% or more), due to the use of multi-coated optics to reduce reflections from glass surfaces and highly efficient mirrors.
How to use it
So, how does this work in SharpCap? Well, you will find a new tool in the 'Tools' menu called 'Light Pollution Measurement'. You can activate this with any camera open, but to get readings you need to meet the following conditions...
- The camera must be in a high bit depth mode (10/12/14/16 bit) - You can't use 8 bit modes for this feature
- There must be sensor analysis data available for the camera, either installed by default as part of SharpCap's sensor library, or by running the separate sensor analysis tool.
- Dark subtraction, flat correction and other image processing tools should be turned off, so the tool can process the image straight from the camera sensor. Activating the tool will disable image processing in SharpCap while the tool is active to try to enforce this.
- The camera must provide information on the pixel size and accurate exposure length - most dedicated Astronomy cameras do this, but industrial cameras tend not to provide the pixel size (Basler, Point Grey, Imaging Source/Celestron). Additionally webcams do not provide pixel size, but they cannot meet the high bit depth requirement either.
With all of these conditions satisfied, you will start to see sky brightness data shown both in the summary area at the bottom and overlaid on the image as a contour plot.
- Screenshot 2026-08-24 154819.png (54.24 KiB) Viewed 711 times
- Screenshot 2026-08-24 154827.jpg (368.74 KiB) Viewed 711 times
It's much more useful to show the information in proper sky brightness units - Magnitudes per square arc second (MPSAS - what SQM meters usually show) or as an estimated Bortle number. Switching to these modes is as simple as selecting the appropriate radio button in the work area at the bottom, although you will need a SharpCap Pro license and also need to adjust the f-ratio, QE and light transmission values appropriately. I'll just point out here that a SharpCap Pro license is a lot cheaper than a dedicated hardware SQM device...
Switching to Bortle for example changes the contour overlay to show Bortle values, and also the status area gives you min/max/average brightness values in the selected unit.
- Screenshot 2026-08-24 155514.jpg (281.06 KiB) Viewed 711 times
- Screenshot 2026-08-24 155542.png (36.97 KiB) Viewed 711 times
How accurate is it?
The biggest issue is going to be not having an exact value for the QE percentage of your sensor - the QE typically varies with wavelength, and obviously the light collected by the sensor is of a range of wavelengths. Probably best to eyeball the manufacturer provided QE graph and try to pick a mean value over the range of wavelengths being measured. That being said, an error of 10% in the QE - ie using 72% when it is really 80% - only leads to a 0.1 MPSAS change in the sky brightness reading.
How is Bortle calculated?
I have used the values for 'Approx SQM' from the Wikipedia page for the Bortle Scale and interpolated between them. It's more 'estimated' than the MPSAS value.
Why the trailed stars in the screenshots?
I spent the whole evening yesterday from sunset to astronomical darkness with the telescope pointing at the zenith (not tracking), taking images every 2 minutes - trying to capture the progression of sky brightness during the whole twilight period. When I have the chance I will process the MPSAS data out of the images and compare to the semi-empirical twilight model I am using in other parts of SharpCap
Can you check old image data?
If your old image data was saved in FITS format using a recent version of SharpCap, and you had sensor analysis data at the time you saved the data then you will find headers like 'BIASADU', 'EGAIN' and 'EGAINSAV' in the fits files. Those provide the information needed to process the image data correctly when loaded from file. Load these images via the 'Folder Monitor' camera and you should be able to work with them using this tool.
cheers,
Robin