- For the mere mortals amongst us: Is there a simplified flux calibration procedure that we can use when imaging with one telescope at one geographic location in one night with a DSLR camera (with only approximately know filter, telescope mirror/transmission and sensor characteristics)? For example calibrating against a known star?
Hi Georg,
The usual way to do this is, as Fernando says, making an exposure of a standard star. Here I see several problems:
First is that we
don't have any catalog of standard stars with photographic filters. All the catalogs we have today are made with photometric filters, which are not valid for our flux calibration.
Second is that we don't use any rigorous methodology for flux calibration. The usual methodology has these requirements:
- First, we need to standardize our imaging system. This requires to image a standard field with several standard stars (again, we don't have any standard field with photographic filters) and make measurements of the atmospheric extinction (again with standard stars at several air masses). Think that our camera never has a flat QE curve, and each filter has an unique transmission curve. With this method we can standardize the flux in a wavelength range between two imaging systems; and, although it seems somewhat complicated,
it needs to be done only one time.After standardizing your system, you can make higher order color corrections. These corrections can be sometimes very large, I'm not talking about a 1 - 2 % correction, but a 20 - 30 % or even more...
- Once you have standardized your system, you can calibrate the color of your object with a much simpler methodology. Here I see two methods:
1.- Make an exposure of your object with one filter and then make a exposure of a standard star near your object with the same object. This way we don't know the atmospheric extinction in these exposures, and we'll simply scale the RGB weights according to the detected flux of the standard star. I think this can be sufficiently precise for our requirements if the object is not too low and is fairly simple to do. It can be even simpler if we have the standard star in the same FOV of our object.
2.- The more precise method is to make the object exposure, then the standard star, and then several standard stars at different altitudes to measure the atmospheric extinction. This is more difficult to do in a regular work and, in most situations, it doesn't justify the extra precision for our requirements... Perhaps it can be useful if we are imaging very low altitude objects (<30ยบ alt).
I see here a long way for the PixInsight platform and community. First of all, I think all the calibrations can be completely automated (at least for the image processing steps). But anyway, as I said before, we
don't have any catalog of standard star with photographic filters. I have been designing from one year ago an all-sky map of standard stars with photographic filters. I have now the correct observational methodology and all the equipment needed. But this map requires also a large amount of software development.
The main facts about the map are:
- Camera: Finger Lakes Proline 16803.
- Filters: Baader RGB.
- Lens: Nikon 105 mm f/2.5. The lens is closed to f/16 in order to be able to measure stars from magnitude 4.5. We need these bright stars because with need
this catalog of nearly 1600 spectrophotometric standard stars.
- Magnitude range: 4,5 to 10 with 1% precision. This would generate a catalog of about half million stars per hemisphere (very rough estimation). Of course, we can go deeper if we assume a higher amount of error in the measurements.
- About 110 fields to cover the northern hemisphere. 10x1-min exposures for each filed; these exposures let us make the map going from the equator to the pole with the same speed as the Earth rotation.
I think it would be easy for me to find the budget to make the same map in the southern hemisphere. Moreover, with this star map we have the perfect platform for going deeper in the future. For instance, I could make a second map with a 180 mm lens, going to mag 14 - 15 (about 20M stars).
Therefore, I ask here the community (developers) if there is interest to go ahead... It would be beautiful if the PixInsight community makes the first photographic sky map. We can open an independent thread.
Best regards,
Vicent.
