White-balancing Optolong LRGB filters with Altair 290M
Posted: Mon Jun 11, 2018 9:32 pm
I wanted to try my hand at color astrophotography using LRGB filters and my monochrome camera. White-balancing is the process of determining the exposure ratio for each color channel so that an image can be rendered accurately and true-to-life. For my filters and camera I determined that the proper balance is 1.0 unit of Red, 1.01 units of Green, and 1.2 units of Blue. Here is how I did it.
A common method is carried out at the telescope and then followed up at the computer by measuring the flux of a Sun-like star in red, green, and blue using specialized software. Note that ZWO sells an LRGB filter set that is specifically balanced for the ASI1600MM therefore eliminating the need to white-balance (this is my understanding.) Since I don't have that camera I decided upon Optolong filters.
Al Kelly has a great tutorial for white-balancing at the telescope here http://kellysky.net/White%20Balancing%2 ... ilters.pdf Unfortunately for me the weather is terrible and has been for too long. I needed a method that I could perform entirely with mathematics. I couldn't find anything online so I invented it:
I began by taking a snapshot of the filters' spectral response from the retailer's website (see attachment "Optolong LRGB Spectrum.jpg".) I converted it to FITS file format using PIPP and then opened FITS Liberator. I placed the cursor over the curves and entered the (x, y) coordinates into Excel every five pixels in the x-axis. Finally I sampled the 'x' and 'y' minimum and maximum values of the grid so that I could convert the (x, y) coordinates to 'wavelength' and 'percent transmittance'. I did the same for the Quantum Efficiency graph from the camera manufacturer (see attachment 'QE of Altair 290M.jpg')
Lastly I needed a white-light source. Our Sun is said to be the standard. I found an online calculator that enabled me to graph and to download to a CSV file the radiance of a Black Body at 5772 degrees Kelvin, the temperature of the Sun's chromosphere (see attachment 'Solar Luminosity vs Wavelength.jpg')
Getting those data points was 99% of the work. The last step was to multiply three numbers for each wavelength: the filter's transmittance percent, the Quantum Efficiency, and the relative radiance of the Sun. Finally for each filter I summed those calculated values over all wavelengths, essentially taking the "area under the curve". Those values are 18.8 for Red, 18.6 for Green, and 15.7 for Blue. With simple rearrangement I converted it to 1.0 units of Red, 1.01 units of Green, and 1.2 units of Blue, in order to achieve a balance of 1:1:1. A practical example might be 30 minutes in Red, 30.3 minutes in Green, and 36 minutes in Blue.
(See next post for Part 2)...
Brian
A common method is carried out at the telescope and then followed up at the computer by measuring the flux of a Sun-like star in red, green, and blue using specialized software. Note that ZWO sells an LRGB filter set that is specifically balanced for the ASI1600MM therefore eliminating the need to white-balance (this is my understanding.) Since I don't have that camera I decided upon Optolong filters.
Al Kelly has a great tutorial for white-balancing at the telescope here http://kellysky.net/White%20Balancing%2 ... ilters.pdf Unfortunately for me the weather is terrible and has been for too long. I needed a method that I could perform entirely with mathematics. I couldn't find anything online so I invented it:
I began by taking a snapshot of the filters' spectral response from the retailer's website (see attachment "Optolong LRGB Spectrum.jpg".) I converted it to FITS file format using PIPP and then opened FITS Liberator. I placed the cursor over the curves and entered the (x, y) coordinates into Excel every five pixels in the x-axis. Finally I sampled the 'x' and 'y' minimum and maximum values of the grid so that I could convert the (x, y) coordinates to 'wavelength' and 'percent transmittance'. I did the same for the Quantum Efficiency graph from the camera manufacturer (see attachment 'QE of Altair 290M.jpg')
Lastly I needed a white-light source. Our Sun is said to be the standard. I found an online calculator that enabled me to graph and to download to a CSV file the radiance of a Black Body at 5772 degrees Kelvin, the temperature of the Sun's chromosphere (see attachment 'Solar Luminosity vs Wavelength.jpg')
Getting those data points was 99% of the work. The last step was to multiply three numbers for each wavelength: the filter's transmittance percent, the Quantum Efficiency, and the relative radiance of the Sun. Finally for each filter I summed those calculated values over all wavelengths, essentially taking the "area under the curve". Those values are 18.8 for Red, 18.6 for Green, and 15.7 for Blue. With simple rearrangement I converted it to 1.0 units of Red, 1.01 units of Green, and 1.2 units of Blue, in order to achieve a balance of 1:1:1. A practical example might be 30 minutes in Red, 30.3 minutes in Green, and 36 minutes in Blue.
(See next post for Part 2)...
Brian