Filter Balance

Sydney has been cloud bound for months. Looking at my observing log there has been 15 nights of observing since August last year. In the mean time I’ve been attempting to work on my configuration and processing.

During the processing I’ve noticed real differences in the relative brightness of the images between R, G and B frames. Combining the aligned frames in a 1:1:1 ratio yields strange results. I found on Don Goldmans imaging site a great tutorial for measuring the filter ratios for my system. I use an LX 200 with a QSI583 and Astronomiks filters (2c).

The process I used for measuring the relative intensities was as follows: download the G2V database off Don’s site and add to the libraries of The Sky. Slew to a G2V star somewhere high in the sky and take a series of images with short exposure times. The short exposure times are needed to ensure the star does not max out the pixel intensities, in my case < 55,000. For the star I chose it was 2 seconds. I use Maxim DL so after taking the image I opened the info window and clicked on my selected star, this gave me the intensity values. I did this for a series of frames made with each filter, recorded the average values and used these averages in my calculations.

The QI583 has a KAF 8300 sensor which is most sensitive in the green so using green as the reference filter I took my average intensitiy values from the previous paragraph and used them to calculate the ratios. I divided the intesity value for green by the value for green (suprise it was 1), red/green and blue/green.

I was very surprised by the results the where for R:G:B 1.02:1:0.60. Now I could live with the R:G ratios perhaps just a small error (BTW it wasn’t an error I’ve retested it on a series of nights and got identical values), but the G:B? I tried it, I took a 300 second image in green and then a 500 second blue and the looked at the results. Smack on :)

A final note focus is very important, get it right first, it might well be, like mine, be different for each filter.