Pier 10: G2V Analysis

The other night the RoboStaff aimed Pier 10 at a Sun-like star in Lyra and captured a series of frames using the red, green, and blue filters. This technique is designed to obtain the proper number of frames in each filter so as to balance the signal-to-noise ratio. Al Kelly is credited with this technique: http://kellysky.net/White%20Balancing%20RGB%20Filters.pdf

The idea behind it is to render the color of Sun-like stars as a shade of gray. I've been using this technique for several years and as a result I never need to do Color Calibration during post-processing. Also as a result I get images that don't look mangled. Faint nebulosity is much smoother. It doesn't look strained.

This particular G2V star is magnitude 12.3. The challenge was finding the exposure that would capture the star without saturation. I used a calculator but the procedure is very much dependent on the estimate of the seeing conditions. Worst case (to my way of thinking) is that seeing is 1 arc-second FWHM. As it turned out seeing is actually around 2.7" FWHM so my frames came out a little under-exposed. I would have liked the star to have been baked in more. I proceeded nonetheless with the analysis.

20x 7-second frames were captured in red, green, and blue. I downloaded flat files from Robo. At the time I didn't have bias frames so I used 30-second dark frames from the calibration library. (This sensor is quite unlike my Atik 314E which has a mean bias of about 230 ADU. Pier 10's sensor has a bias of about 2000 ADU.)

I utilized AstroImageJ for calibration, alignment, and stacking. I do a lot of variable star and exoplanet work so it was a familiar piece of software. I used it to perform photometry on the G2V star for each stack. I made a correction for atmospheric extinction (the red and green frames were captured at 83 degrees altitude but the blue frames were captured at 62 degrees altitude.)

From there I calculated the color balance ratios:
R: 1.39
G: 1.00
B: 1.66

This means that you need to capture 39% more red frames than green in order to balance the SNR, and 66% more blue frames.

For this sensor RoboScopes says that the balance is 1.2, 1.0, and 1.33 respectively. I don't know where those numbers come from but I suspect it comes from just looking at the sensor's QE curve. If that is the case then you still need to adjust for the affects of the filters and the black body radiation of a G2V star.

As a quick test of the validity of my color balance ratios, I created a color image of the star field using 17 reds, 12 greens, and 20 blues. The result is encouraging (see attachment). The star colors match up nicely to what C2A Planetarium software says about the B-V color indices. Next step is to capture a DSO.

Brian