Chris and Peter observed the bright star Markab on the night of September 30 2011. We noticed that changing the size of the Front Iris — from 30 mm setting to the 40 mm setting had no effect on the amount of light coming from the stars. In both positions we were getting peak counts in the star of around 56000, and total of 246000 in a 10×10 box centered on the star.
We were expecting the flux to increase in going to the 40 mm iris from the 30 mm iris by a factor of (40./30.)^2 = 1.8.
Are we misinterpreting what this iris does – or did it not move? We will ask Dave about this cosmic mystery!