One problem one encounters when observing closer to the horizon is so called atmospheric dispersion. While it is a tiny effect, the atmosphere acts on light coming from outer space like a prism, and disperses it into its colors. The effect is about a factor of 200 too small to be seen by the naked eye and goes unnoticed in everyday life. For a high-resolution imager like pODI it can be a significant effect, though. To counter for this atmospheric dispersion, ODI has two large prisms in its optical train that can be tuned to compensate for the distortion of the atmospheric dispersion (hence ADC: Atmospheric Dispersion Compensator). So far we operated the ADC in its neutral configuration, but tonight we observed some first test data, albeit with a poor seeing of about 1.4 arc seconds. At an airmass of about two we took some in the g' band (which is the most affected band) with the ADC in both neutral and in active state. We still need to fine-tune the strength of the dispersion (during better seeing conditions!), but the image improvement by the ADC is apparent in the example below.
For the tuning of the ADC we will use a "UG 5" filter with transmission peaks at 3500 A and 7200 A, but we have to wait until the filter adaptors are ready. At an high air mass, this filter will create double peaks of a single star (corresponding to the separation of the red and blue part of the star's spectrum), and the goal will be to tweak the ADC such that the two stars will merge into one single one. Although one shouldn't cross the beams.
|Image and contour plot of the same star observed in the g' band at an airmass of ~ 2. Left: ADC is neutral, right: ADC is engaged. Note the improvement of the roundness.|