focus scans from IRTF

Here are some images from when we scanned a star across the slit at different telescope focus positions.

The slit runs vertically, as it does in the lab. The scan direction is from left to right. At each point in the scan, we did two integrations each with non-destructive, spin, desructive (ND,S,D) at 1 second framerates. When we finished the second destructive sample, the telescope was commanded to move by 0.25" perpendicular to the slit. The scan started on blank sky and went across the slit.

In the data reduction, I took all the save frames from a scan, computed ND-D (gives positive numbers for more light) for each pair, and then looked at just the second integration from each position. I subtracted the first scan position from each position to get rid of background emission. I rotated the spectrum in software so it ran along rows and compressed all the spectral information from each spectrum so that for each scan position, I had an intensity map along the slit. I subtracted the median of these slit-intensity maps to account for variations in sky brightness during the scan. By putting the scan positions together, I get a full map.

There are two presentations of the same four map postage stamps that come from these scans. The telescope focus is at the top of each stamp. In the first representation, I set the saturation level at the FWHM of each postage stamp. I also have calculated the size of each pixel along the slit in arcseconds and use those units for the map. At far focus positions, the spots are clearly awful. You can see that at 0.96 it is narrower along the slit at each position than at 1.15, but it is spread out more in the scan direction.

The second presentation uses the peak pixel value of all the postage stamps, from focus = 1.15, to define the color scale. Also, the axis are in pixels.