Focus, focus, focus

Focusing a camera attached to a telescope has always been a challenge.  Changing night time weather conditions, such as temperature fluctuations, contribute to the difficulty of keeping stars sharp and in focus.  Dr. Kevin Reil and Dr. Aaron Roodman are two DECam commissioning team members working hard on the focus and alignment (F&A) system for DECam.  First light for the F&A occurred two weeks ago, but the system requires more work and fine tuning.

(Image credit: Dark Energy Survey Collaboration)

This section of a DECam image shows donuts (out-of-focus stars) in the bottom left CCD, but stars in good focus in the neighboring CCDs. The CCD in the bottom left corner is one of the 2k x 2k focus and alignment CCDs.  It is defocussed on purpose by 1.5mm and the resulting donuts will give an indication on how to best focus the camera.

Last night, images much more than 1.5mm out-of-focus were taken.  The top panel of this next plot shows stars +12mm out of focus.  The resulting donuts are enormous!  The F&A team used their code to model these donuts, and a fit of their (simplified) donut model to these images is shown in the bottom panel.  The purpose of using such a large donut, is to make sure that the strange shape of the central obstruction is being well modeled.  Images were also taken at zenith and at a large zenith distance, to see if there are any aberrations from mis-collimation that may effect the location of the central obstruction.

(Image credit: Dr. Aaron Roodman)

Perfecting the modeling of the central obstruction is not a simple task.  As can be easily seen in the +12mm out of focus images, there are quite a few different features that need to be taken into account.