White light PSF

i' PSF

After completing our work with the laser, we switched to a white light source to test the camera’s performance in broad band filters. This is our PSF in the Sloan Digitial Sky Survey (SDSS) i’ filter (a nice set of filter curves is here), which passes light from roughly 0.684 to 0.840 microns. A theoretical Airy pattern is shown for comparison, and Laird calculates our Strehl ratio as 94% – meaning that our optics are very good.

i' PSF
The Magellan VisAO i' PSF

This image is taken without the ADC in the beam. In the laboratory, without the dispersion of an atmosphere to act against it, the residual chromatism of the ADC would slightly degrade the image quality of a broadband source (see Kopon 2008). This “zenith spike” effect was predicted and does not manifest itself on-sky.

It’s Alive!

After a very intense couple of weeks, we have built up the nearly complete VisAO camera in the Magellan AO Lab at Steward Observatory. The images below show the hardware mounted on the board. Missing is the wavefront sensor (WFS) hardware, which is waiting for us in Florence, Italy.

Top down view of VisAO
The VisAO camera in the Magellan AO lab.
VisAO camera from the right side
A side view of the VisAO camera

The numbers label specific components:
1: The input lens, in a temporary holder (the permanent one is awaiting us in Italy too).
2: The ADC mount, containing an early prototype of the custom 2-triplet ADC designed by Derek Kopon.
3: The beamsplitter wheel, a.k.a. filter wheel 1. This allows us to select how much and what wavelength of light is sent to the WFS and to our science camera.
4: The Wollaston prism on its lift. This splits the beam in 2 to enable our simultaneous differential imaging (SDI) mode.
5: Our tip-tilt gimbal mirror. This is a temporary solution, which we hope to replace with a high speed tip-tilt mirror.
6: Filter wheel 2. This wheel contains our main photometric filters, currently: SDSS r’, i’, z’, and a filter which passes wavelengths longer than 950nm.
7: Baffle tube. We plan to add a pickoff occulting spot here, to feed a tip-tilt and Strehl sensing camera which will mount on the platform over the tube. These are planned future improvements.
8: Filter wheel 3: This wheel will contain our SDI filters (2 filters in one cell) and in the future our occulting spots (to block the bright central star light).
9: The shutter. You can see our vibration isolation system (the rubber grommets). These are the only place that the shutter mount contacts the rest of the camera.
10: The CCD47. The liquid cooling attachment, another temporary device, keeps our dark current low.

Everything is now under software control in our lab, and it has even started to feel more like a telescope control room when we are taking data. The image below was taken last week.

VisAO 1 micron PSF
The VisAO point spread function (PSF) at 1 micron.

We took this image at 1064 nm (the main IR line of a double YAG laser) through the input lens, the ADC, using the 50/50 beam splitter, and the SDSS z’ filter. The Wollaston was down, and we first focused the system using our motorized focus stage. This is a log stretch, in which I count 15 Airy rings. The cross (one bright line and one dark line) are very small detector artifacts that are only visible due to the log scale.

Finding Our Focus

On Friday we performed a focus test of the VisAO camera for the first time. We are so far very happy with the results, as it looks like the focus position of our CCD stage is right about where it should be. This experiment was the first time we had the optics, the mechanical components, and the software all working together. A big day!

Derek lines it up
Derek Kopon aligning the pinhole in our telescope simulator.