A Better Closed Loop

On July 21 we closed the loop again (but without wind on the outside of the tower). In these calm conditions we were able to obtain 55% Strelh at i’ (765 nm) which is excellent correction in 0.8″ seeing and 33mph winds. I attach a log10 stretch image below to show the very high contrast PSFs that are obtained with the Magellan AO system. This is slightly better performance then was predicted for an R=8 mag guide star in a 33 mph wind with ro=14cm at 0.55um. Normally at Magellan the seeing should be better (and the wind lower) than what we simulated here, giving us some confidence that VisAO will be an excellent visible AO science camera in >75% of the weather seen at the Magellan telescope.(clockwise) perfect PSF, the MagAO PSF, The AO ON PSF, and the AO OFF PSF (Log Scale)

Note by Jared: due to the way turbulence is simulated in the tower using the mirror itself, there is some missing power in the modes higher than ~585 (the number of actuators on our mirror). This results in an optimistic fitting error during these tests. We can estimate how much lower Strehl at the telescope would be due to this unsimulated turbulence using some AO theory (see Noll 1976). Our most conservative estimate for this correction brings our telescope Strehl down to 37%, from the 55% measured in the tower. Compared to the correction calculated by a more empirical method for the LBT (see Esposito 2010) this correction is probably a little large (resulting in a low estimate for Strehl). Even with this conservative correction, our tower results are exceeding our performance predictions for an 8th magnitude guide star by more than 5% Strehl. This is very exciting!

Our First Closed Loop Visible AO Images

Yesterday, in strong winds on the outside of the test tower, we closed the loop at 400 modes and 800 Hz for the first time. The results were very nice and better than expected. In simulated 15m/s winds with injected 0.8″ seeing we recovered 44% Strehl in i’ (765 nm) and 28 milliarcsec resolution (this is an improvement of 293 times in peak amplitude over the AO loop off image, and 24 times sharper image). The corrected PSF is very high contrast and drops by 1000x inside 0.1″. Overall we are very pleased, the loop is very stable and has yet to crash or open when running. A very exciting day.

Log images of a perfect PSF, and those obtained with the loop on, with and without injected 0.8″ seeing

A linear version of the plot

Our Filters

We are about to start posting some very exciting results, so I thought we should provide some information about our filter system.  We are using Sloan Digital Sky Survey (SDSS) standard r’, i’, and z’ filters as our main bandpasses.  These filters were provided by Asahi Spectra.  Our CCD47 has a near-IR coating to maximize its long wavelength quantum efficiency (QE).   The below plot shows the combined QE of our system (taking into account only the CCD and filters), as well as the mean wavelengths of the filters.

Magellan VisAO Filter Curves
The tranmission of our SDSS bandpass filters and CCD.

Update 22 July 2011: added the transmission of our 950 Long Pass (950 LP) filter. This should be treated with a little caution, as it is from a catalog page, and not a measurement of our actual filter. Also note that these calculations were done assuming our CCD47 QE goes to 0 at 1.1 microns, since the manufacturer’s curve stops at 1.05.

Closing the Loop!

On July 18 we succesfully mounted the NAS on the tower. Then recabled the system. Illuminated the the ASM at the top of the tower with the white light fiber source. Checked the optical alignment (which was unchanged from before). Then we moved the the PWFS on the 3axis stage (while tilting the dummy 50/50 beamsplitter) till the image of the ASM was round and centered on the PI stage (with its 12mm mirror it is the smallest optic). Then we took a 10 mode interaction matrix. Closed the loop! Then we took 50 and 200 mode IMATs with loop closed at 0.05 gain to clean up errors in the tower vibrations on the windy day we were working with (the tower was not floating since this was just an engineering test). Then closed the loop with the 200 modes to yield great images on the CCD47. A great day!Mounting the NAS

NAS mounted on bottom of test tower
NAS all cabled
The very first look at the ASM in laser light

The lowest 50 KL modes from our interaction matrixClosed loop Pupils and ASM Mirror positionsThe Arizona team after the first closed loop (Jared, Laird, Derek)Alfio after closing the loop

This week we finished the optical alignment in the optical lab. We also mounted the NAS on the tower as a test. Then we pulled the NAS off the tower and mounted all the optics into the the NAS for the first time. Then we aligned the flowerpot (calibration point source) to the NAS and the board to the the flowerpot. See photos below

Laird bolting board on NASThe optical board on the NAS (flowerpot at top with calibration source)

A well aligned PWFS

The VisAO PSF at r'