UnISIS Performance

Basic UnISIS System Characteristics:

Telescope aperture 2.5-m
Deformable Mirror (DM): 177-actuator system from Xinetics, Inc.
DM conjugated to telescope pupil with 19.7 cm actuator separation (12.70 actuators across pupil)
Laser Guide Star (LGS) and Natural Guide Star (NGS) wavefront cameras: CCD39 from Marcconi
LGS & NGS Camera Electronics: Dr. Robert Leach, Astronomical Research Cameras, Inc. with high speed electronics (ca. 2002)
AO system speed: 1.48 KHz maximum AO frame rate on LGS side and 1.28 KHz maximum frame rate on NGS side
Simultaneous operation of both wavefront sensors: NGS in closed-loop while LGS in open-loop for calibration
Dual Off-Axis-Parabola optical design. f/30 input, f/77 output, 87 mm diameter collimated beam
Atmospheric Dispersion Correction System based on matched prism pairs of KF9 and LaFN21
Tip-tilt mirror from Physik Instrument runs in closed-loop with NGS CCD camera
Focal plane is imaged onto a plane mirror for coronagraphic applications
Re-imaging system from focal plane to science cameras provides pupil image for coronagrapic applications
Two science cameras simultaneously collect images with a dichroic splitting the light at 1 micron wavelength
Visual light CCD science camera: Loral 2048x2048 CCD with 0.015 arcsec pixels generally run 2x2 binned
Near-IR array camera: NICMOS-3 array, 256x256 with choice of either 0.04 arcsec pixels or 0.08 arcsec pixels
Video rate CCD finder camera to acquire science target
Video rate CCD "scoring" camera to assess real-time AO performance
Fully automated operation from UnISIS Control Room

Laser Guide Star System:

Excimer laser running with XeF
Laser power per pulse generally 75 mJ - 100 mJ per pulse
AO closed-loop operation selection 167 Hz or 333 Hz
In 167 Hz operation, laser produces 12.5 Watts to 16.7 Watts output power
In 333 Hz operation, laser produces 25 Watts to 33.4 Watts output power
Raw laser beam divergence 300 mrad
Laser beam is projected into sky with full 2.5-m aperture
Laser beam focus height 18 km above telescope or 20 km above mean sea level
Laser range gate is flexible but is generally set for 17.5 km to 18.5 km above telescope

Time gating on laser return signal set by Stanford Research Delay Generator in concert with dual Pockel's cell switch

351 nm Laser Guide Star Pupil Image 13 Aug 2004, telescope 20 degrees off-zenith, air mass = 1.20. Image captured just prior to tests on Alpha Ophiuchus	Laser Guide Star System Upgrades
	During the UnISIS development, obtaining sufficient laser guide star pupil brightness became one of the major challenges. As the image at left shows, this problem is no longer an issue. Many improvements were made along the way. The original UV wavefront sensor used only one polarization; it now uses both. The original wavefront sensor had a QE of 49%; a new one was purchased that has QE of 63%. Many very critical optical alignment and focus issues were solved during the instrument development period. Working at a UV wavelength of 351 nm makes the alignment issues a great challenge: the UV light is invisible to the eye and the short wavelength means that the tolerances are very tight. Note: As in all AO systems, the pupil is illuminated with an array of Hartmann images. If the wavefront CCD is moved laterally (in the pupil plane) 0.5 pixels in both x and y axis, the image at left would immediately take on a bright-dark-bright-dark checker board look because the individual Hartmann images would fall in the center of single pixels. Instead, you see a relatively smooth intensity distribution in the image on the left because the wavefront sensor is positioned so that the Hartmann subimages sit at the centers of 2x2 quad-cells. The irregular illumination captured in the image shows the extent of the aberrations present in the wavefront.

351 nm Laser Guide Star Pupil Image
13 Aug 2004, telescope 20 degrees off-zenith,
air mass = 1.20. Image captured just prior
to tests on Alpha Ophiuchus

Laser Guide Star System Upgrades

During the UnISIS development, obtaining sufficient laser guide star pupil brightness became one of the major challenges. As the image at left shows, this problem is no longer an issue. Many improvements were made along the way. The original UV wavefront sensor used only one polarization; it now uses both. The original wavefront sensor had a QE of 49%; a new one was purchased that has QE of 63%. Many very critical optical alignment and focus issues were solved during the instrument development period. Working at a UV wavelength of 351 nm makes the alignment issues a great challenge: the UV light is invisible to the eye and the short wavelength means that the tolerances are very tight.

Note: As in all AO systems, the pupil is illuminated with an array of Hartmann images. If the wavefront CCD is moved laterally (in the pupil plane) 0.5 pixels in both x and y axis, the image at left would immediately take on a bright-dark-bright-dark checker board look because the individual Hartmann images would fall in the center of single pixels. Instead, you see a relatively smooth intensity distribution in the image on the left because the wavefront sensor is positioned so that the Hartmann subimages sit at the centers of 2x2 quad-cells. The irregular illumination captured in the image shows the extent of the aberrations present in the wavefront.

Real-Time AO Images:

Please note that the images shown below -- except for the very last one -- were taken prior to Oct. 2004 when the UnISIS control computer was switched from the old slow system to the new Real-Time Linux system. A significant improvement in performance is expected from the new high-speed AO system under good atmospheric conditions.

Altair: Left Panel Shows AO-Off while the Right Panel Shows AO-On

The two-panel image to the left demonstrates the Natural Guide Star AO performance of UnISIS. On the far left is the AO-off image showing that the natural seeing on this night was about 2 arcsec. The panel on the right shows AO-on with a nice tight image core. Note that this AO-on image was taken with the star positioned near the center of a 2.11 arcsec diameter focal plane aperture. This aperture was created on a glass substrate by NOT aluminizing the 2.11 arcsec diameter aperture. So the AO corrected star light is seen reflecting off the bare glass surface where the reflectivity is in the range of 2.5% whereas the exterior region around the 2.11 arcsec aperture has ~90% reflectivity characteristic of a standard aluminum coating.

Altair: Left Panel Shows AO-Off while the Right Panel Shows AO-On
	The two-panel image to the left demonstrates the Natural Guide Star AO performance of UnISIS. On the far left is the AO-off image showing that the natural seeing on this night was about 2 arcsec. The panel on the right shows AO-on with a nice tight image core. Note that this AO-on image was taken with the star positioned near the center of a 2.11 arcsec diameter focal plane aperture. This aperture was created on a glass substrate by NOT aluminizing the 2.11 arcsec diameter aperture. So the AO corrected star light is seen reflecting off the bare glass surface where the reflectivity is in the range of 2.5% whereas the exterior region around the 2.11 arcsec aperture has ~90% reflectivity characteristic of a standard aluminum coating.

Natural Guide Star AO Images of a test star Alpha Ophiuchus - 13 Aug 2004:

H-Band Image Z-Band Image Explanation

The images on the left provide another example of UnISIS Natural Guide Star AO performance, this time in the dual wavelength mode. On the far-left is an H-band image at 1.65 microns (Strehl 0.38) and on the near-left the Z-band image at 0.85 microns (Strehl 0.20). The natural seeing was variable between 1.2 - 1.4 arcsec during these exposures. The 2.11 arcsec pinhole sets the image scale. Note that the NGS tip-tilt signal comes from the wavefront sensor data.

H-Band Image	Z-Band Image	Explanation
		The images on the left provide another example of UnISIS Natural Guide Star AO performance, this time in the dual wavelength mode. On the far-left is an H-band image at 1.65 microns (Strehl 0.38) and on the near-left the Z-band image at 0.85 microns (Strehl 0.20). The natural seeing was variable between 1.2 - 1.4 arcsec during these exposures. The 2.11 arcsec pinhole sets the image scale. Note that the NGS tip-tilt signal comes from the wavefront sensor data.

Laser Guide Star AO Images of a test star Alpha Ophiuchus - 13 Aug 2004:

H-Band Image Z-Band Image Explanation

Immediately after the two above images were taken with the NGS side of UnISIS, control of the system was switched to the LGS side at 167 Hz. The H-band image (again far-left) shows a Strehl ratio of 0.13 which seems to be characteristic of nights when the raw image quality is low. Only when the natural seeing is 1 arcsec or better does the UnISIS LGS performance reach its "theoretical" limit of Strehl = 0.28 at H-band. Why? When the seeing is poor, the LGS itself is blurred and hence the AO system cannot correct as well. There is no doubt that further performance loss came from the slow (167 Hz) correction rate.

H-Band Image	Z-Band Image	Explanation
		Immediately after the two above images were taken with the NGS side of UnISIS, control of the system was switched to the LGS side at 167 Hz. The H-band image (again far-left) shows a Strehl ratio of 0.13 which seems to be characteristic of nights when the raw image quality is low. Only when the natural seeing is 1 arcsec or better does the UnISIS LGS performance reach its "theoretical" limit of Strehl = 0.28 at H-band. Why? When the seeing is poor, the LGS itself is blurred and hence the AO system cannot correct as well. There is no doubt that further performance loss came from the slow (167 Hz) correction rate.

Natural Guide Star AO Image of Alpha Andromedae: Oct. 22, 2004

Log-Stretch of H-Band Image Line Cut through Image Explanation

After the upgrade to the fast Real-Time Linux AO control computer, UnISIS was run in the NGS mode for a single night. The seeing was terrible with uncorrected image FWHM = 2 arcsec or perhaps even worse. With the AO system running at 622 Hz, the image at the left was obtained on this second magnitude star. Because of the very poor seeing, it was difficult to precisely adjust the AO system and hence the tight image core is surrounded with a set of 4 faint "satellite" images from what is called a "waffle mode". Strehl = 0.2.