Southern African Large Telescope

April 20, 2011

Summary

The imaging performance of the telescope is now near specification, with only minor astigmatism issues in the primary. After a stand-down of over a year, the telescope systems are now operating nominally. The scientific instruments have been upgraded and are now re-installed on the telescope. Normal science operations are expected to commence by mid-2011.

Image Quality

Soon after the telescope was put into operation in late 2005, it became obvious that there were serious problems with the quality of the images. There was a focus gradient over the field of view, and most star images exhibited complicated shapes. All of these effects varied with time and environmental conditions. Through an extensive and exacting program of observation and modeling, Darragh O’Donoghue traced the problem to displacements of the optics of the 4-element spherical aberration corrector (SAC) caused by mechanical stresses transmitted through an over-constrained interface to the telescope structure. A new kinematic mounting system was designed to eliminate these stresses. The SAC was removed from the telescope in April 2009. In addition to installing the new mount, the internal support system of the SAC was thoroughly tested, and the optical figure of all the mirrors was measured; everything was found to be performing at or better than specification. The spherical aberration corrector was re-installed in the telescope in August 2010, using the new kinematic mount, and quickly demonstrated that the image quality problems had been solved. On August 28, in a night of good seeing, the following image was obtained:

SALTICAM, the standard SALT Imaging Camera, was off the telescope for an upgrade, so the image above is a 3x3 mosaic taken with a commercial-grade Apogee CCD system. The obvious ghosts below each star arise within the window of this camera; SALTICAM is free of such artifacts. The image FWHM is 1.1 arc-sec and is uniform over the field.

Testing in the ensuing months has shown that there is some astigmatism in the images, which seems to arise from errors in the software that aligns the primary mirror segments. This is currently being traced, and it is expected that a small additional improvement in image quality will be obtained when the astigmatism is eliminated. Meanwhile, the telescope is fully usable for scientific observations.

SALTICAM was reinstalled on March 28, 2011. The following is a 3-color composite image taken of the first night of commissioning. The telescope was slightly out of focus, and the astigmatism is visible in some of the stellar images. All modes of the instrument have now been verified, and early science observing is beginning, as well as acquisition support for the RSS.

SALTICAM first-light image of NGC 2808. The dark line in the center of the image is the gap between the two CCD detectors.

SALT had a period of good seeing in April, and the following image and analysis demonstrate that the system is delivering sub-arcsecond images:

SALTICAM image of NGC 4052. Image taken on April 12, 2011 in good seeing conditions.

Analysis of NGC 4502 image of April 12, 2011

Robert Stobie Spectrograph

The Robert Stobie Spectrograph (RSS) a versatile instrument, with imaging, long-slit, multi-slit, Fabry-Perot, and polarimetric modes. It was installed at the prime focus of the telescope in October 2005, and commissioning and early science observations were carried out over the following year. One of the design features of the spectrograph is high sensitivity down to the atmospheric UV limit. Although the instrument had good UV throughput when initially tested in the lab, its short wavelength performance on the telescope was disappointing. RSS was removed from the telescope at the end of 2006, and after an extensive series of tests, the throughput problems were found to arise from the chemical interaction of the optical coupling fluid in its lenses with the materials used for the fluid reservoirs. This was rectified, but unfortunately during reassembly of the optics a CaF lens was broken and had to be replaced, delaying the reinstallation of the spectrograph over a year. While the lens was being re-fabricated, all the mechanisms of the instrument were re-worked, and RSS is now highly reliable in its operations. The spectrograph was re-installed on the telescope on April 6, 2011.

RSS being installed on tracker, 4/6/2011

The imaging characteristics of the spectrograph were verified with observations of the Galactic globular cluster Omega Cen, taken on April 9:

“First Spectrum” was taken on April 15 2011. The recurrent nova T Pyx erupted on April 14, and SALT observed it as the first RSS target, using the highest resolution long-slit mode:

RSS is continuing to monitor the evolution of the T Pyx spectrum.

Prior to its removal from the telescope, all the modes of RSS were tested. On September 22, 2006 we used the Fabry-Perot system to scan a Galactic planetary nebula as a flux standard, and to map the Hα velocity field of the barred spiral galaxy NGC 1365.

NGC 1535

Planetary nebulae are the outer shells of intermediate mass stars thrown off into space at the end of the star’s lifetime. The residual hot core of the star ionized the gas, causing it to emit strongly in a few spectral features, with forbidden OIII and H alpha the strongest two lines. Many planetary nebulae in our Galaxy have been extensively studied, and they provide bright, convenient, well-calibrated emission line sources.

The thumbnails below are selected from the scan of NGC 1535. The top row has linear intensity stretch, the bottom row logarithmic. The bright central part of the nebula is about 20 arc-seconds in diameter. The plot shows a quick look of the spectrum. All of these images were prepared by Eric Burgh. Measurement of the total flux of the nebula verifies that the Fabry-Perot throughput is meeting its specifications.

NGC 1365

NGC 1365 is a relatively nearby barred spiral galaxy. These galaxies have flattened disks, with spiral arms containing young blue stars and glowing gaseous HII regions in the disk. Barred spirals have a linear bar structure in their central regions, consisting of mostly intermediate-age and older stars. Several Rutgers astronomers have been focusing their recent dynamical studies on these types of galaxies, exploiting the effects of the bar to pin down the amount of dark matter in the galaxy.

The first image below is a color composite image of the galaxy taken with the SALT Imaging Camera in three bands (blue, green and near infra-red), with a total exposure of 60 seconds in each of the bands. Steve Potter of the SAAO processed the data, producing this (nearly) true-color image. The images were obtained on September 21, the day before the Fabry-Perot first light observations.

The next image is one of the first-light series of Fabry-Perot images of the galaxy. In contrast to the full-color image above, this image was taken through the very narrow

(0.85 nm) spectral bandpass of the low resolution mode of the instrument. The Fabry-Perot etalon was tuned to the center of the H-alpha spectral feature (at the redshift of this galaxy) and the exposure time was 5 minutes. (This image shows the monochromatic intensities at this wavelength; the image is rendered in false color, where the color scale shows the intensity variations.) The image was prepared by Eric Burgh.

The Fabry-Perot is normally used to obtain a series of images covering a spectral region of interest, producing a “data cube” of intensity as a function of position in the image and of wavelength. This dataset is analyzed to extract maps of the physically interesting properties of the object observed. In these first-light observations, we obtained an initial scan of 18 images, each of 1 minute exposure time, covering the (redshifted) H-alpha region of the spectrum. A shorter scan of 8 images, each of 2 minute exposure, was then taken concentrating on the wavelengths close to the line. A final image of 5 minute exposure was taken near the peak of the line. All of the images were included in the data analysis, which was carried out by Rutgers graduate student Ricardo Sanchez.

These data are fit in the wavelength dimension at each point in the image, producing thousands of spectra. The fitted peak heights and wavelength centers in each of the spectra are combined to produce maps of the intensity of the H-alpha emission and of the motions of the gas clouds in the galaxy. The image below shows the map of H-alpha intensity. Only the regions where high-quality fits (velocity precision better than 20 km/sec) were obtained are included in this map. It is false-color encoded as detailed by the color bar at the bottom of the image; the numbers are approximately the number of photons received in a 2-minute exposure.

The final image presents the measured velocity field of the galaxy. The color represents the line-of-sight velocity at each point in the galaxy. Green areas are moving at the average (i.e. systemic) speed of the galaxy, blue regions are coming toward us with respect to the average speed, and red regions are moving away. The color bar at the bottom of the image shows the speed in km/sec corresponding to each hue. The organized patterns of motion show that the galaxy is rotating about its center. The rotation pattern is distorted by the gravitational effect of the bar. The details of this rotation map provide clues to the amount and distribution of matter, both luminous and dark, within this galaxy.

In summary, these images demonstrate that the Fabry-Perot system of the spectrograph was working to specifications while it was on the telescope. Subsequent testing of the instrument has verified that it continues to operate nominally. A team of SALT Astronomers and observers from partner institutions under the direction of T. Williams will recommission the Fabry-Perot system in in the period May 1-14, 2011 and begin normal scientific observations by midyear.