PMAS: Mars in 3D

During the early morning hours of August 2/8, 2003, Planet Mars was observed by German and Spanish astronomers using the 3.5m Telescope at Calar Alto Observatory in southern Spain. The unusual observation of such a bright object (-2.5 magnitudes) with a 4m-class telescope became possible only because thick clouds prevented the team from studying the faint sources of their original observing programme. The clouds, however, provided sufficient attenuation not to saturate the extremely sensitive detectors of the focal plane instrument. Mars is currently closer to earth than ever during the past 60.000 years and very well visible to the unaided eye as a prominent red spot. This unusual proximity has attracted a lot of public interest and activities of amateur astronomers world-wide, prompting the team to perform 3D Spectroscopy with PMAS , the Potsdam Multi-Aperture Spectrophotometer.

3D Spectroscopy (sometimes also called Imaging Spectroscopy ) is a novel technique providing spectral information for each pixel of a 2-dimensional image, rather than only for the limited spatial resolution along a traditional spectrograph slit. Up to hundreds and thousands of spectra are recorded in any single exposure at the same time, depending on the size of the instrument. This innovative technique has only become available recently through the progress in fabricating high quality large area CCD detectors and with cutting-edge optical fiber technologies.

The PMAS instrument has the unique feature of operating 2 cryogenically cooled, high sensitivity CCD cameras in parallel:
the Spectrograph Camera, collecting up to 1024 spectra simultaneously in one exposure, and the direct imaging A&G Camera, which is used for faint target acquisition and guiding. It is the A&G Camera which produced this direct image of Mars :
Mars-PMAS_AG_camera

Technical Information:

The picture was taken on August 2, 2003, with an exposure time of 0.2sec and a narrow-band filter in the green wavelength range of the optical spectrum (central wavelength 510nm, FWHM 3nm). The narrow bandwidth and short exposure time were necessary to avoid the saturation of the CCD detector. The orange appearance of the picture does not respresent the true natural colours of the object, but reflects a false colour intensity scale of the monochromatic image.
Orientation: North left, East down.

On August 8, 2003, the Spectrograph Camera was used to take a total of 2304 spectra, covering the entire visible disk of the planet, which had an angular diameter of approximately 24 arcseconds at the time of the observation. Since the Integral Field Unit (IFU), which samples the spatial image, has a size of 8x8 arcseconds squared (white frame in picture above), the whole measurement required to take 9 telescope pointings to construct a 3x3 tile mosaic image.

The dark circle in the center of the field-of-view has nothing to do with an eclipse of Mars by a hitherto unknown foreground planet !

The optical configuration of the instrument merely creates this persuasive appearance, since the image recorded by the A&G Camera is deflected from the focal plane of the telescope by means of a tilted mirror, whereas the IFU is fed through a circular hole in the middle of the mirror. It is this dark hole that creates the notion of an eclipse.

The PMAS IFU samples Mars in square spatial elements, each measuring 0.5 arcseconds on a side. During each exposure, the instrument records a spectrum for each spatial element of the IFU simulateously. This technique allows one to measure spatially extended objects very accurately, and with high efficiency.

560 nm (yellow) Wavelength ----> 710 nm (red)

This picture displays a fraction of 256 spectra out of the total of 2304, which were recorded for the entire mosaic. Each row of this frame represents a single spectrum, the greyscale code indicating the intensity as a function of wavelength. The wavelength scale of this observation extends from 560nm in the yellow (left-hand side) to 710nm in the red spectral domain (right-hand side). The dark vertical features correspond to absorption lines of the solar spectrum, which is in essence what we see when we observe planet Mars: reflected light from the sun, somewhat modified by the reflective characteristics of the planetary surface ("albedo").

Technical Information:

Observers :	Dr. Andreas Kelz, Dr. Martin M. Roth, Dr. Thomas Becker (AIP)
	Dr. Nicolas Cardiel (Calar Alto Observatory)
Telescope:	3.5 m Telescope at Calar Alto Observatory, Cassegrain Focus
Imager:	PMAS A&G Camera mit 510nm Filter, 3nm FWHM
Spectrograph:	PMAS Integral Field Spektrograph (256 spectra) , spectral resolution R=2700, wavelength range 560-710 nm
Observing Dates:	August 2 and 8, 2003, shortly before sunrise

Further Information:

Potsdam Multi-Aperture Spektrograph (PMAS)

Calar Alto Observatory

GEO-Marsnacht on Saturday, August 23, 2003, at Astrophysikalisches Institut Potsdam

"Tag der Astronomie" der VdS

Created by A.Kelz, translated and edited by M.M.Roth August 30, 2003.