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The Galaxy Gallery Page VII: Infrared Imaging


More Telescope Data Directly to the INTERNET

Data Obtained at Kitt Peak at 3:30 UT March 24

The following three images were obtained at the 1.3-m telescope and then directly transfered to my WWW server as a further example of rapid distribution of data to anyone interested.

These observations used an Infrared Imaging Detector that can simultaneously make observations through 3 or 4 different filters. The very large surface brightness of the sky at near-IR wavelengths is primarily due to emission associated with water vapor and/or OH clouds at high altitudes. As these features are so much stronger than scattered moonlight, the presence of a full moon does not appreciably contribute to the sky brightness at these wavelengths. The three filters used in these observations are J, which is centered at 1.2 microns, H, centered at 1.6 microns, and K which is centered at 2.2 microns. The background at K is also sensitive to the temperature of the observing environment which, presently in the dome is 4.3 degrees C. Colder temperatures mean lower backgrounds, hence the interest in the K-band sky brightness at the South Poles.

The detector is a 256x256 Platinum Silicite array which has low quantum efficiency but very uniform properties. If an optical picture of this galaxy, NGC 1600, an elliptical, were taken with this image area it would fill the whole detector. However, in these infrared bands only the very center of the galaxy can be seen because the sky brightness is so very high. In the J-band the sky brightness is 1000 times that in the optical and in the K and H-bands it is 10000 times brighter. Hence, the J-band extent of this galaxy is much larger than the K-band extent. The first three images are the raw images. One example of a sky-subtracted frame is also provided. Since the brightness of the sky background, especially in the H-band, can vary appreciably on time scales of minutes it is not always guaranteed that the sky reference frame will be the same level as it was when you took a picture of the galaxy. Thus the procedure has to be repeated many times so that these variations can be averaged out. The exposure time for these pictures is 120 seconds. Again the telescope aperture was 1.3-meters.

An example sky subtraction is shown below. This is for the H-band. You should see the the galaxy is now larger (more of it has been detected) after this very high background has been subtracted off.

Bright Objects like Globular Clusters , however, can easily be detected against the very bright background. This is an infrared (K-band) image of M2. The effective band wavelength is about 2 microns which is near the peak of the spectral energy distribution for old, cool stars in a globular cluster. The light from the cluster at 2 Microns is dominated by Red Giant Stars. Since the typical ratio of main sequence to giants stars in a globular cluster is 1 to a few hundred, we are seeing, in this photograph, this minority population of very bright stars. Hence the core of the cluster is not burned out in this image.

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