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Goddard Space Flight
													      Center Sonoma State University
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Technical Parameters

for the
Instrumentation and Observing Site
of the
GLAST Optical Robotic Telescope (GORT)


  • Longitude: 122o 41' 15'' W
  • Latitude: 38o 33' 55'' N
  • Altitude: 335m = 1099 ft
  • Time Zone: PST (-8), PDT (-7)
Telescope The GORT system is a 14-inch f/11 Schmidt-Cassegrain telescope. The telescope is a commercially available system made by Celestron International.
Aperture 14 inches = 356 mm
f/ratio f/11
Focal Length 154 inches = 3910 mm
Field of View 12 x 12 arcminutes
CCD Camera GORT uses an AP47 CCD camera manufactured by Apogee Instruments. The AP47 can normally be set to operate at 40o C below ambient. At Hume this means about -20o C in the summer and -30o C in the winter.
CCD chip Marconi 47-10
number of pixels 1024 x 1024
size of pixels 13 x 13 microns
Pixel Scale 0.68 arcsec/pixel
CCD Gain 1.6 e-/ADU
CCD Readnoise 11.3 e-
Dark Current 0.3 to 0.4 e-/pixel/second
Typical Seeing* 4 arcseconds
Light Sensitivity** V=18 in 120 seconds


Seeing refers to the steadiness of the atmosphere. It is measured in terms of the observed size for a stellar image. All stars are point sources of light because of their great distances from us. In theory, the size of the image of a point source is given by the size of the diffraction pattern for the particular optical system. However, the atmosphere blurs this basic pattern, making the seeing disk for a star, or the smeared out diffraction pattern, many times than the the actual diffraction pattern. This seeing disk is the ultimate limit to resolution for ground-based astronomy.

Real stellar images approximate a Gaussian distribution in brightness, with a central peak and broad exponential wings. At Hume, this Gaussian typically has a FWHM of around 4 arcseconds, so one should not expect to see detail smaller than this in images obtained with GORT; objects this size or smaller can not be distinguished from stars. Furthermore, features this size or smaller on extended objects (planets, say) are also not distinguishable. Given our 12x12 arcminute field of view, the most optimally sized objects to be imaged with our system would have sizes between 5 and 10 arcminutes. Objects on the order of an arcminute or so are easy to identify. Objects smaller than about 10 seconds of arc become difficult to distinguish from stars.

**sensitivity to light

GORT can reach a limiting magnitude of V = 18 with an exposure time of 2 minutes = 120 seconds. The actual limiting magnitude reached will always depend on the specific conditions when the image is obtained. Bright sky or poor seeing or poor focus will diminish the attainable limit. Of course, the actual limit will also depend on how well the telescope system will track the sky. GORT can reliably obtain untrailed single exposures of about 2 minutes. If longer exposures are needed then several exposures must be obtained and then aligned and stacked or co-added to produce longer effective exposures. All the standard image analysis software systems are capable of combining images in this manner.

We can also effectively increase the sensitivity using on-chip binning. This procedure combines several adjacent pixels into one "super pixel" for readout. The sensitivity boost is achieved at the cost of decreased resolution. The angular size of the images remains the same regardless of binning.

Binning Resolution Relative Sensitivity Boost
1 x 1 high 1
2 x 2 medium 4
3 x 3 low 9

Verifying the Field (finding charts)

When everything works as it should, GORT can point to within 1 arcminute of any specified target. (The pointing model formally indicates that this telescope should be able to point to within 10-20 arcseconds of the specified target. Special pattern matching software should ultimately be capable of automatically centering a specified object to within the size of the seeing disk for a star.)

If an object is bright and obvious, it should be easy to detect in the image. If necessary, the image can be fine-tuned or framed by moving the telescope slightly. However, if the target object is faint, or small, or simply a star field, it may be necessary to have a finding chart available that shows the appearance of the star field and identifies the target object.

If you have a question about the GTN, please contact one of the "Responsible SSU Personnel" below.

This page was last modified on Friday 01st September 2017 @ 09:15am

Science Mission Directorate Universe Division

Responsible SSU Personnel:

Dr. Kevin McLin (mclin at universe dot sonoma dot edu)

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