February 28 Telescope Run

On Feb 28, 2001, we opened the dome at Leuschner and imaged a number of point sources with the tv camera mounted to the Thirty. Our objective was to determine the reponse characteristics and point spread function of the tv camera/frame grabber system.

We imaged several calibration stars (see list below) over a range of flux, and several groups of stars in Praesepe (NGC 2632). In addition, we attempted to image the globular cluster NGC 5024 (M53) and the Sb spiral NGC 5194 (M51). These extended sources were too low in surface brightness, and evaded detection (if I may anthropomorphize...)

Calibration stars:

SAO 115756 0.35 F5V (Procyon)
SAO 60198 1.59 A2V (Castor)
SAO 100384 2.83 G8III (Eps Vir)
SAO 82550 4.90 G8III
SAO 82650 5.99 F4V

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Stars imaged with the GBC CCD-500E TV camera.

Below are images of of our calibration stars. All but the 6th magnitude star are saturated. Note that these images are in focus. Properly focused, they should be circular, of course, and fall off like Gaussians (except of course where saturated). Point sources provide our best diagnostic of the optics.

Magnitudes, left to right, are 0.35, 1.59, 2,83,4,90, 5.99. Axes are labelled in pixels.

The plots below show analysis of the 6th magnitude calibration star. The leftmost plots are the (dark- & bias-subtracted) data. The center plots show the PSF (a 2-D Gaussian fit). The rightmost plots show the residuals (on a different scale). The FWHM is 7.8 arcsec. Also of note is the asymmetry apparent in the image. The PSF has FWHM of 7.9" on the vertical axis and 7.7" on the horizontal axis. This, of course is what we need to repair! It should also be noted that the PSF was not nearly so bad originally -- our alignment of the secondary has brought the telescope further out of collimation (temporarily!). Note: the pixel size for the detector was determined by imaging the binary star system Cor Caroli, which has a known separation of 19.4 arcsec. Measuring the offset of the centroids in pixels provided the plate scale of 0.33 arcsec per pixel for the Thirty.

Images of Praesepe fields.

In order to evaluate off axis response of the telescope, we observed several fields in the open cluster M44, also known as Praesepe or the "Beehive Cluster."

Below are two of the three fields in Praesepe, labelled with radec for compaison to the finder chart. The three stars in the first field (left) have magnitudes of 6.5, 7.1 and 7.5. All are clearly present and unsaturated. Note the asymmetry of the stellar images. The second field (right) has stars of magnitude 6.4, 7.5 and 9.5, the latter barely visible. In addition, there is a star with magnitude < 10.0 located approximately 20 arcsec up and to the right of the 6.4mag star. This dim star was clearly visible on the television monitor, but is basically undetectable to the frame grabber. The 9.5mag star is also to faint to give reliable information with this system. Our useful magnitude range is thus about 5.5mag - 7.5mag. Again, note the asymmetry in the 6.4mag star.