Data taking and observing procedures

All data and instruments are controlled with a simple front-end GUI. We have also written a hand- paddle and secondary focus tool to facilitate dithering and focus changes that minimize the need to communicate with the telescope operator. A screenshot example of the GUI is shown in figure 10.5.

All of the imaging data are stored on a data partition of the JCAM computer hard drive. Usually, after the data are written to disk, the remote observer copies the data to the remote machine to inspect the images; this can be done individually or by periodically synchronizing the remote and JCAM data

directories (using, for example, the password-encrypted RSYNCcommand). As the images are rather

small (527 MB uncompressed; 150–400 MB compressed) the transfer times require between 1–25 s per

5 _{Seehttp://gcn.gsfc.nasa.gov/gcn/} 6 _{Seehttp://space.mit.edu/HETE/Bursts/}

10.4. OPERATIONS AND PERFORMANCE Chapter 10, p. 146

Figure 10.5 Screenshot of the JCAM graphical user interface (GUI). In the main window of the GUI, a real-time summary of the current pointing, time, etc., is provided at top. Object name and frame comments are entered by the user and the observing mode (“EXPOSE” for beginning exposures on both CCDs or “SNAP” to begin the exposures separately). The filter, integration time, and frame type are chosen individually for each camera. A status display for each camera shows the CCD temperature, exposure status, and current position angle of the exposure. A small compass rose shows the sky orientation of the CCD, with the longer line pointing North and the shorter line indicating East. Due to the dichroic, there is a y-axis flip between the two CCDs. At the bottom of the main window is a text log, which is also saved with time-stamps to a log file. Offset from the main window are two smaller windows, one to control the rotator angle (top) and the other to aid in small changes in the telescope positioning (dithering) and secondary focus (bottom).

Chapter 10, p. 147 10.4. OPERATIONS AND PERFORMANCE

image, limited only by the bandwidth between Palomar and the remote observer site. The typical transfer time between Palomar and Pasadena is 4 s for an uncompressed image.

We decided not to install the guider camera after realizing, by comparison with the point-spread function of images obtained contemporaneously in the same filter at the 60 inch telescope, that 200

inch Telescope tracks very well over times_∼<300 s. Aside from an occasional smearing due to telescope

jump upon wind-shake (which equally degrades guided-images) the image quality on long exposures is comparable to the image quality of short exposures. Nevertheless, to minimize tracking errors, we typically integrate for shorter time periods than is usual with larger-format guided imaging.

The shorter exposure times, typically integration 100–250s, is warranted by the short read times

(9.5 s) and the fact that the frames are sky-limited after_∼<2 sec of integration time, except inU-band

where the sky dominates after about 30 s; the exact times depend, of course, on the sky brightness contribution from the moon. More frames per field, when dithered between exposures, also allow for the construction of better supersky flat fields and the removal of cosmic rays and CCD defects. When the seeing and/or transmission changes rapidly, more images can also be useful in constructing a higher signal-to-noise summed image of the field (that is, by giving lower weight to those frames with lower signal-to-noise detection of point sources). Note that thanks to the large collecting area of the 200

inch telescope, JCAM imaging is never dark current limited despite being a thermoelectrically cooled

system.

Depending on the science objectives, we typically observe at least 5 frames simultaneously in the

Sloan r0 _{and g}0 _{for 100 s and 110 s, respectively. The 10 s difference in exposure time allows JCAM1}

(Sloan g0_{) to finish exposing just as JCAM0 (Sloan r}0_{) finishes readout. For the other par focal set,}

Bessel I and Bessel U, we typically acquired two 100 s frames in I band while exposing U band for

200 s. (Note that the filtersB/V and R are largely superseded in efficiency by the filters Sloan g0 _and

r0_{, respectively; see fig. 10.3). We have two modes of taking many frames automatically (“Multi” in}

fig. 10.5), one which begins an exposure as soon as the camera is finished reading out and the other which opens the shutters of both cameras simultaneously. The latter mode, when the exposure times of both cameras are equal, is particularly useful when conditions are non-photometric since both images are exposed through the same cloud pattern, preserving the relative flux of objects in the two bandpasses. As a result of shorter exposure times, a given field requires more exposures for a given depth. In a typical full night of science imaging, we have typically generated between 600–800 images. We found that the rate of frame acquisition is too high to adequately log the frames by hand. As such, we wrote an electronic logbook program in Tcl/Tk which automatically creates a new line for each new image that is acquired. The pertinent header information is shown and the user can add comments to each line and then save the logbook to text, Postscript, and graphical output. Aided by the existence of

electronic logs, we have begun to archive each observing run in a uniform set of web pages7_.