With recent advances in Charged Coupled Devices (CCDs), it is now possible to do high speed CCD photometry. Though photoelectric photometry has a rich history and many years of software development, CCD photometry has yet to develop the necessary tools. There are two areas where advancement is needed: decreasing the CCD dead time and producing real time light curves. Here we present the steps we have taken to address these two issues. We have examined various CCD geometries to shorten dead times and written scripts to process the data more efficiently. Our work has produced efficient methods for obtaining and reducing high speed
CCD observations and brings us a step closer to producing real time lightcurves.
When taking data, Charged Coupled Devices (CCDs) are becoming the observational astronomer’s tool of preference. They have many advantages over photoelectric photometers
(hereafter, photometers). CCDs have a quantum efficiency of 80−90%, while photometers are only 30−40% efficient. Photometers have a fixed set of apertures with one photoelectric tube per aperture and each responds slightly different to light. CCDs encompass the entire field of view and the response of different pixels in the CCD is well understood and can be easily corrected.
With their fixed set of aperture sizes, photometers are unable to do crowded field photometry, as unwanted stars may drift in and out of the aperture. Also comparison stars must be chosen ahead of time and cannot easily be changed during the night. With CCDs this is not a problem, as the entire field of view is saved. This allows data reduction using varying apertures that can then be

compared to ensure that only flux from the target star is observed. Also comparison stars can be chosen at the time of reduction, with as few or many as the observer wants.
CCDs do have some shortcomings that photometers do not. Photometer data takes up very little space, storing three numbers per integration. CCD frames take up about one byte per pixel, so every frame on a 512 x 512 CCD takes up roughly 262 kilobytes. The photometer software can also produce real time light curves, adding points as the night progresses. Having real time light curves allows the observer to ensure that the equipment is all working properly.
This has yet to be developed for CCDs, so light curves are often not produced until days after the data were obtained.
Photometers have very short readout times (on the scale of nanoseconds) while CCDs can have long dead times (even several tens of seconds) between exposures while the software saves the frame. Different types of variable stars vary on different time scales. If a star has a pulsation cycle on the order of a couple hundred seconds and a significant fraction of the time observing can be spent waiting for the CCD to readout, the results obtained will not be accurate enough. By cutting down the dead time, more data points can be acquired per pulsation cycle, increasing the accuracy of the results.
The advantages of CCDs as instruments far outweigh the disadvantages. However, CCD software needs to be improved. The basis of this research was to shorten CCD dead time as much as possible and produce scripts that will process the data much quicker, the first step to producing real time light curves.
There are two main things one can do to shorten the dead time between CCD exposures: subframing and binning. Subframing is specifying a region of the chip and having only the selected part saved. By cutting down the amount of pixels to be processed, the readout time is lowered, resulting in less dead time and more time observing the desired signal.

Binning is a method of adding light in adjacent pixels. For example, by binning the chip
2 x 2, the chip reads two pixels on each axis into one, making one pixel out of four which has then four times the amount of light it normally would. Binning allows the same field of view to be recorded, but the chip to be read much faster at the cost of some resolution.
Readout times (taken using Apogee Camera Driver software for Linux) of various combinations of geometry were tested and are given below in Table 1. When binning higher than 2 x 2, the time saved was minimal so we opted on most nights to use half the chip and bin 2 x 2 to maintain spacial resolution.
Bin −>
Subframe
Whole Chip
½ Chip
¼ Chip
1 x 1 2 x 2 3 x 3 4 x 4
15
10
10
6
5
5
4
4
4
8 x 8
3 2
3
3
2
2
Table 1. Various geometric combinations and the corresponding readout times.
Times are given in seconds.
Data were obtained during an observing run using the Fick Observatory 24−inch telescope at Cassegrain focus from June 14−July 2, 2001. Twelve nights of data were obtained during this run. The target star was KPD1930 (Figure 1), a pulsating subdwarf B star in a binary system with a white dwarf companion. The period of the binary system is about 2.1 hours, while the pulsation have periods of approximately 250 seconds. Two software packages were tested:
Apogee Camera Driver for Linux and CCDSoft for Windows.

Figure 1. The field around KPD1930.
The target is marked #1 and the other
6 stars are comparison stars. The field is 5 arcminutes by 2.5 arcminutes.
Advantages for the Apogee Camera Driver software include easy binning and subframing. Also there is the advantage of running the software on a Linux operating system, which tends to be more reliable than Windows operating systems. This software allows the observer to type in information that it puts directly in the images’ header, which leaves less things to be added manually at the time of data reduction. Also, since IRAF is operated under
Linux, it can be run in parallel to the software for real time light curves.
However, during the run, many disadvantages to this software were discovered. First was the high bias level it read from the chip, roughly 35,000. Also, it read out extra lines on the sides of the image. Because of this, it was necessary to trim the images, adding an extra step to

the processing script. We also discovered bugs in the software that would cause the chip to be read improperly during a night, such as blank images, or the image being inverted for several frames. These bad images can lead to jumps in the coordinates because eliminating the bad frames is like a time gap during which the telescope may drift. This problem has to be corrected by stopping and restarting the scripts where data gaps occur. These problems are most likely due to improper recognition of the CCD camera.
CCDSoft recognizes the Apogee AP7 as such and the CCD is consistently read properly.
However, CCDSoft has its share of disadvantages. Being run under windows does not allow
IRAF to be run simultaneously. It also does not have capabilities for zero (bias) frames to be taken in order to correct for the bias level. It can bin the frames, but does not have the option of subframing that is so useful in lowering readout times.
With such an abundance of data, we need an efficient way to reduce and analyze it. We averaged over 1000 frames per night, so manually reducing it frame by frame is not an effective method. Scripts are a convenient tool to run the repetitive tasks such as completing the same processes on many frames.
Reductions are done using IRAF (Image Reductions and Analysis Facilities). During processing we trim the images and edit the headers to add information not included by the acquisition software. Dark and zero (bias) frames were subtracted out to correct for the chip’s dark current and bias level; flat frames were divided out of the object frames to correct for any uneven response to light across the chip due to the telescope.
Once the calibrations frames are applied using CCDPROC, counts for the target and comparison stars are determined using the IRAF PHOT package. PHOT uses aperture sizes

specified by the user to determine star and sky counts. This and other information is then saved to a keep file. In order to run PHOT, coordinates must be set for each star of interest. The first frame is done manually, while successive frames are passed the coordinates of the previous frame, which have been recalculated using IMCENTROID (run within PHOT). If there is a large telescope jump, causing the stars to shift too far for IMCENTROID to find, the coordinates are manually reset and the process begins again. With the information from the mag files, light curves and Fourier transforms of the variable star for each night can be produced. Once multiple nights are obtained, a Fourier transform of the composite data can also be produced.
By taking advantage of Tc and Sed commands, image processing can be done quickly.
Tc shell scripts are written to manipulate the files; Sed scripts are written to edit the files, including automatically updating Tc and Cl scripts. Cl scripts are then used to run the IRAF processing routines.
Our completed scripts can successfully reduce a night’s data in a few minutes time. Future scripts can be written to be used at the telescope while taking data to produce light curves closer to real time, like a photometer.
Once the data were obtained, the scripts written and tested, they were run on each night’s data set. For the first few nights there were several stops due to coordinate jumps. These jumps were caused by moving the telescope to bring drifting stars back into the frame. As the nights progressed, we solved this problem by utilizing the telescope’s tracking rates which correct for and avoid large shifts.
In data sets with no large jumps, the PHOT scripts ran without interruption, reducing 1000+ frames in a few minutes.
Having to process each frame individually would have taken many hours worth of work.
With these scripts and fewer breaks in the data, we are one step closer to producing real time light curves.