EQUIPMENT


Note: This thesis was written in 1993. Although the same CCD is being used, much of the ssupporting equipment, especially computers has been upgraded over the past 15 years. This section describes the equipment and software I used for the project. It is amazing just how far astronomical equipment has come in that time. Much of what was then cutting-edge technology, is now routinely used by back-yard observers!


4.1 Telescope

The telescope used in this project was a 61 cm f13.3 cassegrain situated at the Perth Observatory, some 35 km east of Perth in the Darling Ranges. The instrument can be controlled manually and by using a 386SX IBM compatible computer. For this project both modes were used.

4.2 Charge-Coupled Device Camera

The CCD chip is a Thomson-CSF TH7883, front-illuminated, unthinned chip with an imaging area of 576 pixels by 384 pixels. Each pixel is square, measuring 23 ým per side, which at the focal plane of the telescope, means each pixel covers 0.58 seconds of arc. Therefore the total field of view is 331 x 221 seconds of arc. To minimise dark current the chip is cooled to about -100░ C with liquid nitrogen. Readout noise is about 30 electrons and the gain is 10 electrons per Analog-to-Digital Unit (ADU).

The chip has a spectral response from about 4000 A to about 10500 A so the CCD is sensitive from violet through to the infrared, although the sensitivity of the chip in the blue end of the spectrum is dying away very quickly. This means that blue filter images require substantially longer exposures to reach the same limiting magnitude than the other colours.

A second 386 SX computer controls the CCD camera, looking after such things as filter choice, exposure length, shutter operation and data acquisition. This may be operated in either a fully automatic or interactive mode. Due to some lingering bugs in the automatic mode software, the CCD system was operated in interactive mode for this project.

Image display and storage is handled by a 486 computer. Readout time is approximately 15 seconds, with a similar amount of time to clear the previous image, so the current dead time between images is about 30 seconds for identical exposures and longer if filters and exposure times are changed.

Saturation of the pixels theoretically occurs when the ADU count reaches 65536, however problems with image smear and incomplete charge transfer are expected to become noticeable before then. The filters for the CCD system are a Cousins filter set made from glass.

4.3 Photometer

Observations were also undertaken for comparison purposes using the photoelectric photometer currently in use on the telescope. The photometer was custom-built by staff at the Lowel Observatory. Photon detection is by an EMI 6256b photomultiplier tube which is sensitive to light between 3000 A and 6500 A. It is a DC integrating photometer where incoming photons produce an electric charge which is measured over a preset interval of time and recorded on floppy disk for later analysis. The photometer is fitted with ultraviolet, blue and visual filters. For this project, observations were made in the blue and visual bands. The photometer filters used in this project were standard blue and visual Johnson filters with transmission characteristics very similar to the CCD filters shown below.

4.4 CCD Filters

Blue, visual, red and infrared filters are fitted to the system. Unfiltered images can be made using a CLEAR filter setting which places an empty filter holder in front of the CCD. For this project, only the blue, visual and red filters were used. These are standard Johnson-Cousins filters and the transmission characteristics of each filter used in the project are shown in the diagram below, along with a typical response curve for a Thomson-CSF TH7883 CCD chip.

4.5 Image Analysis Software

Image analysis was undertaken using the VISTA package developed by staff at the Lick Observatory. The original package has been modified and upgraded by R. Martin at the Perth Observatory and it is this modified version that has been used. There are a large number of routines within this package, many of which had no relevance to this project.

Images are read into the package and stored in special memory allocations called buffers. Subsequent analysis is done using these buffers. The most frequently used routines in this project were the arithmetic routines for finding the mean of several images and for flat-fielding; the TV routine for displaying images; the MARKSTAR routine to identify to the computer objects of interest in the image; and the APERSTAR routine to perform aperture photometry on the objects selected using the MARKSTAR routine. It is the APERSTAR routine that calculates the ADU count for the object under study.

4.6 Photometric Analysis Software

Two programs have been used to reduce the image analysis output to a series of magnitudes and coefficients.

Analysis was first undertaken using the PHEX program developed by D. Harwood at the Perth Observatory. This program was originally developed for use with the photometer and investigations were undertaken to examine its suitability for the CCD system.

This program takes as input the ADU count from the observations and converts them into a magnitude. From the time of the observation and the object's position, the air-mass is calculated using equation.

Using observations of standard stars, the extinction coefficients and zero points are derived by one of the methods described earlier. Deviations from the catalogue magnitudes are calculated and stars which have a residual of more than two standard deviations are excluded from the analysis. The coefficients are then used to calculate the magnitudes of other objects and the results are printed out in a list.

A second program by Kaitchuck and Henden called Astronomical Photometry with an IBM PC was used when a problem was discovered with PHEX. This package has a number of routines which are accessed individually, and the order may be varied. First, the ADU counts are converted to instrumental magnitudes. Following this there is a choice of three methods of calculating the extinction coefficients, or the second order extinction may be calculated. Alternatively, the extinction coefficients may be calculated concurrent with the transformation coefficients. These coefficients can then be used to calculate magnitudes and colour indices which can then be printed out.

This package does have a disadvantage in that unless all objects observed have been observed in more than one colour and with exactly the same set of colours each time, the program will not reduce those observations.

4.7 Standard-Star Lists

Two catalogues of standard stars were used in this project. The first of these is a list published by Arlo Landolt in the Astronomical Journal in 1983 and 1992. This catalogue is a list of several hundred stars situated near the celestial equator that are suitable for use as standard stars. The catalogue gives magnitudes and colour indices to three decimal places for the ultraviolet, blue, visual, red and infrared bands.

The second catalogue is the Zenith Star List. This catalogue is a subset of the Photometric Catalogue published by the United States Naval Observatory which covers standard stars which pass close to the zenith as seen from the Perth Observatory. This catalogue gives magnitudes and colour indices to two decimal places generally and occasionally three decimal places, for the ultraviolet, blue and visual bands. Some problems were encountered with imprecise positions being given in the catalogue and so positions for stars observed on the final three nights were derived from the Smithsonian Astrophysical Observatory catalogue.