CONCLUSIONS


My observations have shown that the CCD system at the Perth Observatory can be used effectively for making photometric observations in all colours despite the poor sensitivity of the CCD chip to blue light. This poor blue sensitivity results in a very large (B-V) transformation coefficient which must be applied when transforming the instrumental magnitudes to the standard system. The visual and red transformation coefficients derived are much smaller but can still cause a modification of up to 0.05 magnitudes during the transformation procedure. For this reason, it is concluded that the PHEX program is unsuitable for the analysis of CCD data.

Problems were encountered in trying to derive the various coefficients. One of the biggest problems was due to the computer programs used having no means of identifying deviant observations. These deviant observations can be readily identified when the observations are plotted on a graph, and so this step is recommended as part of the reduction process for all-sky photometry.

The CCD observations show a similar scatter to the observations made using the photometer. Some of this scatter is very likely to be due to the limited air-mass range over which observations were conducted on some nights, however because of the similarity between the residuals for both systems, it is suggested that the limit of accuracy for all-sky photometry from the Perth Observatory is around 0.02 magnitudes. It may be possible to reduce this to some extent with the CCD if the standard stars were observed more often. In order to accomplish this without taking too much time away from observing the project objects, the standard stars need to be as bright as possible, to keep exposures as short as possible. Equations for determining the exposure time required for a given star brightness have been determined and can be used as a guide for determining exposure length.

The use of growth curves to more precisely determine the total ADU count for weakly recorded images was investigated and found to give good results for the majority of cases. Those stars for which it did not work well were those with very poorly defined growth curves. From this it is concluded that the use of growth curves is a valuable tool for analysing weakly recorded objects, especially if there are other stars in the image for which growth curves can be determined also.

The zero points show a decreasing trend, as would be expected, but there is some scatter in the values. From the size of the scatter it is concluded that some of the extinction coefficients have been imprecisely determined, most probably due to these observations being carried out over a limited air-mass range. For this reason, high accuracy is not claimed for these coefficients despite the results being consistent.

With the re-aluminizing of the telescope optics, the system will be more sensitive. This increase in sensitivity will not be uniform but colour-dependent, with the shorter wavelengths being affected most. As a result there is likely to be a change in the transformation coefficients in addition to a change in the zero points, and this will require their re-evaluation. Since the transformation coefficients are stable over a period of days at least, it is recommended that a sizeable amount of time be set aside on three or four nights in succession for observations to be made in order to determine these coefficients.

By making the observations on succeeding nights, variations in all coefficients, apart from first-order extinction, will be minimised and should permit a more precise determination than has been possible in this project.

It is suggested that within the VISTA program, routines be developed to automatically calculate ADU counts for a number of aperture radii and to automatically write the results to a file in a format suitable for analysis. It is also recommended that routines be developed to display in graphical form the values from which the various coefficients are to be derived. This will enable deviant points to be deleted and improve the accuracy of the results.

From a comparison of the observations made with the CCD system and the photometer, it was found that the main advantages of the CCD system was an ability to reach fainter objects, an ability to record a number of objects simultaneously and an extension of the detectable spectrum into the red and infrared. Due to the amount of data generated per image, the CCD system is rather inefficient when observing isolated objects. Another limitation is the dead-time while the CCD chip is being read. This limits its usefulness on objects with rapidly changing brightness. The residuals in both sets of observations were comparable.

A major constraint on this project was the amount of time required to analyse an image. A not inconsiderable time was required to transfer images from the Observatory computers onto floppy disks in order to examine them on the computers at Murdoch University. With each image requiring almost half a megabyte of disk space, only two images could fit on each disk. As a result the images could not always be analysed together due to insufficient disks. Facilities are being provided to transfer images by datatape, which should improve the situation substantially. Attempts have been made to use these facilities, however there have been continuing software problems which have hampered this process.

The development of routines within VISTA to permit ADU counts to be calculated for a number of specified aperture radii in one step and to store the results in a file, and in a form suitable for subsequent analysis, would be a major timesaver. This would eliminate the need to write down by hand and then enter the results from the keyboard for analysis, which is very inefficient and time-consuming and has the potential to be a rich source of typographical errors.