CCD Visual Estimates from the Starlight Express System (Abstract)

Volume 24 number 1 (1996)

Ronald E. Royer

David M. Phelps

Abstract

(Abstract only) About a year ago we realized that we needed something more than photography or the ST41 to image the anticipated subtle comet impact sites on Jupiter. We had heard of an excellent system being used in England and just now available here in the USA, so we bought the Starlight Xpress (SX)2 system, which worked well for us. Initially we had no doubt that our magnitude results were close to visual, since the Sony ICX027BL visual response chip, which peaks at 550 nanometers, was touted as being close to the human eye response. Well, it wasn't long before we realized that star fields without red variable stars seemed fine, but not fields containing them. The red stars were immediately seen as brighter on the monitor screen. So we purchased the tricolor dichroic filters that went with the system, and the infrared- (IR) blocking filter. The following is a brief synopsis of our recent testing, which we consider incomplete because of cloudy weather, and observing commitments. R Leo, at visual magnitude 7.0, came in at about 4.4 on an unfiltered image. The IR-blocking filter brought this down to about 5.4 on the monitor screen, and the computer software for photometry came in at 6.0. The green filter obviously leaks infrared light, coming close to the unfiltered image. The green filter plus the IR-blocking filter gave the best results. While T Pyx just barely showed in the image we took, the suspected variable star in the field came in at half a magnitude brighter than seen visually. Other than that, the image matches well the AAVSO e-scale preliminary chart (dated 2/93). The SU Tau field looked right on our CCD image, taken down to 16th magnitude in a 70-second exposure with a telecompressor at about f/4 on the 12.5-inch Newtonian scope. There was a gibbous moon up. For AL Com, we found that the 17.3 comparison star on the AAVSO e-scale preliminary chart images brighter than the 16.5 star, and the 16.5 star images about 0.5 magnitude brighter unfiltered. It is red! Our conclusions: with the Sony chip in the Starlight Xpress, the green dichroic filter plus the infrared-blocking filter works best for visual magnitudes, but some other set of filters could yet be tried. Though the computer reads out to a millionth of a magnitude, we are lucky to achieve a tenth. We have yet to figure the effect of light pollution on the field. We suspect it has a great effect on the computer analysis, but less effect on the visual scan of the monitor screen. As recommended in the instruction booklet with the Starlight Xpress, we only add images and do normal contrast stretches before photometry, which means no maximum entropy deconvolution. We will keep observing from light-polluted Lakewood as we keep testing such powerful but complicated equipment.