A Search for Distant Satellites of Neptune
Michael J.I. Brown and Rachel L. Webster, PASA, 15 (3), 325
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Contents Page: Volume 15, Number 3

Observations and Analysis

2 nights of observations were obtained on 30 September 1997 and 1 October 1997 with the Mount Stromlo and Siding Spring Observatories' 40-inch telescope. Our detector was a thinned Tektronix CCD with a pixel scale. The primary purpose of the observing run was obtaining photometry of the South Galactic Pole (SGP) so observations of Neptune were restricted to an hour each night when the SGP was low in the sky. Weather conditions on the first night were reasonable with seeing but on the second night thin cloud produced seeing and magnitudes of extinction. Each CCD exposure was in the R band and the total integration times on the two nights were and .

Each image was bias subtracted and flatfielded using IRAF. Fluxes of several thousand stars in each field were then determined with SExtractor (Bertin and Arnouts 1996) and extinction corrections were then determined by comparing object fluxes between images. The images were multiplied with IRAF's imarith routine to correct for extinction and then combined to produce two deep images (one for each night). SExtractor was then used to detect objects in each image and our own code was then used to search for objects moving with rates of motion within per day of the rate of motion of Neptune. Images were also blinked to detect objects missed by the automated analysis and to detect any satellites close to Neptune with apparent motions per day relative to the position of the planet. Neptune and its moon Nereid were easily detected by the automated analysis and blinking but no new satellites were discovered. Triton was not detected as it was only from Neptune and was lost in the scattered light from the planet.

To determine the magnitude limit of the search, artificial objects were added to the data with IRAF's mkobject routine. The magnitudes of the artificial objects were calibrated with CCD images of Landolt Standards (Landolt 1992) taken on 30 September 1997. The same data analysis was used to search for artificial objects as real objects. The detection efficiency for the artificial objects is listed in Table�1. This search is complete to a limiting magnitude of . The relatively low detection efficiency at bright magnitudes () is caused by the large number of background stars due to the low galactic latitude of Neptune () at the time of the observations. If we assume distant satellites of Neptune could have similar colours to Uranian satellites (S/1997�U2's colour is , Gladman et al. 1997), then our search is up to magnitudes deeper than the previous survey by Hogg et al. (1994).