Siarnaq, or Saturn XXIX


Siarnaq was not discovered until 2000 and was given its official name three years later. It is a fairly small body at only 39km across. When this image was created it was predicted to be at magnitude 20.9 and close to the limit of detectability.

To form the images shown above 43 exposures each with a 1-minute duration were taken. Siarnaq is so faint that it is completely invisible on each sub-image.

This was quite a difficult observation to make because at the time, Siarnaq was only a few arc seconds away from a star which is 2.5 magnitudes brighter - a factor of ten times in intensity. The images are (left) stacked on the stars, showing a trailed Siarnaq, and stacked on the predicted motion of the satellite (right), showing noticeably trailed stars and a round satellite, though at poor contrast from light bleeding in from the star. The quality is not as good as I would hope because there are clear guiding errors on the stars. Two remedies spring to mind. The first is to try again later and the other is to deconvolve the image using a bright unsaturated star to estimate the point spread function.

Image information
Date and time of observation  2020-09-13 2200 UT
Telescope 0.4m f/6.5 Dilworth-Relay
Camera Starlight Xpress Trius-PRO SX814 CCD
Filter None
Exposure 62x1m average-stacked on predicted motion of the satellite
Centre of image RA 19h49m08.1s  Dec -21°08'59"
Image dimensions 2.0 arcmin × 1.8 arcmin