The Index of archive/jpg/1996/07/26/ is as follows:
Name Last modified Size Description 07260000.jpg 04-Aug-98 20:21 19K 07260015.jpg 04-Aug-98 20:21 32K 07260030.jpg 04-Aug-98 20:21 5K 07261630.jpg 04-Aug-98 20:21 89K 07261645.jpg 04-Aug-98 20:21 70K 07261700.jpg 04-Aug-98 20:21 5K etc.The image names are a concatenation of the date (at the previous sunset) and the local mean sidereal time (LMST) at the beginning of the exposure. So each file takes the form:
The "Last Modified" field is nearly always within 15 minutes of when the image was taken. The "last modified" date is local (CST or CDT) time at which the Sun workstation reprocessed the data and archived it in the appropriate directory. (One detail: Jpeg images prior to the August 1998 were re-created en-masse, so the "last modified" field just says "04-Aug-98" or a similar date.)
The "Size" field is the size of the file in bytes. Read on for more on that...
If you inspect 07260015.jpg, you will find it shows some stars and some clouds. If you inspect 07260030.jpg, you will find it shows a blank image. The latter image suffered severely from clouds, which reflect a lot of light pollution down toward the camera, and saturate the data. Note that the size (in bytes) of a cloudy image is much smaller than the size of an image that has stars in it. Because of the way JPG images are compressed, images of saturated data usually require less than 10 kilobytes. A nice image of a clear, dark night sky full of stars such as 07262100.jpg, requires 30 KB, typically.
If you inspect 07262315.jpg, then in addition to some clouds, you will see a few vertical lines, each of which is caused by the inaccurate subtraction of the trail of a single hot pixel. The length of the trail is proportional to the exposure time. Every time Stardial takes a picture, it immediately repeats the same procedure but with the shutter closed. Stardial records the difference of the two images (shutter open - shutter closed) as the final image. This process is called dark subtraction, and it removes most, but not all artifacts of the image that are associated with non-uniformities in the dark current from one pixel to the next.
You might notice that 4 images are missing (07261830, 07261845, 07261900, and 07261915.jpg). Those images were lost due to a technical difficulty, which has since been fixed. Because cloudy images take little space and are easily recognized by that quality, we do not intend to delete cloudy images from the data base. Missing images result only from technical difficulties or scheduled down times.
Every JPG image has a uniform band at the bottom. The exposure time of the JPG images is encoded as the height of the uniform band at the bottom of the image. Actually, the height of the band is proportional to the exposure time. The region of the image in which the intensity is ramping up is overwritten with a uniform-intensity band. The intensity ramps up because each successive row is exposed 2.3 seconds longer than the previous row, until the total exposure time has elapsed and the shutter is closed. There are two standard exposure times used by Stardial:
The short and long exposure images have to be scaled differently in the conversion from FITS to JPG to prevent saturation while also providing good sensitivity to faint objects. The FITS data have a polynomial fit subtracted to remove the (variable) background light. The fit consists of a plane in Y (the shorter axis of the image) and a 5th order function in X (i.e. terms up to Y^1 and up to X^5 with no cross terms). More information on the background subtraction is given in each FITS header. Then, the FITS data are linearly transformed, i.e. JPG = A*(FITS + B), where
If the average value of the FITS image is greater than 27000 counts, the entire JPG image is overwritten with white (255) before the uniform-intensity band is added at the bottom of the image. Images such as those are saturated, due to clouds or moonlight, and are useless; they can be identified in the directory listings by their small size in bytes (typically 2 K).
The charts that accompany Stardial images have North up and East to the left; the Stardial images have North to the right and East up, so you'll have to rotate the charts 90 degrees clockwise to make them correspond to the Stardial images. Each Stardial image covers 8 degrees of declination and 20 minutes of right ascension.