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Stardial's data is archived in a simple scheme based upon the date. A typical such directory is

where the "jpg" means JPEG format images; the other choice is "fits". The date is encoded as /YYYY/MM/DD/, which refers to the date of most recent sunset in Urbana, Illinois. The data taking computer for Stardial is locked away in a storage closet; it never gets to see the sunset, but it infers when the sun has set from a nifty public-domain almanac program called Skycalc, written by the person (guess his name and where he works!) at this email address: Defining the date in this way is convenient because then all the images from a particular night will be stored in the same directory; also, the first 4 characters of the filenames will be identical.

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
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 HHMM also provides an approximate right ascension for the image, since Stardial is pointed approximately at the meridian. Actually the meridian crosses each image horizontally about 2/3's of the way down, i.e. closer to the bottom of the image than the top.

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 shorter exposure is used if the sun is between 8 and 12 degrees below the horizon, when the sky is still relatively bright. If the sun's altitude is less than -12, the longer exposure is used.

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

The value overwritten into the pixels in the uniform-intensity band at the bottom of each JPG image is equal to the square root of the average value of the original FITS image. (By taking the square root, we can safely represent all possible FITS values (which are 16-bit data) in the 8-bit JPG format.) The loss of precision is insignificant, because the FITS data are almost entirely dominated by Poisson statistics (aka shot-noise).

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.