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 CCD Astrocamera format .LNX (Spectra Source Instruments)
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 Preamble
----------
The early CCD cameras were for astronomy buffs with a lot of
money to spare, monochrome and quite small. However, they could
be used for exposures lasting several minutes and were thus
capable to gather the light from dim astro objects. Camera
manufacturers used their own proprietary bit map image format to
save the pictures, sometimes there was a different format for
each camera model.

One of them is the PC-Lynxx LNX format. The camera used the
TC-211 CCD chip from Texas Instruments. There are 165 lines
with 192 columns each. The camera digitized the values in each
pixel "bucket" with 12 bits and thus 4096 grayscales were
produced.


 The LNX format
----------------
In the LNX file, the top left pixel value (i.e. the brightnss)
is the first value in the file. The pixel values are written
from left to right, top to bottom.

Please remember that a byte with 8 bits can also be looked at
as 2 nibbles with 4 bits each -- like a 16 bit word consisting
of two 8 bit bytes.

There is the problem with the 12 bit value. The LNX format does
not save the value as a 16 bit 2 byte word, but as two bytes and
two nibbles. There are 3 bytes for 2 pixel values. Consider this
as a simple data compression scheme.

Each byte triplet holds two 12 bit values. The first byte takes
the lower 8 bits of the first pixel value. The low order nibble
of the second byte contains the high 4 bits of the first value
and the high order nibble the high 4 bits of the second pixel
value. The third byte finally stores the lower 8 bits of the
second pixel value. And so on.

        +---------------------+
        Ś                     Ś
        v                     v
+---------------+ +---------------+ +---------------+
Ś    Byte n     Ś Ś   Byte n+1    Ś Ś   Byte n+2    Ś
Ś               Ś Ś       :       Ś Ś               Ś
Ś7Ś6Ś5Ś4Ś3Ś2Ś1Ś0Ś Ś7Ś6Ś5Ś4Ś3Ś2Ś1Ś0Ś Ś7Ś6Ś5Ś4Ś3Ś2Ś1Ś0Ś
Ś               Ś Ś       :       Ś Ś               Ś
Ś Pixel n, Low  Ś Ś Pix   :  Pix  Ś ŚPixel n+1, Low Ś
Ś               Ś Śn+1,Hi : n, Hi Ś Ś               Ś
+---------------+ +-------+-------+ +---------------+
                      ^                     ^
                      Ś                     Ś
                      +---------------------+

The camera contains 165 * 192 = 31'680 pixels. Each pixel value
takes up 1 1/2 bytes. Hence, the file size is always 47'520
bytes.


 REFERENCES
------------


 Author
--------
hans-rudolf.wernli@bluewin.ch
http://mypage.bluewin.ch/horo/
05. August 2002