Clear plate images with enhanced contrast

The full solar image is about 2.5 cm in radius. These images are extracted from below and to the right of the full images and are squares about 1 cm on a side. The grey level has been adjusted so that mid-grey is typical of the clear plate density and the range has been constricted to 1/16th of the full black to white range. The original digitization was at about 12 bits so this approach has extracted 8 bits of information focused on the emulsion itself. The clear plate variations represent the properties of this detector material and will be imposed on the solar intensity variations. The pixel scale on these images is 600 pixels per cm or 17 microns per pixel. The small features on these images are thus about 30 microns in diameter. On the scale of the sky this corresponds to about 1.5 arcsec -- a scale smaller than the spatial resolution of the system.

1916/08/08-08

1917/08/11-21

1915/08/18-18

1918/08/22-39

1919/08/10-20

1920/08/18-34

1921/08/30-50

1922/08/01-01

1923/08/09-16

1926/08/30-88

1925/08/22-43

1924/08/17-37

1929/08/30-87

1928/08/08-22

1927/08/21-65

1931/08/23-57

1930/08/29-81

1932/08/14-40

1933/08/26-72

1934/08/22-53

1935/08/19-51

1936/08/20-52

1937/07/19-46

1938/08/20-55

1940/05/09-34

1939/11/10-28

The images selected above were not previewed except to avoid heavily exposed cases where the scattered light produces a strong gradient on the image and prevents the enhancement of the contrast over a square of the size shown. Image irregularites of note include:

1. A dustlike distribution of speckles. These are distributed more or less uniformly over the clear plate and have a density that varies from plate to plate. The density of these dustlike speckles is indicated in the annual summary files from the number of dust and pit pixels removed. While this algorithm finds and removes the smaller of these speckles, it does not identify the larger ones and leaves them intact. The source of these dust speckles is unclear. They may have come from contaminants in the original plate development chemicals or they may have been introduced when the plates were washed in the 1960's. The strong variation from one group of plates to the next makes it unlikely the dust speckles were part of the original emulsions.
2. A granulationlike mottling of the emulsion. The contrast of this type of feature is low on the images on this page but it can be enhanced to show it is real. A visual inspection of the mottling indicates that it comes from a deterioration of the emulusion in the form of faint surface cracking.
3. A streaking pattern at various angles over the emulusion surface. This appears to be a result of the process of emulsion application used in the plate manufacture.
4. Occasional water spots. The sample for 1935 gives an example of such water spots. There are plates in the collection that have water spots much stronger than is the case for this sample.
5. Scratches on the emulusion surface. These may have come from plate handling due to the sliding motion required to remove a plate from the older envelope storage system. The newer 4-flap housings eliminate the risk of plate scratching from this cause.