(do not confuse with the Gorbachov’s Perestroika, please...)
One of the most strange adventures in which I have participated while taking pictures of the night-sky with the old films, was undoubtedly the one I experienced in the 80’s, when I was using the famous Kodak Technical Pan 2415, "hypered" with forming gas.
As it is widely known, one of the most disturbing defects, common to all films, is the so called "reciprocity failure" (Schwarzschield effect), reason for which not only the emulsion answer to the luminous stimulus is not proportional to the exposure time, but, it starts to decline quickly after exceeding exposure times of some seconds. It is similar to what happens on our skin, when we expose to solar radiation.
This is obviously a big disadvantage in astronomic photography, where very often rather long exposures are needed. There are varied causes for this, but I will just mention two:
When the luminous stimulus is very weak, the effect produced is reversible.
This is due to the molecular agitation, and it increases with temperature.
For this reason it is good to remember that a film temperature over zero degrees Celsius has to be considered as high.
Humidity, always present in the emulsion, is another cause for the loss of sensitivity.
So the methods used to "hyper sensitize" films have to take into account these two facts. How? You might have already guessed it:
On the one hand, cooling films down to very low temperatures (80-150° below zero), what is done with dry ice, with liquid petroleum gases or with electronic devices (Peltier plate).
On the other hand, removing all residual humidity from the film, what is done baking it for around 24 hours under pressure in a hydrogen atmosphere, generally mixed with some inert gas to avoid possible explosions.
A very safe mixture that renders good results is the forming gas (92% nitrogen + 8% of hydrogen).
There are many other ways of sensitizing films, but their description goes further than the purposes of this article.
To take a
picture of the sky with 2415 it was necessary to follow certain previous steps.
The film was generally supplied in rolls of 30 meters, and it had to be shortened down, with due precautions, to the common cartridges of 35 mm.
The second step was to put it in a spiral, the same as used to reveal it, and to place it in a waterproof hypersensitizing chamber in which it was necessary to obtain a high vacuum by means of an electric pump. Afterwards, it was necessary to pump the forming gas into such chamber and bake the film for 24 hours at approximately 60 degrees.
When this process was over, the film was left with practically no reciprocity defect, and, though in the instant photograph did not present any gain in sensitivity, it was fit for long exposures, behaving as a much more sensitive film. Moreover, for being a fine grain film, it kept intact its high resolution of 320 lines/mm and an excellent contrast. What else could be asked for?
for a reason that remained mysterious for a rather long time, and in order to
find it out I had to analyze 23 different possible causes, with much waste of
film (around 130 meters...) and time (between sensitization and long exposure,
you can imagine...), the films came out always stained, destroying the result
of hundreds of hours of work.
Finally I could find out the cause, that was obviously the number 23.
The central tube of the spiral, through which the developer is poured, and that it should obviously be perfectly black and opaque to the light, was in fact only apparently black: seen against the light – something that I found out only by chance, as it was not even thought of – it was in fact seen in H-alfa color, just at the radiation to which 2415 is most sensitive!!! (2415 sensitivity goes further, to the red, compared to that of the panchromatic films).
What else can I
Congratulations to the manufacturer, who, if he manufactured developing-tanks, should have known about the existence of the 2415!
Thanks God … it was a quality product !!
THERE ARE THE 23 CAUSES ANALIZED:
01. Expired film
02. Entrance of light in the cartridge
03. Entrance of light in the spiral
04. Entrance of light by charging it
05. Insufficient vacuum
06. Entrance of light in the gauges
07. Pressure loss
08. Excessive pressure
09. Excessive temperature
10. Too long baking time
13. Inconstant temperature
CAMERA AND SCOPE
11. Entrance of light in the camera
12. Entrance of light in the scope
14. Entrance of light when film charging
15. Entrance of light in the developing tank
16. Darkroom not dark enough
17. Improper developer preparation
18. Improper developer temperature
19. Exhausted developer
20. To long developing time
21. Insufficient fixing bath
22. Insufficient rinsing
23 CENTRAL PART OF THE SPIRAL … TRANSPARENT !!!
Why did CCD had not been invented before ! HOME