[APML] RELATING ASTRO-PHOTOS TO STARS IN SPACE

[glenn shaw <glennshaw@alaska.net>]

Greetings!

Its possible but difficult to relate images of astronomical objects to 
astrophysical quantities. We are photographers and tend to concentrate 
on "exposure" thinking of  numbers, film speed, reciprocity failure 
rates and learned by experimentation what exposure index it takes to 
capture images. For example by using good ASA 400 speed film, an f 
number of around 3.3 and an exposure of a half  hour, you can record 
spectacular images of let us say the Andromeda galaxy that show its 
spiral structure, nucleus and companion galaxies.

Such an exposure index can in principle be related to the individual 
light from the individual stars in the galaxy. The required exposure 
might consist of a large number of rather dim stars or a smaller number 
of bright tracer stars. But one thing is constant: since galactic arms 
or nebulae are diffuse objects it dosen't matter whether they are far or 
near: they photograph the same. Theory shows that the exposure index is 
a function of the general luminosity function and the thickness of the 
spiral arms or the diameter of the elliptical galaxy. This constancy of 
exposure, being the same for near or far galaxies is of interest.

I've spent much time and energy relating our "photographic" units to 
"astrophysical" units and will submit the technical details of 
performing this bridge between astrophysical quantities and our units  
in a technical paper soon. You nerds who want to invest some energy can 
look it over.

In the meantime, here are some rather interesting results that came out 
of this complex analysis:

The predominant light we capture that marks out the outlines spiral 
galaxies or the buldge of light in the center and the edgewise 
luminosity of some galaxies, comes from a high minority of its stellar 
population and clouds of glowing gas. For example the main stars and 
nebulae that mark out most spiral galaxies are in abundance of around 5 
per ten thousand cubic parsecs, while in that volume of space resides 
around 1000 undetected stars!! About 70 of these are as bright or 
brighter than the sun.

While the main markers of spiral structure can be captured and neat 
coloration and morphology can be photographed with a half hour guided 
photograph, calculations show that an exposure of 41 days continuous 
exposure would be required to photograph down to stars as bright as our 
sun. And this is using a fast (f 3.3) optical system!!

Far before you could go down that low, sky fog would mess everything up. 
The "sun-exposure in galaxy" brightness would be around 28 magnitudes 
per second of arc squared.

By using all the highest tech, best films and most modern software you 
could probably not get to this limit. However, by using inexpensive 
(around $10,000) modern CCD's and plenty of pre-planning and modern 
software amateurs  possibly could possibly reach the "sun" type stars in 
distant galaxies. It represents a kind of ultimate challange for our hobby.

Put it another way. Could some organism on some planet on another galaxy 
go down to the photographic limit of being able to "detect" the swarm of 
G type stars, like our sun? It would be very difficult.

I'll have more to say on this topic if anybody shows an interest.

regards,

Glenn Shaw

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