Re: [APML] Petzval hot spot?

[Chuck Vaughn <aa6g@aa6g.org>]

Mike,

> The problem can be corrected for in software
> quite easily because there is exactly the same amount of exposure falloff
> every time.  

Just because the light falloff is always the same doesn't mean the same
correction can be applied everytime. What shows up on the film is dependent
on the gamma of the film. Very different flat fielding curves will have to
be applied to Royal Gold 400, E200 and Tech Pan. In addition, since film
response is not linear, different corrections will have to be applied even
to the same film type depening on length of exposure and light pollution
conditions.

In my mind the notion of "correcting for light falloff" mis-identifies
what is really being done. One is only subtracting out sky fog from the
"hot spot". The areas of light falloff represent a slower f ratio and
thus less information is recorded in that area. There is no way to correct
for that.
   
> They solved the problem by placing a radial gradient filter on the front of
> the objective.

You cannot place it in front of the objective as that would simply slow down
the whole system. It has to be placed in front of the piece of film.

IMO, except for special circumstances like solar eclipses where speed is not
the main issue, using a radial gradient filter for deep sky photography is
ridiculous. Limit yourself to a format that matches the 100% illuminated
circle or find a system with a larger fully illuminated circle.

Chuck <aa6g@aa6g.org>


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