Re: Re: [APML] Petzval hot spot?

[mschex@verizon.net]

Thanks Chuck.  Good point about the non-linear nature of film, the corrections would be much more difficult than I thought.  Many of the systems do place the radial gradient in front of the lens, this does in fact slow down the entire system however -- but you get an evenly illuminated field.  I guess that they do this because it is much easier than trying to place a filter at the film plane.  I agree that this would not be a good idea for deep sky.  I don't know how much it would have to slow the system down though -- it might be a good tradeoff if it were not too extreme.

As far as the Televue scopes -- I would guess that they don't have a focuser that is large enough to allow the problem to be seen.  For instance on the FSQ you don't really see the problem with 35mm or CCD.  Most Televue scopes only have a 2" focuser ( 50.8mm ) -- I really wish that they would make one with a larger focuser.  

Mike
> 
> From: Chuck Vaughn <aa6g@aa6g.org>
> Date: 2002/09/18 Wed PM 12:11:21 EDT
> To: <astro-photo@seds.org>
> Subject: Re: [APML] Petzval hot spot?
> 
> 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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