Re: [APML] rings / reflections - was New M42 Image

[Chris1011@aol.com]

In a message dated 1/22/2002 12:46:41 PM Pacific Standard Time, GB1029@inter-linc.net writes:


> With my setup the filter threads into the ap camera adapter
> which slides right up near the compressor.  This combo might be causing
> some reflections / scattering.?


Scattering will produce an overall haze. What you have is a very clear out-of-focus image of the bright star superimposed on the real image. If your filter is not coated, then 10% of the light will be reflected back to the previous surface. If this surface is the telecompressor rear element, then it will return a portion of that light back to the film plane. The extra distance traveled will render this faint secondary image to be out of focus by that amount. It is definitely NOT your main lens. The only solution is to have both surfaces of the filter multi-coated to less than 1/4%, and your halos will disappear. One other benefit will be a 10% increase in light throughput. 

If this deep sky filter is made with high efficiency multi-layers, then the problem will be much worse, because instead of absorbing these errant wavelengths, they are actually reflected back up the tube. In this case, the result will be that 50% or more of the light goes back to the rear of the compressor lens and then back down to the film. Then, even multi-coating the filter for anti-reflection will do no good at all. 

You can instantly tell that this is the case if you shine a flashlight beam onto the filter and there is a noticeable amount of light that comes back at you. 

Baader makes a Neodimium filter that blocks out sky glow. This filter works on the principle of absorption of the light. In comes in 1.25" and 2" sizes. Perhaps this might be a better bet.

Roland Christen