Re: [APML]: Telephoto lenses

[Chuck Vaughn <aa6g@aa6g.org>]

Peter Ceravolo wrote:
>  
> I would like to know if filtering out the violet light casauses a noticable
> color shift in the objects being photographed. Filtering out light shorter
> than 450nm would make designing big f/2.8 astro-lenses a lot easier.
> 
Off the top of my head I don't remember the exact cutoff for the Lumicon
minus violet filter but I've used one in my 350mm-f/2.8 and 180mm-f/2.8
Olympus lenses for regular photography. The photos looked fine to me.
I originally used it with the old Ektachrome film when shooting at high
altitudes in Yosemite to eliminate that blue haze. The 350mm has to have
at least a clear filter in the slot in order to come to focus so it was
easy leave the minus violet filter in the slot. I ended up leaving it
in even for Kodachrome and the slides still looked fine.

In regards to these telephoto lenses not being so good in white light I
checked the focus of my 350mm once using an H alpha filter and a #47
blue filter by hoding each in front of the viewfinder with the split
image prism installed. The focus was obviously different for the two
colors. Yet I used this lens in Yosemite where the air was very clear
and dry for some telephoto shots and it produced sharper photos then
I got with *any* lens I had used. Yet for astrophotography it's not
that great in white light, 20 um stars compared to 12um with an H alpha
filter.

I think most of us Tech Pan astrophotographers would love to have an
instrument as slow as f/4 that truely worked. Up to f/4 H alpha
photography is still realistic - up to 3 hour exposures. Slower than
f/4 the times are just too long.

Chuck Vaughn  <aa6g@aa6g.org>