Re: (Fwd) Re: [APML] Medium format wide field scopes (blue halo

[Gene Horr <genehorr@texas.net>]

Forwarded:

>  I did not mean to forget the Borg products.  Some nice
> shots, but also blue halos.  Not bothersome to many, but they are to me.

Mike,

I don't know if you are referring to the Borg achromat version, but
I think maybe you are getting the wrong idea based on some shots
by me (using the 100ED) which are on the Hutech website, so I want to
be sure to correct any false impressions!

With the bad weather this season, in desperation I often was
shooting through thin haze.  This scatters light at the blue
end more than the red, so you can still see H-alpha nebulosity
but especially blue stars get that Akira Fujii (or Bill Fletcher) look.

An example is my S-147 shot.  You can see more examples of shots
through more and less haze on my website as well as the level of
desperation this year (see the 3/15 Extreme Astrophotography
adventure)  In better transparency, the blue halos are not there
with the same instrument (Borg 100ED).  And visually there is
no color on bright objects such as Venus or the limb of the moon
for the ED versions of the lens.  Same for the 76ED version
which I use as a guider and for 35mm shots.

In my experience, shooting through glass which is less color
corrected (Pronto), I got purple halos, which I think is consistent
a semi-apo lenses corrected for visual use.  Also the halos are
not as large as when atmospheric scattering is the cause.

Dave Kodama
http://www.eanet.com/kodama/astro/

P.S.  I'm subscribed to the digest version, so please CC me directly
      if you want a quick response to any other question you might have.

>  I did not mean to forget the Borg products.  Some nice
> shots, but also blue halos.  Not bothersome to many, but they are to me.

=========================
kodama@alumni.caltech.edu
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