From: "Sidor . Kurt" 
To: 
Cc: 
Reply-To: "Sidor . Kurt" 


Rick and List,

I have an old ITEK made Petzval lens assembly.  It is 7" aperture and
F # 3.5.  It is made by pairing a  (primary) 7" F # 7 triplet with a
(secondary) 5" F #3.5 doublet.  I did some homework and learned that
the combo is used to reduce chromatic abberation in the final image.  With
the first lens group only refracting the light half the final "cone
angle" before it enters the second lens group less chromatic
abberation is introduced than if you used only one lens group of the final
F # ratio.  My assembly was probably made in the 60's or early 70's and
when coupled to my 35 MM Nikon FM2 with Home Depot plumbing fittings
produced a set of nice chromatic free images of the full moon.  I even put
in a homemade 50 MM eyepiece and got mediocre results, I suspect the
resulting view was more an effect of the F # 3.5 light cone and eyepiece
incompatability.  I have been tempted to split the lens groups up and try
it out as a 7" F # 7 triplet but have not done so yet.  I think I will
thoroughly evaluate its value as a Petzval assembly first.

I looked at your OSLO file briefly and would say that yes the length is
about what you can expect from a Petzval.  However your tube length is
shorter than if you simply made a scope using the front lens group F #
only. I have not studied your file for image quality but from waht I have
read about Petzvals they are primarily designed for photographic use only. 
I think Pentax has a fancy new astrocamera lens on the market now that is a
modern day Petzval design.  A few years ago Roland Christian (Astrophysics
owner) posted a prescription here on the ATM list and suggested some ATM
should try to build it.  His design also used a second lens group, like the
Petzval, to help achieve a more corrected and faster final F #.  Did anyone
ever try building one of these?

There is some info on Petzvals in the book "Modern Lens Design, A
Resource Manual" by Warren G. Smith, published by McGraw Hill.  Edmund
Scientific is still selling this book, they want $91 for it.  Amazon.com
wants $85 new and
$64 for a used one.  I was lucky enough to find mine at a moving sale, it is
a good book but I think $91 is too much, $64? maybee if you REALLY love
lens prescriptions.  The book contains 280 complete lens prescriptions
including eyepieces and Naglers US Patent 4286844 for a 10MM Nagler.  If
you want more info let me know.

Regards,

Kurt Sidor

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