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[ATM] OSC and doublets

  The other day, someone asked a question about doublet available.  After a
little work, I found a Newport (not Newport Glass Works) doublet PAC097
that was close.  It is 3 inches in diameter with a focal length of 750 mm.
I adapted this as 75 mm f/10 and used that for a baseline:

    http://users.rcn.com/rflrs/d04-04x.len

At first, I thought that this design would be rather poor for 
astronomical use.
Taking a closer look, it was that the correction was a little red for my
preference; however, the off-axis correction was particularly good.  A
little digging revealed that, in fact, this lens has nearly nulled the coma.
Plugging this into the GSUM calculator and sliding the correction 
wavelengths
up and down allowed me to get a version which has OSC accurately nulled:

    http://users.rcn.com/rflrs/d04-30x.len

The minimum focus of this lens is at 553.2 nm.  I was correct... the 
correction
was just a little red for my tastes.  I tried a few other glass 
combinations:

N-K5/SF1

    http://users.rcn.com/rflrs/d04-31x.len

which has a minimum focus of 530.9 nm.

N-FK5/F2

    http://users.rcn.com/rflrs/d04-32x.len

which has a minimum focus of 524.6 nm.  Either of these looks pretty good
for Photopic correction, while keeping the OSC nulled.  Going a little
further:

 N-BaK1/SF2

    http://users.rcn.com/rflrs/d04-33x.len

goes a bit too far, with the minimum focus at 492.6 nm.  But what about
the traditional BK7/F2?  This is:

    http://users.rcn.com/rflrs/d04-34x.len

which has a minimum focus at 462.7 nm.  The BK7/F4 combination is
around the same.

What about the other direction (that was too easy!)?
Shifting higher in the band is:

N-FK5/SF2

    http://users.rcn.com/rflrs/d04-29x.len

This last combination has a minimum focus at 583.1 nm.  More importantly,
the two nulls are "roughly" at the e and C lines.  I have heard many 
like this
combination for planetary viewing.

-- 

Rick S.

http://users.rcn.com/rflrs


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