Re: [ATM] 80mm f/5 doublets (was: Surplus Shed objective)

[fhh@npgcable.com]

Re Tom's message:

> From: Tom Conlin <tomc@cs.uoregon.edu>
> Subject: Re: [ATM] Surplus Shed 90mm F10 objective
>
> I have a cheapo 80mm F5 doublet in a cell that I think is sub par
> (not from the Surplus Shed), and I'm not complaining because the unit
> was free but I would be very happy to learn how one would tell if the
> elements are properly oriented.
>
>
> Bob May wrote:
>> Sometimes you get misassembled parts from suych places because it
>> is cheaper for the company to scrap them than to redo them.
>> For your lens, I'd just disassemble it and turn the glass around.
>> While you're at it, you might also check to see if the two lenses
>> are in the cell properly to begin ith!
>> Bob May

Of the one 80mm f/5 doublet from Apogee Inc, and five from Surplus
Shed that are nominally identical, 6 out of 6 had the lenses flipped
in the cell, as delivered.  It took a while for my club and I to
figure that out, as reversed the doublet would still image, but a
second "ghost" image would pop into and out of sight; quite
disturbing optically.

We opened the cell, and found the biconcave element in front, the
biconvex element at rear.  We imagine that at the factory someone
was told "the thick lens goes in front, the thin lens at back" and
then worker(s) followed orders, looking at the **edge** thicknesses
rather than the center thicknesses...

With the elements and spacer slid out as a group, flipped 180-degrees
and closed back in the cell, I am more than happy with the images
given the cost of doublets.  They make great 15x80 finders that do
a reasonable job on larger Messier objects.  The focusable cross-hair
27mm Kellner eyepieces available let you complete the 15x80 finder,
and the helical focusing in the eyepiece is all you need to
accommodate the focus range of typical observers, no other focuser
necessary.

The supplied lens cells are loose in the 4-inch drain pipe from
Home Depot, but adding a cylindrical plastic ring from a Haagen Daz
pint lid around the cell and inside the pipe makes a wonderful
fit.  All such finders now require their sacrificial pint; flavor
at the discretion of the builder.

I urge everyone who has these objectives to check lens orientation:
starlight should first encounter the biconvex lens, then exit via
the biconcave lens to a clean 400mm focus.

No affiliation with Apogee Inc., or Surplus Shed, or Home Depot, or
Haagen Daz, just a satisfied customer, now spoiled by 80mm finders
(and certain flavors).

Fred Harris


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