----- Original Message -----
From: Bernard Miller <bgmiller011@xxxxxxx>
To: az-observing@xxxxxxxxxxxxx
Sent: Sun, 15 Mar 2020 11:15:05 -0400 (EDT)
Subject: [AZ-Observing] Re: Mining Old Silver
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Bernard and others,
Thanks for your nice comments and questions about my 1997 Hale Bopp-photos
taken with an 8 inch Astrograph. Here's some info about the astrograph, more
than you probably want. Yes, this was "Old School" imaging. It was "Current"
or perhaps "Leading Edge" in 1997. And, these images could be taken today with
off-the-shelf cameras and lenses, say a 400mm f/4 DSLR, which I'll explain
below. BUT WE'D NEED ANOTHER HALE-BOPP! Here's a comparison of 1997 vs
today's imaging
.
1997 CAMERA DETAILS: Most is home made:
* Camera (lens): Homemade 8 inch f/3.6 Hyperbolic Astrograph:
93mm diameter doublet 106mm from focal plane, see S&T Dec. 1996 p.83 for
details.
8" f/4.5 hyperbolic mirror, c=-1.812. (e=1.346 )
System EFL = 739mm.
* Focuser, 100mm diameter screw thread, 40 TPI, part of corrector cell,
* Off axis guider built into focuser
* Film Holder: Mamiya 6x7 cm120 roll film back modified with vacuum hold-down
plate, (film paper back removed except for leaders).
* Exposure: 10 minutes.
* System resolution: Ray traced spot radius at 2 degr off axis = 23um or
6.9arcsec , Note: Diffraction limit = 0.67arcsec.
This design was shown in S&T December 1996 p83 article "An 8 inch Hyperbolic
Astrograph" edited by Roger Sinnott. (I made a proofreading error, c conic was
given as 1.346 instead of e. Actual "conic" -e^2 = -1.812).
Also see https://pbase.com/paullind/equipment/8_inch_hyperbolic_astrograph. ;
(or simply pbase.com/paullind)
1997 CAMERA IMAGE
Image rectangle was 65mm x 41mm on on 79mm exit pupil on PPF film.
= 2600 x 1640 pixels at 25um/pixel -> 4.3 Mp image equivalent
Ray traced spot radius at 2 degrees off-axis = 23um spot radius @ 739mm FL,
-> 6.41 arcsec
So, in the old system the spot radius ~= grain size.
MODERN CAMERA SELECTION:
Today's sensors far outperform color film with resolution of say 6.5um rather
than about 25um for hypered PPF color neg film. Also today's ISO exceeds that
of hypered film. Taking that into account, and the fact that the 8" astrograph
is not diffraction limited, a standard camera with 4 degree field could take
these images with a couple minutes exposure.
Modern DSLR with 6.5um pixels easily gets >4.3 Mpix, say 1500 x 3200 px =
4.8Mpix
so, using 3200 *6.5um = 0.028m image=28mm image width
Select lens for 4 degree image: small angle approx: FL(mm) = 28mm/tan(4) =
400mm
Note also: Diffraction Limit of 100mm lens is 134/100 arcsec = 1.34 arcsec..
Plenty of resolution.
So, today it's easy with DSLR with 6.5 um pixels and 400mm f/4 lens.
Clear Skies,
Paul Lind
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What a beautiful old school image. Thanks for sharing.
Bernard
Sent from my iPad
On Mar 14, 2020, at 11:08 PM, Paul Lind <pulind@xxxxx> wrote:
Years ago one of the astronomy magazines (?) used this title to describe
"recent" data recovery from archived B&W observatory plates. That article
came to mind after I found some of my lost 1997 color negatives of comet
Hale-Bopp. These showed little color and no ion tail in the commercial
print on my wall. Coincidentally, my doctor told me to avoid heavy lifting
after my recent successful back surgery, and that's offered wonderful
opportunities to do light work. Since 1997 I had acquired an Epson large
format negative scanner. The film was 6 x 7 cm PPF-2 hypered film. Two of
the processed images show a nice blue ion tail and can be seen at:
https://pbase.com/paullind/fast_moving_objects
I'll check the exposure data later, but the camera was my original 8 inch
f/3.6 hyperbolic astrograph (1996) with hypered Kodak PPF-2 6x7 cm color
negative roll film on a vacuum back. Exposures were about 5 minutes.
Corner-corner images span about 4 degrees (see vignetting in one of the
images) .
Paul Lind
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