| One-Step Photo Exposure Tables - "canned" objects
These tables are in the experimental stage. For the viewing
objects listed, they will generally give calculated results that
agree, within one stop, with the Appendix A measured value tables
published in Astrophotography for the
Amateur,
2d edition, by Michael A. Covington. For more detailed calculations,
use our multiple-step Custom Exposures page.
To use the tables, pre-select the object you will be photographing
from the popup field choices currently offered below. Given the
ISO setting of the film or camera you will be using (across), and
the
total
system
focal
ratio (down), the table outputs calculated shutter speeds (seconds).
Please see detailed notes for more information on using these values
for
astrophotography.
Notes
You can use the tables that this program outputs to determine
shutter speed, for a given film speed and f-stop, for photographing
over 20 built-in solar system objects. This works for a plain tripod-mounted
or
"piggyback"
camera, capable
of manual
exposures, as well as for a telescope with a camera back attached.
1Use the telescope f-ratio field to select the f-stop
that will be used with a camera-only setup. Some popular telescope
models are listed for convenience, but only the f-ratio value is
needed.
If
the optional f-ratio in the popup is selected, the program will
highlight the appropriate row in the table. We have supplied values
for a built-in table range
of f-ratios the amateur astronomer might encounter, and labeled
them for a couple of telescopes we're familiar with.
For photography calculations for filters, eyepiece projection,
afocal coupling, or for custom scenarios and settings, use our
multiple-step Custom Exposures page.
- The Covington tables (for those lucky enough to own the book)
list measured exposure values.
Since no formula calculation will be more "accurate" than
data taken from field measurements, use the book tables when
you
have access to them.
Basic Exposure Calculation
These tables are calculated using the Appendix A formula
t (seconds) = f2/SB
where f is the f-ratio, S is the ISO value or film
speed, and B is the brightness of the object being
measured, in candelas per square foot. "In
practice B is somewhat arbitrary as the subject
spans a range
of brightnesses."
we take the B value from the published table, since
it is available. We'll supply a way to calculate this
(later) if you do not know it.
total system f-ratio must be calculated using published
f-ratio of telescope, and focal length of eyepiece
(if doing eyepiece projection photography), and focal
length of camera lens.
for focal plane photography (camera back, no
lens) f-ratio
is just telescope's f-ratio.
for piggyback and field tripod photography
(no telescope), f-ratio
is just f-ratio of camera lens.
|
common telescopes:
- Meade LX-90: 8", 2000mm focal length, f/10
- Orion StarView Pro 127 (Mak): 5", 1540mm focal length,
f/12
- Meade ETX125 5", 1900mm focal length, f/15 (Mak)
- Please send us other models you'd like included
|
- for film cameras, speeds greater than 1 second need
to be corrected for "reciprocity failure".
This correction is NOT
needed
for CCD cameras, and the one-step tables
do not adjust for film reciprocity failure.
We
build that
correction
factor into the custom tables.
- Tables are generated on the fly in Perl. Print or save them
off for field trips and photo sessions.
- Accuracy is generally within
one stop or shutter speed of published observational
values, well within a range where you would not notice a difference
from
using
a published
value.
- A table entry of "<<" indicates the result
(speed) is too fast for most cameras. Use a slower film or lower
the digital camera
ISO setting. Cutoff is 1/8000.
- A table entry of ">>" indicates the result
(speed) is too slow for reliable calculation. Use a faster
film, or higher digital camera ISO setting. Cutoff is 1800 seconds
(1/2
hour). Longer exposures are certainly stock in trade
for deep-sky photographers,
but here a range of
exposures is a more appropriate result in such cases.
|
