Sky & Telescope - June 2022 - 55

one, so the image produced by a lens at
f/2.8 is twice as bright as one set to f/4,
or 16 times more luminous than one
operating at f/11. A fast lens - one with
a small f/number - is desirable for photographing
extended objects, like galaxies
and nebulae (as opposed to stars), in the
shortest possible time.
As mentioned, telescope focal ratios
are fixed, and usually lie between f/4
and f/15 depending on the optical
design. However, the same rules for
camera lenses apply here as well, so a
" fast " f/4 scope is preferable for imaging
a galaxy's faint light than a " slow " f/10
instrument. Some telescopes are specifically
designed for imaging and are called
astrographs. These usually offer specialized
optics and sturdy construction
among other photo-friendly features.
The specifics of choosing a lens for
astrophotography are beyond this brief
exploration of imaging jargon, but it's
important to know that focal length
and image scale are related. You could,
for example, capture a galaxy with a
50-mm lens, but it will be rendered
very small. A lens with a 200-mm focal
length will show that same galaxy four
times as large. However, lenses with
less focal length excel at delivering wide
fields of view, which is useful for capturing
large swathes of the night sky. In
other words, as with daytime photography,
you choose your lens based on the
One of the delights of the June evening sky is globular cluster M3, in Canes Venatici. This portrait
was captured in just six minutes with a SharpStar 13028 Hyperbolic Newtonian astrographic
telescope working at f/2.8 and a ZWO ASI385MC CMOS astronomical camera.
kind of picture you're trying to make.
Cameras and Chips
Digital cameras use special sensors covered
with a grid-like matrix of millions
of microscopic photosensitive elements
called pixels that convert light to an
electrical signal that is reconstituted,
row by row and column by column, into
a digital image (see page 64). Camera
sensors come in several sizes, but the
two most common are called full frame
and crop sensor (also known as APSC).
Full-frame models have sensors
with the same dimensions as a 35-mm
film frame, that is, 36 mm × 24 mm.
Cropped-sensor cameras have slightly
smaller detectors. Nikon, Pentax,
and Sony cameras feature 23.6-mm ×
15.8-mm sensors, while Canon models
measure 22.3 mm × 14.9 mm. Smaller
still are the 17.3-mm × 13-mm sensors
of Micro Four Thirds (MFT) cameras.
What's important to know is that the
field of view of any given focal length
lens depends on the type of camera it's
used with. That's why a camera's crop
factor matters. For APS-C cameras, it's
1.5×, while Canon's cropped-sensor
cameras have a factor of 1.6×. So, a
100-mm lens in a Nikon APS-C camera
would give you the same sky coverage as
a 150-mm lens in a full-frame camera,
and on a Canon it would be 160 mm.
Perhaps you've heard of some cameras
that have been modified for astroLenses
with fixed focal lengths are generally better performers than
similar zoom models, although a single zoom lens will often be a more
economical way to cover a wide range of fields of view. Shown here
are three fine astrophotography lenses (from left to right): a Canon EF
200mm f2.8L USM telephoto, a Samyang 85mm f/1.4 short telephoto,
and a Samyang 24mm f/1.4 wide-angle.
The author gets plenty of use out of this pair of Canon DLSR cameras.
At left is the crop sensor EOS 550D (also marketed as the Rebel T2i and
Kiss X4 Digital), and at right is the venerable, full frame EOS 5D Mark I.
The 550D camera has been astromodified to make it more sensitive to
the H-alpha wavelengths of light for capturing spectacular images of redhued
emission nebulae.
sk yand tele scope .o r g * JUNE 2022 55

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Sky & Telescope - June 2022

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