Digital camera, CCD, or Film?
Introduction: Which medium is
"best"....digital camera, CCD, or film?
Actually, there are seasoned astrophotographers who still debate
this point. In truth, each medium has its
strengths and weaknesses, and the "best" should be defined as the one that
gives you the most pleasure out of this great hobby. It's also
true
that these are not mutually exclusive methods, so it's possible for an
experienced astrophotographer to use film for some targets, and CCD for
others. Since I've taken astrophotographs using each medium, I
thought that it would be helpful for me to share a few thoughts about
this issue. This is not meant to be a comprehensive discussion of
the digital versus film debate, but it's a personal viewpoint that will
hopefully be helpful to those who need some guidance.
The following comments do not cover the use of webcams, which are
great for planetary/lunar
work and are constantly being perfected for use in deep sky
astrophotography.
Everything is
digital: This
may seem obvious to some, but it's worth stating that all roads lead to
an analog-to-digital conversion at some point along the path to a great
astrophotograph. Whether this occurs in
the conversion of an analog to digital unit (ADU) in a CMOS or CCD
chip, or occurs during the scanning of a slide/print by a typical
desktop
scanner (which is CCD-based), the ultimate goal is to manipulate
digital
data in an imaging processing program like Photoshop. So even
film
can be considered digital, except that the original analog data is
captured by a relatively large silver halide grain on a film emulsion,
as opposed to a more sensitive, smaller pixel on a CCD chip. This
simple fact limits the amount of light and resolution achievable by
film, when compared to CMOS or CCD. Although film is capable of
capturing faint objects, the price to be paid is the requirement for
long individual exposures,
which demands great attention to precise polar alignment and
autoguiding.
Also, because you do not obtain instant feedback with film
(compared
to digital or CCD), you must very sure that your object is well-framed
and in good focus. Otherwise an entire night of astrophotography
could
be ruined.....
So why not just go with a consumer digital camera or
an astronomical CCD camera?: Many
reasons. Film astrophotography is a very cost effective and
satisfying way to take simple astrophotographs, using a camera on a
piggyback mount (on top of a polar aligned scope, for instance).
With the use of a relatively wide angle lens (28-50mm), exposures
in the range of 5-15 minutes each can result in very pleasing photos of
constellations and the Milky Way, and they are more forgiving of slight
inaccuracies
in polar alignment and guiding. For those who aren't
sure
that they want to invest a great deal of money and time in
astrophotography, but would like to dabble for a while, this is great
way to start. Unfortunately, as of 4/04 there aren't good choices
for color negative (print) films
that capture the faint reds of Ha emission, but slide films like Kodak
E200 are still great for this purpose. There is no doubt that for
those who wish to develop the skills necessary for performing
film-based,
long exposure astrophotography, the rewards can be great....although so
can the frustration. Because of the long exposures required,
there
is simply more that can go wrong with film-based work. Michael
Convington's book "Astrophotography for the Amateur," and Jerry
Lodriguss' book "Photoshop for Astrophotographers," provide lots of
useful information.
How to choose
between a consumer digital camera
and an
astronomical
CCD camera: If
you've decided to take the digital plunge, then either a consumer
digital camera or astronomical CCD camera is reasonable. The
choice depends upon your expectations of the hobby. Either of
these choices has greater sensitivity (and resolution) compared to
film, meaning that it's possible to limit your subexposure durations to
5 minutes or so (with subsequent stacking of multiple subexposures to
achieve a final exposure of long duration). This translates into
somewhat less stringent requirements for autoguiding and polar
alignment compared to long exposure film-based work. If you want
to obtain very pleasing, one-shot color images with a "film-like" field
of view, then go with a consumer digital camera like the Canon 10D or
300D (see below for comparison). This approach will cost you less
than half of what would be required for a good quality astronomical CCD
camera, and you can use the digital camera for daytime shots as well.
Software
is now available for image processing of digital camera astrophotos
(like
ImagesPlus), and there are internet groups like the Yahoo
digital_astro
group that provide useful advice. However, keep in mind that
the
sensitivity of CMOS-based consumer digital cameras is generally not as
good as CCD technology, with Ha sensitivity being especially
compromised
in the 10D and 300D. Also, background noise will be higher in
consumer digital cameras, which are typically non-cooled, compared to
cooled astronomical
CCD cameras. Even though the noise characteristics of the
non-cooled
10D or 300D digital cameras are quite acceptable, especially during
the
wintertime in New England, noise will increase in warmer climates and
will limit the exposure duration, ultimately limiting sensitivity for
dimmer targets. Still, it's possible to take very pleasing
astrophotographs of brighter targets during the summertime with the 10D
or 300D, as long
as you keep these restrictions in mind. If, on the other hand,
you want high Ha sensitivity and high resolution images that are not
limited by noise, and if you don't mind a more limited field of view
(CCD chips
are generally smaller than the current CMOS chips in digital cameras),
an
astronomical CCD camera would be the way to go. I will not review
the many CCD choices here, but would recommend Ron Wodaski's book "New
CCD
Astronomy" for additional information. I currently own an SXV-H9
CCD
camera by Starlight X-press and have been very impressed with its
sensitivity,
anti-blooming feature, and low noise characteristics.
Canon 10D or Canon 300D?: The
Canon 10D or 300D are the most popular options for digital camera
astrophotography as of 4/04, because of their low noise characteristics
and proven track record. However, the digital camera field is
moving at a fast pace, so other contenders like the Nikon D70 could be
available soon. Since CMOS-based consumer digital cameras
currently use an IR cut filter to achieve proper color balance for
daytime photography, I would anticipate that they will all suffer from
poor Ha sensitivity, regardless of brand (unless this filter is
removed, which voids the warranty and risks damaging the camera).
Regarding the choice between the 10D and 300D, here are my
thoughts. I own a 10D and love it. I like the feel of the
10D- it has a more durable exterior (I like to take it on rugged hiking
trips), and it feels more balanced when a large lens is attached.
I also like several of the bells and whistles, including the
ability to use flash compensation for indoor shots. Nonetheless,
the 300D uses essentially the same CMOS chip and has similar low noise
characteristics compared with the 10D, and it costs hundreds of dollars
less. I have now seen several astrophotographs taken by both
cameras and am convinced that technically there is no difference, when
operator skill is taken into account. Keep in mind that the
optional Canon TC-80N3 remote controller needs to be adapted to the
300D (no modification is necessary if the TC-80N3 is used with the
10D), although this is quite easy to do and is outlined on Mike
Unsold's Yahoo ImagesPlus site. The 300D does not have mirror
lock-up,
although this
does not appear to compromise the effectiveness of the camera for
deep sky astrophotography. However, the lack of mirror
lock-up might be a
consideration for solar or lunar astrophotography, in
view of the faster shutter speeds involved, especially if your mount is
not rock-solid (but I have seen several very nice lunar shots taken
with the 300D!). Mirror lock-up would obviously come in handy for
daylight, high magnification photography using a macro lens.
Finally, Hutech is selling a modified 300D in which the IR cut filter
has been removed, meaning that it has the potential to be more
sensitive in the Ha (red) portion of the spectrum. I do not have
personal experience with the Hutech-modified 300D camera and cannot
comment on its astrophotography or daytime performance.
Bottom line: If you want a digital camera for both
astrophotography and casual daytime use, and if you are prepared to
modify the TC-80N3 remote, get the 300D. But if you're a
certifiable camera nut, if you
like to take your camera mountain climbing, if you want the mirror lock
up feature for macro work, and if you're really into "serious"
daytime photography, it may be easier for you to justify the cost of
the
10D.
With so many great options to choose from, there's
no better time to get involved in astrophotography.
Good luck!
Steve
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