Sky and Telescope - April 2016 - 68

Celestial Imaging

IMPROVING TECHNOLOGY
Today's digital cameras and highquality commercial optics allow amateurs to produce images that were
simply unimaginable a few decades
ago. This deep image of M45, the
Pleiades (left), was captured with
a 200-mm-aperture telescope and
Canon 5D DSLR camera. Compare
that to the photo above, shot on ﬁlm
with the 60-inch reﬂector at Mount
Wilson Observatory in 1995 - considered a good result at the time.

with what's called dark-frame calibration, but a dark
frame (taken with no light striking the detector) also has
thermal noise. So the cooler the chip is when exposing,
the less thermal noise is generated. Commercial DSLR
cameras do not include cooling because they weren't
designed to make the extremely long exposures required
for deep-sky astrophotography.
In general, a cooled camera will give you much
cleaner (less noisy) images than an uncooled camera
can. While the gain from uncooled to cooled is huge, the
beneﬁt of deep cooling does start to drop oﬀ after a bit.
The gain going from 20°°C to 30°C below the ambient
temperature is not as great as the initial drop to reach
20°C below ambient. So don't lose any sleep if getting
that extra -10° is going to break your budget.
Some of the newer imaging chips (particularly those
produced by Sony) do not require dark-frame calibration
at all. They're rated as having extraordinarily low thermal signal after some moderate cooling, often as little
as one electron over a period of a
lengthy exposure! This small
of a thermal signal creates
negligible noise, so this is
a signiﬁcant advantage.
CATCHING PHOTONS
The pixels in your camera
can be thought as buckets;
small pixels capture
fewer photons than
large ones, and will
ﬁll up (saturate) much
faster than a sensor with
large pixels. S&T: LEAH TISCIONE

April 2016 sky & telescope

Color or Mono?
Color chips are really the same as monochrome chips,
with the only diﬀerence being that they're topped by a
thin, multicolored layer called a Bayer ﬁlter. This is a grid
of microscopic ﬁlters with one red, green, or blue ﬁlter
(in a ratio of 25%, 50%, and 25%, respectively) over each
pixel on the detector. The sensor still records a monochrome image, but it's then processed with interpolation
algorithms to create a full-resolution color image.
There are advantages to adopting a color workﬂow
versus the monochrome/ﬁ ltered workﬂow in astro-imaging, but one factor to consider is that the Bayer matrix
reduces the eﬀective QE of the chip. For example, "red"
pixels do not receive 100% of the red light because of the
Bayer ﬁ lter, and so the base QE is eﬀectively reduced.
The QE of color sensors is generally much lower than
that for monochrome detectors, so the former work best
with fast focal ratios. You might have to work very hard
to get good results from most DSLRs with f/10 optics
on deep-sky objects, compared to using a monochrome
detector matched with high-eﬃciency color ﬁlters.

Determine Your Goal
An obvious consideration when contemplating a camera
purchase is what you want to shoot with that camera.
For imaging the planets, Sun, and Moon, the handsdown winners these days are the CMOS-based video
cameras. The biggest advantage of modern CMOS chips
is the readout speed, critical to the planetary imaging
technique of recording large video streams and stacking
the sharpest frames. However, super-fast readout speeds
aren't warranted unless you have optics that can deliver
enough light to the sensor - not to mention a computer
that can handle high data-transfer speeds.

Sky and Telescope - April 2016

Table of Contents for the Digital Edition of Sky and Telescope - April 2016