Sky and Telescope - July 2015 - 71

The Bayer matrix's ability to collect color in a single
exposure prevents the collection of unﬁ ltered luminance. As each pixel always "looks" through an attenuating ﬁlter, the detector's quantum eﬃciency (or sensitivity to light) is limited by those same ﬁ lters. Because the
Bayer ﬁlter divides the sensor into three images (50%
green, 25% red, and 25% blue), the information is inherently of lower resolution than images captured with a
monochrome camera. The OSC color channels are interpolated to ﬁll in the gaps between the pixels, though the
latest interpolation routines make this diﬀerence nearly
imperceptible. Finally, while narrowband ﬁlters are available for OSC cameras, the loss of light sensitivity when
combined with the Bayer ﬁlter makes imaging through
them a challenge.
Despite these limitations, OSC cameras can be
fantastic performers, particularly when used under dark
skies with fast focal-ratio optics, and they are the perfect
complement to fast astrographs such as Celestron's
Rowe-Ackermann Schmidt Astrograph (reviewed in the
June issue, page 66).

Recording OSC Astrophotos
Capturing images with an OSC camera is certainly
simpler than with monochrome detectors, but most of
the technique is the same. As with all deep-sky imaging,
exposures should be long enough to produce adequate
signal. Depending on your focal ratio, 16 or more ﬁve- to
ten-minute exposures should be adequate to produce a
smooth result.
Image calibration is nearly as important with OSC
images as it is with monochrome cameras. Most recent
models of DSLR cameras and many astronomical OSC
CCD cameras are based around extremely low-noise sensors, making dark frame subtraction (and bias calibration) often unnecessary. However, ﬂat-ﬁeld calibration
frames may still be required. These are exposures of
an evenly illuminated surface that record vignetting or
out-of-focus dust in your optical system. These images
are then applied to your light exposures to remove their
eﬀects from your images.

a process called debayering, which extracts the color
information from the grid.
With your ﬁrst image open in Astroart, select Color >
CCD Color synthesis. Astroart requires users to establish
the order of the Bayer ﬁlters in their sensors by adjusting
the Synthesis, XY Oﬀset ? numbers and clicking OK. In
this new window, a grayscale and color grid representing the ﬁ lter matrix is displayed. Matching the brightness value of the greyscale grid to the color grid should
help to guide you to the proper setting. Try the default
settings ﬁrst by clicking the OK button, but if the result
appears monotone green or blue, these default oﬀsets are
incorrect. Try diﬀerent combinations by changing the
top oﬀset from 0 to 1, and continue until a natural color
image is produced. Once the conversion parameters are
established, close the image without saving any changes;
the software will store these settings until you change
them to accommodate a diﬀerent camera. They will be
applied momentarily.

Calibration and Stacking
Now that you've established your debayer settings, open
the Preprocessing tool (Tools > Preprocessing). In this
window you'll select, combine, and apply your calibration
frames to your light images, as well as execute a number
of other actions that will result in a ﬁnal calibrated stack
of all your images. Beginning with the Files tab, navigate
BAYER MATRIX Color cameras
have monochrome detectors with
a microscopic grid - known as a
Bayer matrix - of red, green, or
blue ﬁlters placed over each pixel.
This information is then re-interpreted as the color image.

Debayering
Before calibrating and combining your images, your
ﬁrst task is to determine your particular sensor's ﬁ lter
pattern. I enjoy using Astroart (msb-astroart.com) when
processing my OSC images, because it includes many
tools designed speciﬁcally for OSC data. Opening an
unprocessed OSC FITS or RAW ﬁ le in Astroart reveals a
grayscale image with a grid-like appearance. As mentioned earlier, the detector in your camera records a
monochrome image with a pattern of tiny ﬁ lters over
the sensor, so what you see here is the diﬀerence in light
sensitivity of your sensor to red, green, and blue light.
This now requires converting into color images through

RAW GRID Unprocessed RAW or FIT astrophotos appear as black-andwhite images with a grid-like appearance.

Sk yandTelescope.com July 2015

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Sky and Telescope - July 2015

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