Sky and Telescope - April 2015 - 68

Astrophotography

Bright Galaxies for Urban Imagers
Object
M31
NGC 253
NGC 891
M77
NGC 2403
M81
M82
M65
M66
M100

Constellation

Mag(v)

Dec.

And

3.4

0h 42.7m

+41° 16′

7.6

47.6m

-25° 17′

9.9

22.6m

+42° 21′

8.8

42.7m

-0° 01′

8.5

36.9m

+65° 36′

6.8

55.6m

+69° 04′

8.4

55.8m

+69° 41′

8.5

11h

18.9m

+13° 05′

9.0

11h

20.2m

+12° 59′

9.4

12h

22.9m

+15° 49′

25.1m

+12° 53′

Scl
And
Cet
Cam
UMa
UMa
Leo
Leo
Com

M84

Vir

9.3

12h

M86

Vir

9.2

12h 26.2m

+12° 57′

8.5

12h

56.7m

+21° 41′

8.1

13h

29.9m

+47° 12′

M64
M51

Com
CVn

pictures, making them appear less noisy. Most DSLRs
today have very little dark current, so imagers often skip
this calibration frame.
Bias frames record the ﬁ xed-pattern noise inherent in
all digital detectors and are taken at the very fastest shutter speed you have. These are likewise subtracted from
your photos, as they help to reduce uneven banding seen
when you stretch an image to reveal faint details during
post-processing.
The last calibration frame is the ﬂat-ﬁeld image. This
is a snapshot of an evenly illuminated surface that records
the vignetting of your lens and dust motes on the detector
that plague virtually every optical system but are more
pronounced when shooting under light-polluted skies.
When properly applied, ﬂat-ﬁeld calibration can greatly
reduce if not eliminate these optical eﬀects.
Flat-ﬁeld images can be recorded in various ways using
the darkening sky during twilight or with an evenly illuminated screen. No matter how you choose to take your
"ﬂats," they need to be recorded in the exact same optical
conﬁguration used for imaging during an entire evening.
It's important to have your telescope focused before taking ﬂats so that you accurately record the location of any
dust motes, and you should not rotate the camera for the
rest of the night.
Flat-ﬁeld images should also be exposed to have the
highest-level register at about 1/3 to ½ of the histogram
display. This is a graph feature in your camera that shows
brightness levels and is plotted from left (black, or no signal) to right. Consult your camera's manual to see how to
display the brightness histogram on your particular model.
68

April 2015 sky & telescope

By shielding his equipment from direct lighting and shooting only when the object was in the darkest part of his backyard sky, the author managed to capture this deep, colorful
image of M33 using an 8-inch f/3.9 Newtonian astrograph
and twelve 10-minute exposures.

Once you've recorded all these calibration frames,
consider picking up an additional computer program that
can apply them to your photos. Although you can capture
your images in your DSLR with only the addition of a
cable release and the camera's Bulb setting, you'll need a
dedicated astronomical processing software program to
properly calibrate and adjust the images.

Managing Your Sky
With all the preparation done, you're ready to start imaging. Spend a little time determining where the best part of
your sky is. Light pollution not only washes out the sky, it
also introduces unusual background hues and brightness
gradients that can be tough to deal with. To top it oﬀ, light
pollution is never uniform across the sky; instead it's more
concentrated in the form of light domes rising up from
the horizon. But a little bit of strategy will usually mitigate
these problems. Set up your scope in a location that best
blocks local sources of light and shoot in the direction away
from the most intense light domes.
When imaging in urban skies, try to plan your evenings to shoot multiple targets that best utilize both your
local conditions and your time throughout a night. Take
advantage of your chosen targets' path across the sky by
planning to image them only when they are away from
light domes, or when they are simply highest. This lets
you avoid following a single subject as it goes from a dark
spot in your sky and into a brighter area later in the evening. While this approach might spread your acquisition
of a particular target over multiple nights, you'll get better
images overall.

Sky and Telescope - April 2015

Table of Contents for the Digital Edition of Sky and Telescope - April 2015