Sky & Telescope - November 2019 - 38

Scrubbing Data

p This image of the globular cluster M5 in Serpens was processed without calibration frames, showing several artifacts that are easily corrected
in calibration.

p Dark frames record electronic signal that builds up during an exposure. A proper dark frame needs to be shot at the same temperature
setting in your camera and must be exposed for the same length as the
light frames it is meant to be subtracted from. The vertical lines are due
to stuck pixels and are completely corrected during dark subtraction.

with multiple ﬁlters, dust motes may be in a different place
on each ﬁlter. This requires a set of ﬂats for each ﬁlter. While
opinions differ on how many ﬂat frames to acquire, I prefer
16 ﬂat-ﬁeld frames per ﬁlter to produce a good master ﬂat.
You should acquire a new set of ﬂat frames every time the
camera is rotated on the telescope, and also when you bin
(group pixels) on the detector. The focus position should be
identical to that used while recording your target images.
Here are three methods for shooting ﬂats, depending on your
setup and level of experience.

This is perhaps the least expensive and easiest method for
producing ﬂat-ﬁeld images, though it can be tricky ﬁnding a
good T-shirt that works well.
Flat panel An electroluminescent panel makes excellent
ﬂat-ﬁeld images. Make sure the ﬂat panel is larger than the
diameter of your objective. Simply point your telescope or lens
straight up and lay the panel on the dew shield or lens shade.
Set the panel's brightness so that it reaches the target level
of illumination with exposures of a few seconds or so. This
method is low-tech and very reliable, but it does has some associated cost: Flat panels can cost up to several hundred dollars.
Twilight ﬂats (sky ﬂats) Beginning immediately after
sunset on a clear evening, point your telescope or lens about
5° east of the meridian (85° altitude, 90° azimuth). This is
the most evenly illuminated area of the twilight sky. You can
also collect twilight ﬂats beginning about half an hour before
sunrise pointed 5° west of the meridian (85° altitude, 270°
azimuth). Turn the mount's tracking off so that stars will trail
and appear in different locations in each ﬂat frame; they will
average out when combined into the master ﬂat. This method
works very well, but it can be challenging since the sky brightness changes rapidly during twilight, and the window for
collecting ﬂats is brief. To simplify collecting twilight ﬂats,
consider using software to automate the process and vary the
exposure times as required to keep the brightness consistent
within your set of ﬂat frames. Compared with the other two

Capturing Flat-Field Images
Recording good ﬂat-ﬁeld frames can be challenging - the
target needs to be extremely evenly illuminated, or else it will
introduce gradients across your light images. When performed carefully, these popular techniques can work well.
T-shirt ﬂats A white or light-gray T-shirt can be used as
a diffuser to produce a ﬂat-ﬁeld by targeting the daytime sky.
Stretch two layers of a clean T-shirt over the objective of the
telescope or lens and fasten it in place, ensuring there are no
wrinkles. Point the telescope towards an evenly
illuminated part of
the sky or other bright
target - an overcast sky
works particularly well.
Adjust the exposure
time as necessary and
acquire ﬂat frames the
same way you would
acquire light frames.
38

N OV E M B E R 2 019 * SK Y & TELESCOPE

t One quick and low-cost technique to record decent flat-field images
involves stretching two layers of a white or light-gray T-shirt over the front
aperture of the lens or telescope and exposing on the sky. New T-shirts
work best, and be sure to smooth out any wrinkles. Use of an electroluminescent panel to record flat-field images can be seen on page 69.

Sky & Telescope - November 2019

Table of Contents for the Digital Edition of Sky & Telescope - November 2019