First Observations with the Polar Bear Telescope

1
First Observations with the Polar Bear Telescope
This telescope will monitor light pollution and
star variance over the North Pole. It is made up
of 3 CCD cameras with 85mm lenses situated on the
roof of Armagh Observatory. The cameras will
point at the North Star (Polaris) and will
collect data about the stars in that region for 3
years. Light pollution levels will be measured by
comparing the brightness of the night sky with a
point of fixed brightness Polaris.
Before the cameras were placed in their permanent
positions on the Observatory roof a number of
daylight tests were run.

• The Angle of View was calculated. The angle of
  view is the angle created between the edges of
  view of the camera and the camera lens. This was
  necessary to position the cameras and to know how
  many stars would fit into each image.
• The edge of view was marked out with pins on a
  board and checked using the focus window.
• The distance between the pins was measured (d).
  As was the distance between the camera lens and
  the board (f).
• a 2arctan( d ) 2f
• The angle was found to be 15.7.

The Little Bear constellation with Polaris as the
tail. As these stars will always be in the
telescopes view, the constellation gives the
telescope its name Polar Bear.
Cameras on their test rig
Dark current is the current detected when no
light reaches the receptor. It is caused by
electrons which break free due to a build-up of
thermal energy in the CCD. This noise can be
removed from images by subtracting a dark frame.
Dark frames are long exposures with the lens cap
on and no light. After taking an image, the lens
cap is put on and an image of the same exposure
time is taken. The dark current depends on the
temperature of the CCD. To measure the
dark current a long exposure is taken with the
lens cap on (the longer the better). A bias frame
can also be taken at the same time. The bias
frame is subtracted from the dark frame so noise
due to bias is removed. This leaves the noise due
to dark current which is the mean pixel value of
the area in the centre of the image divided by
the length of exposure in seconds. Exposure
time 60s Mean pixel value 11.3 ADUs Dark
Current 11.3 0.189 e-/pixel/sec
60
One method of measuring the resolution of the
lens is to measure the Modulation Transfer
Function (MTF). This describes the response of an
optical system to an image decomposed into sine
waves. It is also known as the Spatial Frequency
Response. MTF quantifies the ability of an
optical system to resolve or transfer spatial
frequencies. The units of MTF can be line pairs
per millimetre (lp/mm) or cycles per millimetre.
Lp/mm refers to the number of black lines
discerned by the camera, each line made up of one
black line and one white line, hence the pair.
Cycles per millimetre refers to the number of
sine wave cycles discerned when a sine pattern is
being observed. Often line pairs are used in
place of cycles for a sine wave pattern.
After testing the MTF of the camera the value
was found to be 21.3cycles/mm. This was much
lower than expected as a minimum of 30cycles/mm
is normal.
One night was spent trying out the cameras on the
roof. Unfortunately it was cloudy and as it was
the first time the cameras had been used outside
the focus was not right. As a result the stars in
the images appear as large circles. However,
Polaris is quite distinct as the brightest circle
in the middle. This was a 2 minute exposure.
Many other star-circles are visible when the
image is viewed full size. There is some noise
and an haziness due to clouds.
2 minute exposure of night sky