Transcript MCERLEAN_2007 - Armagh Observatory

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).
• α = 2arctan( d )
2f
• The angle was found to be 15.7°.
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.
The Little Bear constellation with
Polaris as the tail. As these stars will
always be in the telescope’s 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 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 starcircles 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