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 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

Cameras on their test rig

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.

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: 60sMean pixel value: 11.3 ADUsDark Current: 11.3 = 0.189 e-/pixel/sec 60

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.