The M.A.S. Newsletter 17

Venus still dominates the sky after sunset. On the evening of the 11th and 12th, Venus is located close to Regulus. The Moon passes close to Jupiter and then to Saturn in the early morning of the 8th and 9th. On the 15th, the Moon (8% illuminated) is just visible at sunset close to Venus.

Have you ever admired the sky and wondered if you can take your own photo of constellations?

The simplest way is to aim your own camera and shoot! But at night the light intensity from the stars is insufficient to affect the film during such a short exposure. To compensate for the low light level, the film is left exposed to (collecting) the light for a much longer time. Make sure your camera is equiped with a time exposure function (B on the speed selection knob). Now to keep your camera steady during the exposure you will require a tripod. Even with a tripod, you will very likely find arcs rather stars when you have your film developed. This is because the earth is rotating. Due to the rotation of its support (the Earth) the camera's aim shifts steadily through the stars, thus causing the stars to appear as streaks rather than point images. If the lengths of the streaks are so small that they are on average smaller than the size of the stars' images, then they would not be visible. To decrease the streak's length, the exposure time should be made shorter. Also, the length of the streaks will depend on the focal length of the lens being used and on how fast the constellation is moving across the sky (a star near the celestial equator will cover a longer distance than one close to the poles in the same interval of time). Thus, if we are to obtain point images of the stars, there is a maximum exposure time depending on the position of the constellation relative to the celestial equator and the focal length of your camera lens. For those who are not scared of equations, if d is the declination of a star, and f the focal length of the lens being used in mm, then the maximum duration of the exposure T (in seconds) is T = 550/f cos d.

The following table gives the exposure time computed for some objects of interest:

Focal Length	Crux, LMC, SMC, Ursa Major	Andromeda, Centaurus, Lyra, Cygnus	Scorpius, Gemini	Cancer, Delphinus, Leo, Corvus, Canis M, Aquila, Orion, Taurus
35 mm	26 s	18 s	16 s	15 s
50 mm	18 s	13 s	13 s	11 s
70 mm	13 s	9 s	8 s	8 s

Note that to be able to capture fainter stars on film, a very sensitive film is required (ISO 400 or higher sensitivity). I welcome any questions and inquiry at the next meeting.