BBC Sky at Night Magazine
Step by step
Your image scale will be determined by the lens or scope attached to your camera’s front. A lens of 600mm focal length attached to a non-full frame DSLR (eg, APS-C) will deliver a 2˚ (long frame dimension) field. A full frame camera with a 1,000mm lens achieves the same field: 2˚ is the smallest workable size for close approach.
An aligned tracking mount is needed. The M81/M82 field is located near the northern celestial pole. The apparent movement of stars due to Earth’s rotation is reduced compared to what you’d see if the field were closer to the celestial equator. Maximum exposures depend on the setup, but a minimum of 30-60 seconds is achievable.
Set your camera’s ISO to a mid-low value, say ISO 800-1600. For lens-based setups, fully open the lens (lowest f-number). For 30” exposures, use the camera’s 30” setting. For longer values, set the camera to bulb mode and use a remote shutter release to control shutter opening. Use a stopwatch to control exposure lengths.
Accurate focusing is important, so take your time. Pre-focus on a star such as Dubhe (Alpha (a) Ursae Majoris) using your camera’s Live View facility. Set the Live View display to maximum magnification. Approach focus, achieve it and wind through it. Repeat until you recognise what focus looks like, then wind to it.
The night of 23/24 May presents the closest pass with the comet passing M81 by 1.5˚. Align the camera frame so the mid-point of the line joining the comet to M81 is in the centre; having both objects on the frame diagonal works well. It may also be worth noting that there are several other faint galaxies in the vicinity (see page 56).
The comet’s slow speed may allow several minutes of images to be stacked without blurring. DeepSkyStacker can stack and average images, allowing flats and darks to be used. Darks are equivalent exposures with the lens cap fitted. Flats are an image of an evenly illuminated white light source, exposing to 50-70% saturation.