Step by step

STEP 1

Using a Planetarium program (eg stellarium.org) work out the capture field of view for the desired date range. A field calculator can determine the setup to achieve this (see skyatnightmagazine. com/astronomy-field-view-calculator). Work out the centre of the field of view and frame orientation in terms of background stars.

STEP 2

With your gear set up and pointed at the centre of the field of view, take a test shot. For non-tracked shots with, say, a camera on a fixed tripod use the ‘500 Rule’ (left) to determine the longest exposure you can take without star trailing.

If you divide 500 by the attached lens/telescope focal length (mm), this gives you the maximum exposure in seconds.

STEP 3

Set your ISO to a lowish value (eg ISO 400-1600). If using a camera lens, set the f/-number low and then increase it by a stop or two to reduce potential image distortion at the frame’s edge. Use an exposure setting lower than the maximum in Step 2 and take a test shot. Examine for background stars – more than three is ideal.

STEP 4

Take your photo and record setup and settings details, then repeat over several nights. Use the first image as a reference, tweaking settings to achieve a similar number of background stars. Next, load all the shots in date order into a layer-based editor, each as a separate layer, oldest at the bottom. STEP 5

Make all but the lowest layer hidden. Turn visibility on for next layer up and transparency to 50%; adjust its position so the stars align accurately. Set its transparency to 0% and hide. Repeat for all layers using the lowest as the reference. Next, make all layers visible and set the blend mode for all except the lowest to lighten. STEP 6

The background sky of the lowest layer should be bright enough to dominate. If it’s poor, you can swap with a better one but set the lowest layer’s blend mode to ‘normal’, all upper layers to ‘lighten’. If upper backgrounds ‘shine’ through, dim them with the curves tool, applying an S-shape curve adjustment over the histogram.