DIY Astronomy
Construct a home-built accessory to help with wide-field astrophotography
This month’s project is a camera mount to help you produce wide-field images of the night sky using a standard lens. The method involves ‘stitching’ together a series of smaller images to produce a large composition – often called a ‘panorama’. By capturing separate but slightly overlapping shots and using photo-editing software you can create impressive images without the need to invest in expensive wide-angle lenses. The total pixel size of the resulting composite image also means you capture more detail. It is possible to achieve this without the use of a special mount, but if you have ever tried, you will know that it is common to miss small slices of sky or produce a lot of redundant data if your aiming is less than perfect.
To assist aiming, our mount has indexed scales for horizontal and vertical adjustments. The mount holds the camera steady while you take each image. When you are ready to move across or up to the next image you release the clamping screw, turn to the next indexed position, reclamp and shoot. By matching the scales to the chosen camera and lens during manufacture, aligning the shots required for a composition is easy.
Size matters
You don’t need to get involved in maths because the downloadable spreadsheet calculator will do it for you, but thinking about the principles behind it will help you understand how it works. Digital cameras have a sensor that sits behind the lens where the film would be in a traditional camera. A full frame DSLR has a 36 x 24mm sensor, which is about the same size as 35mm film. Cameras with smaller sensors are more common and for a given lens they will pick up correspondingly smaller areas of sky. APS-C format sensors are 23.6 x 16.6mm, so their width is 65 per cent of the full-size sensor.
The focal length of the lens affects the amount of sky captured by the sensor. A long lens will capture a smaller area of sky than a short lens. A full-sized sensor will capture an image 39.6˚ x 27˚ with a 50mm lens. With a 100mm lens this drops to 20.4 x 13.7˚. If you know your sensor width (S) and lens focal length (f) you can use the following calculation to work out the field of view angle (A) as follows; A = 2 x arctan (S ÷ 2f). If maths isn’t your thing, our spreadsheet calculates the values for you and also recommends the indexing for the two ‘dials’, allowing for a 10 per cent overlap for each adjoining frame edge so you should never miss a section again.
Construction is simple using our downloadable drawings, template and protractor. We clamped our finished mount to a garden table, but you could also use a tripod. For tips about out how to process your images using free software visit bit.ly/astrophototips.