DSLR Camera Mounting
DSLR cameras are very flexible in the ways they can be used for astrophotography. This section describes various ways to mount your DSLR to capture different types of astronomical scenes.
Using a simple tripod for star trail photography is one of the simplest ways to photograph the night sky.
Without computer assistance, you need a remote shutter release. This minimizes camera vibration and allows for long exposures. The trick is achieving good focus, particularly since typical autofocus lenses will focus past infinity. Rough focus can be achieved using the viewfinder. Next you take short exposures of a bright star, zoom in on the star to determine how accurate the focus is, and then adjust. Repeat until focus is achieved. Long exposures in the bulb mode will yield arcs across the images as the stars move overhead. The images can be later processed on a computer.
While using the camera standalone has the benefit of portability, software control does have advantages. Software can be used to evaluate the focus using objective measurements. This makes it much easier to determine when you are at best focus. The computer can also acquire a sequence of images automatically, and save them to disk complete with header information. Usually the best results are achieved by stacking multiple exposures.
Piggyback adapters are widely available, which allow the camera to be mounted on top of a telescope with equatorial mount and clock drive. Various tracking platforms are also available, or can be home-built; e.g. the so-called "barn door tracker". If the mount is sufficiently well polar aligned, it is often possible to take long exposures without any guiding corrections.
From a camera perspective, this is fundamentally the same as tripod mounting, except that the stars do not trail. Thus you can take long exposures and get nice wide field shots of constellations, the milky way, and other large objects.
Prime Focus Mounting
There are three common methods to attach a DSLR camera to a telescope: prime focus, eyepiece projection, and afocal.
Prime Focus is the most popular and easiest to use for DSLR cameras. It provides a field of view similar to a low-power eyepiece, which is suitable for deep sky photography. The actual field of view depends on the size of the camera sensor and the focal length of the telescope, with longer focal lengths providing smaller fields.
The camera is attached directly to the telescope using a set of adapters. The first adapter, which is commonly available from camera stores, is called a T-ring. This adapts the proprietary camera mount (Canon, Nikon, etc.) to a standard screw thread.
The T-ring must then connect to the telescope. Various adapters are available; for example, T-thread camera adapters are widely available from manufacturers of popular Schmidt-Cassegrain telescopes (SCTs). It is also possible to get an adapter that connects to a standard 2" or 1-1/4" eyepiece barrel; if your telescope can accommodate it, the larger size is strongly recommended to ensure that the camera's sensor is fully illuminated.
It is important to ensure that the camera can reach focus. This is usually not a problem on SCTs and refractors, which usually have a large back focus and also a large focus range. It may be more of an issue with Newtonian telescopes; it is sometimes necessary to move the main mirror forwards in the tube to achieve focus. One trick is to place a thin sheet of white paper across the back of the focuser, point it at the moon so that it is visible through the paper, and adjust focus until it is sharp. This is where the camera's sensor needs to be in order to be in focus. You need to be able to move the focuser in from this position in order to achieve focus. The exact amount of inward movement depends on the camera and adapters, but you can get a rough idea by removing the lens from the camera and flipping up the mirror. Cameras are a common option on telescopes so your telescope manual should have instructions on connecting your scope to a camera.
A focal reducer or barlow lens can be added to the optical train to adjust the field of view and may sometimes help achieve focus when otherwise impossible on some equipment configurations. Faster (or lower) f/ratios are more desirable for DSLR cameras due to higher noise and the lack of cooling. A faster f/ratio will usually result in a shorter exposure and higher signal-to-noise ratio.
Eyepiece Projection Mounting
As implied by the name, Eyepiece Projection uses an eyepiece to project an image onto the sensor. Typically an adapter is used, one end of which plugs into the telescope, and the other end screws into a T-ring to attach to the camera in place of the lens. Inside the adaptor is a place where a small eyepiece can be inserted, usually held in place by a setscrew. Sometimes the camera is simply mounted in front of the standard focuser using some sort of bracket, although this is less stable.
Eyepiece Projection is used to greatly increase the magnification, compared with Prime Focus. Thus it is typically used for lunar and planetary imaging. The actual magnification depends on the focal length of the eyepiece used, and the spacing between the camera and eyepiece. Although this can be calculated, in practice the distances are difficult to measure and the appropriate configuration is determined by experimentation.
This method is usually used on cameras where the lens cannot be removed. The telescope is focused normally, so that the light exits the eyepiece as parallel rays. The camera is also focused for infinity, and placed behind the eyepiece. In this configuration, the camera is working precisely as the human eye does when looking through the telescope. This is sometimes done with the camera hand-held, although more commonly it is attached with some sort of bracket, or with a special adapter that screws onto the front of the camera lens.
These arrangements are somewhat awkward, sometimes leading to flexure or vibration. Therefore whenever possible the Prime Focus method should be used instead.
Demonstration of Eyepiece Projection Mounting with 1.25inches t mount extension adapter.
Demonstration of Prime Focus Mounting with 1.25inches t mount adapter.
Demonstration of Eyepiece Projection Mounting with digital camera telescope eyepiece adapter.