Choosing astrophotography equipment can be confusing due to the many choices available. When matching a telescope with a camera for astrophotography, there are several things to consider to make sure you have the best match. It is much more than just thinking about budget and what looks nice.
Many beginner astrophotographers fail to consider the image circle when obtaining equipment. And many telescopes that are not designed for astrophotography won’t even give an image circle specification. This does not mean they won’t work for taking images. It just means they aren’t the best options. Telescopes that are called “astrographs” are designed specifically to be used for photography. As you might expect, these telescopes are usually a bit more expensive.
What is an image circle?
An image circle is the area of a telescope or lens’s field of view that can produce a sharp and clear image. It is defined as the circular region within which the optical system can form a focused image without significant loss of sharpness or brightness. The size of the image circle depends on the design of the optical system and is typically measured in millimeters or inches.
The image circle is an important consideration for astrophotography and other imaging applications because it determines the size of the sensor or film that can be used without vignetting or other image quality issues. Vignetting is the fall off-of light at the edges of the image. If the sensor or film is too large for the image circle, the edges of the image may appear darkened or blurred, resulting in a lower-quality image. Therefore, it is important to match the size of the sensor to the size of the image circle to achieve the best possible image quality.
Image circle alone is not enough. The area of the flat field is also important. Because lenses and mirrors are curved, the area of focus can also be curved. In other words, while the center of the image is focused, the edges may not be quite focused, resulting in odd shaped stars in the corners, also called coma. Some telescopes have built in lens designs to create a large flat field. Other telescopes require an optional field flattener to enhance the diameter of the area in focus.
An example of a sensor that is too large for the image circle. The edges of the image will appear dark.
This is an image circle that will fully illuminate the sensor
Other factors to consider
The pixel ratio, also known as the pixel scale or image scale, is an important consideration in astrophotography because it determines the level of detail that can be captured in an image depending on the sky conditions and the optics of the telescope. The pixel ratio is calculated by multiplying the size of the camera’s pixels in micrometers by 206 and dividing the result by the focal length of the telescope in millimeters, and it is expressed in units of arcseconds per pixel. Check out this pixel ratio online calculator.
Why should you care about this number? Depending on your location, detail that is discernible in a telescope depends on several sky conditions in that location, including atmospheric turbulence. You may often hear of the term “seeing” which is a measure of these conditions. Depending on the pixel size of a camera and the focal length of a telescope and the seeing conditions, a star image will spread out over several pixels. Too few pixels and the star image will look blocky. Too many pixels will give a nice round star at the expense of finer detail. Ideally, the pixel ratio and seeing are compared to determine if a camera is a good match for a telescope. Check out this link, where you can enter a camera and telescope combination to see if it is a good match for your sky conditions.
If you already have your camera and you are considering which telescope would be a good match, one of the first things you need to do is calculate the size of your camera’s sensor. This can be determined by either getting the specifications directly from the manufacturer or by multiplying the pixel size by the pixel ratio, for example 6720 x 4480 pixels, which is the size of the sensor in a Canon EOS Ra camera. The pixel size of this camera is 5.36 um (micrometers). This calculation would give a size of approximately 36 mm x 24 mm. This sensor is a full frame sensor, comparable to 35 mm film cameras.
The diameter of full frame sensors is approximately 43 mm, so the bare minimum image circle should be 43 mm. These large sensors have the advantage of giving a larger field-of-view, which is advantageous for large celestial objects. The trade off is that smaller objects can be lost in the large field of view. In this scenario, even zooming in will not generally give the detail that a smaller sensor would give. A lot of this depends on the pixel size vs. the focal length, also called the pixel scale. On the other hand, everything else being equal, smaller sensors result in smaller fields of view, but this is advantageous for smaller objects. Larger objects cannot be imaged in a single field of view. However, that can be compensated for by using mosaics. Mosaics are simply stitching together a grid of overlapping images to compose the whole objects.
There are several considerations when deciding on equipment choice. A lot of the underlying concepts can be confusing and involve very scientific optical topics and calculations. These concepts should be generally considered but should not be deal breakers. Many very fine images can be obtained even when equipment might otherwise be considered mismatched.