Back focus, it's a term you've probably heard of, but what is it, why does it matter, and how do you set it correctly for your equipment?
Back focus is the distance between the last optical element and the focal plane. It's vital for achieving sharp and clear images, especially when using accessories such as filters, reducers, or correctors.
Whether you are a beginner or an expert in photography or astronomy, here are some useful tips and tricks to improve your image quality and avoid common problems related to back focus.
What is focus and how is it different from back focus?
Focus is the point where light rays from a distant object converge to form a sharp image on a plane. In astrophotography, this plane is usually the sensor of your camera. To achieve focus, you need to adjust the distance between the lens or mirror of your telescope and the sensor until the image is clear and crisp.
Back focus is the distance from the last optical element of your telescope or accessory (such as a field flattener or a focal reducer) to the focal plane. Back focus is important because some accessories require a specific back focus distance to work properly and correct optical aberrations such as coma or field curvature.
If the back focus distance is not correct, the image quality will suffer, especially at the edges and corners of the frame.
Focus and back focus are different because they depend on different factors. Focus depends on the focal length of your telescope, the aperture of your telescope, the distance of the object, and the size of the sensor. Back focus depends on the design of your accessory, the flange focal distance of your camera, and the amount of spacers or adapters you use.
How to achieve back focus for your astrophotography?
To achieve back focus you need to consider the following factors:
The type of telescope you are using (refractor, reflector, catadioptric, etc.)
The type of accessory you are using (field flattener, focal reducer, coma corrector, etc.)
The type of camera you are using (DSLR, mirrorless, astro camera, etc.)
The size of the sensor of your camera (full frame, APS-C, micro four-thirds, etc.)
The first step is to find out the back focus distance required by your accessory. You can usually find this information in the manual or the website of the manufacturer. For example, the Orion field flattener for short refractors requires 55mm of back focus. Most ZWO cameras also require 55mm of back focus.
The second step is to measure the flange focal distance of your camera. This is the distance from the flange (the metal ring where the lens mounts) to the sensor. You can find this information in the specifications of your camera or online. For example, the flange focal distance of a Nikon Z6 mirrorless camera is 16mm, while the distance for most ZWO cameras is 6.5mm or 17.5mm with a T-ring.
The third step is to calculate the number of spacers or adapters you need to achieve the correct back-focus distance. You can use this simple formula:
Spacers = Back focus distance - Flange focal distance - Camera adapter
For example, if you are using an Orion Field Flattener for Short Refractors with a Nikon Z6 mirrorless camera and a T-ring adapter, you need to use the following spacers:
Spacers = 55mm - 16mm - 10mm = 29mm
You can use a combination of spacers or extension tubes to achieve the correct back focus distance.
You can try using a digital calliper to measure as an extra step for added accuracy.
The fourth step is to assemble your imaging train with the spacers and adapters and attach it to your telescope. You can use a Bahtinov mask or a focusing aid to fine-tune the focus of your camera. You can also use software like BackyardNikon or APT to control your camera and check the focus on your computer screen.
What are the common issues with focus and back focus and how to solve them?
Star trails
This happens when the exposure time is too long for the focal length and the mount tracking accuracy of your telescope. To solve this, you can either reduce the exposure time, increase the focal length, or improve the tracking accuracy of your mount. You can also use a guiding system to correct the tracking errors of your mount.
Elongated stars
This happens when the focus is not uniform across the field of view. This can be caused by optical misalignment, collimation errors, tilt, or flexure. To solve this, you can either adjust the alignment, collimation, tilt, or flexure of your telescope and accessories or use a field flattener or a coma corrector to correct the optical aberrations.
Donut-shaped stars
This happens when the focus is not achieved at all. This can be caused by incorrect focus distance, wrong back focus distance, or mechanical issues. To solve this, you can either adjust the focus distance, the back focus distance, or the mechanical parts of your telescope and accessories. You can also use a Bahtinov mask or a focusing aid to help you achieve focus.
What are the Common back focusing Setups for astrophotography?
Some of the common back-focusing setups are:
Refractor telescope + field flattener + DSLR camera: This is a simple and popular setup for wide-field astrophotography. You need to use a field flattener to correct the field curvature of your refractor telescope and achieve a flat field of view. You also need to use a T-ring adapter to connect your DSLR camera to the field flattener. You need to set the back focus distance to the value required by the field flattener, usually around 55mm.
Reflector telescope + coma corrector + astro camera: This is a common setup for deep-sky astrophotography. You need to use a coma corrector to correct the coma of your reflector telescope and achieve a round star shape. You also need to use a T-adapter to connect your astro camera to the coma corrector. You need to set the back focus distance to the value required by the coma corrector, usually around 55mm.
Catadioptric telescope + focal reducer + mirrorless camera: This is a versatile setup for both planetary and deep-sky astrophotography. You need to use a focal reducer to reduce the focal length and the focal ratio of your catadioptric telescope and achieve a wider field of view and a faster exposure time. You also need to use a T-ring adapter to connect your mirrorless camera to the focal reducer. You need to set the back focus distance to the value required by the focal reducer, usually around 105mm.
Back focus is a crucial concept in astrophotography that affects the image quality and performance of your telescope and accessories.
To achieve perfect back focus, you need to know the back focus distance required by your accessory, the flange focal distance of your camera, and the number of spacers or adapters you need to use.
By following these steps, you can ensure that your images are sharp and clear from edge to edge.
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