How to Set the Correct Back Focus for Your Telescope (Guide)
Figuring out the correct back focus for your telescope can be a daunting task to beginners and experienced astrophotographers alike. In this article, we’ll go over why back focus spacing is so important to get right, everything to consider for back focus spacing, and how to use adapters and spacers to get your back focus spacing perfect.
What is Back Focus, and Why it Matters for Your Telescope
Back focus, in its simplest definition, is the measurement between the last optical component, such as a corrector or reducer, of your telescope and the focal plane. When using your telescope stock without any accessories, you can easily reach focus because the telescope’s focuser travel is designed to move around this distance. However, when you’re using a corrector or reducer, back focus must be a set distance away.
Measuring Back Focus When Using a Corrector or Reducer
When back focus becomes really critical to get right is when you’re using an optical corrector or focal reducer with your telescope or one that is integrated into the design. Both optical correctors and focal reducers are designed to work best when the focal plane is at a set distance away. If you don’t space the focal plane (e.g. camera sensor) at the recommended distance away from the corrector, then the corrector will not work as intended. This usually results in elongated or misshapen stars, most noticeably towards the corners (off-axis) in the image. It becomes especially noticeable when using large sensors like full frame and/or a fast optical system.
Accessories That Require Correct Back Focus for Your Telescope
The Radian 61's integrated corrector requires 55mm of back focus
Optical correctors and reducers can also be built right into the telescope, which is becoming more and more popular for astrograph telescopes that are specifically designed for imaging. If you use any of the below telescopes, then make sure your back focus spacing is accurate.
Common Telescopes That Have Correctors Built-in & Their Back Focus Spacing
- Radian 61 Refractor — 55mm
- Celestron EdgeHD series telescopes — 55mm (with add'l T-Adapter)
- Celestron 8" RASA — 25mm (with included T-Adapter, incompatible with DSLRs)
- Celestron 11" & 36cm RASA — 55mm (with included T-Adapter)
- Meade ACF series telescopes — 55mm (with add'l T-Adapter)
Back Focus for Fast Telescopes and Large Camera Sensors
Back focus spacing becomes even more critical to get right when you’re using a telescope/corrector with a fast focal ratio, or when you’re using a camera with a large camera sensor such as full frame or larger. The reasons for this are simple. The faster the focal ratio of a telescope (or corrector), the narrower the focal plane is and therefore the more precise focus has to be. Second, the larger a camera sensor is, the more corrected the image field has to be to cover the entire image. If the corrected image field isn’t large enough to cover the full sensor, then stars may appear misshapen or elongated towards the corners of the image since it’s not fully corrected.
Where Do I Measure Back Focus From?
Back focus is most commonly measured from the flat edge of the corrector/reducer facing the camera, not including the threads on the corrector/reducer. You can use a tape measurer (or more ideally, a pair of calipers) to measure back focus. See the diagram below:
My Camera Says I Need 55mm Back Focus. Is This Correct?
Let’s get one thing out of the way: back focus is a property of optics, not cameras. That being said, 55mm is an industry-standard length of back focus for many optical correctors and focal reducers. So, while 55mm is usually correct for most setups, we always recommend double checking the manuals and diagrams for your corrector/reducer or telescope (if it has a corrector built in) to be sure. These can usually be found on the manufacturer's product page. If it's not listed, contact the manufacturer.
A wide (M48) T-ring for use with Canon cameras
I’m Using a DSLR or Mirrorless Camera. Do I Need to Worry About Back Focus Spacing?
Usually not, but it depends. As mentioned above, 55mm is an industry standard for back focus length. You can reach that using a T-ring, which are used to attach cameras to telescopes and other optical accessories. No matter which camera brand you use, T-rings are designed specifically to provide 55mm of back focus spacing from the telescope end of the T-ring to the camera sensor. If your recommended back focus is 55mm, then you'll be ready to go with only a T-ring. However, if your recommended back focus length is longer than 55mm, then you will need to use spacers or a T-adapter to make it longer. If your recommended back focus length is shorter than 55mm, you will want to use a T-minus ring (also known as a short t-ring) if possible to get to the recommended back focus length.
How to Tell if Your Equipment's Back Focus is Off
If you suspect that your back focus spacing is off and it’s producing stars that are elongated in the corners, use our Back Focus Spacing Guide below. If your stars appear to radiate away from the center, then your camera sensor is too close and you need to add more spacing. If your stars appear to be concentric around the center, then your camera sensor is too far and you need to remove spacing. Please note that sensor tilt and optical aberrations, such as coma and astigmatism, can also cause stars to appear elongated on their own and are unrelated to back focus.
This back focus spacing guide is sized to be saved on your phone for easy reference.
How Do I Set the Correct Back Focus Spacing?
To change back focus spacing, you need to add (or remove) spacers, sometimes also called extenders, within your telescope's imaging train. To do that, you'll need to physically thread the spacers into the imaging train at some point in between the camera and your telescope. It usually doesn't matter where, but if you're using filters or a filter wheel of any kind, you'll want to keep the filters as close to the camera sensor as possible. If you're using filters, see the section below on how filters add back focus distance.
How to Use Spacers for Back Focus
Spacers come in many different lengths to accommodate all different back focus spacing needs. They also come in a variety of thread sizes, such as M42 and M48, so be sure to choose the right size threads for your system. To make this easier, many astronomy camera manufacturers (such as ZWO) include the correct spacers to reach 55mm back focus. Spacers can be as short as less than 1mm and as long as necessary. Below are some common spacers, including the common 21mm and 16.5mm spacers that come included with most ZWO cooled astronomy cameras:
Common spacers that come with astronomy-dedicated cameras
To ensure all spacers and components in your imaging train can thread together, you will need to know the size of the threads on the end of your telescope or corrector/reducer. This is most commonly M48 or M42 size threads, but thread size can depend on the corrector, reducer, or telescope. Make sure your t-ring and/or spacers you have can thread together between the telescope, camera, and any accessories in between. ZWO has a helpful back focus guide on how to thread their included adapters together to reach 55mm of back focus.
I Use Filters in My Imaging Train. Does this Change My Back Focus?
Yes, using filters of any kind in your imaging train will alter your back focus slightly. Putting a filter into the optical path always increases the back focus distance. To calculate how much back focus spacing you need to add, take the thickness of the filter and divide it by 3. So, if you have a filter that is 3mm thick, you need to add 1mm of spacing to your imaging train to retain the correct back focus. Therefore, a 55mm back focus with a filter that is 3mm thick added to the imaging train would become 56mm. In practice, however, 1mm off from back focus spacing is usually not very noticeable unless you're using a large sensor or a very fast optical system.
Example of 55mm back focus with an off-axis guider in the imaging train
How do filter wheels/drawers, off-axis guiders, and other accessory in the imaging train affect back focus?
Filter wheels, filter drawers, and off-axis guiders all have thicknesses that will add to the overall length of your imaging train, and these need to be factored in when calculating back focus. Each individual part has their own thickness, which you can usually find in the product manual online. Some manufacturers produce these components with thicknesses that match the spacers included with cooled astronomy cameras, so that you can easily swap out a spacer for an off-axis guider, for example. For that reason, we usually recommend sticking with one brand's product ecosystem as their components are sometimes interchangeable.
I have a Petzval/Quadruplet Refractor. Do I Need to Adjust Back Focus Spacing?
The benefit of the Petzval/Quadruplet refractor design, such as the William Optics RedCat 51, is that the focusing mechanism moves the entire image train. This means that for these telescopes, all you need to worry about is reaching focus, which is rarely an issue, and your back focus spacing will be correct.
Is Back Focus Spacing Important for Planetary Imaging?
While it depends on the sensor size being used, back focus is not nearly as important to get correct for planetary imaging. The reason for this is because incorrect back focus spacing usually effects off-axis portions of the image like the corners. Since most planetary cameras use very small sensors unlike cooled astronomy cameras for deep sky, back focus is not nearly as critical and the effects of incorrect back focus spacing are usually unnoticeable.
Back Focus Spacing Summary
Back focus spacing is important to get right for nearly any deep sky imaging setup, especially those using large sensor cameras or fast focal ratio telescopes. All optical correctors and reducers require a specific back focus length, so be sure to check the manual or diagrams to find out what that measurement should be. Most often, it is 55mm, which is easily achievable with a T-ring for DSLR/Mirrorless cameras or adapters included with cooled astronomy cameras. However, if it’s other than 55mm, you may need to purchase separate spacers for your setup.