Calculating Back Focus/Metal Back Distance

First of all, what is back focus? Or this metal back distance? It doesn't quite have the same use in astrophotography as traditional astronomy, nor is it in reference to a Norwegian metal band. Instead, metal back distance has to do with the spacing requirements when using any sort of corrective optics.

A note about the terminology when discussing this topic: the necessary spacing for an imaging train has been described using different terms, including "Metal Back Distance" and "Back Focus." This can be further complicated by trying to delineate between systems that use a separate corrective element versus telescope systems that either have their corrective elements integrated, or those that don't use corrective optics at all. We are going to simplify it here and say this: what we care about is the distance between the last lens element in your system to your camera chip. For this purpose, we often use "Metal Back Distance" and "Back Focus" interchangeably.

Corrective optics are additional pieces of gear that help fix optical aberrations that are present in most telescope designs. For example, refractors suffer from field curvature, and so need field flatteners to correct for this. Newtonians often suffer from coma, and so a coma corrector is used. These pieces make it so that your stars are pinpoint and your images are sharp across your entire sensor. As a note, telescopes that are designated as "astrographs" have these corrective type elements built in, or are well corrected through their design such that no additional corrective optics are needed.

There is more to consider when planning and connecting your full imaging train. These corrective optics work best at a specific distance away from their glass elements, most commonly 55mm. If your camera chip is too close or too far away, then the elements will not actually perform their intended corrections. If your sensor is say at 48mm or at 67mm away, you may still see curvature, or still have coma affecting your images.

To get the most pristine final images, we can help you figure out the distances in your entire imaging train and get the adapters as needed to ensure your camera is at the ideal distance from your corrective optics. You first need to know the metal back distance requirement of your telescope or corrective optic piece. Then, add up the spacing distances of all of your pieces in your optical train to your sensor. This includes the depth from the front of your camera to the chip itself, the depth of a filter wheel or off-axis guider, and the depth of any thread size changers. If after adding all of that up you are short of the ideal metal back distance, you need additional spacers. If you add all of the above up and you are too far away from your corrective optics, you will need some thinner adapters or accessories, or figure out a corrective optic that provides more metal back distance.

We've created a drop-down tool below to be able to help figure out what your spacing requirements are on your system. It should now be easy to calculate the precise spacing adapters that will unlock the full potential of your imaging train.

Below the drop-down tool is a set of common M42 and M48 spacers to help get you on your way to sharp images. If after inputting all of your values you require some spacing that is not available with the standard spacers, never fear! Contact us and we can help design and order a custom adapter for any imaging train requirements.




  • Hi, I like your backspace calculator. I went to see what my backspace would be for my FSQ-85 with the 0.73 QE reducer attached. Unfortunately, you have many ZWO cameras listed there, but not the ASI183 MC pro. Could you help me find the value for that camera?
    Thank you,

    Dominic Schepis
  • Hello Tim,

    Thank you for the kind words! You are very close on the calculations for back focus requirements. One thing that is a little different than what you have in your question, is that the substrate thickness of a filter actually replenishes the back focus consumption of your imaging train.

    Here’s an example: Lets say your corrective optic requires 55mm of back distance, and your filter wheel consumes 20mm. This means you have 35mm of back distance remaining. Now, say you put in a filter with 3mm thickness into the wheel. This operates on the light cone in such a way, that it gives back 1mm of back distance, meaning you now have 36mm of back distance remaining.

    What we have found in relation to this, is that depending on the filter connection type (internal to a filter wheel, or screwed in directly to your imaging train) that this difference caused by the filter thickness itself can usually be ignored. There are cases when imaging at the absolute far edge of the corrected circle where every millimeter counts, but this can either usually be ignored completely, or you can use a 1mm thick spacer to make up for it.

    So in your case, I think you still hit the nail on the head: 67.4mm required – 6.5mm camera – 20mm filter wheel = 40.9mm required, which can be rounded to 41mm spacing needed.

    In addition to all of this, the spacing and flange distances are also prone to slight manufacturing variations, and so usually flange distances and back focus requirements are given a vale, with +/- 1mm possible variation.

    Hope this helps!

    ~Clear Skies

    Jon Minnick
  • Hi,
    Thank you for your note on back focus (b/f). It doesn’t make everything crystal clear, but its better than most.

    I have a C14, and recently set it up for Hyperstar, so I have a couple of shoes to fill; a rig for Hyperstar imaging, and a native rig.

    Starizona gives 67.4mm as the b/f on the Hyperstar. I’ve got a variety of cameras, but most are either ZWO cooled, or Mallincams, all of which claim a 6.5mm back focus without a nosepiece in place.

    Could you confirm the following calculations, or tell me the correct calculation?

    If I have a filter wheel that is 20mm wide when screwed into the chain, and 6.5mm of camera b/f, that makes a total of 26.5mm of b/f used.

    My understanding of filter details is that each 3mm of filter thickness counts as 1mm of b/f. So, if I use a 1.5mm thick filter, that takes up .5mm of b/f, even though it doesn’t show up on a ruler.

    My b/f stands, then, at 26.5mm physical plus .5mm in lens action, for a total of 27.0mm and given that I need to get to 67.4mm. I need a spacer that takes up 67.4-27.0=40.4mm.

    Thus, rounding up to the nearest .5mm, I order a 40.5mm spacer from Cloudbreak.

    Does that sound accurate? If not, what am I doing wrong?

    Thanks very much!

    Tim McLarney

    Tim McLarney
  • Hello Roger,
    We may include Mallincam cameras in the future. We could not include 100% of all available gear in this tool. If the DS16C specifications state 17.5mm backfocus (also commonly called Flange Distance), then this means the camera consumes 17.5mm of the spacing. For further example, if a flattener states that it provides 55mm of back distance, then using your Mallincam, you would need 37.5mm of distance using spacers and adapters to place your camera at the correct distance (55-17.5).

    ~Clear Skies

    Jon Minnick
  • Very interesting and informative article. I’m just getting into video/digital astrophotography and don’t understand back focus. But, why no Mallincam cameras? My DS16C specifications state 17.5mm back focus. Does that mean that is has 17.5mm or needs 17.5mm back focus.

    Roger Joyner

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