Telescope Back Spacing Calculator
Hit the back focus your corrector wants — with the filter shift, the real factor at your spacing, and a plate-solve calibration when the spec is unclear.
The corrector
The train, from the corrector shoulder back
Camera
Telescope
Spacers to close the gap
Don't know what your Barlow's design distance means? Measure it instead.
A plate solve reports arcseconds per pixel. With your pixel size that gives the true effective focal length, and dividing by the native focal length gives what your corrector is really doing — no datasheet, no guessing which surface the "75 mm design" was measured from. It uses the camera, telescope and spacing you entered above.
Four things that make this harder than adding up spacers
Nobody says what the design distance is measured from. The optics care about the distance from the corrector's own principal plane to the sensor, which no maker can conveniently print — so they publish a mechanical proxy and don't agree on which one. Most flatteners and reducers mean the rear shoulder, the flat face your adapter bottoms out against. Many Barlows mean the top of the barrel, where an eyepiece shoulder would sit; some mean the rear of the lens cell, which can be 25–30 mm different on the same part. If you've found two figures for one Barlow, that's almost certainly two reference surfaces rather than two products — and the panel above settles it from a single plate solve.
Filter glass moves the focal plane. A flat plate of thickness t and refractive index n pushes focus back by t(1 − 1/n), which for ordinary glass is very close to a third of the thickness. A 3 mm filter needs 1 mm more mechanical spacing — so a train that measures a perfect 55 mm is actually 1 mm short, and nothing on the outside of it shows that.
A reducer's ratio is not a property of the reducer. It is a property of the reducer and the distance to the sensor: factor = 1 − d/f. The Celestron f/6.3 corrector is 0.63× at its designed 85 mm and about 0.54× at 105 mm. "0.8× reducer" means 0.8× at one specific spacing and something else everywhere either side of it.
A Barlow works the same way in reverse. M = 1 + d/f, so extending it amplifies more and shortening it amplifies less — a "3×" run short of its design distance is not a 3×, and the shortfall goes straight into focal length and image scale. This is the single commonest reason two people with the same telescope, camera and Barlow get visibly different image sizes, and it always surprises the one getting the smaller image.
- Unscrewing the lens cell is not a free 2×. Threading a Barlow's cell straight onto a camera nosepiece is a very common trick, and it puts the cell perhaps 30 mm from the sensor instead of 75 — so a part marked 2× behaves as roughly 1.4×. It's in the corrector list for that reason.
- A telecentric amplifier is the exception. A Powermate-style design holds its magnification nearly constant with spacing, which is precisely what a Barlow doesn't do. That's what you're paying for.
- A moving-primary SCT has no fixed back focus, and that's the design. You focus a C8 or C11 by shifting the primary mirror, which changes the mirror-to-corrector separation and with it the scope's own focal length — a nominal 2032 mm C8 runs roughly 1900–2300 mm depending on where the mirror sits. Back spacing and image scale are coupled on an SCT in a way they aren't on a refractor. EdgeHD is the exception: the corrector group in the baffle tube fixes back focus, which is why Celestron publishes one for those and not for the plain SCTs.
- The camera's own sensor-to-flange distance is part of the total. 6.5 mm on a cooled ZWO, 12.5 mm on an uncooled one, 17.5 mm on a QHY, 44 mm on a Canon EF body. Taking the wrong one is a whole-centimetre error — and it changes the moment a supplied nosepiece goes on or comes off, which is why every figure here is editable.
- Wrong spacing looks like a corner problem, not a focus problem. The middle of the frame stays sharp while the corners smear, roughly symmetrically about the centre. Which way the stars stretch depends on the corrector design, so don't trust a remembered rule about radial versus tangential — move the spacing a millimetre or two one way and see whether the corners improve.
- Tilt looks almost identical, but is not symmetrical. If one corner is much worse than the opposite one, that is tilt rather than spacing, and no amount of shimming the length will fix it.
- Faster scopes are fussier. Roughly a tenth of a millimetre of tolerance per unit of f-ratio, so about half a millimetre at f/5 and a millimetre at f/10. A reducer makes the beam faster and so tightens its own tolerance. The corrector's datasheet always wins over any rule of thumb.
How to use
- Pick your telescope and camera, or type the figures straight over them.
- Pick the corrector, or type its design distance and the factor marked on it.
- List everything between the corrector shoulder and the camera flange.
- Add your filter thickness — it changes the required spacing and is easy to miss.
- Unsure what the design distance means? Plate-solve a frame and calibrate instead.
Frequently asked questions
What is back spacing and why does 55 mm keep coming up?
It is the distance from the rear shoulder of a corrector to the camera sensor, and a flattener or reducer is computed for one specific value of it. Miss that value and the middle of the frame stays sharp while the corners smear. 55 mm became the de facto standard because it accommodates a T2 thread plus a typical camera flange, so most modern flatteners and reducers are designed around it — but plenty of parts want 56.2, 63.5, 85 or 105 mm, and the datasheet is the authority.
Why does my filter change the spacing?
Because a flat piece of glass in a converging beam pushes the focal plane further back, by t times (1 minus 1 over n) where t is the thickness and n the refractive index. For ordinary filter glass that is very close to a third of the thickness, so a 3 mm filter needs 1 mm more mechanical spacing. This one catches a lot of people: the train measures a perfect 55 mm, nothing on the outside of it looks wrong, and the corners are still soft.
Is my 0.8x reducer always 0.8x?
No. The reduction factor is 1 minus d over f, where d is the distance to the sensor and f is the reducer's own focal length — so the ratio is a property of the reducer AND the spacing. The Celestron f/6.3 corrector is 0.63x at its designed 85 mm and about 0.54x at 105 mm, which is exactly why people extend it deliberately. The number printed on the box applies at one spacing only.
Does the same thing happen with a Barlow?
Yes, in reverse. Magnification is 1 plus d over f, so extending a Barlow amplifies more — a 2x at double its design spacing becomes a 3x. Planetary imagers use this on purpose to reach a target focal ratio without buying another Barlow. It becomes a nuisance when you are trying to land on a specific image scale and cannot work out why your plate solve disagrees with your arithmetic.
How do I tell wrong spacing from tilt?
Wrong spacing is roughly symmetrical: the centre is sharp and all four corners degrade in a similar way. Tilt is not — one corner is markedly worse than the one diagonally opposite it. If your corners are unequal, no amount of adjusting the length will fix it and you need a tilt adapter instead. It is worth checking which you have before spending a week shimming.
Which way do the stars stretch if I get it wrong?
It depends on the corrector design, which is why you will find confident and contradictory rules about radial versus tangential elongation. The reliable diagnostic is the pattern rather than the direction: aberration that is absent in the centre and grows toward the corners, symmetrically. To find the right direction, change the spacing by a millimetre or two one way and see whether the corners improve or worsen — that settles it in one exposure.
What is the design distance actually measured from?
The optics care about the distance from the corrector's own principal plane to the sensor, which no manufacturer can conveniently print — so they publish a mechanical proxy, and they do not agree on which one. Most flatteners and reducers mean the rear shoulder, the flat face your adapter bottoms out against. Many Barlows mean the top of the barrel, where an eyepiece shoulder would sit; some mean the rear of the lens cell, which can be 25 to 30 mm different on the same part. When a spec says 75 mm without saying from what, that ambiguity is the actual problem.
My Barlow lists two different distances. Which one is right?
Almost certainly both, measured from two different surfaces. A 3x Barlow quoted at 88.1 mm and at 61.8 mm is one product with two reference points rather than two products — and the difference matters enormously, because the implied lens focal length is 44 mm in one case and 31 mm in the other. Rather than choosing, plate-solve one frame and use the calibration panel. It recovers the design distance for however your train is actually assembled, and needs no datasheet at all.
How do I find out what my Barlow is really doing?
Plate-solve a single frame. The solver reports arcseconds per pixel; with your pixel size that gives the true effective focal length, since scale is 206.265 times pixel microns divided by focal millimetres. Divide that by the native focal length and you have the factor the corrector is actually delivering. From there the lens focal length falls out, and so does the spacing that would deliver the figure marked on the barrel. It is the reliable route precisely because it measures the thing instead of looking it up.
Why are my images smaller than other people's with the same gear?
Usually because the Barlow is running short of its design distance. Magnification is 1 plus d over f, so shortening the train amplifies less — a 3x at half its design distance is nearer 2x, and the shortfall goes straight into focal length and image scale. A very common cause is unscrewing the Barlow's lens cell and threading it directly onto the camera nosepiece, which puts the cell around 30 mm from the sensor instead of 75 and turns a 2x into roughly 1.4x.
What if my camera or telescope is not in the list?
Every dropdown here fills editable fields, so the lists are a convenience rather than an authority — type your own pixel size, back focus, focal length and aperture straight over whatever they offer. That is deliberate. Back focus figures change the moment a supplied nosepiece goes on or comes off, some models are revised between production runs, and no list of astronomy cameras stays current for long. Pixel pitch is the figure worth getting exactly right, because image scale depends on it directly.
Does my C8 or C11 have a back focus specification?
A plain moving-primary Schmidt-Cassegrain does not, and that is the design rather than a gap in the data. You focus one by shifting the primary mirror along the baffle tube, which also changes the mirror-to-corrector separation and therefore the telescope's own focal length — a nominal 2032 mm C8 runs roughly 1900 to 2300 mm depending on where the mirror sits. EdgeHD is the exception: the corrector group in the baffle tube fixes back focus at 133.35 mm on the 8 inch and 146.05 mm on the larger ones, which is why Celestron publishes a figure for those and not for the plain SCTs.
How close do I need to get?
Roughly a tenth of a millimetre of tolerance per unit of f-ratio, so about half a millimetre at f/5 and a millimetre at f/10. A reducer makes the beam faster and therefore tightens its own tolerance, which is why reduced setups are fussier than the same scope at native focal length. That is a rule of thumb and the corrector's own specification always wins — some designs are noticeably more sensitive than their f-ratio suggests.
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