RigTonightBack focus explained: how to work out the spacers for your imaging train
If the stars in your corners look like seagulls or little comets while the centre is sharp, back focus is the first thing to check. It is also one of the easiest problems to fix once you can do the arithmetic.
What back focus is
A field flattener, reducer or coma corrector is designed to deliver a flat, sharp field at one specific distance behind it. That distance is the back focus, measured from the corrector's camera-side thread to the sensor. For many M48 correctors it is 55 mm. Astrographs with a built-in flattener, such as the Askar FRA400C or SQA55, quote it for the whole scope.
Get it wrong by more than a millimetre or so on a typical f/5 to f/6 system and the corners suffer. Faster optics are less forgiving.
Adding up your imaging train
Everything between the corrector and the sensor uses up part of that distance:
| Part | Typical contribution |
|---|---|
| Cooled camera, e.g. Player One Ares-C Pro | 17.5 mm (its own back focus) |
| Filter wheel | about 20 mm |
| Off-axis guider, e.g. William Optics Elite Drawer OAG Kit | 37.5 mm |
| DSLR body plus T-ring | about 55 mm in total |
Whatever is left over is filled with spacers.
Filters add a little
A flat piece of glass in a converging light beam pushes the focus point back by about a third of its thickness. More precisely, it's thickness × (1 − 1/n), and n is about 1.5 for filter glass. A typical 2 mm imaging filter therefore adds about 0.67 mm. When a filter sits in the train, you need that much more spacing.
A worked example
Askar FRA400C (55 mm back focus) + a 20 mm filter wheel + Player One Ares-C Pro (17.5 mm), with 2 mm filters:
- Needed: 55 + 0.67 = 55.67 mm
- Already used: 20 + 17.5 = 37.5 mm
- Spacers: 55.67 − 37.5 = 18.2 mm, e.g. 16.5 + 2 mm
When the train is too long
A DSLR with a T-ring already sits at about 55 mm. Add a 20 mm filter wheel and you're roughly 20 mm past the corrector's spec, and no spacer can fix a train that is too long. The same happens with an astro camera when you stack a filter wheel and an off-axis guider behind a 55 mm corrector. The answers are thinner parts (a filter drawer instead of a wheel, or a slimmer OAG), a clip-in filter for a DSLR, or dropping one part from the train.
Tips
- Measure your actual spacers with calipers. Real parts vary by fractions of a millimetre.
- Tighten everything before testing; a loose thread adds distance.
- If corners are still poor at the right spacing, check for tilt, which looks different: one side or corner is worse than the others.
RigTonight works this out for every rig you build, using the spacer sizes most manufacturers sell.