Obtaining correct imaging sensor distance - Backfocus calculation tricks

This writeup originated from a couple of friends asking about "rules" :-)

The text applies primarily to optical trains where you have introduced a unit of corrector lens elements in front of the sensor, and this corrector has per design a particular need of exact distance to the sensor. This is the backfocus requirement. Ignoring this will make off-center stars look funny, mostly like not having a corrector. Or even worse.

Visual applications will in most cases not not be affected by backfocus requirements. For example, a doublet refractor just need an eyepiece and focusing. (however, if you are using an variable magnification compressor like the Baader FCC, you also need to consider the corrector to EP distance.
A SCT is a special case, since the corrector elements up the baffle tube does not follow an external (add-on focuser), but you have to first obtain the correct focus with the secondary element (OTA-back knob) focuser, then adjust the imaging train length. (e.g. 146.05mm for the Celestron 11 HD)
Note that some fully corrected refractors like the WO/TS Star71 have no elements moving with the focuser, so all you have to do is focus.

  • Start out with the exact distance as supplied by the vendor. One seems to agree, for several critical applications, that hitting within +/-0,5mm is absolutely necessary.

  • Then check the star-field, manually or with a software tool like CCDInspector.
    (Because, as a contact at Teleskop-Service once wrote back, "..the [Chinese] factories suddenly send something that is made to a slightly different measure..")

If not explicitly stated otherwise, male threads does not count. It's obvious when thinking about it. When putting together imaging accessories, adapters and extenders, you have to count either the length of the male or female threads that goes together. Male threads should be shorter than female threads to avoid getting stuck and having to deal with uncertainty in the functional thread length. So we relate to the female thread length, not the male counterpart.

Use a spreadsheet for calculating total backfocus consumption. (I always write down all relevant comments so I don't mix up)

Choose your extension rings, like these from TS. This is going to cost a little if you need several extensions.

Avoid the adjustable extension tubes, except if you really have no choice or your camera is particularly light.
Instead, use spacer rings of various sizes (0,3/0,5/1,0/1,5/2,0mm) to make fine adjustments leading to a near exact match between the imaging train and the backfocus requirement.
Avoid extension rings shorter than 5mm, as they have female threads shorter than 4mm. Most have 4 or 5mm thread lengths. I guess you can use spacers leaving 3,5mm thread length for the connection, down to 2,5mm if the cam/FW is light, and 2mm if you really have to.
Just don't skip out on making the right distance because a stronger connection feels better!

Measure everything with a good gauge, each part and together if possible. (a gauge with digital readout is recommended)

Finally: Spend some time studying the backfocus details of the various parts. It's not obvious at first what you need. (I've always found all I need by means of spreadsheet deployment) Ask the dealer if it's not totally clear to you how they specify backfocus.