Many astro photographers use field-flatteners or reducer-flatteners to make stars appear more rounded at the edges of the field of view.

Reducers and field flatteners have an optimum spacing between the camera sensor, and the rear of the unit.

The optimum spacing distance for your optics is usually quoted in mm between from the flat face of the camera to the rear of the flattener/reducer excluding any threads (that’s because the threads are buried when the units are tightened together).

To achieve the right spacing you need to know how far inside the camera body the sensor is placed. This is usually referred to as “sensor-flange distance” and it varies between camera models. Refer to sensor to flange spacing for Altair Cameras here.

You then need to subtract the sensor-flange spacing value from the optimum spacing data for your telescope and reducer/flattener combination.

So if you have a Hypercam TEC Cooled camera with 17.5mm sensor to flange distance and a reducer/flattener which requires say 55mm spacing, you would subtract 17.5mm from 55mm = 37.5mm to take up with accessories like filter holders and so-on.

So, let’s say you have an Altair filter holder which takes up 17.5mm spacing in between the Hypercam TEC Cooled camera and the reducer/flattener. Well you’d you need to fill in 37.5mm – 17.mm = 20mm between the camera and the flattener.

So we have: Telescope > Reducer/Flattener (55mm required) > 17.5mm filter holder > 20mm extension > camera (with 17.5mm flange to sensor spacing).

A simple formula is:

Reducer/flattener required spacing (X), minus camera sensor to flange distance (Y) = “Total spacing” to take up (Z).

Then, to take up the gap “Total spacing” (Z) minus “All Accessories” (A) = “Total space to take up with extensions/spacers” (B).

So if we put the values in brackets:

X (55mm) – Y (17.5mm) = Z (37.5mm)

Z (37.5mm) – A (17.5mm) = B (20mm)

Therefore B can easily be achieved by adding a 20mm fixed distance spacer between the camera and filter holder.

Sometimes you will want to fine-tune the spacing between the flattener rear face and the camera using variable spacers, or by stacking fixed spacers (they thread into each other) to make up the desired distance.

Here’s what extension spacers look like:

T2 Extension Spacers

Here’s how extension spacers stack together:

Stacked Extension Spacers

Here’s what a variable spacer looks like:

Variable extension spacer

Fine-tuning your setup. This handy diagram helps you get the camera and the flattener placed better relative to each other:

Altair reducer or flattener spacing star shapes

Altair reducer or flattener spacing star shapes