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 a total of 55mm spacing, you would subtract 17.5mm from 55mm = 37.5mm. That’s 37.5mm space to “fill up” with accessories like filter holders and so-on.
So, let’s say you have an Altair filter holder which takes up 17.0mm spacing in between the Hypercam TEC Cooled camera and the reducer/flattener. It looks like this:
Therefore, you need to fill in (37.5mm – 17.0mm) = 20.5mm between the camera and the flattener.
So we have the following in the “optical train”: Telescope > Reducer/Flattener (55mm spacing) > Filter Holder (17.0mm) > Stacked extension rings (total 20.5mm) > Camera (17.5mm flange to sensor spacing). *
* Remember Hypercam Fan Cooled Cameras have 12.5mm Spacing and TEC Cooled cameras have 17.5mm sensor-to-flange distance:
A simple formula to work all this out, is:
Reducer/flattener required spacing, minus camera sensor to flange distance = “Total spacing” to take up with extension / spacer rings.
In the case above we need to make up 20.5mm spacing distance spacer between the camera and filter holder, so we need extension spacer rings, which look like this:
Here’s how extension spacer rings stack together. Note the easy grip part. This really helps with unscrewing them.
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. The reason is that adding filters changes the sensor to flange distance slightly because they are made of glass. Here’s what a variable spacer looks like:
Fine-tuning your setup. This handy diagram helps you get the camera and the flattener placed better relative to each other: