A Petzval design puts the field-flattening elements inside the objective assembly. When you focus, the whole optical group moves relative to the sensor, so the corrector-to-sensor distance is not a thing you set — it is a consequence of where the focuser ends up.
That is the opposite of a bolt-on flattener or reducer, where the corrector sits at a fixed distance from the objective and the sensor must be placed at a specific distance behind it.
In the manufacturers' own words
Petzval telescopes do not rely on a single, exact back focus distance. Instead, they are designed to reach focus within a supported back spacing range.
William Optics go on to recommend a back spacing of 55 ± 3 mm on that scope — and the reason given is not optical correction but keeping drawtube extension short and reducing the risk of light leakage.
The back focus of FRA400 is 140mm, starting from the base of mounting thread on the housing to the focal plane.
140 mm is not a target to hit with spacers. It is how much room there is between the rear thread and the focal plane. Askar are explicit that you do not need to calculate anything as long as the objective reaches focus.
So what should you actually check?
- That your total train falls inside the recommended window, if the manufacturer gives one.
- That your focuser has enough travel left at both ends, which is the real constraint.
- That you have not eaten the window with a filter drawer, an off-axis guider or a rotator — this is what actually causes trouble on these scopes.
- That nothing is so far out that the drawtube is fully extended, which costs rigidity and invites flexure.
The exception worth knowing
Bolt a reducer onto a Petzval and a critical distance appears where there was none. A reducer is a separate optical group with its own prescription, and it will have a required spacing to the sensor exactly like any other corrector. The scope alone is forgiving; the scope plus a reducer is not.