Three things to distinguish: flattener, reducer, flattening reducer (= reducing flattener? Don't know if this term is used.)
Flattener - as the name suggests - flattens the field. The scope's sharp image is on a sphere but the sensor is flat. A flattener corrects for that. A (pure/non reducing) flattener does only that, the focal length remains the same. A reducing flattener flattens and changes the focal lenght. Now the important point: flattener, reducing or non, must be exactly (within a mm or less) in the specified distance to the sensor. If you move closer or further away, the correction fails, the stars in the corners become bad.
Reducer (precisely non flattening reducer) - as you for sure have guessed - only changes the focal length while the image is still curvy. These things may be placed in different distances to the sensor, within a range of up to 50mm without loss of image quality. This is the commen way to work with Ritchey-Chretien telescopes. Here you have the choice: when you put the reducer relatively close to the sensor it reduces only a little bit. The camera moves closer to the scope. If you put more spacers between reducer and sensor it reduces more and the camer moves further towards the scope, often quite a lot. That in turn means often the reducer is either threaded inside of the focusers draw tube (I do that) or slipped in like an eye piece. With a (non flattening) reducer you don't need to be afraid not to come into focus. Just move it a bit closer to the sensor and you gain some distance. Example from my Setup: adding 3mm between sensor and reducer makes the camera move about 10mm towards the scope.
Now, would you use a flattening or non flattening reducer? If your sensor is small, the field curvature is not a big problem. If your scope is slow it is even less of a problem. The larger the sensor or the faster the scope the better you are off with a flattening reducer. If your refractor is f/7 and with 23mm chip diagonal (ASI294) you can probably live without flattening.
Bottom line: first you have to decide if you want/need the flattening. If yes, there is only one propper spacing. If no, you have options with a given range.
If a non reducing flatener is an option for you, this is the low cost standard item TS offers:
Mind the description: for your focal length you need 111mm between reducer and sensor
This is the "standard" flattening reducer for refractors. Find the spacing in the description, again it depends on focal length bit is far less than the above, 56mm in your case.
Here is a calculator for the distance to move in:
Entering the 56 and f/7 with the WO 0.8 (which I guess is similar to the TS 0.79) you move in by 15mm.
Hope that helps.
Edited by the Elf, 15 August 2019 - 12:29 PM.