Corrector/Focal Reducer position

Does it matter where the F6.3 Focal Reducer/Field Flattener goes in the imaging train? I have it screwed on the back of the SCT (C9.25), followed by a Robofocuser. Will that work, or should it be just before the camera? (after the focuser?)

Alex

Hi Alex,

I think the closer you keep it to the scope the closer you will be to getting the full 0.63x time reduction.Keep in mind you also need to keep the distance from the reducer to the camera in mind too for good results.

I have not used that reducer so I don't know what the spec is.

What kind of focuser are you using?


Jason

Alex,
From my understanding, the question of "Does it matter where it is in the imaging train" is answered by knowing the distance between the CCD image plane and the reducer/corrector. The ideal distance is about 85 mm as others have mentioned in this thread.

The distance impacts the amount of reduction:

Reduction = 1- Dist/FL

where Dist is the distance between CCD and reducer and FL is th focal length of the reducer.

The 0.63 reducer correctors are about 230 mm of focal length: 0.63 = 1-85/230.

There is some ability to work in a range around the designed focal reduction, but you diverge from optimum correction. My experience is that this reducer is very forgiving less than 85 mm and can likely tolerate a little more.

So lets say that there is 35 mm of back focus in your FW / CCD camera. If you put your focuser between reducer and CCD you will have about 85 mm and life is good. But remember, your focuser is changing this length. This means that if your imaging filters require different focus positions, you likely will have slightly different image scale on your exposure sets.

If you put the focuser in front of the reducer, you will have 1-35/230 or 0.84 reduction - so you would have a smaller image scale. Of course with the focuser in front, you could always add spacers between reducer and camera to get back to 0.63 reduction. Now I have used a 0.63 reducer at 0.8 reduction and have been pleased with the results, so this may be an opportunity to have different image scales just by the arrangement of your imaging train.

My 2 cents worth...