Focal reducers typically reduce the distance to the focal plane and require additional in travel of the focuser to achieve focus.
Yes, this is exactly the case.
The spacing on the ASA corrector is something like 55mm, and as with any other reducer, you have to rack the focuser in more than it would require without the reducer.
If the focal plane of the scope is not at least 85mm above the top of the fully racked in focuser, there will not be enough inward travel on the focuser to reach focus and have the necessary spacing between the corrector and the sensor.
If the scope is short of the necessary inward focus, the primary mirror will have to be brought closer to the secondary, but this is only a partial solution. As the mirror is moved closer to the secondary, field illumination falls off so it would be best to have a slightly larger secondary with slightly more offset if it were to work as well as possible.
And last but not least, one would have to check to make sure that the fully racked in focuser tube does not extend into the light path.
I have a 150mm scope that was designed specifically for the ASA corrector. I has to have a 40% secondary to make sure the light cone is big enough to fully illuminate an APS-C size sensor when used at f/2.8. When the scope is in focus, the low profile focuser has only about 8mm of travel left. The focal plane is 90mm above the top of the racked in focuser. The fully racked in focuser tube does not extend into the light path.
So, you really need to have a focal plane that sits 90mm above the top of the fully racked in focuser, and most f/4 imaging Newts don't have this much.
If the scope is only 10mm or 15mm short, it might be possible to get it to reach focus by raising the primary closer to secondary, but many imaging Newts only have about 60mm spacing from the top of the focuser to the focal plane. Again, it would also be important to make sure the focuser tube was not sticking too far into the light path when racked all the way in. Good starting point is to measure the distance from the top of the focuser to the focal plan without the reducer.
Edited by Eddgie, 12 July 2019 - 10:52 PM.