Telescope Reviews: is a mak an APO?

You said 51/2" APO, and ir you wanted to make a direct comparison, then I am going to assume you wanted to say five inch and not five and a half inches.

First, you will never find these two scopes in the same aperture and focal length, and as a result the discussion can only be purely theoretical.

No one makes a 5" f/10 APO, but you can find 5" f/10 MCTs.

In this comparison, if you could find one, the 5" f/10 APO would be a better instrument.

To be f/10, the obstruction of the MCT would have to be rather large, and as a result, there would be two areas taht the MCT would loose ground.

The first would be light transmission. The MCT would not only have less effecient transmission (maybe 90% overall), but it would also have the shading of the secondary obstruction. As a result, the image would be slightly britghter in the 5" f/10 APO. Subtle, but being a theoretical discussion, we are left with splitting paper hairs.

The second would be that the larger central obstruction would decrease the contrast transfer of the 5" f/10 MCT. This means that small, low contrast detail would be more easily visible in the 5" f/10 APO.

Now, you could make the scopes f/15, and the much smaller central obstrucion needed could make the MCT a bit more similar in performance, but why would anyone make an f/15 APO when you can make them f/7.5 or f/8 and still get APO performace

Focal lenght by itself is a neutral in the sense that the focal lenght or focal ration does not change the performance of the scope at the center of the field.

What focal length (again, assuming the same aperture) does is give a scope the ability to give a wider true feild.

And if I had a 5" f/8 refractor, and I put it next to the f/15 MCT, while the MCT would come very very close in performance at the cetner of the field (many observers might not be able to easily see a difference), the f/8 APO would still have the benefit of being able to show a larger field, and hence be able to give a performance capability that the MCT could never match.

But the reality is that no one makes an MCT faster than f/10, and to my knowledge, no one makes a 5" APO slower than about f/8.

Agian, this means that this discussion can only be purely theoretical, and in theory, the 5" refractor (assuming that it is indeed an APO) will have the advantage, regardless of the focal length of the two scopes because it will enjoy both a slightly brighter and slightly higher contrast image.

Edited by Eddgie (08/04/12 11:48 AM)