MCT vs SCT

MCTs have a general reputation as being a good choice for lunar an planetary observation while SCTs seem to be less valued for this purpose. Not being an optics design expert, my question is if both scopes are built to the same basic specs. say an 8" f/12 with a 34% c.o. is there an inherent design advantage for the mak that makes it better or is the difference in reputations because the maks are usually built by smaller specialty optical shops that use better components as opposed to the mass produced SCTs out there?

DanL

An SCT can be as good as a Mak if the design is equal. Generally the design is not equal. SCTs are made with F2 mirrors and 5x secondaries. Maks are normally made with longer mirrors around F3 with 3x, 4x or 5x secondaries, depending on what the final focal ratio is to be. An F2 corrected sphere in an SCT will have large amount of sphero-chromatism (variation of spherical aberration with wavelength) which tends to soften the image. With an F3 primary, the sphero-chromatism of either design is very low, generally being less than 1/10 wave P-V.

The reason commercial SCTs are made with F2 primaries is so that they will fit inside the forks of a fork mount. With an F3 primary, the SCT would be way too long for a practical fork mount. Thus, in order to satisfy the needs for 99% of the users, all commercial SCTs will have F2 primaries.

One other advantage of a Mak-Cass is that it can be designed with zero coma and still have all-spherical components. In order to do that with an SCT (i.e. the Meade design), the secondary mirror must be aspherical, and the corrector power must be approximately double that of an all-spherical SCT. This causes the sphero-chromatism to increase yet again, making this type of design a bit softer still.

Rolando

Another difference relates to production of
the corrector plates. An MCT is just simple spherical
curves, which opticians seem very good at making.
The SCT corrector is made by vacuum deforming
the glass on a frame, and then grinding in a
simple curve. There are various accounts that this
method leaves imprints of defects in the bending
frame, which appears as a pattern of spokes in
the finished glass. There are also issues related
to the use of float glass for the SCT correctors
(ripples and striations), though some are using
better BK7 glass (Celestron XLT series
for example). Probably Rolando can elaborate on
the details.

Dan Lamoreaux said:

MCTs have a general reputation as being a good choice for lunar an planetary observation while SCTs seem to be less valued for this purpose. Not being an optics design expert, my question is if both scopes are built to the same basic specs. say an 8" f/12 with a 34% c.o. is there an inherent design advantage for the mak that makes it better or is the difference in reputations because the maks are usually built by smaller specialty optical shops that use better components as opposed to the mass produced SCTs out there?

DanL

And yet...and yet... the premier planetary imagers...Damian Peach, Chris Go...just to name two, use SCTs.

Maybe that's not a completely fair comparison, since light helps, and larger aperture SCTs are far more affordable than their MCT cousins.

MCTs do have some advantages. One thing is that the corrector is potentially easier to do very well than is the complex curve corrector of the SCT. Also, MCTs can be (but not always are) all spherical systems which, again, potentially can be easier to do very well.

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