Vahe and Jared, I can't remember if Roland gave the focal ratio of the primary but he did post that it was chosen in part to make the image less susceptible to temperature changes.
Edit: Not the post that had the info above, but here is a post he made about the original 10 that touches on some factors he takes into account when choosing the focal ratio of the primary.
"The 10" Mak-Cass was a particular design (aspheric Gregory)
that is not easy to produce. It requires a complex aspheric on the primary
mirror. Aspherics are hard to make smooth and perfect, and in this case it took me
almost 1 year to make just a small number of these scopes. Each mirror took
about 1 week of painstaking hand work (all the optics were pre-polished by our
supplier, and all I did was to do the final smoothing on all the surfaces and
aspherizing on the mirrors).
After that expersince, I took a long break from making that sort of optic
again. By contrast, an all-sperical Simak or Rumak design, which was made by TEC
for a number of years, is simple and straight forward. The main disadvantage
is that for a fully coma corrected design, the Rumak requires somewhat larger
central obstruction for any given F-ratio. The aspheric Gregory has the
smallest possible obstruction for a coma-free design. Nevertheless, if I ever make
any more Maks, they will have all-spherical design, because the aspheric design
is not practical.
Interstingly, the 7" Meade is a form of aspheric Gregory. Unfortunately they
chose to make a huge secondary obstruction > 35%. I believe they achieved the
aspheric surface by deforming the primary during polishing. The main problem
that I see with that approach is that it is almost inevitable that some
astigmatism will result in the primary figure. It is more straight forward to make a
Mak with separate secondary and all surfaces spherical.
You can always do a very small amount of localized figuring to reduce or
eliminate the inevitable 5th order. Just about every optics that uses refractive
elements has some 5th order. All you need to do to bring it to a non-issue is
to reduce it below 1/10 wave P-V. This does not change the reality that an
all-spherical design is the most practical of all.
There are other reasons besides 5th order that causes most Maks to have an
unsymmetrical shadow breakout. The most common reason is due to thermal
instability, which adds a certain amount of 3rd order undercorrection to the figure.
If the thermal design is right, and the optics have been figured correctly,
then you will see pretty close to ideal inside/outside and in-focus images.
Secondly, the amount of 5th order is dependent on the focal ratio of the
primary. The longer the primary, the smaller will be the 5th order component (and
the longer will be the tube assembly). F2 primaries will have large amounts,
F3 and longer primaries will have small amounts. Of course, the longer the
primary focal ratio, the larger will be the obstruction ratio for any given focal
length. Some have suggested longish F-ratios to keep the secondary size down,
but this limits the scope to just planetary observing. Wide field deep sky and
imaging would be compromised. I don't know what the ideal F-ratio is for a
Mak-Cass, but F20 seems to me to be way too limiting. Even the 10" at F14.6 was
a bit long."
Edited by Paul G, 06 October 2017 - 01:55 PM.