Proof that Maks outperfrom refractors

I bought Rutten and van Venrooji's book, Telescope Optics. What grabbed my attention was a chart on page 219 and another on page 221 that shows that contrast is higher in obstructed scopes vs. refractors at the highest magnifications possible with diffraction limited optics. It varies a bit according to central obstruction, but on average at the top 30% of magnification the obstructed scope wins. This seems to be confirmed in comparisons between my perfectly collimated Stellarvue 80mm refractor and LOMO 102mm Maksutov-Newtonian. It has been said you need about 20% to 25% more aperure in an obstructed design to get the same resolution as in a refractor, but in my case the LOMO very clearly outperforms the Stellarvue. The LOMO is without question the superior lunar and planetary scope, due to the increased contrast at high mags that I typically use with both scopes. Of course, for deep sky work at low mags, the situation could reverse according to the charts, but I'm not really seeing much to that effect. The light gathering ability is about equal in both scopes, since the central obstruction does cut down on light throughput a bit in the Mak-Newt. The mirror in the LOMO doesn't cover a 23mm Axiom or 30mm Ultima as well as the Stellarvue does and vignetting is apparent. That's the most obvious difference at low mags.

At least the book exposes the myth that obstructed optics are always lower contrast than unobstructed optics. It is entirely magnification related and also related to percentage of central obstruction with the most obstructed designs getting the best contrast at the highest possible magnifications allowed by a particular aperture. Sort of like wild camshafts in high performance engines. That tends to make the obstructed scopes the true "Ferraris" of the scope world. More limited in scope of usefullness, but unsurpassed in that narrow band of performance they favor.

Interesting...........

Phil

>>I bought Rutten and van Venrooji's book, Telescope Optics. What grabbed my attention was a chart on page 219 and another on page 221 that shows that contrast is higher in obstructed scopes vs. refractors at the highest magnifications possible with diffraction limited optics.>>

That comparison chart is actually wrong. In this chart, both curves are normalized at the low end (i.e. they start out at the same level). The real comparison chart for the obstructed scope would actually start at a lower level depending on the amount of obstruction. For instance, a 33% obstruction would put the obstructed curve at 80% of the unobstructed scope. This 20% reduction is carried all the way to the end point. If you place the curves this way, then the higher contrast of the obstructed scope at the highest magnification disappears, and the two actually become more equal.

What really happens is that the resolution increases a small amount for normally obstructed optics at a lower level of contrast. However, that small gain can quickly evaporate if any atmospheric induced aberration is present. The extra energy in the diffraction rings begins to degrade the resolution quite rapidly.

Some studies were done recently in optics that have extreme obstructions, 99% or greater. These obstructions result in resolutions far exceeding Dawe's limit, but of course with contrast and light grasp approaching zero. The idea then was to figure a way to filter or supress the light from the diffraction rings, leaving only the faint ultra-tiny central Airy disc. With such unorthodox methods, the researchers were able to sparate two point sources that were 10 to 20 times closer than a normal aperture would be able to separate. Essentially creating a 200" out of a 10" aperture.

Roland Christen

Philip Canard said:
I bought Rutten and van Venrooji's book, Telescope Optics. What grabbed my attention was a chart on page 219 and another on page 221 that shows that contrast is higher in obstructed scopes vs. refractors at the highest magnifications possible with diffraction limited optics. It varies a bit according to central obstruction, but on average at the top 30% of magnification the obstructed scope wins. This seems to be confirmed in comparisons between my perfectly collimated Stellarvue 80mm refractor and LOMO 102mm Maksutov-Newtonian. It has been said you need about 20% to 25% more aperure in an obstructed design to get the same resolution as in a refractor, but in my case the LOMO very clearly outperforms the Stellarvue. The LOMO is without question the superior lunar and planetary scope, due to the increased contrast at high mags that I typically use with both scopes. Of course, for deep sky work at low mags, the situation could reverse according to the charts, but I'm not really seeing much to that effect. The light gathering ability is about equal in both scopes, since the central obstruction does cut down on light throughput a bit in the Mak-Newt. The mirror in the LOMO doesn't cover a 23mm Axiom or 30mm Ultima as well as the Stellarvue does and vignetting is apparent. That's the most obvious difference at low mags.

At least the book exposes the myth that obstructed optics are always lower contrast than unobstructed optics. It is entirely magnification related and also related to percentage of central obstruction with the most obstructed designs getting the best contrast at the highest possible magnifications allowed by a particular aperture. Sort of like wild camshafts in high performance engines. That tends to make the obstructed scopes the true "Ferraris" of the scope world. More limited in scope of usefullness, but unsurpassed in that narrow band of performance they favor.

This seems compelling, but I would caution you to interpret the graph carefully. First of all, resolution is shown (in units of lp/mm) along a linear scale. In practice, this is misleading, because detail size tends to distribute itself according to some kind of power law. As a result, there aren't just as many details between, let's say, 120 and 180 lp/mm as there are between 60 and 120 lp/mm; there are more in the latter range. Most of the visible interesting detail on Jupiter falls in there, for instance.

Secondly, even though the MTF is higher for the obstructed scope than for the unobstructed one in the highest frequency range, both functions are very low in an absolute sense. Visually, it's a nonstarter, partly because of seeing, partly because at those highest resolutions, your eye is going to need all the help it can get. You can increase the magnification in order to increase the sampling rate for your eye, but then your brain has a harder time finding the gradient. (These sorts of considerations, incidentally, are what lead to that 50x per inch rule of thumb, stuff like that.) Just keep in mind that neither scope is a good performer on low-contrast, high-frequency details, in an absolute sense; they work better on high-contrast objects like double stars.

Thirdly, as Roland points out, the transmission of the obstructed scope is lower, and this may make features of equal contrast (but lower brightness) more difficult to see, anyway.

In short, this doesn't really mean as much as it seems like it does.

I'm waiting for proof that this thread has come to an end. There is no substitute for aperture.