Hi Hans,
You’ve already been given some good references with “Newt”, Mel Bartels website and Damian Peach’s website. Just remember that a lot of Damian Peach’s comments relate to imaging and not visual astronomy. The effects of central obstruction on imaging and visual astronomy are significantly different, although Damian Peach’s comments on the effects of central obstruction on the Modulation Transfer Function (MTF) curves and spatial frequency, is relevant and accurate. This is also covered very well on pages 49 to 62 of Suiter’s book “Star Testing Astronomical Telescopes”. It’s also covered in somewhat less detail in Chapters 18.6 and 18.7 of Telescope Optics by Rutten and Van Venrooij.
The secondary size of a Newtonian inter alia influences 2 very critical factors:-
1) The size of the Fully Illuminated Field. (FIF)
2) The effect on the MTF curves caused by diffraction.
Unfortunately, the 2 work against each other. As you go to a progressively larger secondary mirror to increase the size of the fully illuminated field, you significantly increase the effects on the MTF curves, which causes image quality degradation through loss of contrast, caused by diffraction. It gets further complicated because as Newtonian telescopes get faster in F-Ratio, they require a progressively larger secondary mirror to achieve the same sized FIF for a given aperture.
When a telescope designer / builder sets their design parameters, they are in most cases assuming that the telescope will be used for a mixture of deep sky observing and planetary observing, so they try to build an “allrounder”. I won’t go into 100% illumination zones and 75% illumination zones, which are covered in “newt” and elsewhere, as they overcomplicate this post. A good general rule of thumb in terms of the size of the FIF is that you should aim for a FIF size of 50% of the longest focal length eyepiece, you are likely to use frequently. In most cases with modern Newtonians generally being between F3 and F7, this means people are looking to obtain a FIF of between 10mm and 15mm.
The other side of the equation is the effect on the MTF curves. For some reason the “magical” number is 20%. When the Central Obstruction (CO) goes under 20% the views will closely approximate the views of an unobstructed telescope. When the CO goes over 20% the effect on the MTF curves becomes progressively greater. In Suiters Book he shows examples of the effect of a 20% CO on the MTF curves, the effects of a 32.5% CO and the effects of ¼ wave of spherical aberration on the MTF curves, which is spherical aberration on a “diffraction limited” optic. The example shows that a 20% CO has minimal effect, ¼ wave of spherical aberration has a slightly greater effect than the 20% CO, but still fairly minimal and the 32.5% CO has a somewhat greater effect than both of those. Of course then you start to “add” the cumulative effects of things like the size of the CO, spherical aberration, astigmatism, seeing, etc etc; iot all goes pear-shaped pretty fast. The “Aberrator Software” is pretty dated now, but you can still use it to view some decent simulations of the effect on image quality of a whole range of things. It’s a free download from here.
http://aberrator.astronomy.net/
With F4 and faster telescopes its difficult to get the CO under 20% due to the steep light cone. With a specialist planetary instrument, which would invariably be slower than F5 and usually F6 to F8, that won’t see much use for deep sky, you can intentionally downsize the secondary to give say a 5mm or 6mm FIF and end up with a 12% to 16% CO. This will make a great lunar / planetary instrument, but the downside will be a clearly visible fall off in image brightness towards the edge of the FOV of longer focal length eyepieces. The other -ve effect is that when you downsize the secondary to this degree you bring the edges of the secondary mirror into play, which is where most secondary mirror aberrations are likely to be.
All of my scopes have been carefully designed and components selected to give the right balance. My 10” SDM has a 1.83” secondary for an 18.3% CO. My 14”/F4.5 SDM has a 2.6” secondary for a 18.5% CO and my 18” Obsession has a 3.1” secondary for a 17.2% CO. My longest focal length eyepiece is a 31mm Nagler and none of the scopes show a detectable drop in illumination near the EOF in that eyepiece, but with all of them having CO’s under 20%, they all perform exceptionally well as lunar planetary scopes. This wouldn’t be achievable with a sub F4 scope, but they have their own inherent advantages in regards to portability and eyepiece height.
In Matt’s case in his 18”/F3.5 he tried a 4.5” (25% CO) and a 4” (22.2% CO) and didn’t see any improvement in contrast. If he had been able to try a 3.1” secondary for a 17.2% CO, I have no doubt there would have been some contrast gains, had seeing and thermal equilibrium all co- operated. The downside would have been a noticeable drop off in image brightness at the EOF in long focal length eyepieces. With the scope being fast at F3.5 there is just no way around this.
The mass produced Asian dobs usually have slightly oversized secondaries at around ~25%. Those with good optics still deliver excellent planetary images. I still have a 10”/F5 GSO that I leave at a friends property to use when I go there and save transporting a scope. It has a 2.6” secondary for a 26% CO and delivers excellent lunar planetary images, considering what it is and what it cost. It takes pretty good seeing before my 10” SDM with Suchting mirror and 18% CO pulls ahead and not be a huge amount. I have no doubt it would be slightly improved by a smaller (sub 20%) high quality secondary mirror.
What are the parameters on the scope you are looking to downsize the secondary mirror on ?
Cheers
John B