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Telescope Multi-Mask and Test
Submitted: Monday, 21st October 2013 by Frank Wright

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Completed Multi-mask assembly showing 6, 8, and 12 inch masks bolted together

Over the years I have upgraded from a 4” to a 16.5” dobsonian telescope. Recently I thought  about these scopes and wondered whether it was really worth the journey of making and using bigger scopes considering that I am just a casual observer.

I do like the bigger view through my f5.1, 16.5” compared to a smaller image in a smaller scope, but a smaller scope does give a fabulous view of the planets while the seeing still leaves the 16.5” with a fuzzy view. I noticed this problem every time I upgraded above 8” diameter, as I went to 12”, then to 16.5”. After using my old 8” for a long time, I bought a new 12”, but there seemed to be an optical problem with it. While an 8” planetary view looked clear, it was less clear when viewed through the 12”.

It was only later I understood the effects of atmospheric seeing, and how my old location of outer Sydney was affected by its position near mountains and sea that created turbulence and poor seeing most of the time. Last year I had about three or four nights of really good views of Saturn through the 12”, and only once did I get to see it in all its glory through the 16.5”, maxed out like my credit card,  at 850x. But when I saw it at that magnification, it was breathtaking! The Casini division looked like a massive gap in the rings. Hmmm, I wonder if I could find a star looking through that gap...   er no. Also I noticed that in Sydney it was nearly impossible to do any star testing due to this atmospheric turbulence.

So after thinking about all this, I got curious and decided to build a set of masks that could be switched in and out easily on the big scope. This way I could see the difference between telescope diameters quickly while conditions were the same for each hole size. This also meant that the same mirror and diagonal were being used for all tests, to make a level playing field. The diagonal was half covered with the 6” hole switched in so as to make things even fairer.

The masks were made out of standard corrugated cardboard from an old box, and sprayed  matt black on the inside and gloss red on the outside for dew protection. A small attachment lug was left around the circumference of each cutout to screw them all together, so they could be rotated out of the way easily. The largest one is under the others and taped to the top of the 16.5” scope. For the 16.5” testing the complete assembly is removed.

The first problem in setting up this test is whether you are comparing apples with apples. For example the maximum magnification differs for each aperture, and for a given brightness. The image starts to get a little dull as the magnification is increased. This is caused by the decreasing exit pupil, which is the area of your eyes' pupil bathed in light from the eyepiece. As the exit pupil  decreases the view dulls, because the smaller light cone is hitting less rods at the back of your eye. For this test, let's assume a 1mm exit pupil as the limit for a minimum comfortable viewing brightness particularly with smaller apertures. I find you always need just a little bit more brightness and magnification than you have, especially at 6” where brightness decreases rapidly with magnification.

The second problem is the clarity versus magnification. Larger magnifications can be used under perfect conditions with larger diameter telescopes. However your magnification is usually limited to the atmospheric seeing  limit before the telescope limitations are reached. This never appears to be a problem in a 6” telescope due to its smaller size and range of useable magnification.

Thirdly, using a big telescope masked down for this test  means that the focal length differs for each diameter, resulting in big focal ratios at smaller diameters. I always thought that the view would be similar to looking through a straw but it didn't seem to be a big problem, even at f13 for the 6” scope it was still OK.

Different aperture characteristics

Mirror Diameter

Focal Ratio

Maximum Magnification













First test used Saturn to get an idea of clarity and brightness versus diameter. I tried the mask out a few times and conditions were not good with stars twinkling fairly frequently. Probably fairly typical conditions for most people who live in cities or near a coastline. With the big scope fully masked down to 6” there were good views at 100 and 170 times magnification. However going up to my next eyepiece size for a 250x view stretched things a bit thin and the view got quite dull. Switching to the 8” mask gave brighter views to 250x but starting to show noise in the form of the image jumping around making viewing not as pleasant at that magnification. With the 8” mask swung out of the way you are left with a 12” hole. We are now entering big scope  territory with their commensurate problems. At lower powers the image was small and very bright, even slightly uncomfortable. At 250x things are OK for brightness but there is a little bit of fuzziness starting to creep in. Things are similar after the mask  is removed to give the whole 16.5” aperture. The image is slightly too bright at 250x but now is little blurrier than the 12”.

Overall, for planetary viewing up to 8” is great for getting a fair degree of clarity in a turbulent atmosphere with reasonable brightness. I have had a number of years of great enjoyment viewing planets and globular clusters with my old 8” and think it is the best all purpose size that can be had at a low price. However I still drag out the big one with the faint hope that tonight the atmosphere will be steady and that great view at 850x will be there, but it very rarely is...

Next test was M8, the Lagoon nebula in Sagittarius. Conditions about the same as for the planet test, what one would call a faint fuzzy night, not a planetary night. Now here the big aperture started to come into play in a big way. The magnification was kept at approximately 170x for all apertures. M8 has a small open cluster of stars close to the brightest part of the nebula but this whole area is radiating light in a really big telescope. However back to reality with a 6” aperture, the star cluster was of course visible, as was a small part of the brightest part of  the gas cloud. Switching up to 8”, there was more  nebulosity visible and also a hint of nebulosity in the star cluster area as well. At 12” the nebulosity was much brighter with the visible parts doubling in size and some cloud detail starting to show. Going all out to 16.5” again showed another step up with more brightness, extent, and detail. It is fascinating to quickly switch between apertures and see the difference. For viewing extended objects like this one, size is everything, so the big boy wins hands down.

So this test agrees with the formulas, brightness is proportional to aperture for a given magnification. For example, if I halved the magnification using the 8” mask, I should get the same brightness of view as with the 16.5”, but at half the size. Similar results were observed when viewing M17. A UHC filter was tried and even at 6” it made a difference, probably like adding an extra 2” diameter to the telescope.

I compiled these results using direct viewing without trying to discern every last nuance possible from each diameter. I thought this method kept the tests more strightforward. I also chose a spot just outside the living room on the deck, with a street light outside, at a location on the Sunshine Coast in Queensland. This location makes the test reflect the conditions most of us are forced to use. Little effort was made to stop light leakage, but the scope did wear a skirt over the struts for the formal testing. Maybe that's why my wife calls it the other woman.

So I think I will keep my multimask for when I am viewing planets so I can switch  diameters to suit atmospheric conditions. For the faint fuzzies, the bigger the objective the better. I must still have a case of aperture fever, as there is already a piece of  19” glass  sitting in the corner waiting to be ground. Anyway, see ya later, I gotta go, I think I hear that new piece of glass calling me...

Article by Frank Wright (frankly). Discuss this article on the IceInSpace Forum.

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