Ultra-Light and Minimalist Dobs

... by Mel Bartels

A new trend in dobsonian design is emerging: the minimalist, ultra light, ultra open dob. Characterized by lighter weight, long truss arms, single flat ring upper ends, very low center of gravity, absence of a shroud, and exposed optics, these scopes carry a larger mirror for a given weight, and are very wind resistant. Steve Overhold, in his 1992 book, Lightweight Giants, proselytized for thin mirror boxes, low rocker heights, very fast mirrors, and low overall weight. He regularly carried his 30" in his Ford Fiesta. Since then several amateurs have forayed into more diminutive scopes.  Today, these ideas are reaching critical mass as more and more of these dobs are being built.

We start with the current state of the art dobsonian design and see where we can reduce.  We can reduce the upper cage to a single ring, we can minimize the mirror box, perhaps even replacing it with a single ring behind the mirror.  We can lower the height of the rocker by using large altitude bearings, and, by placing the feet under the azimuth teflon support pads, reduce the ground board to a 'Y' shaped affair.   A good goal is to reduce the tube assembly weight to that of the mirror.

My newest ultralight mount design I am calling the Trilateral Mount. See trilateral.html

Previously, in my 20 inch, the upper end is a single piece of ½ inch thick apple plywood with a reinforcing sector of ½ inch plywood bringing the thickness to 1 inch next to the focuser. The diagonal is heated: sensors turn on the resistors when the diagonal temperature drops too close to the air temperature. The spider is a commercial Novak spider, but with a twist: it is split in half down the diagonal holder bolt hole. Very exposed large spiders and secondaries have a tendency to quiver rotationally in gusty wind, the result being elliptical star images at high power. Splitting the diagonal into two 'L' shapes, per Texereau's suggestion in his book, How to Make a Telescope, anchors the diagonal holder so that it can no longer rotate.  I also removed large sections in the middle of the spider vanes to reduce weight and to avoid the thermal problems of metal radiating to the night sky where a thin layer of colder air forms around the vanes, leading to increased diffraction.  The 20 inch is ramped in and out of my pickup.

Proper baffling ensures that no unwanted light enters the focuser. There is a baffle just below the focuser and a baffle opposite the diagonal. The focuser baffle is particularly important.  Both are covered with Edmund Scientific black felt. The primary is also baffled, just in front of the glass, and totally enclosed in ultra flat black. It is an impressive demonstration to shine a powerful flashlight on any part of the scope, and discover that your observing buddy looking through the eyepiece with his eye cupped cannot tell you when you have the light on or off. The goal in baffling a minimalist ultra light is to block every ray of light not coming from the primary mirror.  Extra baffling to absorb secondary reflections from the baffles is used in high performance refractors, but this is a luxury we can forego since the flashlight test is quite convincing when using light trapping felt or velvet.

Diagonal size is a tradeoff. Smaller diagonals are lighter, more easily baffled, more easily collimated, but give less illumination at the edge of the field of view of wide angle eyepieces. Lighter diagonals mean less weight on the upper end for a faster harmonic frequency and less vibration in the wind. A smaller diagonal means a tighter cone of light from the field of view to the diagonal, resulting in a smaller baffle opposite the eyepiece, assuming the presence of a focuser baffle. Smaller diagonals collimate easier because the smaller circle is more easily centered. The off-axis illumination profile drops off quicker and deeper, but is usually hardly noticed. The rule of thumb is that it takes a 30% change, or 0.3 magnitude change, in illumination to be barely perceived. In larger scopes, a surprisingly smaller diagonal can give an adequate illumination profile. The issue of contrast is two fold: extended objects that occupy a rather large presence in the field of view and tight low contrast planetary detail. The former is not affected at all by differently sized diagonals (though you may notice differences which are due to baffling issues). The change in the Modulation Transfer Function does not extend very far in angular extent. For more see Suiter's Star Testing Astronomical Telescopes. The latter is not affected by closely sized diagonals. Or better put, small changes in optical quality between diagonals make a greater effect on the image than closely sized diagonals. Do not rely on 'tests' performed with side by side telescopes, each with a slightly different sized diagonal. There are too many variables that will necessarily be different between the two telescopes (optical quality of the primaries, optical quality of the diagonals, baffling, diagonal to focuser distance, smaller diagonal sometimes cutting off light from the primary's not so good edge, the smaller diagonal more likely to involve its not so good edge, to name a few) any of which will affect the image more than the difference in diagonal size. In addition, the 'test' likely was not run double blind, where no one knew which scope had which diagonal. The only valid test that I can imagine, which I have run on a half-dozen telescopes from 6 inches aperture to 24 inches aperture, is to create a series of circular black cutouts that can fit over the diagonal holder's stalk. Appoint a non-astronomical person to alternate the series of circular disks, in the dark, so that neither this person nor you knows which cutout is in place. Observe with eyepieces that give no greater than 2 millimeter exit pupil. A test will likely occupy several hours on a single telescope. You will find subtle changes in field illumination and image quality. Don't be surprised if you find that the larger mask gives a better image. I found this to be true on about half the scopes. I conclude that the explanation is small differences in optical quality between diagonals.

A word about truss tubes and their function.  Truss tubes hold the upper ring by compression.  The goal is to pick a truss tube diameter so that the compressive forces do not buckle the truss tube.   A perfect truss tube design would taper at each each, with the fattest part in the middle.  Several schemes such as filling the tubes with expansive epoxy and pre-stressing them with wire have been dreamt up to strengthen their resistance to buckling.  Since we are aiming for the simplest, most minimalist design, these interesting ideas don't fit our scheme.  Long thin truss arms do vibrate when struck (actually all truss tubes vibrate some).  The key is to make sure that the frequency of the vibration is not a resonant frequency of the overall telescope.

And now a word about vibration, amplitude, frequency, and dampening.  Vibration is the to and fro motion of an object in the eyepiece.  Amplitude is the amount of motion, a convenient unit of measure is arc minutes, or even 'Jupiter' diameters.  Frequency is the speed of the vibration, and dampening is the time it takes for the vibration to die out.  The goal to aim for is an amplitude of less than a Jupiter diameter under 1 second dampening time.  Large amplitude is disconcerting if your scope is sensitive to the wind.  A higher frequency is preferred as 'the pants will shake the ants out quicker'.  A completely hands off approach, with motorized tracking and focusing, and a single upper ring with split diagonal for wind resistance, gives the smoothest views.  A good test for vibration is to knock on the upper end of the scope as you would knock on your neighbor's door.  Rapping your knuckles causes a displacement of the upper end.  The telescope tube swings back at a characteristic frequency.  The mount and ground also 'feel' this vibration, and will reflect or absorb it.  A wide stance of the ground feet will absorb the vibration because the natural frequency of a wide stance is lower.  Since the rest of the scope has a higher natural frequency and will not vibrate at this lowered frequency, the tube's vibrations are transformed into lower frequency vibrations which are dampened or absorbed by the ground.  The result is that a scope that dampens quickly at the eyepiece.  Hard rubber discs under narrower ground feet directly absorb the vibration.  As long as the truss tubes naturally vibrate at a different frequency than the rest of the telescope, their vibration will not be seen at the eyepiece.  The worse case is when the truss tubes do vibrate at the telescope's natural frequency and you have a small ground board: you can actually see and feel the vibration traveling down to the base of the mount, then being reflected back to the eyepiece.  For more in vibration, see my in depth article vibration.html

When the wind blows, it adds energy that shows up as vibration and that is eventually absorbed by the ground.  By the way, an interesting idea is to dangle metal chains from the upper end, the chains' clanking against each other absorbing vibrational energy.  A single upper ring is more resistant to the wind, so exhibits less vibration.  The traditional spider is a wonderful storage vehicle of vibrational energy, particularly when the diagonal mass is large.  At high power when the wind is blowing, stars appear elliptical thanks to the diagonal's rotational vibration.  Tightening the spider vanes only succeeds in increasing the frequency.  Splitting apart the spider into two separate 'V's as mentioned above completely does away with this problem.

Some traditionalists are upset by the absence of a shroud.  With a heated diagonal, and proper baffling, the shroud is superfluous. Some claim that a shroud cuts out stray light. Stray light does not mysteriously diffuse through the air to reach the focal plane. It must reflect off of some surface. Proper baffling will stop all of these reflections. Under bright light situations, if you see glaring then the scope needs to be better baffled. Adding a shroud will be a hit and miss affair, depending if the shroud blocks the unwanted light from the focal plane. Beware of the placebo effect which can cause a shrouded scope to perform 'better' in a bright light situation thanks to more care given at blocking out stray light from the eye at the eyepiece. People will try very hard unconsciously to cause a scope to perform to their expectations. Dewing of the primary is a concern, that's why many of us use a fold up mirror box cover plate or a separate lightweight piece that sits on top of the truss arms extending above and outward from the mirror box. A fan will help keep the primary at air temperature and thus avoid moisture condensation. Mirrors that sit in a totally enclosed lower end rarely dew, but think a moment about what this really means - namely that the primary will take a long time to cool down and give razor sharp images.  One of the unmentioned reasons for off axis masks is exactly this - not lack of optical quality or bad atmospheric seeing, but instead, optics that are forever cooling and never giving those wonderful refractor like planetary images.  Finally, ah, to be a little unscientific, shrouds look, well, ugly, you know, here come the Conastoga Wagons.  Of course, you can throw on a shroud in the dark if it comes to that - the shroud issue is really not at the core of minimalist ultra light designs.  I will say that I have never used a shroud in my dobsonian career, and I have been building and using large aperture dobsonians since 1980 (that's when my amateur astronomy life was turned upside down when I met John Dobson at Crater Lake National Park, and got to use his 24").

Even with the ultra open design of these dobsonians, mirror cooling is still called for.  The layer of air clinging to the face of the mirror must be removed.  Perhaps side facing fans are the easiest, though best is a forced air system.  Chuck Dethloff attaches a fan just above his mirror face, aimed at the mirror face, and held in place by wire that lies in the shadow of the diagonal vanes.

The goal of ultra light and minimalist dobsonians is to get more aperture into the hands of amateurs.  By incorporating these ideas, and hopefully improving upon them, you will be able to shuttle more aperture and enjoy it better.

Speaking of adding aperture, my experience since 1990 with tracking dobsonians leads me to make the following conclusion: one can see as much with a well made high quality tracking as a larger scope.  For instance, I can see about as much with my 20" tracking scope as with a standard 30" dob. The ability to use higher powers, as much as 1000x to 2000x, as well as keep the object in the center of the field of view for extended periods of time, makes all the difference in the world.

an email from Tom: Subject: Sixteen inch f/6 - second light at Mt Magazine Star Party

Despite intermittent high winds and high cirrus, I got in about six hours of observing over two nights at the Mt Magazine Star Party last weekend with the sixteen inch f/6 - call it second light.  Here are the major findings:

Upper ring (instead of a secondary cage/cylinder).  It's frequently breezy (or worse!) on the mountaintops, and the upper ring design is fantastic for this!  Even in light breezes the 36 inch nearby was moving and bobbing and required hands-on steering to counteract the breeze. . .but my scope was barely moving (about 1/4 of Jupiter's diameter in light breezes. . .at least less than the planet's diameter in most breezes).  In higher breezes. ..when many big scopes (especially those with shrouds) were having trouble staying put - the sixteen inch could still be used at lower powers.  In really high winds the secondary vanes (or maybe the truss tubes?) start fluttering/buzzing and the image moves so rapidly that it's blurred into a useless mess.  The flutter problem in high winds is the limiting factor here, not the upper ring/low wind profile design.  Great concept!  Thanks to Mel, Bruce, and others for this idea!

Focuser and upper ring baffling:  Works well!  After the sun dropped below the horizon I observed the four day old moon. . .I didn't side by side compare with a solid tube newt, but the contrast appeared good.  By pulling my eye back about four inches from the eyepiece (use a long focal length eyepiece for this test) I could see the secondary and primary mirror in focus. . .and baffling material. . .and nothing else. . .which is what you want.  By moving my head to the side (to examine the edge of the field of view) I could see if the baffling was the only thing visible at other angles.  It was.  The baffling layout works well if you don't try to use a low profile focuser setup.  In case I observe in a street light infested area I've made my focuser bottom baffle aperture removeable.  I can insert a smaller baffle that allows the eyepiece to "see" only the secondary mirror. . .and very little else.  It will limit the size of the unvignetted field a good bit, but may help improve contrast in tough street light situations. For observing in daytime I'd probably also rig a sun shade (or short/half shroud near the mirror box) to make sure no sunlight falls on the mirror and inside of mirror box.)

Mirror box baffling.  I've noticed that some large dobs with open/tailgate mirror cells don't baffle the back of the mirror box.  I've temporarily taped some strips of flocking paper across the back of the mirror box so that you can't see the ground/grass from the eyepiece.  I didn't get a chance to compare this to other non-baffled scopes, but it was easy to do and certainly can't hurt.  Still plenty of room in back for air flow.

Filter holder.  I mounted my helical focuser to the upper ring by making a short wood box to provide mounting points for the focuser base and baffle just below the focuser.  In between them I set up a system for quick change filter capability by cutting two grooves in the wood box.  The two inch filters (colored glass for planets and nebula filters too) are mounted on squares that slide in/out.  This may not be as fast/easy as a long slider bar with several filters, but that can get rather large with four or more two inch filters.  I think it works pretty well. . .especially when "blinking" the O-III filter to find planetary nebulae or showing how much more visible the Veil nebula is with/withouth the O-III

Tape measure/protractor setting circles.  Work well!  After aligning on the crescent moon I dialed in Vega and Altair in bright evening twilight.  Also found Uranus in darker twilight.  At one point in the night I was observing with a 170x eyepiece and four objects (in a row!) I dialed in on the setting circles were in the eyepiece field of view!  Who needs a low power eyepiece? ;-)

Star testing the optics.  Seeing was not very good, but this mirror looks better than my tench inch f/7. . .especially in the outer zones where the most mirror area lies.  (My ten inch tests as a bit better than 1/6 wave and less than 1/8 wave.  The sixteen inch's outer zones star test as clearly/obviously better in the outer zones)  The very inner zones of the sixteen inch (only out to a radius of about 2 - 2.25 inches, which is a small portion of the entire mirror's area) are a bit overcorrected but beyond that area the star test is very good.  I still need steadier skies for a more definitive result.  This is a good mirror!  I may not star test again until spring/summer or whenever I get much better seeing.  This mirror is plenty good enough to enjoy right now and at this point additional star testing may only be good for the ego and bragging rights around the water cooler ;-)

Collimation.  Using only a cheesy, home made sight tube and peep hole this f/6 system only may need one minor tweak of colimation adjustment with the star test. . .if the seeing is good enough.  A laser collimator is not needed.

Spring counterweights.  Continue to work well!  A lucky break:  Since I use two springs (one on each side of the rocker box) I've found that when I unhook one of the springs and put a light/1.25 inch eyepiece in the focuser. . .it's almost perfectly counterbalanced!

Observing ladder.  I use a six foot ladder.  I added about three additional rungs between the bottom three rungs, so now there's a step every six inches or so.  Once I got over the initial strangeness of a ladder with such close spacing it was very comfortable to use!  I don't remember any object/observeing height where I could not find a rung to step on that gave a comfortable observing position.  There was no neck craning or stooping needed!  Also, a six foot ladder leaves enough ladder above me that when doing high mag observing I can lean/rest my torso/shoulder on the upper parts of the ladder for steadier viewing.  A good ladder helps for longer/comfortable observing.

(Off topic, but this is why I build em. . . .)  Observing highlights:  Comet Giacobini-Zinner sporting a tail about 1/4 degree, maybe a bit longer.  M-33 and its brightest H-alpha region (NGC 604?).  Veil nebula.  Horsehead. Rosette.  Biggest surprise that I didn't expect:  color differences on Jupiter and Saturn are so much more enhanced in this scope compared to my ten inch, and the cloud banding on Saturn with medium blue filter.  Ray D. also helped point out to me spokes on Saturn's ring system. . .my first time seeing them.  Best long-term payoff:  My son wants to take another camping trip to dark skies in three weeks!

Major Tom Krajci B-52 Intelligence Officer