Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making
There is little consensus on how light an “ultralight” dobsonian telescope ought to be. The label is thrown around
with little regard for the actual weight ranges of the different structural styles of dobs; some telescopes have been
claimed by their builders to be ultralights when, in fact, they were clearly quite massive for their apertures.
This paper examines the weights of a number of commercial and owner-built scopes to establish an “ultralight
zone” of total weight across a range of apertures. The data for commercial scopes was drawn largely from Harrington
(1998). Data for a few recent brands were gleaned from their commercial websites. The remaining (all ultralight)
owner-built scopes) are described in personal webspages accessed largely from Ray Cash’s and Mel Bartel’s
linkpages on ultralight/compact scopes. I examined 100 telescopes from 32 product lines/designs.
Three styles of dobsonian telescope
Classic “sidewalk” telescopes. The classic style resembles a circus cannon with its long (usually cardboard) tube, mid-
tube altitude-bearing box, and tall rocker. Based on older designs pioneered by John Dobson, the classic design first
became popular among amateurs in the 1970s, and remains the most common style of smaller (12.5 inches
aperture) reflectors. Built of cardboard concrete-form tube with plywood or particle board, classic dobs are cheap,
steady, and serviceable. They are wonderful examples of “small-is-beautiful” appropriate technology at its best.
However, compared to other designs they are quite heavy, which limits practical classic dobs to apertures well below
Truss-tube dobsonians. To make larger telescopes practical, in the early 1980s a number of builders abandoned the
heavy cardboard tube for a light triangulated truss of (usually eight) aluminum tubes. This lowers the center of gravity
to the point that the altitude bearings can be attached to a mirror box at the base of the optical tube, which in turn
permits a lighter and lower rocker. The secondary mirror and focuser are attached to a light framework of rings and
struts, forming an airy upper tube assembly (UTA). A cloth light shroud keeps stray light from entering the optical
path. Truss-tube dobsonians are usually designed to break down into several components, making transport and
storage much more manageable than for heavier-and bulkier classic-style dobs of similar aperture. Even relatively
large truss-tube dobs can be unloaded and setup by one person. In the late 1980s Dave Kriege wrote about his
Obsession design in Telescope Making, which has become so widely imitated that today it is the standard pattern for
commercial and owner-built truss dobs.
Ultralights. A third revolution in dobsonian telescopes began in the 1990s as some builders began to pare down the
truss-tube design to save weight, making them even more portable. While there is as yet no “standard” ultralight
design, these telescopes incorporate one or more of the following weight-saving features:
• a very lightweight upper end, often reduced to a single ring
• thinner—and sometimes fewer—truss tubes
• abandonment of the all-enveloping shroud in favor of smaller light baffles around and behind the secondary
• a shallower and lighter mirror box (which is occasionally omitted altogether)
• larger altitude bearings, permitting a much lower rocker
• reduction of the ground board to a light triangular or Y-shaped arrangement.
The ultralight approach results in telescopes as large as 30” that can be carried in a small car and set up by one
The weight of dobsonian telescopes
Overall, the weight of dobsonians increases as a power of their aperture—a telescope of twice the aperture is likely
to be considerably more than twice as heavy (Figure 1). There is also a clear evolutionary trend towards lighter
telescopes—matched aperture-for-aperture truss dobs tend to be lighter than classics, and ultralights are lighter than
trusses. Figure 1 also plots the increasing weight of a 1.6” thick pyrex mirror at different apertures (this thickness is
not universal, but is typical of ultralights).
The heavier weight of the classic design limits their size—a 20” classic dob would weigh about 400 lbs, comparable
to that of a 30” truss. The weight disadvantage of the classic design is especially apparent in Figure 2 which plots the
weight of the heaviest component which must be handled while assembling the telescope (for classic dobs this is
usually the optical tube-altitude bearing assembly; for truss and ultralight designs it is the mirrorbox). Component
weights above about 60-70 lbs means that it will take two or more people to unload and set up the telescope. This
limit is reached for classic dobs around apertures of 14” and for truss dobs around 18”. Note the clear advantage of
ultralights—their light mirror boxes mean that telescopes 22” and larger can be handled by one individual.
Figure 3 reexpresses total (structure plus optics) weight as a multiple of the assumed mirror weight. We see overall
that the larger the aperture, the smaller the weight of the telescope relative to that of the optics alone. Conversely,
the smaller the aperture, the smaller the proportion of total weight represented by the primary mirror (and likely other
optics as well). This relationship makes it difficult to define a specific relative weight for ultralights (as suggested by
Bartels, who recommends an optical tube weight of twice that of the optics alone). Rather, the target weights for
smaller ultralights should be heavier relative to mirror weight.
As we would suspect, matched by aperture the relative weights of classic dobs are higher than those of trusses, which
are in turn higher than those of ultralights.
Figure 4 compares the total weights of different brands of classic dobsonians. All show a similar general trend with
aperture, which is generalized algebraically in Figure 5. Figure 6 plots the size-weight relationships for product lines
of truss dobsonians, which show considerably more scatter than the classic designs. Figure 7 shows the generalized
trend for Obsession-style truss designs (by Obsession, Starsplitter II, and Litebox). (I think this narrower scatter
among Obsession-style designs reflects their relatively standardized proportions—earlier truss telescopes were
structurally more diverse and generally heavier, as heavy in one case as similarly-sized classic designs.)
Figure 8 compares these generalized trends to the observed weights of ultralight designs (the black filled circles).
Note that at apertures of 10” and below there seems little (if any) weight advantage to truss designs over the classic
“circus cannon.” We also see that almost all the ultralights are below the weights set by the generalized curves for
classic and truss designs. The sole exception is instructive—it is an aluminum “ultralight” by Sayre with apparently
arbitrarily-designed smallish altitude bearings, requiring 40 lbs of counterweights to maintain balance. Even without
these weights, however, the scope would still be slightly heavier than truss dobs of comparable aperture. Thus, not all
“ultralights” are really all that light, even when they incorporate common weight-saving strategies like single-ring UTAs
and much-reduced mirror boxes as in the Sayre scope.
Surprisingly, we don’t see much difference in f-ratio among the three telescope designs (Figure 9). Rather, there
is a generalized trend towards faster primaries (lower f-ratios) with increasing aperture. At very low apertures long f-
ratios are best because (in addition to optical advantages) they place the eyepiece at a comfortable height off the
ground. As the aperture grows, however, f-ratios must be reduced to keep the eyepiece at an accessible height,
stabilizing at f-ratios around f/4-f/5 at apertures of 14” or so. Since a further increase in speed would bring significant
degradation in performance, larger telescopes stay within this range, even though the longer absolute focal lengths
may require ladders to reach the eyepiece.
The ultralight zone
Figure 10 shows the generalized relative weight-aperture curves and the ultralight examples. We see that all but one
ultralight is below the curve for standard truss designs. Thus, this gives us the upper weight limit of the “ultralight
zone.” The lower weight limit is set by the weight of the primary mirror (until future developments in mirror-making, of
course). The lower aperture limit is around 10”; telescopes smaller than this are sufficiently light that there is little
sense in sacrificing stiffness and stability for such modest weigh savings. The upper aperture practical limit for one-
person ultralights is probably around 22” at this time, since larger telescopes entail quite heavy mirror-mirror box
assemblies, beyond the comfortable lifting capacity of one individual. This ultralight zone is diagrammed in Figure 11.
Consider a telescope in the 16”-18” aperture range. Built according to Obsession-style proportions, such a telescope
would be expected to weight just about 100 lbs, the primary constituting slightly less than a third of that total. Thus,
the ultralight zone for this aperture range would be about 30-100 lbs for a 1.6” thick mirror. Figure 12 summarizes the
observed weight ranges for the three styles of dobsonians; these are shown graphically in Figures 13 and 14. A
typical classical dob would weigh about 175 lbs, or about 6X the weight of the mirror. Truss designs vary somewhat;
the average of those surveyed weighs about 125 lbs (4X mirror weight), although as mentioned Obsession-style
trusses would be a little lighter than this. The three ultralights are considerably lighter than these, averaging only 75
lbs (2-1/2 X mirror weight). Within this aperture range Bartel’s target of 2 X mirror weight (for the optical tube, at least)
should be reachable.
Copyright 2009 Ross Sackett
|How Much Should Ultralight
[Figures to be added]