wpo - optical configurations & origami
astro-optics page
text & images [c]Maurice Gavin 1989/2001-
BAAJournal
-1989 Dec v99,#4, p162/163
Letters to the Editor - BAAJ
Dear Editor,
Queries like the following can be posed:
How far apart should the mirrors be in a Newtonian?
How small can the flat be in a long-focus Newtonian for planetary work?
Using plane mirrors to fold a refractor to one third of its normal length, what size and where should these mirrors be?
Answering all these questions is simple. Try some origami i.e.
paper folding! Most systems can be tested in a practical and
graphic way without computation including quite complex designs.
Two examples are shown in the adjacent sketches although for clarity only
the Newtonian is described below. Reference to Cassegrain optics
(where the beam from the secondary mirror is altered) is also omitted.
Using a large
sheet of paper (the reverse of wallpaper is ideal) plot the vital dimensions
of your system to full size. Figure 1 -
start with the primary mirror diameter and its focal length on the optical
axis. The focal length quoted by the supplier may only be approximate
so it should be measured accurately otherwise all the tolerances incorporated
into the design may be absorbed. Do so by pointing the Newtonian
primary directly at the Sun using a card mask so that only slithers of
sunlight on opposite sides of the mirror are illuminated. The twin
images so produced will merge when in focus. Measure accurately the
distance between the mirror and image. Mark the field diameter
at the focal plane. This will vary with your needs. For
a 35-mm photographic format, a 44-mm diameter field equal to the
film diagonal is appropriate. (Some vignetting is still likely from
the mirror box of most SLR cameras and unavoidable without purpose-made
cameras.) If the telescope is to be used predominantly for
deep-sky visual work with low powers, typically in a Dobsonian mounting,
then 25-mm to 30-mm diameter field is recommended as matching the field
lens of the eyepiece. If the instrument is solely for visual lunar and
planetary work, typically at higher powers, then perhaps even 10-mm diameter
will suffice. Opt for larger diameters up to the maximum quoted if
your interests are not specific. Lines drawn from field diameter
at the focal plane to the edge of the primary form a cone.
Using a sharp knife cut along the lines of the cone and fold the paper
back so that it projects beyond the tube walls. This is where the
final image will come to focus. It should project just sufficiently
to accommodate say a camera body and the rackmount in mid position.
If the telescope is to be used only for planetary work, the focal plane
can be kept as close to the tube wall as convenient. In this
way the size of the flat is kept to a minimum.
Check that the focusing rackmount matches the f-ratio of your
primary. Some older designs are unsuitable for modern 'fast' mirrors.
If necessary use a modern 'low profile' design with a wide throat to avoid
vignetting. If astrophotography is amongst your future needs,
it is better to allow for it initially. It is easy to adapt for visual
use by replacing the camera body with a short tube for the eyepieces but
virtually impossible to do the reverse satisfactorily without major surgery.
SLR teleconverters of x2 or x3 can be screwed-in before camera or eyepiece
and will increase the focal length of the system.
They are usually optically superior to the Barlow lens.
The fold-line
represents the size (major axis) and location of the flat. The minor
and major axes of the mirror and its distance from the primary can then
be accurately measured as can the minimum inner diameter of the main telescope
tube. The latter must be not less than the primary diameter plus
the field diameter. Add a further minimum of 30-mm to the tube diameter
if it is not fully cross-ventilated i.e. an open tube, and thus prone to
tube currents.
At this stage it is possible to judge if the overall tube length can be reduced - effectively the flat is placed near the top of the tube - and what, if any, extraneous light can flood the focal plane and where baffles would be useful. The smaller the field diameter, the smaller the flat, especially at higher (slow) f-ratios. This can help improve resolution by minimising the diffracton caused by the central obstruction in visual planetary work for example. For lower f-ratio wide-field astrophotography, the larger central obstruction has little effect in resolution terms. Usually, the flat directs the cone of light out of the tube at 90o but is by no means essential as long as the rackmount is square to the focal plane itself. The folded refractor in the figure 2 is such an example.
Yours faithfully,
Maurice Gavin - Worcester Park - Surrey