"My God, it's full of stars!" --- 2001, a Space Odyssey. Star fever, nebula fever, aperture fever, field fever!
Astronomy is like life. The more you open your eyes the more
surprises you’ll see. The most profound surprises are the silent ones
that unexpectedly tap your shoulder. The panoramic composition of
multiple deep sky objects backed by considerable aperture leaves visual
memories that I can never forget. I call the experience Wide Angle
Large Aperture observing, WALA!
13.2 inch [34cm] f/3.0 is conceived to squeeze as much aperture as
possible into a compact package that fits into a back seat of a car,
yet can be carried as a single unit in my arms to an observing spot. It
unfolds from its travel configuration to its observing configuration in
about one minute. At a recent star party, a gentleman came by and asked
if it was an eight inch or ten inch scope. When I explained
it was a 13 inch, he apologized. No apologies needed – that was a
I finished the mirror in 2008 and built the mount over the winter.
The first image is from the Oregon Star Party 2010 Telescope Walkabout.
The second image is of me observing
during the Oregon Star Party, August 2011. It's a 20 second exposure
light illumination of foreground for a couple of seconds. Image by
Craig Stott. The third image is my 2nd generation design: one fewer fold with less volume.
Focal Ratios Over the Years
The evolution of Newtonian focal ratios closely
mirrors eyepiece technology. In fact, it can be said without
exaggeration that the ever increasing sophistication of eyepiece design
is an enabling technology. As better corrected eyepieces yielded better
views in fast Newtonians, even faster telescopes were built. This gave
new opportunities to sell better corrected eyepieces of wider fields of
view. Look at the following graph.
surprisingly, the consensus fastest focal ratio has steadily decreased
uniformly over the past 40 years. Most surprisingly, if the trend
continues, we’ll be at f/2.5 by the end of the decade!
Look at the growth of corrections in the eyepieces and their apparent
fields of view:
Plossl, Erfle, Orthoscopic
corrector + Nagler
coma corrector + Ethos
Telescopes and Reports
telescopes are beginning to make their appearance, for instance, an
f/3.3 telescope project is described in Miller and Wilson’s Making and
Enjoying Telescopes, published 1995. The breakthrough for me came when
the French group, ADIA, built a 40 inch f/3.0 several years ago (see
notable F3 scopes below). The accomplished, expert
French telescope maker and observer, Frederic Gea, reported marvelous
views and pinpoint images to the edge of the field of view, as long as
a coma corrector was in place. I determined that I had to see for
myself, and set about designing and building a new f/3.0 telescope.
Recently Mike Lockwood, Steve Swayze, Kai
Kretzschmar, among others, have made
f/3.0; Lockwood ventured down to f/2.6. The reports are uniformly
positive and exciting.
Fields: Not All Focal Ratios are Created Equally and Why F3 is the
Ultimate Focal Ratio for Richest Field Observing
The old rule of thumb that telescopes with a range of focal ratios
can achieve Richest Field performance as long as a suitably matched
eyepiece is used is no longer valid. Eyepieces of extreme apparent
field of view are now available, but only in shorter focal lengths.
Here's a table showing how eyepiece apparent field of view and focal
length impact the RFT experience for varying focal ratios.
Table generated for aperture
13 inches, exit pupil = 6mm.
Telescope focal ratios optimized for several popular eyepieces.
||Coma corrector X
||Eyepiece Focal Length mm
||Apparent FOV deg
||Telescope Focal Length inches
||Eyepiece Field Stop mm
||Actual FOV from Field Stop deg
||Actual FOV from Field Stop with Coma Corrector X
||FOV area deg^2
|3.6||ES 100||1.15||25||100||47||43 ?||2.1||1.8||2.5||55|
Notes on derivation:
Most columns are published values from the manufacturer.
The "Coma corrector X" is the magnification factor built into the coma
exit pupil is the eyepiece's focal length divided by the focal ratio,
further divided by the coma corrector magnification factor.
The "Actual FOV from Field Stop deg" is given by the formula: field
stop in inches / focal length in inches * 57.3
There are three keys that work in concert:
Shorter eyepieces allow faster scopes to maintain 6mm exit pupil.
2. Wider apparent fields of eyepieces allow shorter eyepieces to
achieve the same field stop as longer focal length eyepieces.
3. Since the field stops are essentially the same, the faster focal
results in a shorter telescope focal length which results in a larger
are the widest fields possible (each at 6mm exit pupil) for
focal ratios through 13 inches aperture observing M31 (image from
|F/2.5, F/3 or F/3.6- 2.5 square deg field
100 deg Ethos/ES
or F/5.2 - 1.5 square deg field
82 deg Nagler
- 1.1 square deg field
70 deg wide field
Another interesting way to look at it is to calculate the maximum
for different focal ratios given a field of view. The focal ratios are
optimized for widest angle eyepieces.
field of view = 1.8 deg, exit pupil = 6mm
Focal Length mm
Field Stop mm
|3.6||ES 100||25||100.0||43 ?||1.15||12.8|
down to f/3.6, f/3.0 or f/2.5 means jumping up in aperture from 10
inches to 13 inches. In other words, what we could see previously with
8 inch scopes and wide angle Erfle eyepieces in the 1960's to 1990's
and with 10 inch scopes with Naglers in the 1990's and 2000's is
now seen with 13 inches aperture. This increase in aperture
increases the limiting magnitude by a whole number.
Formula is: mirror diameter = eyepiece field stop * exit pupil * 57.3 /
(field of view * eyepiece focal length * 25.4) (from: field of view =
eyepiece field stop / telescope focal length; focal length = focal
ratio * mirror diameter; eyepiece focal length / exit pupil = focal
For more on Richest Field Telescopes, see my web page http://www.bbastrodesigns.com/rft.html
New Relationship: Maximum Aperture or Field of View Based on Varying
Focal Ratio While Holding Exit Pupil Constant
Increased Aperture on Visibility
The impact of 13 inches of aperture for the same field of view as
an 8 inch is drammatic. Objects like the Horsehead, barely detectable
in the 8 inch, are readily detectable in the 13 inch. Otherwise
invisible galaxies pop into view. Instead of a faint object or two,
many objects are visible at once.
Problem with Standard Mirror Blanks
However, for large thin mirrors to be
made at f/3, something has to be done about the shrinking center
thickness. A 40 inch diameter mirror at f/3 has a sagitta or central
depth approaching an inch. That leaves precious little thickness at the
center of the mirror. I determined to try a meniscus mirror, where the
entire blank is curved to the appropriate shape by softening in a kiln,
resulting in a mirror of constant thickness.
Meniscus Shape to the Rescue
meniscus mirrors are quite strong. The nine point support I was
envisioning wasn’t needed; instead a simple three point back support
and two point edge support supports the mirror without observable
distortion. The analogy goes like this: pick up a piece of paper and
wave it. See how it bends over? Now cut a pie section out of the paper
and tape the remaining paper into a cone shape. Waving it around
doesn’t bend it at all: the cone is a stronger shape.
full thickness 12 inch or a thinner 13 inch mirror require a 9 pt
support. Deformation is clearly seen in the star test if a 3
support is used. And high power images are heavily
very thin 12 inch mirrors made from flat glass. Not a trace
deformation can be seen at highest powers in the star test with the 13
inch meniscus mirror on a 3 pt support. The meniscus mirror
an equivalent thickness of a mirror made from a flat piece of glass 1.4
inch thick where the concave curve is not only ground into the face but
also a convex curve is ground into the back side of the mirror.
The weight reduction is key: less weight means less
Taking into account the weight reduction and the equivalent
thickness means that this meniscus mirror is twice as stiff as a full
thickness blank. For example, one cannot use PLOP to calculate
deflection using a thickness equal to the sagitta plus edge thickness
because the weight reduction is missing as a factor: PLOP's deflection
estimate is too severe (remember that PLOP is designed for flat backed
mirrors; for more go to David Lewis'
PLOP) . A simpler support is fine; how much simpler
needs to be determined empirically.
the Lightweight Mirror
mirrors are lightweight because the curved glass is relatively thin.
Consequently the glass cools quickly. Typically I don’t need a fan
while observing. Taking the scope outside
requires about a 15 minute cool down period, after
which the star images are very stable. Since the glass is thin and I
have a fan for backup, I can
substitute inexpensive plate glass. Plate glass further cools faster
than Pyrex so the plate glass reaches equilibrium quicker than
Pyrex. Plate glass runs 20x cheaper than
Pyrex. The downside is that when making your own mirrors, plate glass
is a a pain during figuring because care must be taken to ensure
that the glass is in equilibrium with its ambient surroundings. It's
necessary to take active steps to equilibriate the mirror. I
fans blowing air in an insulated room on the mirror during the indoor
star test. Thanks to active mirror cooling and an insulated shop, I can
do a half dozen
figuring sessions each evening.
Slump Meniscus Shapes
a sense, computer controlled kilns promise to be an enabling
technology, allowing thin mirrors to be slumped to very large
diameters, no longer being limited to sheet Pyrex’s width of 40 inches
or so. Instead of grinding a curve into the mirror’s face, the mirror
is placed upside over a convex mold and heated until the glass softens
and folds down over the mold. The kiln is then directed by computer
control through the annealing cycle, cooling the glass over a period of
several days. The cost of a kiln plus plate glass is competitive with a
Pyrex sheet glass blank, with the bonus that the kiln can be used again.
Slumping Reduce Effective Aperture?
No, not at F/3. See the slumping
Part of my interest in the 13 inch f/3 was to see how
difficult grinding an f/3 mirror truly is. Well, I can report that it is
fundamentally no different than grinding any other mirror, except that more
parabolization is pushed into a smaller aperture and the error tolerance
tightens a bit. I used standard mirror making techniques with excellent
results. I gauge the difficulty of the 13 inch f/3 to be roughly equal to the
difficulty of grinding a 20 inch f/5. Curiously, the 13 inch f/3 has about the
same degree of parabolization as the 20 inch f/5. So if you can make a somewhat
larger mirror, then you can be confident in attempting a somewhat smaller f/3
My goal in grinding, polishing and figuring a mirror is to make it
indistinguishable from perfect. Consequently, I most need mirror tests that
qualitatively reveal defects. I have less of a need for quantitative tests that
yield numbers because I don't really care if the error is 1/8 wave or 1/8.5
wave. The error has to be removed. A key question in my mind was, "Could I
detect miniscule deviations that could prove injurious at high magnifications given the ultra-fast F3 focal ratio?"
I used the Ronchi matching bands test along with the indoor star test. The
Ronchi test proved quite sensitive, able to show the slightest zonal defects
and overall paraboloidal shape. I conducted indoor star tests using my 20 inch
F5 mirror, a proven highest quality optic that gives a perfect star test at
highest magnifications. I used a fine point needle and make several pinholes of
varying size in aluminum foil placed against an aluminum block. The foil then
went into the focuser, placed at the center of the focal plane, along with a
bright light above it. The 13 inch mirror in its wooden test frame along with
diagonal, focuser and high power eyepiece was aimed horizontally into the 20
inch, also placed horizontally. Since my shop is insulated, the air was very
stable and I was able to conduct high quality star tests. The varying pinholes
provided artificial stars of different brightness and size. A final check with
the outdoor star test of several hours of careful back and forth focusing at
high power with the 13 inch in its test frame aimed at Polaris confirmed my
For my online Ronchi matching bands test, see http://www.bbastrodesigns.com/ronchi.html.
For my mirror making webpages in progress, see http://www.bbastrodesigns.com/JoyOfMirrorMaking/JoyOfMirrorMaking.html.
For more on my telescope making, see http://www.bbastrodesigns.com/tm.html.
Telescope Changes Thanks to F3
quickly realized that such a short telescope called for a new telescope
mounting design: the incredibly stubby truss tubes as commonly built
begged to be replaced. It’s important when considering the patterns of
telescope design to allow the design to grow organically. If one
component varies from tradition, then it is likely that surrounding
components will also.
Center of Gravity
An f/3 with a lightweight mirror places
the center of gravity farther up the tube than what is customary. This
results in benefits such as balance insensitivity, small footprint, and
a smaller eyepiece swing from horizon to vertical.
insensitivity means the telescope can move smoothly in altitude despite
today’s heavy eyepieces: no counter-weighting need apply here. Small
footprint means that the scope rotates in azimuth within the smallest
possible circle. This has particular impact on observatories, which can
be horrendously large with Dobs that balance close to the
For instance, a 16 inch Dob might need a roll-off roof observatory 12
feet x 12 feet. By moving the center of gravity to the mid-point of the
optical tube assembly, the building size can be greatly reduced. Roll
off roof observatories are so much easier to build when the roof size
is small. Minimizing eyepiece swing means that I can observe with the
13 inch sitting in a chair regardless of whether the scope is pointed
at the horizon or is pointed vertical.
the Folding Design
It’s my observation that
assembly time and difficulty has a large impact on how often the scope
is used. It’s not so much how fast a scope could be assembled. Instead,
it’s more of, “Do I have the energy tonight to drive somewhere and set
it up?” The key is to avoid assembling the scope at all.
I designed my first folding scope, the 'Tri-Dob', in 2001. It
featured folding altitude rims. I wanted more though, to fold the
entire telescope such that it would fold for transport and unfold for
observing like Origami. I
studied a number of folding arrangements, which essentially squeeze air
out of the telescope as it folds up. Folding is much quicker and most
importantly, easier than assembling. In the end I chose a three-fold
arrangement that squeezes the telescope into a small cube, easily
picked up and carried by hand (total weight is about 25 pounds). From
the bottom up, the folds are 30 degrees, 42 degrees, and 108 degrees.
Note that the folds add up to 180 degrees, which results in the upper
end folded down against the primary mirror, about as compact as
Google Sketchup model is available from the online repository at http://sketchup.google.com/3dwarehouse/details?mid=447e8896bc0fb328b47a4848f4241ae0
More on folding and sliding telescopes can be found on my web page, http://www.bbastrodesigns.com/FoldingScopes/FoldingTelescopes.html
Unfolding the telescope
at the Oregon Star Party Telescope Walkabout, 2010
Iteration II Design
improved the design with a follow-on second generation iteration. There
is one fewer fold and the folded volume is 1/3 less than the first
generation model. I use folding upper trusses made from ApplyPly; as is
all the wood on the telescope. They are held in position by the mirror
cover that doubles as a dew shield. This is proving very rigid, though
the telescope could survive without the upper trusses because of the
folding altitude rim engineering. Other notable features include a
return to the single upper ring that I first began building in the
early 1990's and a very light flex rocker with centerless bearings.
Diagonal size continues to be a 3.1 inch [78mm] .The Google Sketchup
model is available at http://sketchup.google.com/3dwarehouse/details?mid=f1eaaf10973bdc94bfd7348943bf41c&ct=mdsa
Compare to the first iteration design: the new more compact design is on the left.
Better Telescope Design: Multipurpose Parts
always look for ways to create telescope components that serve multiple
purposes. With the center of gravity in the middle calling for sweeping
altitude bearings, it occurred to me that the reinforced altitude
bearings can take the place of most of the truss tubes, and provide
folding pivots to boot. This works quite nicely in practice – the
telescope is rock solid with no hint of any vibration.
I use two
tests to determine the design integrity of the optical tube assembly.
First is the optical alignment test: can alignment be maintained
exactly as the scope swings between horizontal and vertical? Finally,
can I grab the upper end and twist the mirror end out of the rocker?
This design passes both tests with flying colors.
might note the wire spider. I've been using wire spiders for many
years. They work wonderfully in that there's no diffraction except
short spikes around the brightest stars and the diagonal stays rock
solid and unmoved, allowing perfect optical alignment from horizon to
zenith. Curiously and counter intuitively, I discovered that the
tension in the wires is
irrelevant to supporting the diagonal properly. Mathematical
analysis reveals that tension drops from the picture. Further, wire
spiders have less springiness than tradional spiders, particularly when
rotating the diagonal holder back and forth. On very large scopes, wind
blowing through the upper end can create a never-ending vibration of
the diagonal, visible as astigmatism at high power. Properly designed,
both traditional and wire spiders need not suffer this ailment. The key
is to break apart the spider at the hub into two
like '>-<', separated by a hub that holds the diagonal
and resistance to flexure and twist is determined by the geometry of
the wires. Hence my wires cross in broad 'X's with the two 'V's on
either side of the hub. The wire spider is a single piece of wire,
wired into position by supporting the hub on a removable jig. You can
see the drilled holes that hold the jig in position at the top of the
upper end. Once the wire is strung into position, I tighten the eye
bolts until there is no slop in the wire then remove the jig.
Focusing range is a little discussed topic amongst amateurs.
However, it is a primary factor determining how sharp is the image.
Focusing range is the distance that the focuser can be moved on either
side of the theoretically perfect focus yet not change the
The formula for focus range where the optical path
difference is limited to quarter wave is: focus range = wavelength of
light / index of refraction times the sin squared of the angle of
the edge ray (focus range = λ /N' sin^2 U'). This
simplies to 0.0001 * focal ratio squared. See Conrady's Applied
Optics and Optical Design, volume 1, pages 136-7.
While a F5 scope has a focus range of 0.002, a F3 scope has a much
tighter focus range of 0.001.
procedure is to obtain the ultra finest focuser, then carefully focus
back and forth, stopping in the middle. If disatisfied, repeat. The
difference at F3 is between a so-so Saturn and a great Saturn.
Here are (unexpected) lessons learned from designing and building a
very short focal ratio telescope.
optimal diagonal size is the minimum size. Calculate the minimum size
by dividing the focal plane to diagonal distance by the focal ratio.
For this scope, the
focal plane is 3/4 inch above the racked in focuser. Positioning the
short height focuser as close to the tube's bottom or edge results in a
focal plane to diagonal distance of 9 inches. Dividing by F/3 means
that I need at least a 3 inch diagonal. Diagonals are commonly
available in 2.14, 2.60, 3.10 and 3.5 inch sizes. I selected the 3.10
inch size as the closest fitting diagonal. The illumination fall-off
from diagonals is very gradual at F/3, much more gradual than at slower
Don't forget that larger diagonals block additional light across the
field for all eyepieces: you can fix the edge alright but accidentally
ruin the center. It's better to maximize illumination in the center
of the field where much of the observing is done and where small exit
pupil/high magnification eyepieces operate by selecting the minimum
sized diagonal. Also, the field stop or field size to illuminate
adequately is smaller compared to slower
focal ratios. The largest field stop that can be used with a F/3 scope
is the 100 degree 21mm Ethos eyepiece from TeleVue or the 20mm 100
degree eyepiece from Explore Scientific. These eyepieces have a field
stop just under 1.4 inches diameter. The illumination fall off at the
extreme edge of these eyepieces with an optimized minimally sized
diagonal is 0.2 magnitude light loss - not noticeable to the visual
observer. Further, this light loss is only at the edge of the field of
view at low magnifications - if in doubt, move the object closer to the
center of the field. Finally, the eye's response needs to be considered
since the light drop affects both the object and the background
that surrounds the object, meaning that the contrast stays constant. My
tests show that objects remain equally detectable towards the edge of
the field. Consequently
there's no need to go big with diagonals in a F/3 scope, which because
of their increased size block additional light across the entire field
of view.This will be a
new and unexpected experience for telescope designers. See my diagonal
calculator at http://www.bbastrodesigns.com/diagonal.htm.
Baffling the diagonal and focuser is very important - see my baffle calculator.
small optimized diagonal size means that the diagonal shadows a
standard sized upper end. The upper end can be made smaller. Further
discussion can be found on my web page, http://www.bbastrodesigns.com/SmallerUpperCage/SmallerUpperCage.html.
This reduces size and weight, since the upper end often dictates the
mirror box size which dictates the rocker and ground board size. The
upper end can then act as a baffle, joining the diagonal baffle and
focuser baffle along with the primary mirror baffle to block stray
light from the view.
- Calculating the diagonal baffle can be difficult because of the coma corrector.
I settled on an empirical method where I aimed a flashlight at the
scope to determine baffle size. Here the baffle size is sufficient to
block light at narrowly aimed angles into the focuser from the opposite
side of the tube.
Center of Gravity is nearer the middle of the tube assembly.
makes the telescope stabler, the eyepiece swings in a shorter arc
and the overall footprint of the telescope as it swings 360 degrees in
azimuth is greatly reduced in area. Though the Center of Gravity looks
like it is radically changed, it is only one mirror diameter up from
the mirror cell. There is little change in the mirror box and flex
rocker compared to more standard focal ratio scopes. The
perspective may take some getting used to.
folding design needs to be optimized by iterating from a starting
design, and adjusting the pivot points until equal folding with minimum
volume is achieved. This takes time and patience.
The tube is so short that simple solutions like breaking the tube into
two sections become attractive - the transport volume when collapsed is
- The scope may need to be elevated because the eyepiece
distance from the ground is short. I became tired of observing
my knees so I place the scope on a folding platform. An adjustable
height chair also looks promising. Steve Swayze uses one on his 18 inch
f/3 - it's very comfortable to observe for long periods of time.
most critical aspect during observing is accurate focus. Focusing at
f/3 at small exit pupils/high magnification is touchy to say the least,
even with a high end precision focuser. Buy or build the very best
focuser that you can possible manage.
alignment or collimation must be perfect. A precision focuser,
precision laser collimator, exact centering of the white ring that goes
on the primary, exact centering of the laser spot in the white ring,
and the return dot must all be perfect. Otherwise small exit pupil/high
magnification images will not focus to a pinpoint. As the tube is swing
from horizontal to vertical, the laser dot must remain exactly centered
in the primary's white ring. Many designs and telescopes will not
measure up to this required standard. Rigid upper end and rigid
connectors to the mirror box are mandatory. If the telescope is
transported, expect to make very minor but necessary (~1/12 turn of a
mirror mount bolt) adjustment every time.
- The TeleVue P2 coma corrector is compulsory. With it the
images are pinpoint
to the extreme edge of well corrected eyepieces; according to the
literature, the coma is reduced to that of a f/12 telescope. Without
it, the images are hopeless, the stars become seagulls a third of the
way to the edge of the view. I have not tried the Keller corrector from
Germany; it looks to be of high quality.
- There is no vignetting using the P2 with a 21mm TeleVue Ethos At
f/3 and f/2.7. Looking at the light path and P2 barrels, one does
wonder. Here is a simple test that I use for any type of telescope. I
defocus the star just enough to see a disk of light. The mirror's edge
support clips denoting the mirror's edge should be visible if there is
no vignetting. More precisely, the calculated point off-axis where
vignetting should begin to occur should match the actual off-axis
position in the field of view where vignetting actually begins to occur
as the defocused star image moves from the center of the field towards
the edge of the field of view.
- The TeleVue P2 coma corrector with the TeleVue 21mm Ethos
requires a baffle on the far side of the diagonal. But what size?
Experimenting in the dark with a flashlight demonstrates that the
required baffle angle is about 60 degrees or one radian.
thin plate glass meniscus mirror performs marvelously. A few minutes
with the fan is all that is necessary to get perfect star test images.
Experienced observers are none the wiser that the scope has a plate
glass mirror - the images are rock steady, very sharp and bright.
- The dew shield, a flat black sign board, nicely protects
the mirror from dewing.
The telescope frame can be dripping in dew yet the mirror stays dry.
Consequently I do not bother with a full on shroud. The upper end is
designed to baffle the field of view from stray light, obviating a
shroud. The test is to look through the focuser without an eyepiece -
you should only see mirrors and black baffling.
the telescope are wonderful. Incredibly wide fields at low power with
pinpoint stars to the edge give way to high power views with excellent
resolution and contrast. Dark nebulae have never looked better,
showcase objects are wonderfully framed and globular clusters are
resolved into tiny pinpoints of starlight at magnification. Here are
early observations, images representing the field of view through the
13 inch from Microsoft's WWT. Keep in mind that these fields
of view are with a 13 inch telescope!
|M31: spectacular aggregate
view: entire galaxy along with companions
fit into the field of view; striking multiple dust lanes; details in
galaxy arms at the extensions and in the companions
|Horsehead, Flame nebulae:
in one view the Horsehead is faintly visible
(no filter) with good detail in the Flame nebula; NGC 2023 and IC 435
are bright; all this despite a very bright Zeta Orionis
||Pleiades: all of the
extremely bright stars fit into a single view;
extensive nebulosity everywhere, particularly detailed next to Alcyone
with extensive sweeping from Merope to edge of view, along with some of
the general nebulosity that surrounds the Pleiades
||M42 region: entire loop of
M42 seen with lots of detail with some
color; the green nebulosity embedding the Trapezium is quite striking,
field of view extends from the open cluster NGC 1981 through NGC
1973/5/7 up past NGC 1980.
Chain, the Virgo Cluster, M84-M86 area. The image is a good match to
the view through the eyepiece (though the stars are missing)
|Lagoon and Trifid nebulae both fit into the same field of view, but with 13 inches aperture. Very nice!
Other interesting observations:
Sh2-264 (bubble around Lambda
Orionis) is surprisingly. I noticed some parallel banding
on the east side and general glow in towards the center. I was able to
about 2/3 of the way around it's perimeter. The brightest portions,
larger, where of similar magnitude to the bright nebula that the
(the Phantom Nebula) is a large distinct brightening with large dark
patch ala Horsehead (start with Horsehead, go past M78, continue on
line past Barnard's Loop).
Loop. Was able to trace out the loop for about 10 degrees. There was
detail in the loop including unexpected dark patches southwest of M78.
See my online
visual detection calculator.
the best object of all in my 13 inch f/3.0 during the 2011 Oregon Star
Party was the North American Nebula and the nearby Pelican Nebula using
an OIII filter. The unexpected detail and great contrast was amazing -
of all the views of the NAN/Pelican over the many years, the view
through this scope in OSP skies came closest to photographic and
digital images, showing the streakiness in the 'Floria' and 'Central
America' areas, the jet black 'Gulf of Mexico' and dark nebula in
'Northern Canada' and good detail in the Pelican.
I have changed how I describe my telescope: old style: 13” telescope at
55x; new style: 1.8 degree actual field, 100 degree apparent field, 6mm
exit pupil, use the visual detection calculator for limiting star
magnitude and to ascertain detection for extended objects. See my
New Way to Look at Things.
Observing at Oregon Star Party, 2010 (note the reflection of stars in
more on f/3 observing experiences, check out Frederic Gae et al large
f/3 telescopes along with Lockwood's observations. Run, don’t
walk to your nearest f/3 telescope, clear your mind of preconceptions
and find out for yourself!
Conclusion: pluses and minuses
field at lowest magnification (Richest Field) compared to f/4 and
slower: excels at very low contrast difficult objects, excellent at
high power - pinpoint images.
required (with TeleVue P2 coma
corrector, the coma is equivalent to that of an F/12, so better than
standard Dob). There is no vignetting - see my comments above.
ladder" eyepiece height
mirror takes extra skill to produce
every eyepiece may perform well
stiffer tube assembly holds optical
alignment (collimation) better
same as f/4.5 scopes due to
flatter field illumination profile
alignment (collimation) tolerance is 0.2
|Ultimate star hopping
telescope with its extra wide field of view
|Focusing is very touchy at
pupils/high magnifications: need a precision focuser that resolves to
better than 1/1000 inch. Focusing
is probably the greatest practical disadvantage in the field.
tube with Center of Gravity mid-range
smaller observatory footprint
eyepiece swing from horizontal to vertical
|No need to counterweight
glass because of meniscus shape and less
weight means simpler mirror cell
meniscus shape equilibrates quicker and shows no overcorrection during evening observing while temperature drops rapidly.
to design folds so that there are no collisions
|Setup time in seconds
||Less diffraction than
||Need jig for initial
|Inexpensive: a few dollars
for the wire
|Stronger than standard
spiders because of the wider geometry
Christensen sent this image of the Small Sagittarius Star Cloud taken
with his 6 inch f/2.7. It illustrates the stunning tack sharp field
possible with super fast scopes.
last updated November 2013