Fall 2006: No. 14
PULSAR
THE NEWSLETTER OF THE KITCHENER-WATERLOO CENTRE OF THE ROYAL ASTRONOMICAL SOCIETY OF CANADA
Schmidt-Newtonian Astrophotography
The APO alternative for modest budgets
In this Issue:
- see page 6
Starfest 2006
The popular expression
“it’s all good” was sorely tested at Starfest this
year when the weather
gods refused to cooperate!
Italian Astronomy,
Renaissance Style
Join our globe-trotting
Doug Bulgin as he visits
a few of Italy’s famous
astronomical “shrines.”
Plus, a few fun facts
about the history of
Italian astronomy.
Schmidt-Newtonian
Astrophotography
The unintended brainchild of Bernhard
Schmidt and Sir Isaac
Newton makes a decent
astrograph for far less
money than an APO
refractor of equivalent
aperture.
Editor’s Corner
M31, the Andromeda Galaxy, with its attendant dwarf galaxies M32 and M110, by
Alen Koebel. A 30-minute exposure on Fujichrome Provia 400F through a Meade
LXD55 SN-6 6-inch f/5 Schmidt-Newtonian OTA, processed by Brady Johnson.
For some of us, fall (or autumn, if you prefer)
is the best season of the year. The temperatures
outside are still comfortable and the scenic backdrop of the colourful, changing leaves is without
compare. The night skies can be among the best
of the year too, with plenty of impressive celestial objects to view through crisp, transparent air.
However, seeing stars through that crisp air
assumes a clear sky. Alas, we haven’t had very
many clear nights this fall – heck, all year really!
What we have been getting is rain – a lot of it!
Solid cloud decks and raindrops caused many
club observing events to be cancelled (explaining
why there’s nothing about them in this issue!).
Even big-time events were not immune: Starfest
this year, for example, was a soggy mess.
Of course, if it’s raining or just cloudy you can
always do some reading. Maybe that’s why
you’re looking at this issue of the Pulsar right
now. Hey, whatever it takes!
RASC
K-W Centre
Alen Koebel, Editor
http://kw.rasc.ca
R O YAL A S T R O N O M I C AL S O C I E T Y O F C AN AD A
PULSAR
Fall 2006: No. 14
Star Parties
Starfest 2006 – Fellowship
Under the Rain
“I am a man of constant sorrow. I
have seen trouble all my days.”
Those are the opening lines of the
song “Man of Constant Sorrow” that
was most recently popularized in the
Coen Brothers' 2000 film “Oh Brother
Where Art Thou?” In that movie, the
song is supposedly sung by the three
lead actors, George Clooney, John
Turturro and Tim Blake Nelson, who
appear in a talent competition as the
Soggy Bottom Boys.
The song could just as well serve as
the theme for this year’s Starfest, the
annual star party hosted by the North
York Astronomical Association, which
was one of the wettest since the event’s
inception in 1982. Constant sorrow may
well have described the feeling the
clouds and rain brought to so many
hopeful amateur astronomers. And no
doubt the event’s perpetual locale – the
River Place campground north of Mount
The ever-popular swap meet coincided with one of the week’s few brief spells without rain.
Forest, Ontario – was filled with a lot of
soggy bottoms (on both boys and girls).
Starfest this year occurred from Monday, August 21 to Saturday, August 26.
However, the best night turned out to be
the Sunday before the campground
officially opened for the event. “You
should have been here!” was what visitors who arrived on the opening day
heard. Monday night itself was mostly
clear, clouding up around 1:00 A.M. but
clearing again around 4:00 A.M. Tuesday night was very similar, with high
SkyNews editor Terence Dickinson could be considered Canada’s official astronomical celebrity!
2
clouds coming in around 2:00 A.M and
clearing by 4:00 A.M. The rest of the
week was a write-off, weather-wise,
with almost constant rain and clouds.
In years past Starfest founder and
chief organizer Andreas Gada used to
claim a special affinity with the weather
gods, pulling clear skies out of a hat on
many occasions. On this, Starfest’s 25th
anniversary year, the weather gods
opted for some payback. “Fellowship
Under the Stars” was the official theme
of the event this year; while the fellowship surely happened, the stars were nowhere to be seen for most of the attendees.
The bad weather actually had a few
positive side effects. With almost constant rain on Friday the attendance at the
talks that day inside the shelter of the
“large” and “small” tents was probably
at an all-time high. And no doubt the
food truck and the restaurants in town
did good business (who wants to cook in
the rain?).
Unfortunately, the astronomy vendors
on-site didn’t fare as well, since the rain
made open-air shopping under the canopies they erected a bit problematical.
A hint of sun on Saturday afternoon
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Starfest was the debut for club member Peter Pekurar’s amazingly lightweight 25-inch Dobsonian.
At 71 Story is one of the most accomplished men on the planet. He has six
academic degrees, including an M. D.
and most recently a master of arts in
literature. He has logged over 1200
hours in space beginning in 1983 in six
Space Shuttle missions. Story attributes
his success at NASA not to his impressive academic achievements but rather
to the experience he gained fixing
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things while growing up on a farm in
Massachusetts.
Story’s greatest accomplishment at
NASA was as the Payload Commander
for STS-61, the Shuttle mission to repair the Hubble Space Telescope in
1993. He engineered many of the special tools and the complicated spacewalk “choreography” required to achieve the repairs, not to mention actually
performing three of the five spacewalks
himself with fellow crewmember
Jeffrey Hoffman. More than any other
individual, Story could be credited with
the success of the mission, which saved
NASA’s then much tarnished reputation. However, in characteristically
humble fashion Story himself has never
made such a claim.
During his two hour talk, illustrated
with photographs he took himself from
space, Story held the audience spellbound and at the talk’s end received a
standing ovation to thunderous applause.
In light of the abysmal weather this
year, Story can now be credited not
only with effectively saving the US
space program but Starfest as well!
Club member Jeff Collinson demonstrates the
proper use of this huge lens as an exercise aid
to prospective buyers during the swap meet.
was most welcome and very fortunate
for the swap meet, but the rain returned
with a vengeance that evening. The
downpour on the roof of the main tent
threatened to drown out the inspiring
and much anticipated words of the featured speaker, retired astronaut-scientist
F. Story Musgrave.
Fall 2006: No. 14
Story Musgrave autographed portraits of his younger self for dozens of admiring fans.
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Fall 2006: No. 14
Travel
The Astronomical
Treasures of Italy
by Doug Bulgin
Last March I traveled to Libya to see
a total solar eclipse on an expedition organized by Ralph Chou of the Toronto
Center. This was my first total solar
eclipse and it was, in a word, awesome.
[See the spring 2006 issue for more
about Doug’s Libyan adventure. – Ed]
But Libya was only the first portion
of my trip. Of the original group of 25,
fifteen of us went on to Italy. We spent a
total of seven nights there, touring many
well known cultural landmarks, some of
them important to the early history of
astronomy.
Instantly upon arrival in Italy we noticed a change in how people treated us.
In Libya, we truly felt like guests,
whereas in Italy we were just another
group of tourists. There were lots of
churches and cathedrals. All of them
were very beautiful and very old. Unfortunately they quickly started to look the
same. As well, I never thought I would
grow tired of pizza, but I did. But only
for a few days!
On our first night in Italy we had a
tour of the Vatican Observatory, one of
the oldest astronomical research institutions in the world. It is located at the
papal summer residence in Castel Gandolfo, outside Rome. There are three
telescopes at the observatory today: a 40
cm f/15 refractor, a 40 cm f/5 astrograph
for photography and a 98 cm f/2.5
Schmidt camera for photography. The
observatory is rarely used now since the
opening of the Vatican Advanced Technology Telescope at the Mount Graham
International Observatory in Arizona in
4
The University of Padua.
1993. The Castel Gandolfo observatory
also has a large collection of meteorites.
[Learn more about the Vatican Observatory on page 5. – Ed.]
On Saturday we had a brief tour of St.
Peter’s Basilica and then spent the day
on a walking tour of ancient Rome. On
Sunday we moved on to Florence with a
side trip to Pisa to climb the city’s
famous leaning tower. The tower was
closed to the public in 1990 but fortunately was reopened a few years ago
after reinforcements were made. Galileo
is alleged to have performed an experiment dropping cannonballs of different
weights off the tower to prove that they
fall at the same rate, but historians say
it’s unlikely that he actually tried it.
On Monday we toured Florence, including the Science Museum where
Galileo’s original telescope is kept. It is
amazing what he managed to do with a
telescope that was smaller and looked to
be flimsier than one of today’s cheap
“department store” telescopes! [See
page 5 for more about Galileo and his
telescopes. – Ed]
Tuesday we moved on to Venice with
a stop at Padua to visit the University of
Padua. The second oldest after the University of Bologna, it was founded in
1222 by Jesuits and students who left
the University of Bologna in search of
academic freedom. From the fifteenth to
the eighteenth century, the university
was renowned for its research, particularly in the areas of medicine, astrono-
my, philosophy and law. Galileo taught
there from 1592 to 1610.
We toured an ancient lecture hall
where they taught medicine. Because
dissecting a human was strictly forbidden, they had a special table built where
they lifted the body up from the basement. Someone watched for the authorities and if there was any sign of trouble
the body could be dropped down and a
pig carcass brought up to replace it!
Wednesday we toured Venice and
Thursday we moved on to Milan, the
last city on our tour before returning to
Canada. Milan is home to the only
cathedral in Italy built in the late gothic
style, which is also the second largest
Roman Catholic cathedral in the world,
bested only by the cathedral of Seville.
(Saint Peter’s Basilica in Rome is also
larger, but it isn’t a cathedral.) Construction began in 1386, but the last details were not completed until the late
20th century. (And you thought road
construction in Ontario was slow! - Ed.)
Italy proved to be a very worthwhile
addition to my eclipse trip and I’m glad
I went.
Two of Galileo’s famous telescopes.
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The Vatican Observatory
History
Galileo and his Telescopes
Contrary to popular belief, Italian
astronomer Galileo Galilei did not invent the telescope. Nor was he even the
first to aim a telescope at the heavens.
But he was the first person to publish his
observations. He was also the first to see
the moons of Jupiter and to identify
them as objects in orbit around the
planet, among many other important
discoveries.
What we call today the Galilean telescope was actually invented in Holland
in 1608. Then and now, it consists of a
convex lens in the front of a tube and a
concave lens in the rear.
A drawing of the moon by Galileo.
Galileo Galilei (1562 – 1642).
Nuncius (The Starry Messenger), which
was published in 1610. He later saw the
phases of Venus and the rings of Saturn
(the latter, however, only as unresolved
“appendages”).
Englishman Thomas Harriot was
actually the first to use a telescope astronomically, viewing the moon in 1609
several months before Galileo and even
drawing reasonably accurate maps of its
surface, but he never published his observations.
Galileo’s well published observations,
on the other hand, put him in trouble
with the Roman Catholic Church because they flew in the face of official
doctrine, supporting the heretical Copernican view that the Earth was not at the
center of all things. His troubles culminated in house arrest from 1633 until his
death in 1642 at the age of 78. Only
recently, in 1992, did the Church
“rehabilitate” Galileo, in effect apologizing for its condemnation.
News of its invention spread quickly
throughout Europe and it didn’t take
long for Galileo to work out the principle of its operation and to build one
himself. His first instrument magnified
only eight times. With a refined version
that magnified twenty times he discovered the moons of Jupiter and saw
craters and mountains on the moon,
observations he described in Sidereus
Castel Gandolfo near Rome.
Fall 2006: No. 14
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The Vatican Observatory is one of the
oldest astronomical research institutions
in the world. Directed by George Coyne,
a Jesuit priest with a doctorate in astronomy, it employs ten Jesuit astronomers
who split their time between the Observatory’s headquarters in Castel Gandolfo,
near Rome and the top of Mount
Graham, near Tuscon, Arizona.
Dr. Coyne was on the Pontifical Commission that concluded a 13 year study
in 1992 with an apology for the Roman
Catholic Church’s treatment of Galileo.
However, the very existence of the
Vatican Observatory demonstrates that
the Church has for some time agreed
with Galileo that science and religion
need not be at odds.
The history of the Vatican Observatory goes back to at least 1797 when the
Tower of the Winds within the Vatican
was fitted with meteorological instruments and named the Specola Vaticana.
After many years of continuous meteorological observations, the Observatory
was abandoned until being refounded in
1891 by Pope Leo XIII, at which time it
was relocated to a hillside behind the
dome of St. Peter’s Basilica where it
established itself as a major centre for
astronomical observations. In 1933 it
was moved to its present location at
Castel Gandolfo, 32 kilometres (20
miles) outside of Rome, to escape the
glare of Rome’s lights.
Of course, that reprieve was only temporary – in 1981 the Observatory decided to build a second site on Mount
Graham in Arizona. Construction of the
1.8-metre Vatican Advanced Technology Telescope began in 1989 and
was finished in 1993. The telescope is
used to perform a wide variety of research programs, including the spectral
classification of peculiar stars and the
study of mass-exchanging binaries.
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Fall 2006: No. 14
Feature
Astrophotography with
Schmidt-Newtonian
Telescopes
by Alen Koebel
Read a book or a magazine article
about astrophotography today and you
will likely reach the conclusion that all
serious deep-sky astrophotographers use
refractors. And not just any refractors,
only apochromats, the most expensive
kind. You might well then decide that
such a pursuit is something you’ll put
off until after you win the lottery.
Of course, not all apochromatic
refractors (APOs for short) are prohibitively expensive. Recent models from
China employing FPL-53 extra-low
dispersion (ED) glass, such as the SkyWatcher 80ED are every bit as good as
Flourite doublet models but sell for a
fraction of the price.
Currently, however, these Chinese
APOs are not available with reasonably
fast focal ratios in apertures larger than
80mm. With astrophotography it’s all
about image scale and photographic
speed. An 80mm APO with a typical
focal length of 600mm is well suited to
large celestial objects like the North
America nebula and the Pleiades, but
not a good choice for smaller objects,
such as the vast majority of galaxies.
With a typical focal ratio of f/7.5, an
80mm APO is also not photographically
very fast. That can be solved by using a
focal reducer – a garden-variety 0.63x
reducer meant for SCTs reportedly
works very well – but that reduces the
focal length by the same factor; in this
case to 378mm (600mm x 0.63).
That’s actually not as bad as it sounds
if you’re using a digital SLR. The
6
NGC 6992, the eastern arc of the Veil Nebula in Cygnus. This is a 30-minute exposure on
Fujichrome Provia 400F through a Meade LXD55 SN-6 6-inch f/5 Schmidt-Newtonian OTA,
processed by Brady Johnson. All images in the article are by the author except where noted.
sensors of popular models like the
Canon EOS 350D (Rebel XT) and 20Da
have only 40% of the area of 35mm film
(so-called “APS” size). So, a focal
length of 378mm is equivalent to using
600mm with 35mm film.
But what if you’re photographing
small objects or you’re one of those
Luddites who still uses film? In either
case, you’re going to want a longer focal
length. Can you afford a photographically fast telescope – meaning a focal
ratio of f/5 or lower – with a focal
length over 600mm? At f/5 that would
mean an aperture in excess of 120mm
(4.7 inches). An APO that large, even
one made in China, is going to be expensive. A few innovative refractor
designs have been announced that promise to achieve near-APO performance
without resorting to exotic glass, and
hence should be cheaper, but so far none
have reached the market. Fortunately,
there are a few alternatives.
Off the Beaten Path
M42, the Great Nebula in Orion, a 10-minute
exposure on Kodak Elite Chrome 200 through a
Meade MTS-SN6 6-inch f/5 Schmidt-Newtonian.
One obvious alternative is the Newtonian reflector. While the APO has the
highest cost per inch of aperture among
telescope designs, the Newtonian has
the lowest. Suitable models with fast
focal ratios of f/4 and f/5, and with apertures from six to ten inches, are very
affordable and can be mounted adequately on popular and reasonably priced German equatorial mounts.
The main problem with a Newtonian
is an optical aberration called coma,
which can severely compromise the
quality of star images away from the
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sometimes available (e.g., Astromart).
This is not the first time Meade has
produced Schmidt-Newtonians. They
offered 6-inch f/5 and 8-inch f/4 models
in the late 80’s and early 90’s under the
MTS (Modular Telescope System)
name. These differed from today’s models primarily by using a helical focuser
and an unusual fork-arm equatorial
mount. Meade also produced for a short
time a very fast 6-inch f/3.6 OTA called
the Comet Tracker in anticipation of the
return of Comet Halley in 1986. This
was Meade’s answer to Celestron’s
popular Comet Catcher, a 5.5-inch
Schmidt-Newtonian, also f/3.6.
Choose your Weapon
NGC 1449, the California Nebula. This is a 30-minute exposure on Kodak Elite Chrome 200
through a Meade MTS-SN6 6-inch f/5 Schmidt-Newtonian.
optical center, spreading them out into
fan-like shapes. The faster the focal
ratio, the worse the coma.
An optical accessory called a coma
corrector placed in the focuser is one
solution. But few of these provide full
illumination for a 35mm-sized frame
(they might work fine, however, with an
APS-sized sensor).
Another solution is to use a catadioptric variant of the Newtonian design that
employs a corrector plate or lens at the
tube opening. Two designs have been
commercialized: the Maksutov-Newtonian and the Schmidt-Newtonian.
Most Maksutov-Newtonians are optimized for visual observing. Their very
small secondary mirrors maximize contrast, which is important for planetary
viewing in particular, but can’t fully
illuminate a 35mm-sized frame. Another
consequence is that the focal plane ends
up very close to the tube, leaving insufficient room for a camera body.
The only easily available MaksutovNewtonian designed expressly for astrophotography is the 7-inch f/4 MN74
from Intes. It represents very good value
Fall 2006: No. 14
for its aperture at $2,700 US for the
optical tube assembly (OTA), but it is
still not what many would consider inexpensive. The Intes MN55 (5.5-inch
f/5.5) and MN65 (6.5-inch f/5.5) OTAs
are more affordable at $1,200 US and
$1,750 US respectively but are a compromise photo-visual design with a
secondary that is still a bit undersized.
That leaves the Schmidt-Newtonian.
While never a popular design, a few
manufacturers over the years have considered it worthy. The most notable is
Meade, which currently makes three
OTA’s – 6-inch f/5, 8-inch f/4 and 10inch f/4 – each offered on a decent German equatorial mount. Current models
bare the LXD75 name, superseding the
older but optically-identical LXD55
versions.
The LXD75 ‘scopes are very affordable, with current list prices of $1,000,
$1,200 and $1,400 US for the 6-inch, 8inch and 10-inch models respectively.
The OTAs are not officially available
from Meade without the mount, but
dealers sometimes sell them alone for
about $300 US less. Used OTAs are also
All of the aforementioned SchmidtNewtonian telescopes are well suited for
astrophotography. For one thing they
have fast focal ratios, which keeps
Meade’s LXD75 SN-6 6-inch f/5 SchmidtNewtonian. Image from Meade web site.
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Fall 2006: No. 14
`
The author’s old MTS-SN6 telescope on its
fork-arm mount. While it looks a bit unusual,
it’s surprisingly stable for astrophotography.
exposures tolerably short. For another
their secondary mirrors are relatively
large, which improves the uniformity of
illumination from the center to the corner of the image frame. Finally, all but
the Celestron Comet Catcher employ 2inch focusers with enough back focus to
accommodate a camera body. (The
Celestron does have enough back focus,
but the focuser is only 1-1/4 inch, which
will vignette a 35mm-sized frame.)
When choosing between these telescopes, the faster f/3.6 and f/4 models
are very tempting because they minimize exposure times. However, they
also exhibit a noticeable amount of
coma near the edges of the frame. (In
fact, a popular nickname for the Celestron ‘scope is Coma Catcher!)
That may be surprising. Doesn’t the
Schmidt corrector plate eliminate coma?
It would if it were placed at the radius of
curvature of the spherical primary mirror, as it is in a Schmidt camera. But in a
8
Schmidt-Newtonian it is placed at less
than half that distance. This results in a
tube length about the same as a standard
Newtonian but at the cost of some coma,
roughly 55% that of the Newtonian. The
smaller the focal ratio the larger the
coma. Technically, the coma increases
in inverse proportion to the square of the
focal ratio, hence an f/4 telescope will
show much more coma than an f/5.
If one were trying to decide between
the Meade 8-inch f/4 and 6-inch f/5
models, which have similar focal lengths, lower coma would be one reason to
choose the smaller 6-inch.
Another reason would be weight. The
LXD75 German equatorial mount can
easily handle the 6-inch OTA’s six kilograms (13 lbs). But the 8-inch OTA’s 11
kg (24 lbs) is a bit too much for it photographically, although it would be fine
for visual use. The 8-inch OTA needs a
beefier mount, such as the Sky-Watcher
EQ6. The EQ6 might also carry the 14
kg (30 lbs) 10-inch OTA adequately,
although it is much closer to the
mount’s specified limit (about 18 kg or
40 lbs).
(e.g., a Barlow lens), then the optical
axis of the guidescope could shift with
respect to that of the imaging scope over
the course of the exposure. Known as
flexure, this condition will result in oblong stars or obvious trailing.
An OAG completely eliminates this
potential source of guiding error. It inserts between the camera body and the
focuser and contains a prism that intercepts light from the telescope just outside the imaging frame, redirecting it to
a guiding eyepiece or an autoguider.
Besides eliminating flexure, an OAG is
considerably lighter than a guide telescope, easing the load on the mount.
However, it is generally much harder
to find a guide star with an OAG, especially without adversely affecting the
framing of the target in the camera. And
some telescopes don’t have enough back
focus to accommodate an OAG. This is
unfortunately the case with Meade’s
LXD55 and LXD75 OTAs. Meade’s
older MTS OTAs, on the other hand,
have just enough back focus for certain
OAGs, including Lumicon’s 2-inch
Newtonian Easy-Guider.
Guiding Principles
One of the fundamental choices an
astrophotographer must make after
selecting a telescope is how to guide it
during a long exposure. There are two
common methods: a separate guide telescope mounted “piggyback” on the main
scope or an off-axis guider (OAG).
The first requires that the guidescope
be firmly mounted to the main ‘scope.
The mounting method should also at the
same time allow some degree of angular
adjustment so that a guide star can be
selected without moving the main telescope, which would compromise the
framing of the target in the camera.
If there is any play in the mounting,
or sag in either the tube assembly or the
accessories inserted into the focuser
The author’s current guiding set-up utilizing a
diagonal, a 2x Barlow, a 2.8x Barlow and a
14mm illuminated reticle eyepiece inserted
into an 80mm f/5 short-tube refractor.
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Mounting Concerns
Another difference between Meade’s
various Schmidt-Newtonian models that
affects astrophotography is the quality
of the mount sold with the OTA. The
LXD75 models come with a fairly good,
mid-sized German equatorial mount
with ball bearings in each axis. It tracks
relatively smoothly with reasonably low
periodic guiding error. Although it lacks
an autoguider port, the GoTo version
can reportedly be autoguided using a
webcam and a computer.
The older LXD55 mount is of lesser
quality and has a greater amount of periodic error. Its stock tripod is also much
less stable. On the whole, it’s not as well
suited to prime-focus astrophotography.
The suitability of the mount supplied
with the old MTS models is not as clear.
This unusual creation employs fork
arms riding on a German polar axis
attached to a short (24-inch) pier. While
it looks a bit awkward it’s actually quite
sturdy, at least when supporting the 6inch f/5 OTA.
The concerns come when you try to
guide it. Its periodic error is on the same
order as the LXD55 mount and since it
uses an AC drive system, it doesn’t
readily support an autoguider. However,
that doesn’t mean it can’t be used for
long exposures. You just need a lot of
patience and care. The author used a
slightly-modified MTS-SN6 (6-inch f/5)
for several years, manually guiding
dozens of exposures up to 45 minutes
long with good success.
That said, the better the mount the
easier the job. The author has since
switched to a 6-inch f/5 LXD55 OTA on
an auto-guidable Vixen GP-DX German
equatorial mount equipped with GoTo,
and doesn’t intend to switch back!
Performance Anxiety
What kind of results can you expect
with a Schmidt-Newtonian? Since this
Fall 2006: No. 14
A Lumicon 2-inch Newtonian Easy-Guider.
This off-axis guider works well with the old
Meade MTS-SN telescopes but the newer
LXD55 and LXD75 models don’t have quite
enough back focus to accommodate it.
article is proposing it as a more affordable alternative to a large apochromatic
refractor, let’s compare the theoretical
performance of the two designs.
First, because the secondary of a
Schmidt-Newtonian is supported by a
corrector plate, the images are free of
the diffraction spikes that are typical of
images from a plain Newtonian, which
are caused by the vanes that support the
latter’s secondary mirror. In this respect,
the images from a Schmidt-Newtonian
are very much like those from an APO
refractor.
It’s true that some people consider
diffraction spikes to be aesthetically
pleasing, even going so far as to artificially introduce them into refractor images by placing crossed dowels in front of
the objective. But there’s no denying
that they are artifacts.
Because of its correcting plate, a
Schmidt-Newtonian should show no
spherical aberration. It should also be
practically free of chromatic aberration.
Although the corrector is a refracting
element it is far too weak to introduce
RASC
any noticeable false color. With a
refractor, this level of colour performance requires the best (and most
expensive) apochromatic designs.
On the minus side the Schmidt-Newtonian’s secondary blocks some light
from hitting the primary, as is the case
with all Newtonians. This reduces the
effective aperture of the system and increases its effective focal ratio. For instance, the effective aperture of Meade’s
6-inch f/5, which has a 36% obstruction
by diameter, is 5.6 inches and its effective focal ratio is f/5.36. The central obstruction also reduces contrast and resolution. However, none of that seriously
affects the design’s performance for
deep-sky astrophotography.
Of greater importance are parameters
such as coma, astigmatism and field
curvature. As stated earlier, a SchmidtNewtonian does suffer from coma.
While it is noticeable at f/3.6 and f/4, it
is for all practical purposes not present
at f/5, its effects just visible in the
corners of a 35mm-sized frame.
A Schmidt-Newtonian also has some
astigmatism, about 30% that of a plain
Newtonian, for the same reason it has
coma. On the 6-inch f/5 models the
author has used the effect appears to be
minor. Even the smallest star images
look properly round and undistorted (on
correctly guided exposures!) except in
the very corners where coma is visible.
Some evidence of astigmatism might be
apparent in a DSLR image but even then
it shouldn’t be obvious unless you
enlarge the image (or zoom in on it)
greatly.
The Schmidt-Newtonian optical
design also has field curvature, about
40% that of a Newtonian. But it is not a
serious concern at f/5 if one is careful to
focus 1/3 of the way from the center
toward one of the short edges of the
frame. This is the point of best compromise for a spherically curved focal
9
PULSAR
Fall 2006: No. 14
Astrophotography with Schmidt-Newtonian Telescopes
surface, dividing the focus error as evenly as
possible across the image. The technique, of
course, also works at faster f-ratios but the
effects of the compromise – less than perfect
focus in the center and corners – is more
noticeable because the curvature is stronger.
Most refractors also suffer from field curvature, which is usually solved using an optional
field-flattening lens in the focal path. Such a
lens also typically reduces the focal length.
More Pudding, Please
It’s one thing to talk about performance but
the proof, as they say, is in the pudding. And
the “pudding,” so to speak, is the astrophotos
included with this article (and on the cover
page). They were taken by the author on slide
film, either Kodak Elite Chrome 200 or Fujichrome Provia 400F, through either a Meade
MTS-SN6 6-inch f/5 telescope on its original
mount, or an LXD55 6-inch f/5 OTA on a
Vixen GP-DX mount. The MTS-SN6 was
manually guided with an OAG; the LXD55
with a piggybacked 80mm f/5 refractor.
These photos are not being presented as the
best that can be achieved with a SchmidtNewtonian telescope. Use of an autoguider
and especially a filter-modified DSLR camera
rather than film would almost certainly have
produced superior results.
But would these photos have been any better
through an APO? That depends! Telescopes
are a bit like golf equipment. While better
Want to join our club?
To become a member please
call Anita Burns at
519-885-9740 or send e-mail to
[email protected]
continued…
clubs might lower your score, you have to learn
to swing them properly before they can help.
Similarly, a better telescope (and admittedly,
APO refractors are better) will help with astrophotography only after you’ve mastered all of
the essential techniques.
Even if an APO refractor is your ultimate
goal, you can learn the ropes on an inexpensive
Schmidt-Newtonian while you’re saving up. In
the process you might just find that it’s all the
telescope you really need!
K-W RASC Executive
President: Mike Burns
Past President: Brady Johnson
1st Vice: Dave Garner
2nd Vice: Peter Clarke
Secretary: Phil Lacasse
Treasurer: Anita Burns
The K-W Centre of the Royal
Astronomical Society of
Canada usually meets on the
second Friday of every month
excluding July and August.
Meetings are held in the
Science Building at Wilfrid
Laurier University in Waterloo,
Ontario. All residents of the
Milky Way are welcome but
must supply their own
transportation.
PULSAR Staff
Editor: Alen Koebel
Assistant Editor: Jeff Collinson
Layout Design: Brady Johnson
The Pelican Nebula (IC 5070), a 30-minute exposure on Kodak Elite Chrome 200 through a Meade
MTS-SN6 6-inch f/5 Schmidt-Newtonian, processed
by Brady Johnson.
In Coming Issues…
How to cure Telescope Addiction
World’s lightest 25-inch Dobsonian?
Barn-door adventures
and lots more…
RASC
K-W Centre
http://kw.rasc.ca
R O YAL A S T R O N O M I C AL S O C I E T Y O F C AN AD A
Other Editorial Contributors:
Darryl Archer, Kate Baker,
Doug Bates, Doug Bulgin,
Ralph Chou, Peter Clarke,
Duncan Class, Jim Failes,
Brian Hollander, John
Kulczycki, Phil Lacasse,
Bernd Mueller, George Peer,
Peter Pekurar, Brenda Purdy,
Brent Spencer, Tim
Spiegelberg, George
Tomesch, Maryanne Weiler
 2006 Kitchener-Waterloo
Centre of the Royal
Astronomical Society of
Canada. All rights reserved.