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* The Dream Astrograph primary mirrors are currently f3.5 to
f3.75. These are slower than typical Cassegrain primaries, which
are normally f2.75-f3. There are numerous advantages in this
one factor:
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- The optical figure of the Newtonian primary is easier to produce
because it is slower and therefore not as deep as faster mirrors. |
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- They are not as severe a figure, when compared to a hyperboloidal
(Ritchey-Chretien Cassegrains). |
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- The transitions between the zones of the primary are smoother
on the Newtonian, for both of the above reasons. This helps produce
a higher quality image at the focal plane. |
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* The secondary on a Newtonian is flat. The secondary on a Classical
(CC) and Ritchey-Chretien (RC) Cassegrain are both hyperboloidal.
Again, there are numerous advantages in this one factor:
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- Flats are easier to produce than convex hyperboloidal figures. |
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- The flat has no magnification like the Cassegrain secondary.
The magnification of the Cassegrain secondary means that any
errors in its figure and errors in its physical placement in
the optical system will greatly affect the final optical quality
of the system. |
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- Because the Newtonian's flat secondary is close to the focal
plane, the system derives most of it's quality from the primary
mirror, along with the excellent correction of the 3" coma
corrector. Extradinarily high optical quality (1/20 P-V wavefront
or better) in the secondary flat is not needed. |
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* For many of the above reasons mentioned, both the primary and
secondary mirrors are less expensive to produce. This means the
corrected Newtonian offers the largest "bang for the buck."
In fact current pricing (12/06) shows that Dream's 24" costs
$14,000 less than a 20" RC Cassegrain and will weigh roughly
half of what a 24" RC weighs. With the 1.8x barlow utilized
on the Dream Astrograph it comes within 300mm of the focal length
of the 20" f8.1 RC, yet the light gathering capability of
the 24" Dream Astrograph is roughly 1.5 times greater than
the 20" RC. You can see what an uncorrected RC looks like
here. You can see how the 1.8x
barlow compares to an uncorrected RC here.
Keep reading this page to learn why larger aperture is better. |
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* The Newtonian is extremely fast for a system focal ratio.
Cassegrains are typically f8 (RC) to f10 (CC) and slower. This
yields high magnification but much smaller fields of view. Most
large Cassegrains are being used with a focal reducer to give
wider fields. Not using them reduced puts tremendous performance
requirements on the mount and the user. The Newtonian starts
with a wide field. If a longer focal length is needed, increase
the aperture, say from a 16" to a 24", or the 1.8x
barlow can be utilized. |
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* The secondary on Dream Astrographs is smaller than a typical
f8-f10 Cassegrain. Don't be fooled by misinformation about central
obstruction. Some Cassegrain manufacturers list the size of the
secondary mirror, not the size of their baffling coming off the
secondary. It is the baffling on a Cassegrain that determines
the largest central obstruction or true obstruction, not the
secondary itself. Often they will quote the obstruction is 38%
but in reality, after fully baffled, the obstruction is 50% (16"
f8.4 RC using f3 primary and 14" of back focus). Some manufacturers
undersize this baffle in an attempt to lower their central obstruction
numbers and/or to decrease the pyschological affect of seeing
such a large obstruction. This is in detrimant to the optical
quality however as contrast is compromised by allowing starlight
to directly strike larger CCD chips off-axis. Because of the
optical design of the Newtonian, no baffling is coming off or
making the secondary larger than the glass itself. Baffling is
keenly important in any optical system but a Newtonian is baffled
differently. More light is striking the primary mirror because
of the smaller central obstruction. This does not mean 50% central
obstructions produce "bad" images. Among many things,
it means less area of the primary mirror is being utilized by
the Cassegrain design. This means that not only can a larger
aperture be purchased dollar for dollar (compared to Cassegrains)
but that the smaller central obstruction also reduces
exposure times (reaching a given saturation point). |
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- It should be mentioned that Dream fully evaluates and then
designs each size telescope that we offer. We do not cut
corners by using sub-sized secondaries that will cause vignetting.
This would be less expensive and yield a lower central obstruction
number, but it is not something we are interested in. A fully
optimized system is our goal. Quality over quantity and therefore
performance over sales. |
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- Vignetting can be caused by a whole list of items. The secondary
is but one of them. |
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- For Newtonian's: as you decrease the inner diameter (ID) of
your OTA structure, you can use a smaller in secondary size.
This is because the distance from the secondary to the focal
plane was reduced, thus allowing a slightly smaller secondary
size to be utilized. This is at the penalty of many things though.
Vignetting in the corners of the CCD chip due to the front ID
of the OTA now being too small for the field being imaged. A
second issue is that making the ID of the structure this small
leaves no room for proper baffling. So not only will vignetting
now occur but because baffling is inhibited, contrast suffers
as well. Overall quality is lowered by chasing a few percentage
points of central obstruction. It's not worth it and it is not
how a performance-based product is made. Dream will not compromise
the image in this way. |
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- The effective focal ratio of a 16" f8.4 RC with 50% obstruction
is f9.7. The effective focal ratio of a 16" f3.75 Dream
Astrograph with 34% central obstruction is f4.0. If the Dream
Astrograph had a 50% central obstruction, like an average RC,
its effective focal ratio would be f4.3. This is 1/5th of a stop
of difference and is yet one more reason the design is "faster." |
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* The physical diameter the airy disk of a specific wavelength
is based on it's focal ratio. The angular diameter of the airy
disk gets smaller as the aperture increases. A f8 (say a RC Cassegrain)
has an airy diameter of 10.8 microns. A f4 system has an airy
diameter of 5.4 microns. If we compare a 10" f8 with a 20"
f4 system, the two have the same focal length. However, the 20"
gathers light four times faster and has a (physical diameter)
airy disk half the size of the f8 system. Because the systems
are the same focal length, the field size for a given CCD chip
is identical. The 20" f4 system will get there faster and
produce a star that is smaller in size. The resolution is therefore
superior to the f8 system. |
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* For low contrast objects larger aperture truly does get the
job done better. Even in light polluted areas. It has to do with
MTF (Modulation Transfer Function). Again, this is beyond the
scope of this discussion but basic information involving this
should be mentioned. It is a common misconception that telescopes
are seeing limited. In other words, if your skies are 2-4 arc
seconds, the old rule was to not go larger than 12-14" in
aperture. However a MTF shows that 12" of aperture is just
simply not enough resolution for low contrast objects. Also consider
this: if you double your aperture, you can accept twice the background
sky brightness in the same exposure time and still have the same
S/N ratio. Please look at this astrophotographer's light
pollution situation. He uses a STL-11000 on his 16"
f3 paraboloidal primary with the same 3" coma corrector
Dream is offering. Yet here is a false
color image of the Veil Nebula from his location. This
page shows easy to understand information about airy disk,
aperture size, high and low contrast subjects and how they relate
to resolution. |
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* All of the above benefits mentioned increase throughput compared
to slower systems with smaller apertures. "Throughput"
is (generally defined) as the effeciency of a system or the amount
of time the telescope has its CCD chip exposing an object. A
system that is larger and faster will capture data much more
rapidly than a smaller aperture system. The wider fields
also help reduce the magnitude of mount induced errors, compared
to similar apertures with double the focal length. Again, increasing
throughput. Because of Dream's extensive use of carbon fiber,
the entire OTA has a lower mass. This means less stress on the
mount, which in turn means less down time. It also means the
OTA can be slewed at faster rates. Because of Dream's extensive
use of carbon fiber and sandwich core technology, combined with
the use of lighter weight glass types, both the structure and
the primary mirror equalize faster. Thermal issues are reduced,
thus making them much easier to deal with. Reduced down time
due to excessive flexure is yet another advantage. Throughput
can be increased by magnitudes compared to an average Cassegrain.
It also means that in certain Dream Astrograph sizes expensive
mounts are not a requirement. Thus reducing investment costs
even further. |
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* Optics are a very complex subject. The number of designs is
daunting and cannot be touched upon on this web site. However,
we should mention that other designs were evaulated against the
Newtonian, more than just CC's and RC's. Refractors and folded
optic systems that use full aperture (front) correctors are both
heavy, hard to manufacturer above ~20" in diameter and are
very expensive. A lens-based system, one who's lenses start an
optical system, inherently cause chromatic aberration issues
(differential alignment of light at different wavelengths). Additional
elements and/or exotic glass types have to be used in order to
take out the chromatic aberration the systems inherently puts
into them. A mirror-based system does not suffer from this (chromatic)
problem. Adding a corrector (lenses) does cause chromatic aberration
but because the Newtonian corrector Dream uses is employed near
the focal plane, it is easier to correct out this aberration. |
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* this size no longer offered |
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