What makes Dream's telescopes mechanically different?

There are a great many advantages to a corrected Newtonian (Dream Astrograph) over other optical designs. This page will go over some of the basic and more complex reasons this particular optical system is so powerful. After reading this page you should come to understand why Dream chose to marry this optical system with our state of the art carbon fiber structures.



* 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:

- The optical figure of the Newtonian primary is easier to produce because it is slower and therefore not as deep as faster mirrors.
- They are not as severe a figure, when compared to a hyperboloidal (Ritchey-Chretien Cassegrains).
- 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.
  * 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:

- Flats are easier to produce than convex hyperboloidal figures.
- 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.
- 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.
  * 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.
  * 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.




* 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).

- 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.
- Vignetting can be caused by a whole list of items. The secondary is but one of them.
- 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.
- 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."
  * 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.
  * 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.
  * 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.
  * 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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