COMPREHENSIVE TELESCOPES DESIGNS GUIDE

Posted by Daniel Amado on

The types of telescopes are basically 3, refractors, reflectors, and compound telescopes. Refractor telescopes are comprised by lenses, reflectors have mirrors and compound have both. There is no optical design that is perfect, so each design is more suitable for the specific astronomical subjects that are going to be observed, photographed, or studied.

 

Glossary of terms used to describe a telescope optical features and capabilities:

 

Aperture

 

Size of the telescope’s objective generally referenced by its diameter’s length. A bigger aperture will provide a greater light gathering capability and higher resolution.

 

Bigger Aperture = better visibility of smaller and fainter details

 

Focal length

 

Distance between the objective and the focal plane where the concentrated light rays converge projecting a focused image. The focal length alters the magnification and the field of view of a telescope.

Shorter focal length = wider field of view and lower magnification

Longer focal length = narrower field of view and higher magnification

 

Focal ratio

 

 

Is the proportional relation between the focal length and the aperture of a telescope. Calculated dividing the focal length by the diameter of the objective. The resulting number determines the photographic speed. The higher the number the slower, and the lower the number the faster. Also known as “f/number” or “f/stop”

 

To collect the same amount of detail in photography:

 

Lower f/number = Shorter exposure

Higher f/number = Longer exposure

 

The focal length of a telescope can be modified with a focal reducer to shorten it, and a focal extender or barlow to enlarge it. By reducing the focal length, it will also change the focal ratio to a lower number. Increasing the focal length will also raise the focal ratio number.

 

On-Axis: The exact center of the field of view.

Off-Axis: out of the center of the field of view.

 

Diffraction-limited optics

Image quality is mainly affected by the original wave properties of light and the residual wave error induced by the optics aberrations is one-quarter of wavelength of light or smaller.

 

Optical aberrations

Are figure imperfections on the optics that distort the original wave patterns of incoming light to the objective. The most common are spherical aberration and astigmatism.

 

Spherical Aberration

In this aberration the converging light rays from the center and out the center of the objective do not focus at the same distance over the center line (optical axis) of the focal plane, resulting in multi-focal points distances between the objective and the focal plane along the optical axis blurring the projected image.

 

Astigmatism

When the lens or mirror is not perfectly round with a figure tending toward an oval shape that is not rotationally symmetrical, it makes the stars appear to have a “cross like” shape. Astigmatism aberration impacts the image sharpness.

Spherical and astigmatism aberrations affect the entire image field, from the exact center to the very edge.

 

Field curvature

Is the natural spherical curve produced by the projected beam of light due to the optical surface geometry of the lens or mirrors. In prime focus to an image sensor, the light rays projected from out of the center of the objective (off-axis) will come into focus before reaching the sensor’s surface. This is not an issue when the focal plane is a curved or spherical surface, such as the human eye. For this reason, field-flattener corrector lenses are used in astrophotography to produce flat-field images.

 

Astrograph: Telescope design optimized or dedicated for astrophotography.

 

 

Telescope’s usage purpose

 

For observations of bright objects such as planets or the moon, whether from an area with light pollution or without pollution, a refractor telescope with 3” to 4” inches of aperture’s diameter or a reflector or compound telescope of 4.5” to 6” inches of aperture is more than enough for observing planets, the moon, double stars, or open stars clusters.

The observation of bright objects such as Jupiter, Saturn or the Moon, is not affected at all by the pollution produced by streetlights in cities with dense population. Short focal length refractors and small Newtonian telescopes have a wide field of view suitable for bright deep space objects observations at low magnifications.

 

For faint deep sky objects observations, Newtonian reflectors, or compound telescopes with apertures of 8” inches and bigger are recommended, especially when used in locations with little or no light pollution. These last-mentioned telescopes with big apertures also offer spectacular and highly detailed views of the planets and the moon.

 

Regarding astrophotography, apochromatic refractors are usually one of the best choices for wide field imaging of the night sky. Schmidt-Cassegrains and other Cassegrain variants telescopes are suited for deep field imaging of distant galaxies, nebulae, and planetary astrophotography as well. Some apochromatic refractors with Petzval optical design like Takahashi FSQ-106 and the Corrected Dall-Kirkham telescopes produce a large corrected flat-field image circle that can illuminate completely a medium format sensor without vignetting.  Ritchey-Chretien telescopes are the most suited for scientific astronomical research.

Lens elements on refractors telescopes are commonly made of borosilicate glass. Mirrors on reflectors and compound telescopes are made of borosilicate, soda lime or pyrex glass with an aluminum layer as the reflective material covered by silicon dioxide for its protection and durability.

Carbon fiber optical tubes have a reduced thermal contraction/expansion with temperature variations minimizing focal point position changes.

 

The most common telescopes designs commercially available today are described in depth below.

 

Achromatic refractor

 

 

The achromatic refractor telescope has two glass or lens elements called crown and flint, that refracts or bends the light converging its rays to project an image at a focal plane, enlarging or magnifying any object’s size visually like a magnifying glass for reading or crafting will do. Refractor telescopes are the best known historically since Galileo Galilei used the single element refractor for his study of the moon, the planet Jupiter, and solar observations. They are commercially available from 50mm diameter to 150mm (2.4 to 6 inches). A refractor telescope is the icon of what most people know as a telescope and what it should look like. They have good a correction for chromatic aberration in telescopes with focal ratio from f/12 to f/15. This aberration consists of blue/purple light wavelength not converging at the same focal plane as red and green light wavelengths resulting in blue hues out of focus noticeable around bright objects like the Moon, Jupiter, Venus and bright stars. Known brand names makers of achromat refractors are Explore Scientific, Celestron, Meade, Vixen and Orion. They are available in apertures from 50mm to 150mm (2” to 6”)

 

Apochromatic refractor

 

 

 

They offer significantly better correction of chromatic aberration than achromatic refractors due to an extra-low dispersion glass element, traditionally composed of calcium fluoride, known as “fluorite”. The substantial color correction is done by reducing the difference in refractive index of blue wavelength spectrum related to red and green wavelength spectrums focusing on the same focal plane. Blue or purple halos on bright objects such as the moon or planets are not noticeable even in telescopes with medium or fast focal ratio (f/5 to f/7). They can have 2 elements (doublet) or 3 elements (triplet); the best apochromatic telescopes typically have three elements with one extra-low dispersion glass element sandwiched between the other two glass elements. Known apochromatic refractors manufacturers are Explore Scientific, Meade, Stellarvue, Sky-Watcher, Takahashi, William Optics, etc. Typically available in apertures from 50mm to 165mm. (2” to 6.5”)

Apochromatic refractors deliver the best quality views with the highest contrast than any other telescope design.

 

Petzval Refractor design

Is a quadruplet refractor astrograph with two front elements group and two rear elements group. When one or two of the optical elements is made of fluorite or ED glass, the overall color correction achieves full apochromatic performance.  It is highly corrected to reduce spherical and astigmatism aberrations with a fast focal ratio (typically around f/5) producing a large image circle with flattened field curvature. The Takahashi FSQ-106 is the most known top-notch fast refractor astrograph with this optical design. The William Optics Red Cat has also the Petzval lens configuration.

 

 

Solar Refractor Telescope

 

Solar refractor telescopes are achromatic refractors dedicated to observe and photograph the sun, which may have one or two etalon solar filters built into the optical tube, one in front of the objective and one before the focal plane. Etalon is a filter made of one or two flat crystals with 2 reflective surfaces that deflect a significant percentage of the light spectrum. They are designed to isolate and transmit a narrow bandwidth corresponding to a specific wavelength for the emission of hydrogen alpha. With this telescope is possible to see sunspots, prominences, filaments, granulations and flares. They are mainly made by Meade Coronado, Lunt and DayStar brands.

 

Pros:

 

  • Unobstructed light path for higher contrast views than reflectors and compound telescopes
  • Round stars with no diffraction spikes compared to Newtonians or other Cassegrain reflectors
  • Best telescope design for lunar, planetary and double star observations
  • Well-suited for terrestrial observations as a spotting scope
  • Apochromatic refractors deliver the best quality views with the highest contrast
  • Excellent for wide field observations and astrophotography
  • Practically never will require optics collimation (on collimatable models only) if the optical tube has been severely dropped or bumped
  • Dedicated reducers or field flatteners are usually available from the same telescope’s manufacturer or 3rd party brands. Generally suitable for full frame imaging sensors.
  • Refractors’ lenses do not require recoating like reflectors’ mirrors usually do after 10 years
  • No mirror shift/flop issues
  • Better choice than Newtonian telescopes for large color cameras or monochrome imaging with filter wheels because the focuser is installed on the rear end of the optical tube with better balance

 

Cons:

  • Highest cost per inch of aperture among all telescope’s designs
  • Due to the most expensive optics design and difficulty to manufacture, refractors are limited in apertures usually up to 6 inches
  • Because the aperture limitation, are less appropriate for faint deep sky objects observations like far away galaxies and nebulae
  • Longer and heavier optical tubes than Newtonians and Schmidt-Cassegrain telescopes of the same aperture size
  • Dew is prone to form over the objective lens in humid environments. Heaters might be required to reduce or eliminate condensation
  • Tube current issues on significant temperature changes with bigger apertures. Takes longer to reach thermal equilibrium (cool down) than newtonian telescopes or other open tube reflectors due to the closed tube design
  • Uncomfortable low level eyepiece viewing position when pointing straight up with long focal length refractors
  • With focal lengths longer than 1000mm, the optical tube might require a tripod pier extension to avoid movements range limitation with tripod legs
  • Solar dedicated refractors telescopes can be used only to see and photograph the sun
  • Short tube refractors might require risers blocks to avoid the camera or imaging train obstruction with the optical tube dovetail

 

Newtonian reflector

 

 

 

Invented by Sir Isaac Newton in 1668, the Father of the theory of gravity and celestial mechanics, it consists of a concave (shaped inwards) primary mirror of spherical or parabolic figure that reflects the light magnifying, enlarging or “Zooming in” the image like those magnifying mirrors that women use for face cleansing and make up. The light rays reflected from the primary mirror converge at a focal plane projecting an image through the reflection of another flat secondary mirror placed in the path of light being held by 3 or 4 spider vanes structure. This secondary mirror directs the light at a 90 ° angle towards the focusing tube where an eyepiece for observation or camera for astrophotography is attached. Available between 3 to 12 inches in aperture’s diameter (76mm to 305mm) to be used with Alt-Az mounts (small apertures) and equatorial mounts (up to 12” f/4 astrographs). Newtonian telescopes with Dobsonian mounts are commercially between 4.5 to 25 inches in aperture’s diameter (114mm to 635mm). Common brand names of Newtonian telescopes are Celestron, Meade, Orion and Sky-Watcher.

 

 

Pros:

  • The best bang for the buck with the lowest cost per inch of aperture
  • Significantly better for faint deep sky objects observations than refractors telescopes because the bigger apertures availability
  • Absent color aberration (Zero Chromatic aberration without relay lens or correctors)
  • Somewhat Shorter optical tube than refractors telescopes
  • Faster cool down with open optical tube design compared to refractors and compound telescopes
  • Significantly reduced dew issues because there are no glass elements at the optical tube opening
  • Wider field of view than Schmidt-Cassegrain and other Cassegrain variant telescopes
  • With usually fast focal ratios (f/4 to f/6) a focal reducer is not necessary to shorten exposures’ length for deep sky astrophotography
  • F/4 imaging Newtonians are the best value for astrophotography. With a coma corrector are suitable for full frame imaging sensors
  • Being fast telescopes, are also an excellent value for EAA (Video Astronomy)
  • Reduced mirror shift compared to Schmidt-Cassegrain and Maksutov-Cassegrain telescopes because the primary mirror is fixed to the rear cell of the optical tube
  • Available in carbon fiber truss tubes open optical tube structure, for even faster cool down and reduced thermal contraction/expansion with temperature variations minimizing focal plane position changes

 

 

 

Cons:

  • Contrast reduction due to the objective’s central obstruction caused by the secondary mirror
  • Diffraction spikes effect visible on stars caused by the secondary mirror spider vanes structure
  • Newtonian telescopes require frequent collimation of the mirrors if they are often transported to dark skies observing locations. When shipped, most of the time arrives with the mirrors out of collimation.
  • Subject to coma aberration where stars out of the center towards the edge of the field of view have a “comet like” shape. This aberration is more noticeable with f/6 and faster focal ratios
  • Usually not suitable for terrestrial observations due to the inverted upside down and backwards image
  • Uncomfortable low level eyepiece viewing position when pointing lower towards the horizon with dobsonian mounts
  • Newtonian telescopes bigger than 12” or 16” inch of aperture will require a ladder to observe through the eyepiece when pointing straight up
  • Significantly longer optical tubes than Schmidt-Cassegrain and other Cassegrain variant telescopes.
  • Open optical tube design exposes the mirrors to dust and moisture
  • Reflectivity ratio reduced after 10 years. Eventually after that time might require mirrors recoating service.
  • Requires a coma corrector for a flat field image circle in deep sky astrophotography
  • 3X to 5X focal extender required for planetary imaging
  • Optical tube unbalance issues when imaging with a large camera or long imaging trains. May require additional counterweights for the optical tube to properly balance the mount’s Declination axis

 

Maksutov-Newtonian

 

This telescope is basically a Newtonian reflector, incorporating the concave maksutov corrector lens which holds the secondary mirror. It typically has a fast f/5 focal ratio and offers a wide, substantially corrected field of view and delivers high contrast in planetary and double star observations quite similar to an apochromatic refractor telescope. For Astrophotography, a coma corrector is not mandatory to flatten the image circle because the Maksutov corrector lens already reduces coma and spherical aberrations. Due to the manufacture difficulty of the Maksutov corrector lens, Maksutov-Newtonians are generally available only in 6 or 7.5 inches (150mm or 190mm) of aperture diameter from Sky-Watcher, Explore Scientific and Orion.

 

Pros:

  • Mimics the apochromatic refractor views with excellent contrast on lunar and planetary observations for a fraction of the cost
  • Corrected wide rich-field ideal for observations of large deep space objects and open star clusters
  • Reduced spherical and coma aberrations. No coma corrector needed for deep space astrophotography
  • Like refractors, stars look round with no diffraction spikes compared to Newtonians or other Cassegrain reflectors telescopes
  • Closed tube design protects the telescope’s mirrors against dust and moisture

 

 

 

Cons:

  • Maksutov-Newtonian telescopes are limited in apertures availability up to 7.5 inches
  • More expensive than Newtonian telescopes
  • Dew is prone to form over the corrector lens in humid environments. Dew shield and heaters are required to reduce or eliminate condensation
  • Tube current issues on significant temperature changes. Takes longer to reach thermal equilibrium (cool down) than newtonian telescopes or other open tube reflectors due to the closed tube design

 

 

 

 

Cassegrain based telescopes designs

 

 

 

The original Cassegrain reflector design was attributed to Laurent Cassegrain of France in year 1672. The Cassegrain based telescopes design have a concave (shaped inwards) primary mirror (that “Zooms in” or magnify the image) converging the light rays with a focal ratio of f/2 to f/2.5 and a convex (shaped outwards) secondary mirror with a focal ratio of f/4 to f/5. The secondary mirror “zooms out” the image diverging the light rays to increase the telescope’s effective focal length just like cars’ side mirrors with the “Objects in mirror are closer than they appear” warning do. Both primary and secondary mirrors are squared facing each other frontally. The secondary mirror reflects the concentrated light from the primary mirror perpendicularly through a hole in the same primary mirror and the focal plane is behind of it.

 

Cassegrain based telescopes share these common advantages and disadvantages:

Pros:

  • Very compact optical tube due to the “folded” light path reflection design
  • Having a long focal length are excellent for lunar, planetary and double stars observations and astrophotography with large angular size
  • Well suited for deep field observations and astrophotography of small faint objects like distant galaxies and nebulae because the big apertures availability and the long focal length
  • Lower cost per inch of aperture than refractor telescopes
  • Comfortable eyepiece level observing position when pointing at any direction (straight-up or towards the horizon)
  • Better choice than Newtonian telescopes for imaging with large color cameras or monochrome cameras with filter wheels because the focuser is installed on the rear end of the optical tube with better balance

Cons:

  • Contrast reduction due to the objective’s central obstruction caused by the secondary mirror
  • Narrower field of view than Newtonians and refractors telescopes because the longer focal length
  • With generally slow focal ratios, a focal reducer is required to shorten the astrophotography exposures’ length
  • Higher cost per inch of aperture than Newtonian telescopes because the secondary curved mirror is more difficult to figure than a flat mirror
  • Might require occasional or frequent collimation of the mirrors if they are often transported to dark skies observing locations
  • Reflectivity ratio reduced after 10 years. Eventually after that time might require mirrors recoating service.

 

 

Schmidt-Cassegrain

 

They are comprised by a corrector plate and mirrors. The most popular compound telescopes are the Schmidt-Cassegrain design, a variant of the Cassegrain reflector telescope with a Schmidt corrector plate. They generally have a spherical or parabolic primary mirror and a spherical secondary mirror. The corrector plate has an aspherical figure and also holds the secondary mirror. The focal ratio of a Schmidt-Cassegrain telescope is typically f/10, and its corrector plate reduces spherical aberration in green light, which is the most noticeable wavelength spectrum for the human eyes’ sensitivity. These telescopes are typically available from 5 to 14 inches apertures from Celestron and Meade brands.

 

Pros:

  • Most versatile design as 3 in 1 telescope with Hyperstar (f/2) for widefield astrophotography, f/6.3-f/7 for deep space observations and astrophotography and f/10 for terrestrial, planetary/deep space observations and astrophotography
  • More accessories available for observations and astrophotography than any other telescope design
  • Like refractors, stars look round with no diffraction spikes compared to Newtonian or other Cassegrain reflectors telescopes
  • Higher-end models have built-in correctors for a flat coma-free field of view suitable for full frame sensors
  • Dedicated reducers are available from the same telescope’s manufacturer
  • Closed tube design protects the telescope’s mirrors against dust and moisture

 

 

Cons:

  • Mirror shift or flop because focus is achieved by moving the primary mirror
  • Dew is prone to form over the corrector plate glass in humid environments. Dew shield and heaters are required to reduce or eliminate condensation
  • Tube current issues on significant temperature changes. Takes longer to reach thermal equilibrium (cool down) than newtonian telescopes or other open tube reflectors due to the closed tube design
  • Standard Schmidt-Cassegrains exhibit coma aberration on prime focus imaging and the projected image circle is not suitable for full frame sensor cameras

 

 

Maksutov-Cassegrain

 

 

 

Another variant of the Cassegrain reflector is the Maksutov-Cassegrain. It incorporates the concave, negative figured spherical corrector plate that corrects spherical and coma aberrations, and its focal ratio is typically f/12 or f/15. In the Maksutov-Cassegrain telescope, the secondary convex mirror is just an aluminized spot in the center of the corrector plate. The Maksutov corrector plate is difficult to manufacture in big sizes. These telescopes are available typically from 3.5 to 7 inches apertures.

 

Pros:

  • Most compact optical tube among all telescope designs. Best for backpack portability.
  • Well-suited for terrestrial observations as a spotting scope
  • Like refractors, stars look round with no diffraction spikes compared to Newtonian or other Cassegrain reflectors telescopes without a corrector plate
  • Are mostly collimation-free with the optics elements aligned at the factory
  • Closed tube design protects the telescope’s mirrors against dust and moisture

 

 

Cons:

  • Maksutov-Cassegrain telescopes are limited in apertures availability up to 7 inches
  • Mirror shift or flop because focus is achieved by moving the primary mirror
  • Dew is prone to form over the corrector lens in humid environments. Dew shield and heaters are required to reduce or eliminate condensation
  • Tube current issues on significant temperature changes. Takes longer to reach thermal equilibrium (cool down) than newtonian telescopes or other open tube reflectors due to the closed tube design
  • Slower native focal ratio than Schmidt-Cassegrain telescopes
  • No dedicated reducers available from the same telescope’s manufacturer
  • The projected image circle at prime focus is not suitable for full frame sensor cameras

 

 

 

 

 

Ritchey-Chretien Telescope

 

It was developed by astronomers George Willis Ritchey and Henri Chrétien in the early 1910s. This design is a variant of the Cassegrain reflector telescope with both primary and secondary hyperbolic mirrors with usually f/8 or f/9 native focal ratios. It is highly corrected for spherical aberration and coma. The Ritchey–Chrétien telescope is the most chosen optical design by professional observatories for astronomical scientific research. The Hubble and Spitzer Space telescopes are configured with the Ritchey-Chretien optical design. Commercially available in apertures from 6-inch to 24-inch.

 

Pros:

  • Excellent Astrophotography performance with a coma-free field of view appropriate for full frame sensor cameras
  • Suitable for Astrometry, photometry and spectroscopy
  • Faster native focal ratio than typical f/10 Schmidt-Cassegrain telescope
  • Zero chromatic aberration without any refractive element like reducer/corrector lenses
  • Most accurate reading from ultraviolet to infrared light wavelength’s spectrum research on a pure mirror to monochrome sensor imaging configuration
  • Increased infrared sensitivity on Ritchey-Chretien telescopes’ mirrors with golden coatings
  • Significantly reduced dew issues because there are no glass elements at the optical tube opening
  • Reduced mirror shift compared to Schmidt-Cassegrain and Maksutov-Cassegrain telescopes because the primary mirror is fixed to the rear cell of the optical tube
  • Faster cool down with open optical tube design compared to refractors and compound telescopes
  • Available in carbon fiber truss tubes open optical tube structure, for even faster cool down and reduced thermal contraction/expansion with temperature variations minimizing focal plane position changes.

 

 

Cons:

  • Less suitable for observations due to the large central obstruction (over 45% by diameter)
  • Heavier than Schmidt-Cassegrain telescopes because the larger secondary mirror size and support structure. Also, the external focuser sums additional weight.
  • Demands perfect or near perfect collimation. A slight misalignment of the mirrors will cause a significant impact in the optical performance because the tight tolerance of the secondary mirror optical axis
  • Diffraction spikes effect visible on stars caused by the secondary mirror spider vanes structure
  • Field curvature of image circle is not flattened because the lack of built-in field flattener lens
  • No dedicated reducers or field flatteners available from the same telescope’s manufacturer
  • Open optical tube design exposes the mirrors to dust and moisture

 

 

 

 

Corrected Dall-Kirkham (CDK) Telescope

 

The CDK telescope has an elliptical primary mirror, a spherical secondary mirror, and corrector lens group near the focal plane. This optical design configuration delivers an image free of coma and off-axis astigmatism with a flat field. The corrected field is considerably larger than the Ritchey-Chretien telescope without a corrector lens. Native focal ratio is usually over f/7 to f/8. Known telescopes with this optical design are Hubble Optics CDK, Planewave and Takahashi Mewlon. Available in apertures from 10-inch to 1 meter.

 

Pros:

  • Excels as Photo-Visual instrument for both observations and astrophotography performance
  • Optical design produces a large 50mm to 70mm image circle corrected for off axis aberrations with flattened field curvature suitable for astrophotography with medium format or larger sensors
  • Forgiving collimation tolerance compared to RC telescopes because the spherical secondary mirror has no optical axis
  • Superior optical performance than Coma Corrected Schmidt-Cassegrain telescopes
  • Faster native focal ratio (over f/7) than Schmidt-Cassegrain telescopes
  • Better color accuracy than Schmidt or Maksutov-Cassegrain telescopes and Apochromatic refractors because the corrector lenses are placed near the focal plane instead of being placed at the opening of the optical tube like corrector plates.
  • Significantly reduced dew issues because there are no glass elements at the optical tube opening
  • Reduced mirror shift compared to Schmidt-Cassegrain and Maksutov-Cassegrain telescopes because the primary mirror is fixed to the rear cell of the optical tube
  • Some CDK telescopes focus by moving the secondary mirror, saving back focus space and optical tube weight.
  • Dedicated reducers are usually available from the same telescope’s manufacturer
  • Available in carbon fiber truss tubes open optical tube structure, for even faster cool down and reduced thermal contraction/expansion with temperature variations minimizing focal plane position changes.

 

 

Cons:

  • Less suitable than Ritchey-Chretien telescopes for Astrometry, photometry and spectroscopy because the corrector lens refractive element placed on the light path introduces spherochromatism (Chromatic Aberration)
  • Significantly more expensive than high-end Schmidt-Cassegrain telescopes
  • Diffraction spikes effect visible on stars caused by the secondary mirror spider vanes structure
  • Open optical tube design exposes the mirrors to dust and moisture

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