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photo:telescope_types

telescope types

summary

  • if photography through the telescope is your main aim, then you will be best off with a telescope that is optimised for this purpose and thus has fast f/ratio (f/5.6 or faster), minimal aberrations and a flat field (ie. same level of brightness throughout the field) and should be mounted on a high quality German equatorial mount such as the Losmandy or perhaps a LXD75, in descending order of excellence are:
    • the very expensive scopes:
      • very fast Schmidt cameras; R-C scopes
      • APO refractors (incl. expensive APO/ED camera lenses)
    • less expensive, optimised scopes:
      • Schmidt-Newtonian
      • non-APO refractors used with filters to minimise chromatic aberration
    • other scopes that can be used but tend to have more significant aberrations:
      • SCT, Maksutov - these will require focal reducers to get fast f/ratio with loss of flat field & increased aberrations
      • fast Newtonians - have coma aberration & poor flat field
  • for planetary & moon observation, any will do, but for best contrast, a good refractor may be best
  • for deep sky observation such as nebulae, a large aperture scope which can readily be transported to a dark sky site is best, such as a 10“ Newtonian perhaps on a Dobsonian mount if photography is not needed.
  • as a versatile, reasonably portable scope a LX200 8-10” SCT may be adequate to be used for most of the above purposes but is not the best choice for each individual purpose.

Refractor:

  • similar to a camera lens, expensive, unwieldy and limited to small apertures
  • very low maintenance but highest cost per inch of aperture
  • needs a star diagonal with its attendant problems:
    • loses 10% of light
    • must be accurately made to avoid decentration of the field on rotation
    • must be completely enclosed to avoid observer's breath, & this is undesirable for good definition
  • must use star diagonal to view objects near zenith
  • high f ratio limits usefulness for photography
  • have the best image contrast and are usually considered the best telescopes for planetary observing and binary stars
  • cannot be used for BVRI photometry as the lenses filter out some of the wavelengths

types of objective lenses:

  • achromatic refractors:
    • designed to minimise chromatic aberration but lacks complete achromatism
    • good quality 5“ on equat.mount $A2200, 6” $A2400
    • Meade LX55:
      • motor drives are very noisy & diagonal needs replacing but very good optics otherwise
  • apochromatic (APO) refractors:
    • very expensive, apochromatic lens to minimise chromatic aberration and spherical aberration
    • allows faster f/ratios suitable for astrophotography
    • offer the BEST resolution & seeing for any telescope of same size and are even better than Schmidt-Cassegrains of larger diameter!
    • 4“ APO refractor gives comparable views to a 6” MCT and a 8“ Newtonian
    • extraordinary dispersion (ED):
      • Megrez II 80mm f/6.25 refractor ED $A1100;
      • modern camera lenses:
        • eg. Olympus C8080 with TCON14D = 75mm diam f/3.5 and equiv. focal length 196mm
    • fluorite apochromatic:
      • extremely expensive, apochromatic lens to check aberrations almost completely
        • Megrez II 80mm f/6.25 refractor flourite APO $A2750;
        • Megrez 110mm f/6.5 flourite APO refractor $A7000
        • the “FC” series of fluorite doublets introduced in the 1980's were typically f/8 refractors (and, in one instance - the FC-100 N - an f/10).
        • the “FCT” series of flourite triplets introduced in late 1980's & early 1990's were f/5.6 to f/7
        • FS series:
          • the new “FS” series came out in the late 1990's, and the only real change from the FC was in the location of the fluorite element (in front rather than the rear.) Doublet focal ratios remained at f/8, and is still found in the FS-78 through the FS-152.
          • FS-60C 60mm diam f/5.6, compact & wide FOV
          • Sky90 90mm f/5.6 $US1850-2300
            • see pic of Rosette 
            • highly recommended; compact;
          • FS-102 102mm f/8 $US1900; 
            • Takahashi 106NS see fantastic pic of Rosette
          • FS-128 f/8 $US4500
            • Takahashi 130mm diam - see pic of M42 
        • Borg 76mm diam - see pic of M42
    • super apochromatic exotic glass:
      • Takahashi TOA-130 130mm f/7.7 triplet with much reduced spherical aberration

Newtonian reflector:

  • simplest & cheapest reflector design - lowest cost per aperture size ⇒ cheap to get good light-gathering power
  • no chromatic aberration as with cheaper refractors
  • uses a parabolic mirror with focus point set at right angles via a small 45deg. flat mirror
  • may have better on-axis performance than SCT's as smaller central obstruction from the mirror
    • the relatively chroma-free Newtonian reflector however, does have such a blockage, a blockage that prevents high quality on-axis light from entering the eyepiece. The effect of such a blockage (or obstruction) is similar to that noticed when stereo speakers are separated so widely that the music no longer “synthesizes” in the minds ear. The result is a type of neuroprocessing confusion that detracts from the quality of the experience and makes interpretation difficult.
    • The effect of a central obstruction also causes a purely physical problem. When light passes around any transitional boundary, it is slightly diffracted. As such, the waveform is disturbed and its ability to re-integrate itself is hampered. Effectively, photons that would normally come to a point (in an airy disk for example) are dispersed into the diffraction rings around that disk. As a result, clarity is lost due to the dispersion of photons into a larger area.
    • however, it is well established amongst the astronomical community that scopes with linear central obstruction percentages less than 20% the aperture of the scope are virtually indistinguishable from unobstructed scopes in terms of contrast and fine detail.
    • Although obstructed scopes reveal detail comparable to unobstructed scopes of roughly 2/3rds aperture, obstructed scopes of high quality frequently offer an advantage in terms of image scale. Thus a 6” MCT is able to operate at significantly higher magnification than a well-refined 4 inch refractor - even when turned on low contrast Jupiter cloudtops.
  • ideal size for amateurs: f ratio 7 to 8 with aperture as large as possible (max. useful for amateurs 30cm, most are 15-20cm)
  • large apertures such as 10“ Dobs are great where quantity of light is more important than quality - through such scopes galaxies and extended nebulosity gave better detail direct than a 6” MCT revealed on the very finest of nights employing every degree of averted sight feasible. These scopes often made obvious to the eye what was vaguely hinted at through 6 inch MCT.
  • cons:
    • most difficult to use, clumsy, need to rotate tube when moving from one part of sky to another
    • need to adjust optic collimation - even more tedious with faster scopes such as f/4
    • mirrors need to be re-aluminised after 10-20yrs (depending on atmospheric salt levels) but this is inexpensive and can be done locally
    • produces inverted image
    • mirrors, esp. fast parabolic,  are subject to off-axis coma aberration (where the edge of the field of view appears distended) - options to correct coma include:
      • Celestron/Baader MPCC:
        • focus needs to be racked out extra 10mm - maybe this helps when using off-axis guiders
        • does not change the focal length and gives nothing short but superb results on the scopes I have used it on
      • TeleVue Paracorr:
        • I use the Paracorr on an f4 reflector and find no trace of coma over the
          field of my CCD (about 14 mm).  Have not tested it with larger format
          sensors but suspect that it will be pretty reasonable, bearing in mind that
          asking any reflector less than f12 to cover a 35 mm format is pushing it.
          The Paracorr has a T-thread which will allow direct coupling to the camera.
          Spacing is critical and should be in the range of 55 mm to 59 mm, measured
          from the Paracorr T-thread shoulder to the focal plane.

          According to the literature, using an f6 reflector with the Paracorr will
          give you spot sizes below the diffraction limit out to a semi-diagonal of 20
          mm or so.  This means you should be able to cover a larger format sensor.

          Have a look at the Televue site, it's done quite well.  You'll see its
          performance at other f ratios also.
        • At the same time, the Paracorr is a 1.15x multiplier, so it turns an f/5 system into
          an f/5.75 system.  You lose a bit of “speed” and a bit of FOV.  
      • Lumicon LVP-CC
      • Schmidt-Newtonian, Mak-Newtonian & Wright telescope (ellipsoid) designs
    • reduction in image contrast due to the secondary mirror placement in the central light path 
    • regular maintenance of mirrors needed for peak performance  
    • larger reflectors are bulky - a fast 8“ is the limits of portability
    • often have limited inward focus travel and thus may be difficult to use off-axis guiders, hence possible solution is:
      • replace the stock focuser with the reasonably low-profile JMI NGF-DX3 (or the -DX2 or -DX1, depending on how much you want to spend)
      • get the Lumicon or Paracorr Coma Corrector
        • “I've not tested the Paracorr against the Lumicon but I have tested the Paracorr against standard Ross correctors (which may well be what the Lumicon is).  The Paracorr was clearly better.  It also boosts the effective focal length by 15% extracting the focal plane a little, which may mean that you will not need to buy a low-profile focuser.”
      • then get the Lumicon 2” Newtonian Easy Guider
    • long tube on equatorial mount thus:
      • requires heavy counter-weights for balancing
      • vibrates more in wind thus not as good for high magnification viewing
      • tube needs to be rotated within mount so eyepiece is in suitable position for eye when changing from viewing northern aspect as to southern aspect
      • more bulky to transport 
    • long tube on Dobsonian mount:
      • cheapest but must alter both altitude & azimuth to combat rotation of earth during viewing
      • bulky for transport

Schmidt-Newtonian reflector:

  • Newtonian design but uses a spherical mirror instead of a parabolic one to reduce off-axis coma aberration by ~40% and has a front corrective lens to correct for the flattening of the outer rim of the primary mirror and correct for spherical aberration.
  • optimised for photography and gives better images than a fast Newtonian (but images not as good as a f/8-f/10 Newtonian?)
  • 2003 prices:
    • Meade f/4: 6“ = $A1850; 8” = $A2350; 10“ = $A2700;
  • tend to be f/4-5 so better for prime focus astrophotography, although relative obstructive impact of the secondary mirror is more significant with short focal lengths and maximum visual magnification is decreased
  • cheaper and longer tube than Schmidt-Cassegrain
  • cons:
    • as for Newtonian, although shorter tubes (usually f/4-5 rather than f/8-10) (10” requires 3x10lb counterweights)
    • requires front corrective lens which is subject to dew and damage (see Schmidt-Cassegrain)
    • still has field curvature and off-axis astigmatism

Schmidt-Cassegrain reflector (SCTs):

  • parabolic reflector with focus point directed back through hole in main mirror, thus compact
  • pros:
    • probably the best all round telescope for the serious amateur who can afford one
    • easiest to use & most compact for the aperture
    • produces highest quality image as well as being an upright image
    • least vibration
    • relatively portable & maintenance free as long as looked after carefully
    • new computer-driven models can self-align using GPS & search for objects
  • cons:
    • requires corrective lens:
      • thus expensive, although usually these require the least maintenance
      • must not be cleaned other than air or distilled water wash without rags) as easily damaged
      • requires dew cap to minimise dew
    • usually f/10 thus limits usefulness for photography but can usually be increased to f/3.3 using special eyepiece adapter
    • mirrors need re-aluminising after 25-30yrs as enclosed tube helps them last longer (but usually need to be sent back to manufacturer ie. in USA, thus consider buying new one instead!)
    • must use star diagonal to view objects near zenith
    • may need to attach weights to counterbalance any attached camera
  • models:
    • Meade LX90 series:
    • Meade LX200 series:
      • fork mounted OTA's with autoguider port & better periodic error correction
      • 2003 prices:
        • 8“ = $A2995 on equatorial mount; $A3995 on fork mount on tripod; $A6830 for LX200;
        • 10” = $A8000 on fork mount;
    • Celestron:

Visac:

  • Vixen Sixth-Order Aspheric Cassegrain - a SCT with added elements for further optic correction
  • the VISAC optical system is composed of a concave primary mirror, a convex secondary mirror and a three element field corrector. Especially noteworthy is its sharpness and flatness at the edge, which far exceeds that of Schmidt-Cassegrain systems of the same aperture. It provides a pinpoint star image less than 15 microns, even at the edge of its 42mm image circle.
  • the open tube design eliminates the dew problem that is common with conventional Schmidt-Cassegrain systems.
  • not readily available, marketed by Vixen
  • examples:
    • Vixen VC200LDG ($US1600 OTA) 200mm diam. f/9 but opt. f/6.3 focal reducer for photography; 6kg;
      • Chris Venter of Malvern owns one - see a pic of M11
    • NB. the Vixen VMC200L is different, being a unique design that employs a meniscus corrector lens along with two high-precision spherical mirrors to help eliminate spherical aberration and yield a sharp image with no hint of false color.

Maksutov reflector (MCT):

  • similar to Schmidt-Cassegrain but convex secondary mirror
  • usually f/15 so good for planetary detail but usually not for astrophotography
  • 2003 prices: 6“ f/15 = $A2200; 7” = $8000?
  • Intes:
    • Intes MK69:
      • see pic of M17 at f/6
  • Meade ETX series:
    • plastic German equatorial motor driven mounts
    • 125 has problem with a large secondary mirror causing 40% obstruction, thus many feel the 105mm is better
    • Autostar buttons require too much pressure
    • future models will only be available in UHT coatings which add 20% light transmission
    • very portable and usable but not strong enough mounts for astrophotography

Ritchey-Chretien hyperbolic reflectors:

  • must be professionally made to exacting standards as complex hyperbolic primary & secondary mirrors
  • very expensive (cheapest is $A12,000)
  • no coma aberrations
  • flat focal plane
  • the best design for robotic installations
  • examples: Hubble Space Telescope, AustroAnglo Observatory;
  • see pics of  NGC681NGC6888 , HorseheadJupiter, Saturn 
  • also Takahashi make a Baker RC wide field scope for CCD use only 10“ f/5 $US10,500
photo/telescope_types.txt · Last modified: 2014/05/18 09:53 by gary1