8 inch Aperture Telescopes part 2 – Schmidt-Cassegrains

An article in the author’s Astronomy Digest http://www.ianmorison.com

In 1930 an Estonian optician, Bernhard Schmidt, invented a camera – called a Schmidt Camera – that was able to take wide field images of the sky.  The primary mirror was spherical and Schmidt realized that he could eliminate the spherical aberration by using a thin, very weakly curved aspheric lens (called a ‘Schmidt corrector plate’) at the front of the camera.  This aspheric lens has a complex curve that cancels out the mirror’s spherical aberration and also corrects for ‘coma’ at the edges of the field of view.  He employed a very clever method to make the corrector plate; which involved carefully warping a parallel glass plate under a partial vacuum into a slight sagging curve and then polishing the upper face flat.  After release of the vacuum, the lens springs back into the desired shape needed for the camera.  Both the spherical mirror and appropriate corrector plate are thus easy to make so making Schmidt-Cassegrains affordable.

In 1970 Tom Johnson, who had founded ‘Celestron Pacific’, introduced a ‘C8’ 8 inch diameter, 2,032 mm focal length, f/10 telescope, the first of a new line of Schmidt-Cassegrains that could be built at low cost.  These immediately became very popular and with very minor changes until the Edge HD series, all standard Celestron 8 inch telescopes made since then have been much the same.

Meade, founded by John Diebel in 1972, began to make Schmidt-Cassegrains as a competitor to Celestron in 1980.  Their telescopes have also proved very popular.  The company is now owned by the USA company, Orion Optics and Binoculars.

These Schmidt-Cassegrains are, perhaps, the most popular of all 8 inch telescopes – and with good reason as they can be used both visually or for imaging with a very wide range of celestial objects.  As in the Schmidt Camera, they use a spherical primary mirror whose aberrations are corrected with a corrector plate at the front of the telescope tube.  This, neatly, also supports the secondary mirror so there are no diffraction spikes.  Those made by both Meade and Celestron have a focal ratio of f/10 giving a focal length of 2,032 mm but can be used with a x0.63 focal reducer to give a wider field of view and shorter exposures when imaging. 

Schmidt-Cassegrain design features

As bought, the focusing of a Schmidt-Cassegrain is carried out by slight movement of the primary mirror within the telescope tube.  As the single speed focuser – which is a little coarse – is to one side of the mirror, focussing can cause what is called ‘image shift’; the image moves a little in the field of view as the telescope is focussed.  This is a particular problem when imaging planets when a very small image sensor is used for ‘lucky imaging’ to remove the effects of atmospheric turbulence.  The focusing can be improved  in two ways.  The single speed focuser can be replaced by a dual speed version making achieving fine focus easier.  Particularly useful for planetary imaging, the adding an external focuser at the rear of the tube assembly will remove the problems of image shift.   The German company, Teleskop Service, sell either Crayford or (better, but considerably more expensive) Rack and Pinion focusers for this use.  A second possible result of the focussing method in the original telescopes is that when, for example, a meridian flip is made by an equatorial telescope mount or one does a large slew across the sky the mirror can move slightly in what is called ‘mirror flop’ – so slightly changing the field of view. 

One effect of the corrector plate is to increase the effective width of the incoming light cone.  If the primary mirror is the same size as the corrector plate not all the available light will be focused.  Meade make their primary mirrors larger than the nominal size (for example – 8.25 inches diameter rather that 8 inches) to capture all the light entering the telescope.

The secondary mirrors have a 34% obstruction which some say will affect planetary observing and imaging.   This is not really a problem as attested by the fact that the world’s top planetary imagers use Schmidt-Cassegarins.  The Jovian image below was taken by the author with a 9.25 inch Celestron.  [In the UK, the atmosphere tends to limit the resolution, so this image is no match for those made using 14 inch telescopes from locations, such as Barbados, where the ‘seeing’ is exceptional but it is representative of what can be easily achieved with a smaller telescope here.]

The corrector plate is prone to dewing up so dew shields and dew heaters are often required.  With a closed tube, cooling times are longer than for open tube telescopes.  (A fan system such as the ‘Asterion Cooler Cat’ can be bought from First Light Optics for £109 to pass filtered air through the eyepiece holder into the interior  to reduce the cooling time.)

When viewing with a very wide field eyepiece or imaging with a large camera sensor, some coma will be visible towards the edge of the field using the original Celestron and Meade designs.

Advanced versions

Both Meade and Celestron have produced new versions of their Schimdt-Cassegrains to remove coma and give a larger flat field.  The Meade telescopes are called ‘Advanced Coma Free’ and use a hyperbolic secondary mirror and new corrector plate design and are the only versions now manufactured.  To achieve the same objective, Celestron Edge HD versions employ a dual element lens mounted in the primary baffle tube.

Once focussed, the position of the Celestron and Meade primary mirrors can be locked to eliminate mirror flop, very useful if astroimaging is to be a common use of the telescope.

Celestron Telescopes

Celestron  Schimdt-Cassegrains can be bought as just a tube assembly or with a wide variety of both Alt/Az or equatorial mounts such in the Celestron Nexstar 8 SE Telescope using a single arm computerised Alt/Az mount for around £1,300.  

The new NexStar Evolution 8 mount, costing ~£1,800, enables one to cоntrоl the tеlеѕсоре wіrеlеѕѕlу frоm an іОЅ оr Аndrоіd ѕmаrtрhоnе оr tаblеt.  The Alt/Az mount includes a rесhаrgеаblе lіthіum-іоn bаttеrу wіth еnоugh роwеr fоr 10 hоurѕ оf соntіnuоuѕ оbѕеrvіng. 

 An equatorial wedge can be acquired for astrophotography use as, then, ‘frame rotation’ is not a problem.

The telescope can also be bought with a computerised equatorial mount for around £2,300.   The Edge HD version costs £1,900 for the tube assembly and can be bought mounted on an equatorial mount for ~£2,500.

An interesting feature of the latest Celestron versions is that the secondary mirror can be replaced by a Starizona Hyperstar optical system to allow a camera to be placed in front of the corrector plate providing a focal ratio of f/1.9 allowing very fast wide field imaging with maximum field of view of ~4 degrees.

Meade Telescopes

At the present time in the UK, Meade telescopes are only available new from Meade UK (https://www.meadeuk.com/) and only the new ACF versions are available.  At the time of writing, their 8 inch ACF Optical Tube Assembly can be bought for £1,127 (nominally £1,499) but, due to electronic components being in very short supply, no mounts are currently available in the UK.  It is hoped that this will be rectified soon.  

There will then be three OTA/mount combinations available.  Their latest is a single arm alt/az mount, the LX65-8acf , which will cost around £1,500.

The second is the ‘classic’ LX200-8acf  fork mount, at around  £4,500 with tripod, which has been the mainstay of Meade Telescope mounts for many years.  For around  £270 an ‘equatorial wedge’ can be bought to make the mount better suited to astrophotography.

The third is the LX85-8acf computerised equatorial mount which will cost around £2,000.