wpo - review : Celestron Fastar SCT

astro-optics pagetext & images [c]Maurice Gavin 1997/2001- Astronomy Now 1998 March issue page 23/24/25

The new Celestron Fastar SCT is a Schmidt Cassegrain telescope with a difference  - a CDD camera can replace the secondary mirror.  review by Maurice Gavin

A Schmidt Cassegrain Telescope (SCT) combines a typically f/10 optical system within a short sealed tube. The 8-inch Celestron Fastar is a breakthrough in optical design and adds a new dimension to the ubiquitous SCT.

Deja vu? My SX CCD review  picture  for AN Sept'94 predates the Fastar principle !

Removable mirror:  In the Fastar a CCD camera can be placed at prime focus of the main mirror to produce a f/2 optical system of 400mm focal length.  It covers 25 times more sky than in standard f/10 mode and exposures are proportionally reduced.  This is similar to the original Schmidt camera invented by Bernhard Schmidt in the 1930s designed to cover relatively wide portions of the sky.  Celestron marketed such a camera in the 1970s using 35mm format film.

'I'he Fastar mimics these principles but uses a modern CCD camera whose imaging chip is tiny compared to film.  Celestron have liaised with the reputable Santa Barbara Instrument Group (SBIG) to market the low profile PixCel 255 CCD camera to couple to the Fastar. The PixCel 255 CCD camera was reviewed in the March 1997 issue of Astronomy Now.    Normally a SCT secondary mirror, centred on the front corrector plate, is permanently installed with no user options except for minor retilting to collimate the telescope.  The Fastar is different in that the secondary mirror is held in place with a locking ring. This can be unscrewed, the mirror removed and the Fastar lens assembly for supporting the CCD camera locked into its place.   The secondary mirror is stored in its own case, and the whole system is perfectly balanced by a heavy counterweight attached to the rear eyepiece boss.

Simple installation: Guided by the manual, installation was very simple.  Reinstating the secondary mirror is equally simple: its cell is notched to return to the same position on the telescope to leave collimation undisturbed.  It works very well.  Naturally great care is needed when working in such close proximity to the delicate corrector plate which bears all the weight of the additional optics and camera.

Focusing:  Removing the secondary mirror removes a vital element in the overall correction of the optical system.  The Fastar assembly tube, that substitutes the secondary mirror projects forward of the telescope and contains a lens system to reapply the necessary corrections. The corrective optics appear to be a fully coated four element lens in two groups.  To focus a SCT the main mirror is moved backwards and forwards. The same applies in Fastar f/2 mode and when an object is sharply in focus (as seen on the computer monitor) then all the optics have the correct separation. Although designed to work at infinity, the Fastar would focus down to 12m.   The PixCel camera is designed to mate precisely with the Fastar lens assembly to produce the sharpest images.  However, to see if the f/2 corrective optics would cover a larger field than the tiny PixCel imager, I offered a second CCD camera (SX-L8) of identical weight onto the Fastar.   This camera covered seven times more sky [check this Fastar SX image] with good images but could not be placed at the critical distance from the lens system without exchanging the Fastar draw tube.  This should be a relatively simple task.

Drive system:   The Fastar drive can be powered by internal 9V PP3 batteries, via a car battery or a mains adapter. The telescope was delivered with the wrong power adapter, and the batteries lasted less than half the eight hours quoted in the manual.  The drive comprises a stepper motor  turning a spring loaded worm onto a 14cm diameter 266 toothed wheel about the RA axis and is totally silent in operation.   This proved deceptive when I ran out of replacement batteries and plugged into the car battery via the cigar lighter.  I had a dead car after a few hours!  The 230V AC mains adapter seems the preferred power source.  The drive unit has a single switch (power on/off) and a button for cycling through the various pre-set drive rates.   These rates are stellar (sidereal), solar, lunar and King.   King mode accounts for atmospheric refraction.  Internal DIP switches cope with reversing the drive for the southern hemisphere.  A separate button starts the periodic error correction (PEC).  If the telescope is accurately guided on a star for one cycle of the RA worm (about six minutes) the manual states the tracking errors are reduced by 70%.

Like the King mode, PEC is only of value during long guided exposures at f/l0 where some manual guiding may still be necessary.  Unfortunately PEC is lost when the power is disconnected and must be retrained every time the telescope is set up, which is a chore.  When used visually at f/10 the stellar drive rate without PEC was near perfect, with stars and planets locked to the centre of the field for many minutes at high power.   All the accompanying images were taken at stellar rate and unguided without PEC.   The smoothness of the drive is impressive.

Hand control unit:  The base unit has two additional phono sockets for plugging in the hand controller lead and the declination control.  The hand controller is fairly basic with buttons for directing the telescope east/west or north/south at 8x sidereal rate.  An internal battery activates an LED readout for date/time, stopwatch, alarm and ambient temperature.  This telescope is pointed manually at the target, aided by the bright 50 x 9 finder and clamped in two axes with knobs.   I found this very quick, convenient and positive.  A newcomer will need a good star atlas.  Thumb wheels control manual slow motion in both axes with a long tangent arm on the declination axis.  Sockets allow a CCD camera to autoguide the Fastar when accurately polar aligned.  Unfortunately the PixCel camera cannot guide and image at the same time.

Viewing through the Fastar:  Although eager to test the telescope as the Fastar f/2 camera, the first few clear evenings were used to view through the telescope in regular f/10 mode.  Initially the collimation was found to have been disturbed and a floater (tiny piece of tissue) was deposited on the surface of the secondary mirror.  This was quickly rectified with a puffer brush and a tweak of the collimation screws.   The telescope thereafter performed perfectly.  Whilst testing the Fastar f/2 mode the secondary mirror was removed and replaced numerous times but the telescope's collimation remained perfect.

Amongst the targets viewed was Jupiter,  low in the southern sky with its cloud belts and four primary moons. Sometimes a tiny black point of a satellite shadow was seen in transit.  High overhead was the Ring Nebula  (M57) like a small smoke ring.  No central star was visible although this is easily seen with a few seconds CCD exposure at f/10.  The fine globular cluster M 13 in  Hercules was unresolved with the single supplied 26mm eyepiece but broken into a myriad of stars with my 9mm eyepiece.  After midnight, Saturn at a respectable altitude was spectacular - a crisp creamy globe and well tilted ring system.  The shadow of ring on globe and visa versa were clearly visible, and five satellites.  Various close double stars were tested,  such as the beautiful Albireo in Cygnus, the Double Double in nearby Lyra, alpha Herculis and gamma Arietis.  Each was well separated with delicate, tight diffraction patterns and contrasting hues.

Telescope assembly:   The telescope, complete with fork and drive assembly, tripod and numerous accessories comes in protective cartons. A high grade travel case is supplied for the PixCel 255 CCD camera and the Fastar removable optics.  A similar telescope case is an optional extra.
Assembling the telescope is very straightforward.   The tripod is in one piece and adjustable for height.  The equatorial wedge is secured to the tripod with three bolts.  The tripod legs are spread and locked into position (one leg remote from the wedge is pointed due north) and adjusted until the spirit bubble is centred.  The wedge is tilted and locked to the observer's latitude .  A single bolt is initially screwed into the  telescope base and offered onto a notched slot on the wedge.  This is sufficient to support the telescope whilst the two remaining locking bolts are located.  Set-up is complete and more than adequate for all visual observations.  An upgrade kit for precise alignment is supplied.

The f/2 imaging mode:   The Fastar f/2 mode has no other application except for the CCD camera and cannot be used visually or with a film camera. All these applications can, of course, be used at the normal f/10 focus at the rear of the telescope where a focal reducer or a Barlow lens may also be added to change the effective focal length.   The Fastar variant of the SCT exists to prove the worth as an f/2 imaging system of unparalleled 200mm clear aperture.   It is sheer aperture that counts when recording faint stars and a fast f/ratio that records extended objects in the minimum exposure.  This incredible speed and relative short focal length makes it ideal for larger deep sky objects preferably from a dark site.   The Fastar unfiltered and unfettered can snap stars to magnitude 15 in a few seconds exposure.  There are no photographic camera lenses to match it. [first-light M27 mono+colour images]

There is a second issue.  Unlike regular films, CCDs have extended infrared (IR) sensitivity and camera lenses are rarely corrected to focus this radiation.  The Fastar showed slight enlargement of stars with an IR excess.  A regular camera lens would record bloated star images unless an IR block filter was used which in turn kills the CCD speed.   The f/2 Fastar proved well matched to the PixCel camera and unequalled in producing tight star images in monochrome.   For colour the f/2 Fastar uses the PixCel's optional RGB colour filter wheel (CFW) in combination with a IR rejection filter.  The speed of the system despite filtration was still remarkable with faint objects captured with high colour saturation in under five minutes, a fraction of the time needed at f/l0.

However I find the f/2 colour option flawed.  The supplied software cannot merge RGB to sub-pixel accuracy and even with the best efforts, brighter stars have unpleasant red/blue fringing.  Fainter stars have garish colours probably due to receiving uneven RGB exposures through under sampling with each pixel covering five arc-seconds of sky.   There appears no quick solution.  Close inspection of the sample colour images supplied with the software show similar problems.  There was also evidence of field rotation (again showing as colour fringing) on the RGB images unless the telescope was perfectly polar aligned.

Build quality:  Generally I was happy with the Fastar build quality with its all metal construction.  The optical tube assembly was excellent.  Only two points jarred: a cheap plastic cap covering the central RA axis boss and excessive  movement of the slip rings on the declination tangent arm. Celestron are to be commended for acknowledging the exposed front corrector plate needs protection from dew by including as standard a compact cone shaped lens hood which snaps onto the Fastar.  Air movement from the CCD secondary fan cooling also probably helped reduce dew formation on the front corrector.  Another useful accessory is the focusing knob counter which allows precise resetting of the focus position when equipment is exchanged.

Summary:   The Fastar SCT only makes sense if purchased with the PixCel CCD camera for use in f/2 mode and I was impressed with the monochrome images.  The telescope performed very well in the regular f/10 mode.  'I'he Fastar principle is really too good to be hogged by a single telescope.   Perhaps Celestron will consider marketing all their 8-inch SCTs with removable secondary mirrors.  'I'hen the Fastar optical set would be a powerful optional extra to the Celestron range with a camera of the user's choice.

Note: the f/2 optical set is now (2000) available from David Hinds Ltd for other Celestron SCTs including C14 with the larger SBIG ST-237 CCD with a 4.7mm x 3.6mm chip. [check this Fastar SX image]

Available from:
David Hinds Ltd., Unit 34, The Silk Mill, Brook Street, Tring, Herts HP23 5EF
Tel: 01442 827768;  Fax: 01442 890763
Price [incl VAT] 20cm f/10 Fastar with f/2 optical set; £2449; Pixcel 255 CCD incl software £1649; CFW £359
reviewed late 1997; published Astronomy Now / March 1998 p23/24/25

Maurice Gavin is the vice president of the  British Astronomical Association with a special interest in CCDs and optics.