wpo - review : Celestron Pixcel 255 CCD camera
astro-optics pagetext
& images [c]Maurice Gavin 1997/2001- Astronomy
Now 1997 March issue page 18/19
Maurice Gavin reviews the Celestron Pixcel 255 Advanced CCD camera, introduced by Celestron to complement their range of telescopes.
Celestron
are the second American telescope manufacture to offer their own CCD (charged-coupled
device) camera to complement the extensive range of equipment for their
Schmidt Cassegrain telescopes (SCT). Unlike Meade, who have chosen
the difficult path of designing their own products, Celestron have liaised
with the reputable Santa Barbara Instruments Group (SBIG) to offer a camera
called Pixcel 255 Advanced CCD.
The Pixcel camera is of high specification and excellent build quality.
This is reflected in the price. For the UK buyer there is a US dollar
to UK stirling price penalty. SBIG market a similar camera for industrial
user in the States.
Chips and the camera Most photographers using film covering many square inches find the diminutive CCD imaging chip something of a joke. This is misplaced. Through the CCD's supreme sensitivity even on a smaller telescope it regularly out-performs the photographic behemoth. And results are seen immediately. Despite increasing computer memory demands many CCD users still crave for even bigger chips often before they have mastered the smaller versions! The Pixcel CCD camera uses the same Texas Instruments TC255 imaging chip as the earlier SBIG ST-5 camera now discontinued. The TC255 is described as a full frame devise with a second but masked area to store the image prior to download to the computer. The chip is of match-head proportions measuring a tiny 3.3mm x 2.4mm with ten micron square pixels. This is less than 1% the size of 35mm film format so the telescope and camera combination must be aimed with precision. This can frustrate the novice who is advised to initially couple the Pixcel 255 to a regular camera lens to cover a wider view. This is easily achieved by unscrewing the T2 mounted 1.25-inch push-fit adapter (for coupling the camera to the telescope) and replacing it with a T2 mounted camera lens. These adapters are available from camera and telescope dealers.
Small
pixels give improved spatial resolution but tend to be of lower sensitivity
(speed) than larger pixel cameras. I was agreeably surprised to find
the Pixcel very sensitive, recording the central, magnitude 15,
star in the M57 Ring Nebula in a mere two seconds exposure with a 12-inch
aperture telescope. This
closely matches the manufacturer's estimate of magnitude 14 in one second
exposure with an 8-inch telescope. With longer exposures utilising
the camera's full dynamic range (l6-bit i.e. 64,000 grey levels) the images
were pleasingly photographic in quality - amongst the best I've seen. [Space
did not permit inclusion in the AN review of any colour images including
this early four colour LRGB M57 image taken
during these tests during November 1996 using my 30cm Meade LX200 SCT].
The camera incorporates a miniature fan placed centrally in the spiral fins at the rear of the casing. This secondary cooling help stabilise the chip's electronic cooling and regulation system set from the PC keyboard. The temperature dropped a rapid one degree Celsius per second indicating the commendably low mass and inertia of the design. Some CCD cameras can take 20 to 30 minutes to stabilise. The TC255 chip seemed, to me, relatively noisy in the absence of darkframe subtraction and chip cooling so these features, automatically invoked by software, are essential in maintaining the ecellent image quality. Although the Pixcel can be used with any telescope or camera lens, its low profile (80mm diameter) allows it to be placed at first focus of the projected Celestron Fastar SCT where it replaces the secondary mirror and converts the telescope into an f/2 Schmidt camera of 400mm focal length. The Fastar will also work at the standard f/10 secondary focus and f/20 (or greater) via a Barlow lens or eyepiece projection.
Optional
extras All CCD cameras (unless using
camcorder chips incorporating a filter matrix) work in monochrome.
To create a full colour image, three exposures through red, green and blue
filters are later combined in the computer. The optional motorised
Colour Filter Wheel (CFW) and associated software makes the process simple.
The CFW is of jewel-like quality some 48mm diameter with four 12mm
diameter RGB interference filters and a clear window for regular black
and white imaging. A fifth blank space caps the camera for darkframes.
Located immediately before the imaging chip the CFW replaces the
standard darkframe shutter. An IR-rejection filter is supplied
with the CFW to maintain good colour balance and screws into the eyepiece
adapter.
Interface box Some five metres of lead connect the camera to the interface box and a further four metres lead links the box to the PC RS232 port. This effectively is the maximum separation of camera to PC although longer leads can be made up. The interface box is big, almost A4 size; sufficient apparently to act as a base for a laptop computer.
Software The software looked very familiar after my absence of some years and remains essentially unchanged from its 1990 launch for the first (ST-4) camera. It has been amended and customised to suit a growing family of cameras - in this case the Pixcel 255, but remains as robust as ever. Clearly SBIG have got it right although I think its DOS implementation looks dated compared to Windows. DOS does however remain very fast in operation and doesn't need a Pentium PC. The user is not restricted to downloading the whole frame but can define a selected window of much smaller file size which greatly speeds up the save-to-disk time. This is ideal for repeated imaging of a planet or lunar crater (for the best seeing) or for making a timed movie of perhaps a planet's rotation or a lunar occultation. Using the smallest subset of pixels the soft ware will take, darkframe subtract and save-to-disk in black and white mode at the rate of a frame per second. Impressive.
Tricolour
imaging Colour has a magnetic appeal.
Unless one's interests are confined to scientific pursuits like astrometry,
photometry, supernova hunting etc. then monochrome with its extra turn
of speed will suffice. Having used a Starlight Xpress single-shot
colour CCD camera (with camcorder chip) for some years, I vowed never
bother with tricolour imaging. This camera could change my mind !
It gets as near to fully automatic RGB imaging as can be reasonably expected.
The technique at the computer keyboard is simple and straight forward. From the menu select Track and key-in exposure duration (e.g. four minutes), select Color image and press Enter key. The software and hardware (camera and filter wheel) now cycles through a complete sequence of six exposures (red, green and blue including a darkframe calibration exposure for each) and saves the result to disk. The software even allows for progressively increased exposures through the green (x 1.5) and blue (x2) filters to match the CCD's spectral response and ensure the images are of comparable density. All the parameters can be amended. Later the three images are merged via supplied software to make a full colour image. Tricolour imaging is time consuming but the results are very worthwhile. This all assumes your telescope can precisely track the stars for the duration of each exposure and hold the target in the general field for 40 minutes or so - a surprisingly difficult task.
There is hope for lesser telescopes: SBIG's patented Track and Accumulate (T&A) mode. This allows a series of snapshots (say eight snaps of 30 seconds) to be automatically added together to create a longer eflective exposure. The result can be virtually indistinguishable from a single long exposure of equal duration. For colour there is an additional benefit for results can be seen sooner. There is wasted time in the RGB sequence when the camera is taking the dark calibration frame subtracted from each starlight exposure.
These must be of equal duration to the light frame itself. In the examples above the total wasted time for dark frames is 18 minutes (4+6+8 minutes) and 2.5minutes (30+45+60 seconds) in T&A mode, respectively. With T&A, whether capturing images in colour or monochrome (unfiltered), the first snapshot is down-loaded and displayed. Selecting a guide star in the picture with the cursor ensures the remaining snapshots overlay precisely. This star is then displayed greatly enlarged and side panels show its drift over the chip in X and Y coordinates (measured in pixels) and provides useful information on the telescope's tracking. If the star has remained centred and circular within its box during the whole exposure then the final image should be sharp. T&A greatly improves the CCD potential of amateur telescopes in this exacting field.
And finally What do you get for your money? A quality camera of diminutive size (including a small chip) capable of excellent results.
Astronomy
Now / March 1997 p18/19
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 £1545; CFW £359 [early 1997].
Maurice Gavin wishes to thank Michael Barber of SBIG and David Hinds Ltd for supplying technical data for this review