COMPARISON OF MONOCHROME AND COLOUR VERSIONS OF MINTRON PD 2285C-EX /ORION STARSHOOT CAMERA

We are often asked how the Mintron PD 2285C-EX (monochrome) camera, compares with the colour versions. , such as the Orion Starshoot Deep Space Video Camera As far as live video is concerned, the colour version does have the advantage of live colour video - the monochrome version does mono video only. On the other hand, the monochrome camera (PD 2285C-EX) gives sharper video, and goes deeper (is more sensitive) than the colour version. This is illustrated by the images on the right which show live video stills of same small area of sky taken at the same time with a colour Mintron (top) and the equivalent monochrome Mintron (bottom).

As far as imaging is concerned, the better resolution of the PD 2285C-EX will enable it to produce better colour images than even the colour camera, through the use of technique called LRGB image compostion. This involves capturing separate red, green, and blue images to compose the colour. Even the US makers of the colour camera admit that -      "When it comes to producing the very best astrophotos, most imaging hobbyists will readily acknowledge that you must compose shots taken with a monochrome camera, through separate LRGB filters".

So, live colour video apart, the monochrome PD 2285C-EX is the better camera, both in terms of the sharpness of its output, and its sensitivity. This is not a peculiarity just of Mintron cameras. By the very nature of digital imaging, any monochrome camera will be more sensitive and have better resolution than its equivalent colour camera. The reasons for this are discussed below. REASON 1 - BAYER MASK FILTRATION When light falls on a CCD chip, it is collected by a matrix of small potential wells called pixels. The image is divided into these small discrete pixels. The information from these photosites is collected, organised, and transferred to form an image for display.

Monochrome Camera The image at right shows a set of 4 pixels in a Mintron monochrome camera. The recording of light is based on a photoelectric effect and therefore each of the 4 pixels in the set records all the light falling on it. The pixels cannot distinguish between colours, so the light is recorded as white light at different intensities (monochrome).

Colour Camera The image at right shows a set of 4 pixels in a Mintron colour camera. In a colour camera, we require that the colours MUST be distinguished, and so the CCD chip incorporates a mosaic filter (Bayer mask) to separate incoming light into a series of colours. The image on the right shows the filter arrangement for each set of 4 pixels.

EFFECT ON COLOUR CAMERAS This Bayer mask, while it allows the CCD chip to recognise colours, has two unfortunate side effects. Firstly, much of the light is filtered out, and never reaches the sensor. So a colour camera will always be less sensitive than the equivalent monochrome one. Secondly, since the colour information is averaged over groups of 4 pixels, colour CCD chips have inherently lower resolution than monochrome ones.

REASON 2 - INFRARED CUT FILTRATION

Unfortunately for colour cameras, there is another factor which can further reduce their sensitivity for astronomical uses. This is due to the fact that the CCD Sony Ex-View chip (as used in the Mintron cameras) has sensitivity, not just to the visible spectrum of light, but extending well into the infrared. Now, the human eye can see light radiation in the range of wavelengths from 400nm (violet light) through to 700nm (deep red). As the chart on the right shows (orange line), the Sony Ex-view chip has sensitivity extending beyond the visible spectrum at 700nm, out to 1000nm.

This extra infrared light presents no problems for the monochrome PD2285C-EX. The extra sensitivity is a bonus, enabling the camera to display fainter objects. But for the colour camera, such as the Orion Starshoot Deep Space Video Camera, the extra-infrared light, which must be recorded as red light, distorts the colour balance of the image, which shows as "too red".

FILTRATION TO CORRECT COLOUR BALANCE So colour cameras have IR-cut filters to correct the "too red" bias of extended-red sensitivity. Two important types of IR-cut filter are - 1. STANDARD IR CUT FILTER This is the standard filter used in most CCD cameras. Its effect is shown by the dotted red line in the chart above. For astronomical purposes, the big disadvantage of this filter is that it filters out much of the Hydrogen alpha (Ha) emission line at 656.28 nanometers - important for photographing emission nebulae such as the Ring Nebula, Barnards Loop, Veil Nebula, etc. A colour camera with such a filter is not so good for deep sky imaging. 2. BAADER UV-IR FILTER The answer to this problem is to replace the standard IR-cut filter with a filter which will cut infrared light, but allow passage for light at 656nm. Such a filter is the Baader UV-IR filter. Its effect is also shown in the chart above (purple line). But whatever type of correction filter is used, it's effect will be to reduce the overall sensitivity of the colour camera.

FILTER SWITCH TECHNOLOGY A recent development in video technology is the use of CCDs with filter-switching capabilities. This enables a colour camera to operate in two modes, colour and monochrome. In colour mode, the IR-cut filter is in place for correct colour balance. In monochrome mode, the IR-cut filter slides out of the way to enable monochrome imaging using exteNded infrared sensitivity. Some colour Mintrons such as the Orion Starshoot Deep Space Video Camera, have this facility. But please note that, even with the IR-cut filter removed, colour cameras will still suffer the sensitivity loss caused by the Bayer mask filtration.

PD2285C-EX FULL KIT The full kit offers exceptional value for money. Everything you need is included - 1. The latest version PD2285C-EX camera. 2. Body cap and 5mm spacer ring for C-mount lenses. 3. Nosepiece to connect to any telescope 1.25" focuser tube. 4. Regulated 12V mains power unit. 5. 5 meter cable to connect to any TV/flatscreen. 6. Video capture device for viewing video and images on PC/laptop. 7. Software to capture video and images on any PC/laptop (XP, Vista, Windows 7). 8. Small lens so you can practice set-ups indoors, and get great wide sky views outdoors.

MINTRON PD2285C-EX Camera and Full Kit £299.00 (UK post free) DHL Europlus Extra charge £15.00