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Astroimaging in practice - a quick history

Astrophotography

How did amateur astronomers start taking digital images of the stars ?

The very first amateur digital astroimaging was done using a Web-Cam (the Phillips TouCam). Whilst the resolutions and sensitivity were insufficient for deep sky imaging, the results achieved for planetary imaging were a revelation.

Many people seeing the first results could not believe what they were seeing and were convinced that professional telescopes must have been used - however soon some of the results being obtained were BETTER than the 'professionals' still using film

Was the web-cam Hardware modified for Astrophotography use ?

Yes, and over the years considerable effort was been put into optimising the performance of such 'low end' camera's by making simple circuit changes (to reduce amplified noise, control exposure times etc).

Unfortunately, some individuals efforts were turned into 'a business', with the result that specific web-cam modifications have became 'commercial secrets'

So, to avoid trouble with those wanting to keep their "secrets", I'll say no more about modifying web-cams

You are invited to Google for yourself .. one place to start would be the 'Steve Chambers SC1 mod' or you can read more about modifying web cams here

Can such cameras be purchased 'ready modified' ?

Yes, a number of 'main stream' makers of telescopes in the USA, having seen what was being achieved by the amateur, manufactured 'Astrophotography' versions of the web-cam using the same chip set (but at more then double the price :-) ) = again, I'll say no more about this, other than to suggest you look on eBay for a bargain rather than line the pockets of the commercial vendors.

A comprehensive list of Web Cams for astroimaging can be found here

What's the advantages of CCD sensors ?

The main advantage of CCD technology is that it is very immune to noise (indeed, it can be up to 10x better than CMOS). The main disadvantage is that CCD's are expensive to make and consume more power (typically 4 to 5 times CMOS).

What's the advantages of CMOS sensors ?

The main advantage of CMOS sensors is that they are cheap (which translates into (many) more pixels at each price point). The main disadvantage of CMOS is that it suffers from more noise than CCD's, especially 'thermal' noise.

What's today's situation ?

Only in the last couple of years have CMOS chips started to become as sensitive as CCD's. The sensitivity of the latest CMOS DSLR cameras (as measured by the maximum supported ISO) now rivals that of dedicated professional astroimaging CCD sensors

What's best, CCD or CMOS ?

As is usual with debates of this type, what 'is best' rather depends on what you want to do with it :-)

What's best for Planetary imaging ?

For planetary imaging, we typically have lots of light to play with .. and the 'size' of the image that reaches the sensor is typically very 'small' (i.e. it does not fill the field of view, even if 'eyepiece projection' and a Barlow is used). Thus the large sensor arrays of DSLR camera's would be wasted whilst the smaller sensor arrays of Web Cams are ideal.

Further, the effect of atmospheric distortion is MUCH more significant when imaging the planets. Distortion causes the planetary 'disc' to distort and their features to 'swim' about.

As with all photography, the way to 'freeze' a moving image is to use short exposure times. So, to image the planets, you need to take large numbers of short exposure images - in other words, a movie.

Web-cams are thus ideal for planetary imaging. Their small sensor sizes (typically no more than 1/2" (12mm) square) is no disadvantage when the image is so small anyway.

A 'movie' of thousands of frames is processed using special software that throw away the worst distorted frames and 'stacks' the rest (using, for example, RegiStax ).

What's best for Deep Sky Imaging (DSI) photography ?

Here we need sensors with large numbers of pixels to capture nebula and galaxies .. and exposure times will be much longer (typically 5 minutes at a time) - and whilst atmospheric distortion still causes stars to 'swim around', these are 'point objects' and stacking software (such as DeepSkyStacker) have evolved to cope.

How did the Canon 300/350D become the 'standard' for Deep Sky use ?

In amateur deep sky Astrophotography, cost and number of pixels outweighs the noise. So when Canon started selling a low cost CMOS sensor DSLR camera (the 300D, in Aug 2003), it was quickly adopted for amateur deep sky imaging. As a result, the Canon range of DSLR camera's (especially the 350D) have became the 'de-facto' standard of deep sky imaging.

What changes were made to the 300/350D ?

The presence of the IR blocking filter in all DSLR's is a big disadvantage when it comes to imaging faint nebulae. Although Canon did manufacture an Astrophotography version of the 350D (without the IR filter), it was much too expensive for 'amateur' use (and the 'semi-professionals' stuck with their bespoke CCD's) .. so it failed to find much of a market and has since been discontinued.

However, after Canon 'pointed the way', a number of enterprising individuals started to modify their standard 300D & 350D's and these modified camera's occasionally appear on eBay. If you search via Google you may still find some-one that offers a modification service. Most** will replace the IR filter with optically clear glass (so the camera can still be used with it's normal lens), and, if a custom 'white balance' is used, even take 'normal' daylight photo's.

**some will replace it with a specialised OII 'nebulae' (or 'Baader') filter that cuts out much of the 'sky glare' caused by (high & low pressure sodium) street lights, although it is also possible to purchase specialist 'clip in' filters for the Canon's

Why Canon not Nikon ?

Nikon CCD sensors camera's were initially a lot more expensive than the Canon's, so despite the advantages of CCD V's CMOS they have never really caught on to the same degree. Even so, many amateurs prefer the Nikon's over the Canon's due to the inherently lower noise and greater sensitivity.

It is to be noted that the higher pixel counts, Nikon's are now equipped with CMOS sensors, as CCD's seem to have reached a limit at about 10 mega-pixels.
Image
**higher figure only supported if the CHDK firmware 'hack' is used.

What about CCTV security cameras ?

The CCTV security industry is indeed a source of good low-light performance cameras.

Many of the 'low end' CCD 'security cameras' aimed at the consumer market can be found on eBay. Some are specifically designed to operate in the IR region (and are equipped with IR LED illumination so they can operate in total darkness) and produce a 'B&W' image.

Unfortunately, when operating in 'daylight' mode (and generating a colour image) their non-IR (low-light) sensitivity is often very poor.

1 Lux is typical (1/4" SHARP chip), although the 1/3" inch Sony Super HAD Colour CCD goes down to 0.5 LUX. Some of those using a 1/3" SONY chip are claimed to operate down to 0.001 Lux however very few vendors publish any real specifications (and discovering the actual sensor chip make / type number before purchasing (and dismantling the camera) is an almost impossible task).

What's the disadvantages of CCTV camera's ?

The main drawback of these simple 'CCTV' CCD camera's is that they switch from 'daylight' to 'IR' mode automatically. To be useful, it is necessary to gain control over the CCD by modifying the circuitry (which may be almost impossible given the highly integrated / minimal chip count nature of theses units).

More useful are the camera's designed to work in very low light conditions (without switching into IR mode) and equipped with control interfaces (usually serial link) - for example, the PC164CEX-2 which delivers 600 line resolution at 0.0001 lux for $140. Such cameras fit neatly in price above web cams but well below the 'semi-professional' Astrophotography cameras.

The final 'gotcha' is that most will output only an analogue 'video' signal, thus necessitating some sort of 'digitising' step (fortunately low cost USB devices designed to digitise TV video are still generally available)

What about commercial astronomy Cameras ?

Web cams were quickly 'commercialised'. Such cameras are almost all B&W only (although some come with an 'integrated' filter wheel), and, despite using sensors with 10 to 50 times fewer pixels than that of current DSLR Camera sensors, prices are, indeed, astronomical :-)

The major advantage such units offer is cooling. Almost all have heat-sinks, many have fans and some have active cooling using Peltier Devices

As mentioned already, smaller chip sizes and pixel counts mean the camera is best suited to planetary imaging although sensor sizes are increasing and current designs are being successfully used for deep sky imaging.

Some 'commercialised' Web Cams (note tiny sensor sizes and minuscule pixel counts) DMK 21AU04.AS (Sony ICX098BL), 4.6x3.97mm CCD 0.33 mega-pixels (31.36 m2)
DMK 31AU03.AS (Sony ICX204AL), 5.8x4.92mm CCD 0.80 mega-pixels (21.62 m2)
DMK 41AU02.AS (Sony ICX205AL), 7.6x6.20mm CCD 1.40 mega-pixels (21.62 m2)

What's the Optimum pixel size ? (see also link)

There is no advantage in having pixels smaller than the resolution limit of your telescope's optics. This is given by the diameter of Airy Disk (in mm) = 2.43932 x ? x Focal Ratio (where '?' = Wave Length of the light you wish to image, in mm (e.g. mid visible light, 546nM = 0.000546mm)

A typical 5" Refractor (127mm dia lens, 1000mm focal length) = f8 & we have Airy Disc dia. approx 10 um (= area of about 31 m2 ). A typical 10" Reflector (250 mm dia. mirror) will be f4 and have a limit 5um = area 15 m2.

As will be seen from the tables above, many modern camera sensors are getting close to these limits.

How do Colour 'sensors' work ?

All CCD / CMOS sensors simply measure the intensity of light falling on them. To generate a colour image it is necessary to use RGB filters. With a B&W web-cam or B&W security camera, you exposure 3 (or more) images using a different filter for each exposure.

Note - most astronomy filter wheels have 4 positions - this is because any filter will reduce the available light falling on the sensor so it is normal to expose one image in 'B&W' mode i.e. 'without' a filter (to get the overall 'intensity' of light) and then 3 filtered images (r, g, b) to get the overall colour.

In a DSLR, a colour result is obtained in a single exposure. To achieve this, tiny colour filters are built into the sensor array above each individual pixel. These filters are arranged in 2x2 arrays known as a 'Bayer Matrix'. Each individual sub-pixel has it's own built in colour filter (part of the 'micro-lens' structure above each sensing element that is used to maximise the light falling on it).

How are the colour filters arranged on a typical DSLR sensor array ?

The 2x2 sub-pixel colour filter 'Bayer Matrix' array is arranged with 2 Green, 1 Red & 1 Blue filter as follows :-
G R
B G

Thus, whilst an '8 mega pixel' camera outputs an 8 mega pixel RGB image and really does have 8 million sensor pixels, they actually ONLY have 2 million Bayer Matrix 2x2 sets i.e. there are 4 million Green sensors and 2 million Red and 2 million Blue.

How are 8 million full colour pixels generated from 4 million G and 2 million each of R & B ?

To 'convert' the 2 million Bayer Matrix (2x2=4 pixel) sets into 8 million RGB pixels, the camera performs INTERPOLATION to 'guess' the colour / intensity of each RGB pixel from the nearest individual Bayer Matrix pixels

Each (3x8 = 24bit) RGB pixel in the resultant image is thus derived from 3 to 5 'sub-pixels' in the sensor. Cameras that allow the user access to the "RAW" sensor data will output the actual individual (10, 12 or 14 bit values) found at each of the underlying sensor sub- pixels.

Needless to say, interpolation is not what we really want when trying to maximise the sharpness of point objects (such as stars) :-)

What you really need for astroimaging is to use the RAW data yourself (before the camera starts interpolating it).

If you want to go straight to RAW processing and skip the 'capture' step, click here for the What, Why and How of RAW data.

For a detailed Q & A of setting up your telescope and image capture, continue with 'Next>>' (in the navigation bar, left).

Next page :- Setting up your telescope

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