Introduction.

Over the last couple of years I have been a member of QCUIAG and involved with the hardware modifications of webcams and video cameras for use in ‘non-planetary’ astronomy.  18 months ago it was enough to be able to record any thing at all of deep sky objects.  However with every development the unconventional imaging devices get a little closer to the performance of conventional astronomy cameras.  The 1004JG modification by Jon Grove brought us similar sensitivity to standard astronomical CCD and Etienne Bonduelle brought the webcams close by substituting the colour CCD in a webcam for an unfiltered one.

M42 taken with the ICX424 modified webcam through a ETX70.

Here is a tale about 2 types of CCD cameras converging.

 Webcams and dedicated astro CCD cameras are very different beasts.  As costly astro CCD's are optimised for deep sky imaging their performance on bright targets (Moon and planets) is compromised.  So many (or most) of the best amateur pictures taken of Jupiter, Saturn and the moon are taken with webcams

The differences between these two types of camera fall in to a numbers of areas.

The size of the CCD determines both the field of view and the number of photons which can be collected.  For planets this is not an issue as there are small bright objects.  However, when imaging large faint objects, a large CCD is preferable.  

The pixel size determines the resolution and to a large extent the sensitivity of the camera.  For planets small pixel size and high resolution is good while sensitivity is the most important criterion for deep sky imaging and is met with larger pixels.  

Binning is the technique of combing imaging pixels into larger virtual pixels.  When done in hardware before digitisation this offers nearly all the advantages of even larger pixels and more sensitivity at the expense or resolution; again only useful when deep sky imaging.  

As well as collecting signal from arriving photons CCD's also develop signal from thermal electrons.  This dark signal is temperature and time dependant.  To minimise the effect of longer exposures the temperature should be reduced.  Also different types of CCD's have very different amounts of dark signal.  

Planetary disks tend to be quite bright over their whole surface while deep sky objects such as nebulas and galaxies can have very bright cores and outer regions which are many orders of magnitude fainter.  To capture all the 'dynamic range' of a planet in one go a 8 analogue to digital converter (ADC) is largely sufficient and in line with the detail our eyes can pick up.  However to generate the kinds of images of galaxies where detail is observable in both arms and core much more dynamic range is required and 12-16 bit (ADC).  

Using CCD's with built in filters provides instant colour images.  This again is important to planetary imaging where rotation can seriously limit the time available to collect individual coloured frames.  Of course there is a trade off.  The filters on the surface of a CCD reduce its sensitivity markedly and also impact on its resolution.  

Exposure is controlled differently in the webcams and astro cameras.  The webcams electronic shutter makes very short but accurately timed exposures possible which can have the effect of 'freezing' the seeing.  However there hardware limits the maximum exposure to 1/5^th of a sec which is too short to start recording much detail in deep sky objects.  

Webcams contain some very high tech hardware to transfer images to a PC and at the PC end the CPU of modern computers has been optimise to handle high speed picture data.  By making use of this webcams can record many more images per sec than a standard astro camera.  The downside is some compression is used which can have a negative impact on quality.  

Finally cost.  Webcams can be manufactured on very large scales as consumer items which brings down the price.  Compared to astronomical CCD cameras they are very good value.  Astro cameras start at about 10 times the price of a webcam and easily go up to 100 time the price.  For this kind of money you do get a nice camera but its still not as good as a webcam for planetary imaging!!!

So if you want to take a picture of Jupiter though a telescope than you need a webcam.  If you want to use the same webcam for deep sky imaging then you loose out to astro camera users in all areas except cost!

Members of QCUIAG and Astrocam have been increasing the webcams performance for deep sky imaging over several years.  Progress has come in a number of jumps which relate well to the categories describe.  The first deep sky capable webcan was the Connectix B/W as modified by David Allmon.  As I don't know too much about this camera discussion will be limited to stages in the development of the current Mod.

The first development was that of video integration.  The stacking of a large number of frames.  People important in the development of this technique include Jürgen Liesmann and Bev Ewen-Smith. There developments were primarily aimed at increasing sensitivity and limits of detection,  this we now achieve with different methods.  The main advantage this technique offers the current mods is to increase image depth or dynamic range.  By stacking large number 8 bit frames a final image can have a dynamic range of 12 or more bits.  The ability to generate images with sufficient dynamic range is extremely important in deep sky imaging and provides a method to compete with astro cameras with better analogue to digital converters.

The best method to increase sensitivity is to take longer exposures.  The technique used to get the webcams to take controlled exposure pictures is describe here and is now very well established and supported by a very wide range of software.

This document describes the process of replacing the ICX098 CCD in the Phillips Toucam, vesta (untested!) and Logitech QC3000/4000 with a ICX424.  The new CCD is a 1/3" device with square 7.4 um pixels.   The QE of this CCD are shown in this graph from Mathias Rimkus

All charts show only typical values. Differences of more than 25% between several chips are possible. The ICX sensitivity function is extracted from absolute Qe statements of other companies and relative diagrams of SONY. Matthias Rimkus

The ICX424 in yellow can been seen to have a very similar curve compared with other CCD used in astronomy KAF400,KAF400E = Kodak (z.B Audine, SBig), TC256 = Texas Instruments (z.b Cookbook camera), ICX084,ICX424,ICX285 = Sony (z.B MRxxx cameras).  In addition the ICX424 has less dark frame noise and hot pixels than the other CCD listed here.

Also described in the instructions is a method to allow 2x1 on chip binning which can deliver a 2 fold increase in sensitivity.  Taken together with development in the last few years on the software side and the hardware control of exposures the ICX424 webcam is very much comparable with some astronomical CCD cameras and still has the advantage of being able to stream video at 5 frames per sec to help with finding and focusing.

M1 Taken with a ICX424 Toucam using 2x1 binning.

As I very much hope that these developments can form the basis of a commercial product which should bring down  the cost of CCD astronomy to the non DIYer, I do need to protect this work

I order to obtain the instructions to make this modification for yourself  I ask that you agree to the following terms and conditions.

1. You will not distribute these instructions.
2. You will not place information derived from these instructions into the public domain.  If you have any improvements or tips you would like to give others I will be happy to add these to the instructions distributed, and acknowledge your input.
3. You will not sell a camera modified in the way described for any more than the value of the parts, unless all extra money is passed to MSF.
4. You will not sell a camera modified as described as part of a larger deal.
5. The instructions are provides 'as is' with no guarantees of being correct, accurate or even working.  You agree to be responsible for any injury or damage that results.

If you can not agree to these terms I quite understand.

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