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WEBCAM TUTORIAL FOR ASTROPHOTOGRAPHY
USING THE PHILLIPS TO U CAM PRO 840. by Phil Reed
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What is a webcam used for? I think if I was to ask the question to people in the street most would say, 'it is a small video camera that sits on top of a computer that allows you to see and be seen all over the world on a thing called the Internet'. Indeed that is exactly what it can do but very few people would know it can also be used as a camera for astrophotography. It will image the brighter planets Mercury, Venus, Mars, Jupiter and Saturn with ease and it can even image Uranus and Neptune if your Telescope is of sufficient aperture. It can image the brighter stars and will image the Sun as long as you have the correct filters and take great care.
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The Moon is a favourite target,
especially with beginners as it easy to find and can be imaged in black and white, no colours to worry about. Focussing is also quite easy if you aim for something on the terminator. But webcams do have limitations, most are not capable of exposures longer than around 1/5 th of a second, which makes them unsuitable for deep space imaging. Having said that you can modify or buy modified webcams that can give you longer exposures. Modifying your own is not recommended unless you have a good understanding of electronics and a steady hand. As for buying a modified one, well that is up to you. Personally I would rather buy a camera that was build for deep space such as one from the starlight xpress range but these cost a minimum of £600 for monochrome and a £1000 for colour. You can also use a 35mm camera but this can be time consuming and frustrating and even though the camera itself can be picked up cheap, the developing side can be a little expensive, but if you can master the techniques you can achieve amazing results. Even a camera that bridges the gap between webcam and deep space camera such as the Meade deep space imager will cost around £250. Also available are ready-made imagers such as the Celestron Neximage
which is more or less a To U cam circuit board inside a purpose built camera,
or the Meade LPI, but as to date I have had no experience with this type of cameras.
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Celestron Neximage |
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But good news for the planetary worker as Webcams are very powerful pieces of kit and they reign supreme in astrophotography as far as the planets are concerned. CCD cameras can not match them and the reason why is because the webcam can freeze the atmosphere and capture that split second of perfect seeing, imaging at 5 or 10 frames per second. Where as the CCD camera takes a single exposure that then has to be sent to the computer so you can adjust focus but is it out of focus because of poor seeing or is it just plain out of focus to start with, very hard to tell the difference, but with a webcam you can focus using thirty frames a second
, so focus can be adjusted in real time and seen with no delay
on a PC monitor, not that any planetary imager is ever happy with focus. So to sum up it is the sheer number of frames per image that these tiny webcams can give you. up to a thousand or more that makes them so powerful and along with a processing program such as Registax, that can stack all these individual frames into one image, it is this partnership that will give you results that astronomers could only dream about10 years ago and gives amateurs a chance to turn out images that are beyond belief.
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So, which webcam do you choose?
You will find these cameras have two main differences one is the transfer speed of data and the other is the type of chip that they have onboard. The transfer speed of data depends on which type of interface you have
USB1, USB2 or Firewire. USB 2 has the fastest transfer rate followed by Firewire and then USB 1. The faster the transfer rate the less compression and the better the image quality will be. You may be enticed to buy a webcam by the frames per second advertised on the packaging but do not be misled as although it may say 60 frames per second on the package, USB1 which most webcams are cannot handle high frame rates. If you try to run a webcam at 30 frames per second all that will happen is the camera will compress the image, because USB1 can only transfer data at 12 megabits per second and a single frame of video can be around 10 megabits. So you are trying to squeeze (30 x 10) 300 megabits down a cable that can only transfer 12 megabits per second. This may be ok for terrestrial video conferencing but unsuitable for such a small images used in astrophotography. You must also take note of the cameras maximum exposure times, which should be at least 1/25th, or 1/30 th of a second,
which by the way is to be totally ignored in manual mode as it is the frames per second that govern the shutter speed e.g. at 10 fps the shutter speed is 1/10 sec or at 5fps shutter speed is 1/5sec as tests have revealed by some of the worlds best astro imagers.
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There are also two types of chip, CCD and CMOS, the most popular by far is the CMOS (Complimentary Metal Oxide Semiconductor). These cameras are cheaper for manufacturers to make and are smaller in design than camera's with a CCD chip. When light hits a CMOS chip, the charge is converted to a digital signal on the chip itself and then sent straight to the computer. The other type of webcam has a CCD chip (Charged Couple Device). When light hits this chip the charge is sent as an analogue signal to the back of the camera where it is then converted to digital and then sent to the computer. You may think by comparing these two that the CMOS chip is the better option, but in this case you need to choose a camera with a CCD chip. This is because the two types of chip are only a few millimetres in size and because the CMOS chip contains all the hardware needed to convert photon to electron to voltage on the chip, this takes up valuable room on the chip itself, which makes it less sensitive whereas the CCD chip does most it's converting off the chip. Typically a CMOS chip will image down to lux 5 and a CCD chip will image down to lux 1.
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The most widely recommended webcam with a CCD chip was the Phillips ToUcam Pro 840. These webcams are sadly now out of production and have been replaced by the SPC 900NC (also known as the To U cam Pro III) The problem
with the 900
is where the 840 has an adapter that screwed into the body of the camera that enabled you to connect it to the Telescope focuser tube the SPC900 adaptor screws on to the circuit board which houses the chip, which in turn makes it very fragile, one tug on the cable could pull the camera apart and in the darkness of an observatory dome tripping over electrical cables and other bits and pieces is so easily done as I am sure any astronomer will be able to tell you. Another problem with the 900 is an obtrusive white light that illuminates about a foot in front of the camera.
This camera is a little more complex than the 840 to make ready for astrophotography but with a little patience and the correct focus tube adapter I hope this camera can become every bit as good as the 840. . A full guide on how to adapt the 900 can be found on Robert Reeves web site at www.robertreeves.com/900NC.htm
If you do come across a Phillips 840 for sale then grab it as they are becoming hard to find. These cameras will cost you about £60 - £80. If you buy from E-BAY be warned that there are many scams out there and some webcams are sold as new but are second hand and could be faulty, try to buy from established sellers who have got good feed back and if possible in this country. You will also need an adapter to enable the camera to fit into the telescope focuser these can be bought at almost any astronomical supplier such as Broadhurst, Clarkson, and Fuller in Kent where I buy most of my equipment from. All webcams must have there original lenses removed and with some webcams this actually means taking the camera apart there by invalidating the warranty, the Philips ToUcam 840 unscrews quite easily, without dismantling the camera but great care must be taken when screwing in the eyepiece adaptor as the thread on the camera is only plastic. When you screw in the adaptor keep the camera thread pointing to the ground, this is because small slithers of plastic can fall on to the chip as the adaptor is screwed in. You must then carefully take the adaptor out and clean it and also very carefully clean the threads inside the camera once again making sure none falls onto the chip then replace the adaptor again.
If you do happen to get some dirt or a foreign body on the chip you can use a computer air duster can, but do not shake before use and keep the can upright or you may get some liquid residue on the chip.
If you do not intend using the camera on the Internet it is best to leave the adaptor screwed in. Also when buying a webcam you need to make sure the resolution is at least 640 X 480 or above, this figure represents the number of pixels on the chip by row and by column the more pixels the more detail can be captured.Some webcams come with far less resolution but come with software interpolation, which boosts the image electronically but this is of no use in astrophotography.
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A useful accessory you will need is an infrared filter; this is because CCD chips are sensitive to infrared emission. This is not a problem when working on the moon as the moon can be imaged in black and white but if you are imaging in colour your images may take on a violet tint, but with an IR filter in place this magenta cast is much less noticeable. You may still have to adjust the camera controls to get the colour to what you think it should be but this is much easier with a filter than without it. Another bonus for having a filter in place is you will be able to protect the chip from dust and dirt.
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Another useful accessory you may want is an image mate. This acts as a Barlow lens but with much superior optics. You can buy them from BCF for around £60 and they come with varying factors of magnification. This is important because you need to boost your focal ratio to ensure the image is of sufficient size to show planetary detail.
e.g. A 203mm aperture telescope with a focal length of 2,000mm is an f10 (2,000mm ÷ by 203mm), with a 4x image mate it becomes an F40 (4x10). This gives you a working focal length of 8,000mm. Although 6,000mm focal length will be ok.
You will also need a good achromatic Barlow lens, 2x will suffice, this will come in handy when the seeing is poor making it impossible to use magnifications as high as the 4x image mate would give you.
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