Webcams for Astro Work?
Before digital cameras were invented, astrophotography was done using film cameras. Nowadays, it is more usual to use digital cameras. Until recently, digital cameras that are suited to astrophotography have been expensive, particularly those designed especially for astro work.
Astronomers are an inventive lot, so it wasn't long before humble and inexpensive webcams were pointed skywards. Initial results were encouraging.
Webcams are significantly less expensive (approx £50) than purpose made digital astro cameras (approx £500 or more), so can provide an economical way into practical astrophotography.
Webcams are designed to use only very short exposure times, as needed to be able to capture images at rates of 5 to 15 frames per second, giving the impression of acceptably smooth moving images. How can this be used to produce better results than are normally obtained by taking single long exposures using a conventional camera?
The trick is to select the best images, or frames, from a webcam 'movie', and to process these via a computer so as to superimpose these frames to produce a single static image. This can be done by manually aligning and stacking frames using Photoshop or something similar (and much cheaper), but is a very tedious process. Fortunately, software has been developed to automate the process, so it's relatively easy to select, align and stack several hundred or even thousands of frames.
By aligning these short exposure frames before stacking them, we can effectively reduce the effects of atmospheric turbulence which inevitably blurs long exposure photographs. This technique can also be used to cater for inaccurate tracking and even for 'field rotation' which occurs when using Alt/Az (Altitude/Azimouth) mounts.
Aligned and stacked images can show fine details that can rarely if ever be seen by other means.
Stacking frames can average out much of the digital 'noise' that is present in individual frames.
Perhaps the most striking results are those showing images of the major planets, Mars, Jupiter and Saturn, which are typically viewed at high magnification. Other suitable subjects are the Moon and solar details such as sunspots.
For all of these subjects, webcams can be used in their normal, unmodified form. All that is really needed is an adaptor which replaces the original lens of the webcam and fits into the eyepiece holder of a telescope. In addition, it is strongly recommended that an infra red/ultra violet blocking filter is fitted to the adaptor. This filter is needed to correct for the high sensitivity of webcam sensors to infra red light. (IR filters are built into lenses that are supplied with webcams)
Adaptor and IR filter Assembled adaptor, filter and webcam |
Unmodified webcams can be used to produce high quality static images of bright objects, but are unable to properly record feint nebulae and other deep sky objects. To do this requires the use of long exposures. Again, the inventive nature of astronomers came to the rescue.
Perhaps the best known pioneering work in modifying webcams for use in astro work has been presented by Steve Chambers and forms the basis of the well known SC mods.
It is worth mentioning that the SC mods were first developed on the old Philips Vesta webcams, which are now obsolete. However, these mods are also applicable to the later Philips webcams such as the various versions of ToUcam (also now obsolete) and the SPC900NC. These cameras are similar in terms of electronic circuitry in that they all use the NEC uPD16510 chip and so can all be modified in similar ways.
The SPC900NC uses a very different physical layout of components from it's predecessors and as a result, is very much easier to modify than the earlier models. It is no longer necessary to lift pins of the 16510 chip. It's almost as if Philips provided for modification when designing the SPC900NC.
Steve Chambers was not alone in modifying webcams. Several other astronomers have proposed other mods, mostly based on Steve's original work.
For those wishing to modify an SPC900NC, do have a look at the mods reported by Mattheus Meijer. Make sure that you have a CLOSE look at the images (left click on them) under section '5. Long Exposure Mod.'
All of the mods refered to above were intended to be controlled via the parallel printer port of a computer. Most recently made laptop computers make extensive use of USB ports and are not provided with a parallel port.
USB to parallel adaptor are available, but DO NOT WORK with modified webcams. Also PCMCIA to parallel adaptors are not usable for this task. This topic has been discussed on the QCUIAG forum.
USB to serial adaptors are available and these can be used to control modified webcams via yet another adaptor. The down side is that serial ports provide only two lines which can be used for control purposes (ie RTS and DTR). This means that it's not possible to provide full functionality for an SC2 mod (see below) via a serial port. However, it is possible to control basic long exposure and amp-off operation via USB/serial operation. Refer to Steve Chambers circuit. This arrangement provides an interface between a serial port and a conventional SC1.5 or SC2 (again see below) modified webcam, but long exposure operation must be enabled/disabled via a manually operated switch.
So in the absence of a genuine parallel port, the procedure is to modify a webcam in the usual SC1.5 or SC2 (again see below) way, make up a serial to parallel webcam adaptor, fit a USB to serial converter to the computer, and then plug them all together.
Various SC mods are possible with the main types defined on Steve Chamber's website. SC1 and SC2 mods are the original and main reference mods.
SC1 provides basic frame transfer, shutter, and long exposure operation, with long exposure operation being enabled/disabled via a manually operated switch.
SC2 provides for frame1/frame2 transfer, shutter, long exposure, and amp-off operation, controlled entirely via a computer.
Actually, SC2 is uncommon, and SC1.5 is more likely to be seen. As far as I have been able to discover this is essentially SC2, but with the two frame transfer controls connected in parallel as in SC1.
The later mods, SC3 and SC4 (replacement of the sensor chip), and SC5 (still under development) are later additions, and need not concern us here, except perhaps to note that SC3.5 is fairly common among advanced webcam users.
Sensor chips used in webcams suffer from 'amp-glow', which can give the impression of light pollution, typically emanating from the top left hand corner of images. This is causd by electroluminescence, generated by on chip circuitry within the sensor, and becomes significant during long exposures.
'amp-glow' can be reduced by temporarily reducing the voltage that is applied to the sensor chip during exposure, and then reapplying the full voltage before reading out the image data. This is refered to as 'amp-off' operation. The SC2 mod includes a transistor arranged to do this, and as for long exposure, it is controlled via a computer.
The benefits of 'amp-off' are very apparent for exposures of a few minutes or longer. Alternatives to the original SC2 amp-off circuit have been described which are claimed to work somewhat better.
Two alternatives that have been documented are by Martin Burri and Greg Beeke
Greg Beeke's solution has the advantage that no additional components are introduced into the main circuitry. Don't be put off by the reference to the icx414. The mod applies equally well to the webcams using the icx098 sensor chip as used in unmodified ToUcams and SPC900NC. I can recommend it. Greg's website shows the connection point for ToUcam webcams. Connection should be at the same point in the circuitry of the SPC900NC, but the different physical layout might lead to confusion.
Connection point on the SPC900NC for the Beeke amp-off mod. |
All of the mods referred to above apply to webcams made by Philips. There are many other makes and models of webcam, and some have been used/modified for astro work. Steve Chamber's website includes a list of other webcams which have been modified, but none has the history of the Philips webcams. Also, many other webcams are either too 'noisy' and/or are not sensitive enough for astro work. A helpful summary of webcams is presented here.
Users of the Linux operating system might prefer Phillips webcams because a well tried Linux driver is available for these webcams.
More recently, Michel Xhaard has taken it upon himself to write Linux drivers for many webcams, but I have no personal experience of these.
Note: For all of the mods refered to above, transfer of images from webcam to computer is done via the original USB connection present on the unmodified webcam. Control of long exposure, amp-off, etc is done via an ADDITIONAL parallel or USB connection.
So, there is a brief and basic background to the modification of (Philips) webcams for use in astronomy. My own experiences are described in 'Practical Implementation'.
The story cannot be complete without a discussion about computer hardware and software needed to operate modified webcams. These are discussed in 'Computers, Hardware, etc' and in 'Webcam Software'.