The specific details of the modifications are best gained from the Web sites given in Table 4.1. Here, we will only cover the subject in broad terms. Also bear in
Table 4.1. Web sites with background information on long-exposure modified webcams.
mind that astronomical cameras based on these designs are available commercially if soldering to surface mount components is not one of your skills.
The first stage is acquiring a suitable webcam. While this sounds trivial, time spent researching the best webcams for modification will be very worthwhile. At the time of writing the Philips ToUCam II Pro is regarded as the best with the Logitech Quick Cam Pro 4000 and Creative's Ex Pro also worthy. All these cameras feature CCD sensors rather than CMOS devices. While some digital camera CMOS chips have been found to be very capable of astro-imaging, those currently used in webcams invariably have low sensitivity. This might change in the future, so it is worth checking Web sources such as QCUIAG to find the currently favored webcams.
Without modification, the cameras are limited to relatively bright objects such as the Moon and major planets. This lack of sensitivity is not due to inherent limitation in the CCDs used in the webcams. Indeed, the quantum efficiency of the best webcam CCDs is on a par with dedicated astro cameras. The sensitivity problem is linked to the webcam producing a moving image consisting of at least 5 frames per second. To image deep-sky objects we really need to take pictures with exposures of 30 seconds or more rather than the 0.2 second offered by the standard webcam. The solution developed by the authors was to place some of the webcam's internal timing directly under the control of the PC. The electronic circuits needed to achieve this are very simple, but working on the surface mount components requires some skill and experience with a soldering iron. Good advice if you are considering doing this work yourself is to find a scrap circuit board with some surface mounted chips to practice on. Because the specifics of the modifications vary from camera to camera, these are best referenced from the Internet (see Table 4.2). Once modified, the webcam's exposures can be set to any length the user wishes by using software that is compatible with these modifications.
The modification to control length of exposure is fundamental to adapting webcams for deep-sky use. Further modifications to the camera's hardware are optional but offer additional benefits. The CCDs used in the webcams feature on-chip amplification circuitry. This significantly increases the quality of the images produced as it keeps the amount of electrical noise added to the image to
Table 4.2. Sites giving camera-specific modification details.
http://www.philip.davis.dsl.pipex.com/tcp2_mods.htm ToUCam 2 by Phil Davis http://mypage.bluewin.ch/bm98/l3k/modification.htm QC3000 by Martin Burri http://www.foley-tax.com/Astro/modz/ Creative Ex Pro by Jack Reed
Table 4.3. Web sites detailing CCD replacement modifications. http://www.astrosurf.com/astrobond/ebvpnbe.htm Use of 1/4-inch B/W CCD.
http://www.pmdo.com/wsc3.htm 1/3-inch CCD. Steve Chambers
http://www.greg.beeke.btinternet.co.uk/icx414.htm 1/4-inch CCD. Greg Beeke a minimum. However, this amplifier also emits photons by a process termed electro-luminance. In exposures lasting 30 seconds or more this results in an objectionable glow in a corner of the image corresponding to the part of the CCD array closest to this circuit. A solution is to drop the voltage to this circuit while the CCD is collecting its image and then to restore it when it is needed for reading the image out. This modification is, slightly confusingly, known as the "amp off" or "amp switch" modification. Also possible, although not particularly popular, is a modification to the webcams that allows half the CCD to be read out at a different speed from the rest of the array. This can be used to simultaneously guide a telescope using exposures of around one second while the main image is built up for a minute or two.
CCD-based webcams tend to use 1/4-inch color CCDs. The color information is gained from tiny red, green and blue filters that are located on top of the CCD structure. The arrangement is in blocks of 4 pixels, each having 2 green, 1 red and 1 blue filter. This allows the webcam to take a full color image without requiring external filters but, as the filters only let though a single color, the sensitivity is significantly reduced. Recently, amateur astronomers have successfully replaced the standard CCD with unfiltered black-and-white versions (see Table 4.3). It is also possible to swap the standard 1/4-inch CCD for one rather larger like a 1/3-or even a 1/2-inch CCD. When bought in small quantities, CCD chips can be quite expensive, so a 1/2-inch CCD will probably cost more than the original webcam. However, as the light gathered is proportional to the surface area of the sensor, these chip-swaps can be very worthwhile.
The final stage of modifying a webcam is often building a new case for it (see Figures 4.2, 4.3, and 4.4). While it is possible to fit a modified webcam back in
Figure 4.2. SC
modded ToUcam, by Ashley Roeckelein.
Figure 4.2. SC
modded ToUcam, by Ashley Roeckelein.
its original case, there are advantages to using a bigger box that allows for better air circulation and some cooling. When a webcam is left running, it consumes electrical power and produces some heat, which, if left to build up in the camera, will raise the CCD temperature and increase the thermal noise it produces. Simply allowing air to circulate and take away this heat is surprisingly effective, especially on a cold night. For greater cooling, Peltier coolers are able to reduce chip temperatures to 40 degrees or more below ambient. This does generate a whole new set of problems in stopping the chip from dewing or even icing up.
The standard lens in a webcam typically has a focal length of 7mm and a focal ratio of about f/3. This is ideal for capturing whole constellation pictures, maybe including some foreground subject. To move onto specific deep-sky objects a method for coupling the camera to a telescope is required. Probably the most versatile method is to incorporate a camera macro extension ring into the box design because this will allow both 1.25-inch focuser adaptors to be
used for prime focus telescope imaging and camera lenses to be employed for wide-angle shots.
After modifying the webcam hardware for use in deep-sky imaging, it is also possible to change the settings for the camera to make the best use of these alterations. The standard drivers for webcams tend to concentrate on producing the best high frame rate images of brightly lit subjects by using high compression and artificially emphasizing edges in images to give the appearance of sharpness. By writing directly to the memory chip within the webcam, it is possible to override these settings in favor of ones that are able to better represent the image formed on the CCD. This has been shown to give a very great improvement for webcams retaining their standard color CCD, with image artifacts much reduced. For webcams with black-and-white unfiltered CCD, the improvements are even better as higher resolution processing techniques can be employed (see Table 4.4).
A webcam that has been modified to take long exposures doesn't work very well anymore with off-the-shelf webcam software. Astrophotography is much more
Figure 4.6. M51, Whirlpool Galaxy. Captured with SC3 B&W Vesta and SC color Vesta by Etienne Bonduelle with a Meade LX-90 scope and a 0.63x focal reducer. 53 x 90 sec IRB (Infared Black), 42 x 60sec IRB. Astrosnap, Registax, Iris and Paint Shop Pro 7.
Table 4.4. Web sites describing methods for altering the webcam's factory settings.
http://groups.yahoo.com/group/twirg/ Yahoo discussion groups on
http://groups.yahoo.com/group/qcuiag/ webcam reprogramming http://www.foley-
tax.com/Astro/modz/Advanced.htm#ULTRA Jack Reed's guide to webcam firmware
http://www.astrosurf.com/astrobond/ebrawe.htm Etienne Bonduelle's guide to
demanding for a number of reasons. The first is that ordinary software no longer drives the camera properly because it has no way of controlling the new exposure control circuit.
There are a variety of freeware and shareware programs that control cameras modified according to our design. These programs go to great length to acquire the cleanest data possible by providing histograms that allow you to ensure that you are not saturating any part of the image and by performing no compression on the image before it is saved to disk. The most common capture programs are listed in Table 4.5.
The actual task of capturing images requires some understanding of how such images will be processed at a later time. Unlike film astrophotography or most professional CCD astrophotography, webcam astrophotography requires a process called image stacking, explained in greater detail later. For now, realize that image stacking means capturing numerous virtually identical images of an object during one session. Consequently, another function of the programs mentioned is batch image capture, in which a series of long exposures are captured one after the other, perhaps for an hour or more at a time.
Once your webcam is centered on the object in question, the next step is, of course, acquiring your focus. Focusing is tricky with dim objects. If you have a computerized scope, we recommend slewing to a nearby bright star and doing your initial focus there. Medium brightness stars provide better focusing information than really bright stars because the latter produce a huge washed out disk. Once your focus is approximate, we highly recommend using a Hartmann mask or diffraction spikes to refine it even further. Remember to refine your focus
Table 4.5. The most common programs used for capturing images from long-exposure modified webcams. Maxim CCD AstroArt
K3CCDTools for Windows Iris for Windows Astro-snap for Windows AstroVideo for Windows Keith's Astrolmager for Mac Equinox for Mac Qastrocam for Linux Many more
throughout an imaging session as changes in temperature and the angle of the OTA tend to throw the focus off slowly.
Once you are focused, you are pretty much ready to go. Different programs have slightly different interfaces, but you basically need to find the exposure time that suits your purposes and start capturing images. As we will see, there is a balance to be struck between capturing a lot of short exposure images and a few much longer ones. The balance will differ between different subjects but as a rule of thumb collecting lots of short exposures will give high image quality at the expense of detecting faint objects and vice versa.
So, how many images is enough? Webcams are not precision instruments and suffer from a large degree of random noise and detrimental artifacts that result from such causes as warm temperatures, electrical interference, and the on-chip amp. These problems can be reduced in a variety of ways, but ultimately no single raw image will be very impressive.
Stacking multiple images offers several advantages, one of which is reducing noise. Additionally, webcams do not have a very good dynamic range because, by the time they transfer their images to the computer, the images are reduced to only 8 bits. This means that objects with a wide dynamic range (such as most nebulae and galaxies) will be unobtainable in a single image. Bright parts of an object will saturate before you have recorded any discernable signal from dim objects. Stacking can reduce this problem by increasing the dynamic range.
You will get smaller and smaller improvements for greater and greater amounts of stacking. However, when possible, a huge number of images is still the best. With deep-sky imaging, you might take images with exposure times ranging between 15 seconds and 3 minutes. Clearly, you cannot capture more than a certain number of frames while the object is well placed in the sky. In short, the more frames you have the patience to collect, the better your final results will be.
Before you take your scope down for the evening, it is important to take some dark frames as well. A dark frame is an image taken in complete darkness, say with the telescope cover on, or simply with a tight cover over the end of the camera-telescope adapter after removing the camera from the scope. The dark frames should match both the exposure time and the temperature of the actual frames. Dark frames suffer from the same maladies of noise as do actual images, so it is a good idea to take several dark frames and stack them to produce a single final dark frame that can be applied to the raw frames later. In addition to a dark frame, it is a good idea to take a flat field frame. This needn't be done each session. Once will be enough. A flat field is most easily captured against a twilight sky and represents an even illumination of the camera's CCD. Flat fields can be
used to counteract the effects of vignetting (a dimming of the edges of a image) and artifacts caused by dust on the CCD, though it could be argued that keeping the CCD clean is easier than processing out the dust artifacts!
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