Strictly speaking, the detector is just the circuit board inside the webcam: the CCD and the electronics. It is turned into a camera by putting it into a case and adding the means to connect it to the optics. An obvious choice of optics is your telescope. Take the lens off the webcam and replace it for the eyepiece. Most people use an old film container, cut off the bottom and fix the resulting tube in front of the webcam detector. The diameter of these containers closely matches that of eyepieces.
What you're doing there is often misunderstood: You do not replace your eye with the webcam. Nor do you replace the eyepiece with the webcam. You replace the webcam lens with your telescope lens or mirror.
Personally I prefer to use the M42 threads of the old SLR cameras. Both my SLRs have this thread, as do my lenses (f = 28 mm, 50 mm, 135 mm and 400 mm). I also have a 2x adapter and a set of spacers, either of which screw between camera and lens. A long time ago I also had a ring made that screws onto the outside of my telescope's eyepiece adapter and has an M42 thread on its outside. This allows me to put an M42 SLR body (or a webcam) into the prime focus of the telescope. But it actually leaves the eyepiece adapter functional, so that I can insert an eyepiece and add the camera behind some spacer rings. This is perfect for eyepiece projection (cf. Optics).
If you want to go down the M42 route and use ordinary photo lenses, make sure you can focus them. The back end of the lens mount needs to be about 40 mm from the detector. If the distance is too long you cannot focus. If it is shorter than that you can focus, but have to set the distance on the lens to something nearer than infinity.
Before you stick anything in front of your webcam, you have to remove something: The original lens. On some models - like the Philips Vesta and ToUcam models - the lens can simply be unscrewed from the case. For others - including the Logitech QuickCam models the case has to be opened, the lens unscrewed, and the case re-assembled.
Like other authors, I should perhaps say that by opening the webcam case you may violate the terms of the warranty. If you break the webcam the warranty doesn't cover this, anyway.
As one of the pictures below shows, for my QuickCam VC the ideal M42 spacer to glue onto the webcam case would have been 18 mm. Having to choose between 11 mm and 20 mm I must take the 11 mm spacer and hope that all lenses can themselves lift the remaining 7 mm from their infinity setting. This is no problem for long tele lenses, and also not for my macro-capable 50 mm lens. But the 28 mm lens can't do this.
It is very simple to turn an 11 mm spacer ring into an adapter for all my lenses. The case of the webcam is a ball and glueing the spacer ring onto it leaves no gap for stray light to get in. One has to take care, however, that the ring plane is parallel to the plane of the detector.
You can file away part of the lens's plastic frame such that it can be removed and replaced without opening the ball case. That way the webcam can still be used with its original lens. Tests show that there is not much astronomy one can do with that, the aperture is rather small.
The Philips cameras are more difficult to attach to anything. Their Vesta webcams have the shape of a squashed drop, while the ToUcam webcams look like an egg with the yolk flowing out (the orange lens frame). The ToUcam's also have the detector and circuit board not aligned with the symmetry plane of the case.
With the experience from the QuickCam VC I considered some more features for this camera:
- M42 photo thread. As before, this allows to use a variety of lenses. Also of my telescope in prime focus and in eyepiece projection.
- Correct thread to detector distance. The previous camera relies on the photo lens being able to lift at least 7 mm. This time I want to be close to the correct separation, so that a wider variety of lenses can be used, including a 2x photo adapter.
- Correct alignment of the detector with the optical axis. Previously I had not paid particular attention to the detector being exactly perpendicular to the axis of the spacer ring and hence the lens or telescope. This time I want to set up a lab bench to make sure the image is in focus in all corners of the detector at the simultaneously.
- Mirror and light shaft. The detector is very small, which makes it difficult to find the objects. The field of view that an SLR light shaft affords is almost 10 times larger. Being able to redirect the light away from the webcam detector and into an SLR light shaft can help immensely finding the objects. 'Real' astro-imagers may well have flip mirror units on their telescopes when using an SLR or a CCD camera. But these units work only on the telescope and not on M42 photo lenses. Including this function into the webcam to M42 adapting mechanism would be neat.
- Tripod thread. This is not required when the webcam is used behind the telescope. A long tele photo lens will have its own tripod thread to mount it on a stationary tripod or piggyback on a telescope for guiding. But normal photo lenses do not have this. Either the webcam's own mechanism needs to be used, or the apparatus that attaches the webcam to the lens needs a custom tripod thread. The ToUcam tripod 'thread' is useless, in particular for piggiback mounting parallel to another optics.
- Use of original lens. For some purposes the lens that comes with the webcam may be useful. The detector is very small compared to your regular 35 mm film, so that only the webcam lens will give you a large field of view. This feature is not difficult to accomplish, because the lens is small enough to fit through the M42 ring and also it goes into the ToUcam without dismantling the webcam case each time.
- Eyepiece tube. This is what most webcam astro-imagers go for. The webcam without lens sits at the end of a 1.25 in diameter tube that goes into the telescope instead of an eyepiece.
Once I had taken some images with a 20 mm M42 spacer ring duct-taped in front of the ToUcam I went to the - electronic - drawing board. The proper spacing of the M42 thread and the detector leaves quite a bit of space to put a mirror that deflects the light into a light shaft. But the ToUcam case limits this space. Also, how to insert and remove the mirror?
Not shown here is the deactivation of the LED. The ToUcam Pro has a red LED inside, the light of which is transmitted by clear plastic to the top of the case. This light comes on when the computer recognises the USB device. Some of the LED's light goes astray and may end up on part of the detector. This happens in particular when the webcam's original lens is in place. So I have opened the webcam and removed the LED from the circuit board.
Copyright © 2003 Horst Meyerdierks
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