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| Very home made! |
Summarizing: the cam will take images of the sky and send them to a guiding program, for example PHD Guiding (for windows). You select a star to keep centered and the program calculates how your telescope mount should be moved in order to keep the selected star stationary by calculating the apparent motion between the images it receives from the cam.
This method is better than inertial guidance (standard tracking in telescope mounts: the mount moving based on it's position and internal clock) as optical tracking doesn't suffer drift.
Pretty neat eh?
Again, you can buy these off-the-shelf, but they cost quite a bit of money (usually upwards of a hundred dollars like this Celestron unit). Why buy that when you can make one with some inexpensive parts you've probably got lying around the house?*
So let's make one!
In this post I'll explain what I did to the webcam and how it's mounted to the lens.
*Disclaimer: this is just one of the probably many mounting possibilities and strongly depends on what kind of webcam you have. Consider it for inspiration, unless you have the same parts of course. Also I have to admit that I'm assuming you have some sort of netbook or laptop to run the guidance and webcam software on. The Celestron unit is able to operate standalone (though that strikes me as being rather lonely for the guider.. poor thing).
So you will need:
- An old lens: preferably something with a focal length > 135mm. If you don't have one, you can find plenty of lenses on ebay or marktplaats (dutch) for cheap, sometimes for just 5 euro's. The quality of the lens isn't that important as we are only going to use it for guiding the mount rather than observation or taking pictures.
- A rear lens cap for said lens.
- An old webcam of reasonable quality: be prepared to take it apart beyond repair as we are going to strip it down to the bare sensor. Compact is good as the final product will need to fit inside an old-fashioned 35 mm film container. It'd be nice if the cam has a resolution of 800x600 or higher.
- An old-fashioned 35mm film container.
- (optional) Spray paint
- A laptop / netbook to run webcam and autoguider software on
As for tools, these are what you'll use:
- A drill
- Something drill-like to drill 20mm holes with.
- Glue
- Sanding paper
My victimized webcam is a Microsoft Lifecam NX-6000.
The challenge is to mount the webcam sensor at the flange focal distance of the lens. Click the wiki link to see what that distance is for your lens. For old Olympus OM lenses it's 46mm and for M42 and Pentax K it's 45.46mm.
So that's basically what we are going to make: something to mount the webcam sensor at the right position behind the lens.
Webcam
Here's a picture of my cam before butchering. | |
| The victim |
We want the webcam sensor to be in the projection plane of the bigger lens. The "optics" already in front of the cam act as a very wide angle lens. This means we need to get rid of all that plastic or the lens won't be able to focus properly. So first you need to take apart the webcam.
Unfortunately, I forgot to take pictures during the process, but it's a simple process. Basically it means stripping the cam of it's housing. Be careful not to harm the internal component.
Removing the top half of the housing allowed me to rotate the sensor 90 degrees so it is now perpendicular to long axis. I removed the useless bit of plastic, but I kept the lower half of the housing as a base for the electronics and the sensor.
After wrapping the whole thing in duct tape (or gaffer tape, whatever you like best), it should look something like this:
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| Stripped, modified and duct taped |
Here's another view from the side; you can probably make out the lines of the old housing.
And here's a close up of the sensor. Note that I fixed the sensor in a right angle to the piece of plastic that used to be the housing with some super glue (a little visible on the right side of the black frame around the sensor).
Most importantly is that it will fit inside a 35 mm film container now with the sensor facing forward.
Mount for the lens
The lens mount is very simple: it's a rear lens cap with a film container glued to it and a 20mm hole in the middle. Here's how to:1. Sand the bottom of the container and the top of the lens cap. If your container as a little thickened edge on the open side, like the one pictured here, you'll want to cut it off (I'll get to the why in part 3 of this tutorial). A utility knife works well, but be careful not to cut yourself. Afterwards sand the edge smooth and clean.
2. Glue the cap and container together with the closed sides towards each other. Make sure they are aligned ok. I used a construction adhesive. It takes about 24 hours to dry fully, but it works great because it also acts as a filler.
3. When they are dry (depending on your glue), punch a hole in the center of the filmcontainer. This is to guide the drill.
You can skip this step if you have a lens cap with a center indication inside the cap.
4. You can now drill a small hole (4mm) at the spot where you've just punched the hole, drilling from the lens cap side. This helps to guide the larger drill which you will use next to make the big hole.
Note the type of drill I used (20 mm wide).
5. Remove burrs by sanding the inside of the hole smooth. It now should look something like this.
Optionally, but recommended: now would be the time to paint this piece of plastic. I've coated mine in a matte black.
6. Mount on the back of the lens and insert webcam package with the sensor forward. It should now look like the photo at the start of this post. If the fit is a little loose, use duct tape to fatten the package.
Make sure everything is dust free before using it. You don't want dust on your sensor or in your lens.
Next post I will explain a bit on how to "calibrate" this astro webcam and what kind of optical performance you can expect.
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