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I teach physics at a community college, and for use in our lab course we have a nice set of 1960s spectrometers that each contain a telescope. (For a description of the optics, see my lab manual, lab 14, p. 62.) The telescope has an eyepiece that fits into a 49/64 inch diameter hole and seems to have a focal length of about 2 mm (I haven't been able to find an exact figure). The manufacturer is still in business and sells the eyepiece as a microscope eyepiece (their part number M240).

The students line up the telescope by eye on a spectroscopic line using crosshairs, and read the angle from a vernier scale. I think I could actually save a lot of time and effort, and get a better educational experience for my students, if I could replace the eyepiece with a digital camera. I would take a double exposure of a calibration spectrum and the unknown spectrum, for a case where the calibration lines were visible in the same field of view as the lines with unknown wavelengths that are to be determined.

So the only thing is that I need some kind of inexpensive digital camera that can be adapted physically to a hole of this size and has a ~2 mm lens, manually adjustable focus, and manual control of exposure. The supplier sells something like this for $850, which is way out of my price range. Would it be possible to adapt a $50 USB microscope or el cheapo telescope camera for this purpose? It seems that 49/64" is not a standard size for a telescope eyepiece. Cheap USB microscopes seem to be designed for little kids to hold them in their hands and look at leaves and bugs.

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You don't want a camera with a 2mm lens. you're not trying to use it like an eyepiece and get parallel light rays coming out of the back of the lens - you need something that will focus the image of the spectrum on the camera sensor.

The normal way would be to use a standard microscope USB eyepiece camera without a lens, and adjust the focus of the telescope so that the the image of the spectrum is focused on the camera sensor (i.e. further out than normal), instead of at the crosshair. However, you might end up needing to remove the crosshair if it causes a shadow problem on the sensor. But I don't know if the telescope focuser has enough in focus to get the image far enough out to focus past the end of the eyepiece holder (which is where the sensor is likely to be positioned).

If you could find a camera small enough to slide into the eyepiece holder tube, then you'd have the option of removing the crosshair and sliding the camera in so the sensor is where the crosshair used to be.

Alternatively, you might be able to try a digital camera with a very close focus macro capability, take the eyepiece out, and focus on the crosshair.

Or, you could just try afocal photography - set the camera/webcam to infinity focus, and line it up with the eyepiece lens. This usually needs a camera where the camera lens is smaller than the eyepiece lens - so a phone camera might work well. buy a cheap phone case, and glue a tube that's a snug fit over the eyepiece/eyepiece tube to hold it in the right position.

By the way, if you have a search around, there is at least one shareware spectrographic program (originally intended for astronomical use with a diffraction grating, webcam, and spacer) that may be useful for what you want to do.

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Consider using a telescope-camera adapter. You can search your favorite auction site for "telescope camera adapter" along with the name of your camera or mount.

Many come in two pieces. Telescope to T-mount. T-mount to camera mount.

telescope adapter

Here is a picture of a microscope adapter. It's the same concept, and works the same way.

microscope adapter

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