Freedom Freedom Freedom, Oy!

Reading through John James’ Spectrograph Design Fundamentals it became increasingly clear that while a Czerny-Turner spectrograph has excellent properties with regards to resolution and elimination of some of the many abberations, it also presents a daunting number of degrees of freedom in its design.

I normally don’t think the phrase “too much of a good thing” applies to freedom, but there’s of course always an exception to the rule.

To put it briefly; I’ve decided upon a much simpler design. I don’t even know if it has a name, but I see that some astronomers employ a similar construction.

The schematic for my spectrograph is shown here (not as pretty as the diagrams on the astrosurf page, but you get the idea):

drawing11The ligth from the slit is collimated by an achromatic doublet (Thorlabs AC254-040-A) one focal length from the slit. Everything is mounted a 2″ long lens tube, greatly reducing the degrees of freedom on this side of the grating.

The focusing lens is an m-rokkor 90mm f/4 that I had lying around. It’s basically a Leica lens in disguise. It doesn’t image the spectrum of interest (500-4000 cm⁻¹) on quite the entire CCD – that would require a slightly longer focal length.

The grating sits in a rotating mount. I don’t plan to do any rotating though, except for calibration. The design of the rotating mount is rudely stolen from here.

And here are the pictures:

DSC01958and

DSC01963Some modifications are pending; I would still like to anodize the aluminium base black, but I’ve not come across a way to do it (that I like). The grating adjustment mechanism might also be changed to something simpler ..think a screw in a piece of plastic..

And of course I’m waiting for the circuit board for the TCD1304 to arrive, so until then you’ll have to use your imagination for that particular component.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s