The Classic spectroscope I built in 2006, although providing great experience and useful results, was not perfect. My workmanship let me down and there are remaining issues of camera attachment and grating movement.
I spend some time making up a stepper motor drive for the grating; works well, but I still wanted a more compact, lighter version.
I've therefore built a Littrow version with a 200mm collimator and micrometer adjustement of the grating. I've also made up a couple of grating holders and purchased a new 2400 line per mm grating ( Now have 600/ 1200/ 2400 gratings)
Incorporated a neon reference lamp ( Chris Hyde has offered to make up a low voltage driver circuit - make it easier, and safer!)
Found there were a couple of undocumented issues with the Littrow design:
The inclination of the grating causes the image of the spectrum to move down the CCD as you go from zero order to red. This "slant" is caused by the inclined cylinderical curve that the grating follows. If not corrected, the spectrum can easily move off the CCD chip entirely.
The way to address the problem is to slightly rotate the grating in the holder, I ended up about 2 degrees rotation. This averages the movement of the spectrum, so although there is still some up and down movement, it's pretty well centred.
Having rotated the grating you also have to rotate the slit to maintain alignment!!!
I also found the getting the neon light into the slit, at the same time as taking and exposure, was not easy. In the LiHiresIII design, the lamp is moved infront of the slit between exposures ie on a different frame. I didn't like this idea; depends on the grating/ CCD not moving relative to the spectrum image.
Ended up using a small 9mm prism to direct the neon into the top half of the slit. Now I can get a reference image at the same time.
The documentation of the design and construction is on the project page of IceIn Space.
Earlier this month I purchased a 1 1/4" Chinese cube beamsplitter to replace the modified Meade OAG.
The hope is to get getter alignment with the star, and brighter images. Not tried yet.
Also invested (!!) in a QHY5 guide camera, should be more sensitive than the webcams or the LPI.
