A Simple CD or DVD Spectroscope
I recently made one of these and added my spare Vcam webcam to it to do some IR filter comparisons.
See the effect of a low quality IR filter here.
A CD works well, especially a CD-R with a very shiny mirror surface and NO COLOUR TINT.
It gives a nice 1st ORDER and an even nicer 2ND ORDER spectra (thanks Robin for the definitions)
A DVD-R works even better, and with some slight modification of the cardboard test bed, the camera
can face further down and look at the 2ND ORDER spectra better.
This spectroscope works quite well and is fairly linear too. Being linear lets you make some quite easy
calibration measurements as fellow webcam astronomer, Robin L taught me, see below.
My neon spectra has quite a few calibration lines in it which Robin was able to measure and put in an Excel table.
The aim of all of this is to do filter comparisons in the near IR or IR regions for the modified webcams.
Some of my results are shown here below:
This is a quick CD-R Spectroscope view of some of my scavenged IR Filters.
They have come from webcams and old handicams etc....anything with a CCD or CMOS sensor.
You can see they do a slightly different job of cutting off the IR end of the spectrum.
A good commercial IR cutoff filter has a very sharp cutoff frequency which stops all or most
of the IR from getting through.
Some of these filters stop Hydrogen Alpha (Ha) red light from getting through.
This Ha light from nebula and especially objects like the Horsehead Nebula is what defines
the appearance of them and if you cut it off, it simply won't get through.
So any of these cheap filters that let Ha through but cut off just above is what you want.
Any filter that does that but the cut off is slow and gradual lets too much IR through and
makes stars appear bloated and unfocussed.