I was playing around with an $8 plastic handheld spectroscopes, one of the triangular black ones. It is graduated with a 10nm scale. It is not well calibrated in terms of absolute wavelength, but 10nm seems about the right width per line.
I am updating my narrow band filter collection so I have a number of filters available. I was surprised how accurately I could estimate the FWHM, within 20% (or about 2nm) on five filters. Estimating the transmission is tough; hard to tell even a 2x difference. And hard to tel if you are just making or just missing a line without a reference, but easy to go back and forth between filters and compare placement.
For $8, it teaches you a lot about your filters. You can easily see which ones pass red light in addition to their main wavelength. You can tell which kind of old Lumicon you have. You can tell exactly what your UHC-like filter is passing.
Some predictable results:
I got my first DGM NPB filter. In daylight it is mostly red to me. The spectroscope shows its passbands. In the telescope on M57, as related by other viewers, it seems to perform just like UHC filters. The big red passband did not brighten the sky background to my eye (due to poor red sensitivity in dim light). I look forward to testing it out on objects with H-alpha or other red emissions.
My Celestron OIII is very narrow, as demonstrated in the spectroscope. In the telescope, it really dims the view of not just the sky and stars, but also the OIII emissions themselves. It really is a photographic filter that should be marketed as such. It was still better than no filter on M27 and M57, but noticeably inferior to my other visual filters, OIII and UHC. Nice price though.