Spectrometer and measuring light wavelengths
Using a spectrometer to test astronomy filters and determine the appropriate filter
The spectrometer measures wavelengths within it's range limits of UV, Visible and Near Infrared spectrum from about 360nm to 950nm. For lab and precision testing a certified full spectrum light source would be used. I'm using the rather broad spectrum of the Sun. The spectrometer that I'm using should be calibrated with a fluorescent light source through the software. If an astronomy filter is placed between the light source and the spectrometer's sensor the band pass for that filter will be displayed s long as the source light contains the wavelengths that are being allowed to pass through. A filter can only block light, not add it.
Alternately, if you were to measure the light from the night's sky you should be able to see the areas that may need a filter to be blocked.
The Sun's visible wavelengths and how the spectrometer sees them. I use this as my filter test spectrum.
The Spectrometer
The spectrometer that I'm using was bought new on eBay. It has a 1000 line diffraction grating and a USB connection to the PC. This was a pretty low budget project for what it does.
The calibration process involves using a fluorescent light to align the software with the 436nm and the 546nm spikes. That's the left blue and the middle green peaks shown here. Three coils of the CFL bulb can just be seen peeking out to the left of the notebook monitor and the spectrometer is in the bottom left.
Testing the Optolong L-Pro's wavelengths
This is a light pollution filter that clips many of the man-made lighting wavelengths. The setting Sun was the light source.
Testing the Optolong L-eXtreme's wavelengths
This is a dual band Ha/OIII filter. The setting Sun was the light source.
Spectrometer Filter Adapters
I quickly realized that stray light could get around a filter held in front of the spectrometer, especially the 1.25" size. I tackled this from three angles. First was a 1.25" threaded adapter. Second was a 2" threaded adapter. Lastly was a holder that accepted the ZWO gen 2 filter tray. It was suggested that I make a double gen 2 tray holder to see what different combinations of filters would allow to pass through.
These items only fit the spectrometer in the previous link. In the photo starting in the upper left going clockwise:
➤ Single holder for the ZWO gen 2 filter tray.
Set for supports everywhere. You may need to file off any rough support areas so that the tray will fit smoothly.
➤ Double holder for the ZWO gen 2 filter tray.
Same notes as the single tray holder.
➤ Fluorescent bulb used for calibration from Home Depot.
➤ 1.25" threaded filter holder.
➤ 2" threaded filter holder.
➤ Calibration gap with a stray light shield.
The link on each of the above is to the STL file in case you wanted to 3D print it yourself.
The Gen 2 tray holders use two 5mm x 2mm magnets
for each tray slot epoxyed in place. In reality they are 1.78mm thick and the STL files are made for this thickness.
If the Gen 2 holder could be used without magnets if you are careful not to allow the tray to slide out.
➤ Dust Cap (Not shown).
