Two of my friends have recently bought a set of Optolong narrowband filters: Ha (7nm), SII (6.5nm), and OIII (6.5nm). When one of them tested by taking pictures around his house using a modified DSLR (IR-cut filter removed) mounted with either Ha or SII filter, he noticed that there are other colors besides red. He was curious and requested me to do some testing with my Baader Ha (7nm) and SII (8nm) filter. I noticed that I only have red but no other colors in my images. We decided to do a simple comparison between Optolong and Baader filters.
We use a RGB color template as the test subject. The template is displayed on a LED screen. Then using a modified Pentax K-5 (IR-cut removed) camera to image the color chart.
Figure 1: Test Setup
Figure 2: Color Template
Here is the comparison between Optolong Ha (7nm) and Baader Ha (7nm):
Here is the comparison between Optolong SII (6.5nm) and Baader SII (8nm)
According to the spectral curve on Optolong's website, the Ha filter has 80% transmission of Ha (centered around 656nm) and only 0.1% off-band transmission. Here is the official spectral curve:
The Optolong SII filter has 80% transmission of SII (centered around 672nm) and only 0.1% off-band transmission. Here is the official spectral curve:
From our rudimentary tests, the Optolong filters seem to have extremely high off-band transmission as compare to Baader filters. Both the Ha and SII should only appear as red while most green and blue should be cut off as in the Baader filters.
Is our method of testing wrong or something is wrong with the Optolong filters? Can anyone conduct independent tests with similar filters?
Edited by Fongky, 17 December 2017 - 11:27 AM.