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ANTLIA 3 nm SHO filters transmission spectra (really centered around emission lines?)

Equipment Filters Acquisition H-alpha Narrowband OIII SHO SII Optics Deep Sky Help Needed
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lumendo avatar
lumendo avatar

I was looking for transmission spectra for the ANTLIA 3 nm 2” SHO filters (standard version, non-highspeed). I did not find any spectra on the antlia website. However, for the 50 mm ummounted version I found this:

📷 image.pngimage.pngSources:
http://www.antliafilter.com/pd.jsp?fromColId=137&id=96#_pp=137_607

http://www.antliafilter.com/pd.jsp?fromColId=137&id=61#_pp=137_607

http://www.antliafilter.com/pd.jsp?fromColId=137&id=91#_pp=137_607


So it seems for OIII and Ha the transmission window is exactly centered around the corresponding emission line. This would mean that approximately half of the transmission window is not used because angles of incidence larger than zero lead to a blue shift of the transmission window. I thought that narrowband filters (even non-highspeed versions) are usually slightly red-shifted towards the emission line to compensate for the blue shift of larger angles of incidence.

This seems really odd to me. And it is really strange that they only show the spectra for this particular filter size. I am wondering whether these spectra are actually correct. I have the following questions:

  • Does anyone know whether the transmission windows are actually dead center on the emission lines? Does anyone have spectra for the actual mounted ANTLIA 3 nm 2” SHO filters (normal version)?

  • Would there be any reason for this filter design?

  • Might it be that these spectra are already “effective spetra” taking into account the angular distribution of typical telescopes (e.g. f/5)? So the do not show the spectra for a collimated beam?

Well written Respectful Engaging
andrea tasselli avatar
andrea tasselli avatar

lumendo · Sep 20, 2026, 09:09 AM

blue shift of the transmission window

Blue shift of the emission line. You could ask them since they test every one of them or so they claim.

lumendo avatar
lumendo avatar

andrea tasselli · Sep 20, 2026, 10:47 AM

lumendo · Sep 20, 2026, 09:09 AM

blue shift of the transmission window

Blue shift of the emission line. You could ask them since they test every one of them or so they claim.

Hi Andrea, I am not shure whether I am interpreting your reply correctly (maybe it is a matter of a different wavelength references between the two of us). Generally, die transmission window of a dielectric multilayer is blue-shifting for angles of incidence larger than zero. Furthermore, the emission line has absolute wavelength and is not determined by filter orientation. Could you explain your statement?

I actually asked them via email and got a first response. However, that was not really targeting my questions. So I will try again and let you guys know, if I got helpful information.

andrea tasselli avatar
andrea tasselli avatar

I guess you’re correct although for practicality you shift the line and keep curve the same. Good luck with your quest. From an optical design point of view, with high enough refractive index substrate they don’t really need much shift at all.