D2 filter taking twice as long for flats - less efficient than L-eXtreme?

Optolong L-eXtreme Filters Filter Wheel Calibration Duo-Narrowband
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Benjamin DeHaven avatar
Benjamin DeHaven avatar

I’m currently taking (as in as I write) flats with a cheap LED tracking pad. I just for the first time used my new EWF with an L-eXtreme and an Askar D2 Oiii Sii filter installed. When taking automated flats with my Air camera and wheel for the first time, it occurred to me that at the same, untouched luminosity level, the D2 took nearly twice as long for the auto exposure to hit proper illumination of the flat as the L-eXtreme. Does this mean it’s letting in half the light, as in much less efficient? Or is something else at play here? Would I find better transmission with the L-synergy filter in place of my D2? Thanks!

Benjamin

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Tony Gondola avatar
Tony Gondola avatar

The D2 has narrower band passes so you’d expect to see a difference when shooting flats, I am surprised it’s that much though as it’s just 6nm verses 7nm band passes. I wouldn’t see that as less efficient though. It’s just more efficient at blocking unwanted wavelengths.

Benjamin DeHaven avatar
Benjamin DeHaven avatar

I guess I forgot to say, the L-eXtreme was 8.59s and the D2 was 14.34s.

andrea tasselli avatar
andrea tasselli avatar
The effective passband (also) depends on the focal ratio. If you were using a fast lens/scope then you efficiency might as well drop more than the ratio of nominal passbands. D1/D2 shouldn't be used on anything faster than f/4 and at f/4 they loose more in the OIII than in the longer wavelengths. This is also compounded with the relative size of the secondary, if using a reflector (bar schiefspieglers and such). Given the numbers my best guess you are losing around 30% of the incoming light due to bandpass shift.
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Benjamin DeHaven avatar
Benjamin DeHaven avatar

andrea tasselli · Jan 18, 2026, 09:05 AM

The effective passband (also) depends on the focal ratio. If you were using a fast lens/scope then you efficiency might as well drop more than the ratio of nominal passbands. D1/D2 shouldn't be used on anything faster than f/4 and at f/4 they loose more in the OIII than in the longer wavelengths. This is also compounded with the relative size of the secondary, if using a reflector (bar schiefspieglers and such). Given the numbers my best guess you are losing around 30% of the incoming light due to bandpass shift.

I’m using a Petzval type refractor, an Apertura 75Q. The focal ratio is 5.4…

andrea tasselli avatar
andrea tasselli avatar
Then probably your flat panel doesn't emit in the deep red as much as in the Ha band, besides issues of lower QE at the SII wavelength. At f/5.4 your bandpass shift should be minor (but not zero).
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