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Anyone using Astronomik's MaxFR 6nm filters?


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It’s been a while since I’ve posted on here, so here goes.

I’m currently using a RASA 11” with the Baader F2 HighSpeed filters (which I believe to have a FWHM of 10nm) and while they’re fine, I am based in Bortle 8 skies and have found that they pass just too much of the light pollution as well. This means I need rather high integration times to reach an acceptable SNR and even then, with so much LP the integrated masters are usually low in contrast.

In doing research for a new set of narrower filters, I’ve discovered that finding filters with a small enough FWHM and a high enough transmissivity is difficult for such fast f ratios due to the centre wavelength shift at those extreme angles. While I believe most of the standard filters with a bandpass between 3 and 6nm for instance would produce an image, from my research I’ve come to believe that the signal loss caused by CWS at the steepest light rays would effectively mean one was no longer taking advantage of the telescope’s full aperture and would now operate at a higher focal ratio.

I’ve narrowed it down to Chroma’s 8nm and Astronomik’s MaxFR 6nm filters. I spoke to Chroma and they’ve been very helpful and even sent me spectra graphs of their 3, 5 and 8nm filters at the 12.5-degree incidence angle for the RASA. The 3 and 5nm filters definitely suffer considerable transmissivity loss but the 8nm filter doesn’t look too bad. After seeing the graphs, I was seriously considering buying an 8nm Chroma filter to try out but then I found Astronomik’s new MaxFR range which are specifically designed for very fast optics.

On the Astronomik website they claim their 6nm filter has a guaranteed transmissivity of about 90% at F2 and the 12nm one about 85% at F1.4. After speaking with them they confirmed that the filters are preshifted and backed up the transmissivity claims.

The thing is I’ve only found about two people using the 12nm MaxFR filters but no reports or comparisons to other filters. However, I haven’t found anything on the 6nm ones.

So on to my question, is anyone out there using the 6nm filters? How are you finding them, any issues? I’m not too concerned about reflections but rather performance differences in terms of SNR when compared to other filters such as the Baaders on fast scopes.

I’m really tempted to just buy one and drop it in my filterwheel next to the Baader one and compare them during a session, but given their current price I’d like to hear from anyone that is using them before doing that.

Any input is greatly appreciated as always

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