When do you start considering F ratio when choosing "fast" filters? (e.g., F/4, F/3, F/2?)

From what I've seen, above f/4 and really above f/3.5 is more or less the cutoff for normal filters. Anything faster than that needs the highspeed versions.

For me bellow f/3 or equivalent. F/4 could be called already fast, but to be honest most narrowband filters can manage at that speed. Bellow that is a problem.

Regards,

Cesar

Markice Stephenson:
You will normally be directed to either regular or fast filters, whichever is consistent with your focal ratio. 

In the case of fast filters, some manufacturers show where the transmittance decreases by focal ratio (IDAS) while others just give you a range where the filter is expected to perform according to specifications (ANTLIA ALP-T, f/2.2-f/4).

Here's an example of the IDAS NBZ II filter that shows the focal ratio performance by transmittance%: 

Perfect thanks!

The narrower your filters, the more impact optical speed will have. 

If you're shooting 12nm filters, you won't notice much between f/5 and f/2, and even 7~6nm filters, there is a difference down at f/2 ~ f/2.8, but you're probably losing 8~10% transmission at best. If however, you're shooting 5 to 3nm band pass filters, you're going to want 'fast' specific filters any faster than f/3, and as mentioned, even at f/4, you're going to see SOME transmission shift at 3nm, but not bad enought ot warrant 'fast' filters.

I'm shooting Astronomik 6nm SHO filters with my 1600MM Pro on a 130mm f/2.8 newtonian and don't see the need to move to 'fast' filters for it… if I were using 3nm filters, I would either have to buy fast filters, or ditch the 130/2.8 for narrowband work.

Most filter manufacturers publish the ratio they are good for, and if you are using wide NB filters like 12nm, I doubt it will matter much. I try to buy fast filters that are good to f2.8 whenever Isave on buy filters to  having to upgrade later.

Hi everyone,
I use the Antlia 2.8 nm at F3.75 and get very good results with it. 
I used my Newtonian at F5 and F3.75 (with Nexus Reducer) and captured the same halpha region. The increase in brightness was only slightly below the expected increase. This was not a highly scientific experiment, but it did ease my conscience.

There is a white paper from Baader on this topic that explains the problem and also contains diagrams that you can use as a guide: https://www.baader-planetarium.com/en/downloads/dl/file/id/1794/product/0/whitepaper_narrowband_filters_on_astronomical_telescopes.pdf


cs
Andreas

And, for added fun, unless you have a transmission curve for your filter, your mileage may vary………

Tony Gondola:
And, for added fun, unless you have a transmission curve for your filter, your mileage may vary.........

That's the tough part, you don't know where in the filter lottery you sit. I was looking at buying a spectrometer to test them so I would know for certain, but they are not cheap.

Yes, that's a tuff one. My personal approach may be different from many, but it's what I have adopted after 19 years of imaging.  
1. You generally get what you pay for.  It is not to a manufacture's advantage to over price in a small competitive market. It takes many, many layers to build up the rejection of particular wave length and only accept a very small 6 to 3nm of wave length. Harder yet is get that centered on the main wave. Now, throw in the ability to alter that acceptance towards the outer steep light cone. It doesn't surprise me they are expensive. It also doesn't surprise me that cheaper filters have a lower tolerance of quality controll.  That is true with any product.

I have see YouTube video we're a knowledgeable imagers (sorry, I forget the vediographor's name) did spectrograph testing on some filters. Disappointing that budget usually limits such testing to cheaper filters. But, maybe those cheaper filters are the ones you want tested. Anyway, he found that some let a small percent of the designated wave length through.   So, the avatatage of a camera rendering 80 to 90% QE can be greatly compromised by a filter reducing the designated wave length.  The more narrow the band pass, the harder to maintain high tolerance.   Go to a wider band pass, not a cheaper brand.

When Don Goldman was still owner of Astrodon filters, his web site claimed his filters transmission were in the nineties percent (forget exact percentage).  He also claimed to have tested other brands making similar claims, and found they could be as low as 70%.  After he sold the company those tolerance could no longer be maintained. It's my understanding that Astrodon filters are no longer made. 

2. Your system is only as good as your weakest link. It's hard to spend $1000 on equipment you can put in your shirt pocket. But cheating out out on filters can diminish investment in quality optics and camera. Not to mention acquisition time.

Anyway,  that's my thinking and approach to the filter issue. I don't have the ability/knowledge to test filters and get into a debate with a manufacturer over transmission of a filter. I only have the choice to put my faith in the manufacture. I now buy Astronomik and Chroma.

Lynn K.

FWIW, Altair sells all their narrowband filters with individual spectrographic reports. Their costs are super reasonable and I've been happy with mine.

Just to follow-up.  Chroma got back to me and sent transmittance curves of their standard 3nm filters @ f3.6 for Ha, S2 and O3.  They said all is good - no cutoff.  I read elsewhere that Chroma uses optical glass with a high index of refraction and this helps with faster scopes.  The same post showed data for Antlia filters that faired worse in this respect and the post mentioned the lower index of refraction being the cause.