Fast optics with regular speed filters?

Hi,

What will happen if using not a highspeed designed filter with fast speed optics such as f/2 or f/1.8, f/2.8? What could be the problem if doing that?

you just end up with far less transmission through the filter than it is designed for. There is a detailed explanation in this Baader document:

Baader white paper on narrowband filters

I wouldn’t necessarily say “far less” transmission. It depends greatly on the filter. For instance, the standard Hutech NBZ filter claims to lose only about 5% to 8% transmission when using their standard filter with an f/2 system. Simply put, some filters require more shift than others.

But Jon’s overall statement is correct. Nothing evil will happen, but you will collect varying degrees of less data depending on the filter and the optical system.

Fyi - I do it all the time.

yeh, probably a poor choice of words on my part but, if you decided to use a standard 3nm bandpass H-alpha filter on say a RASA, then you could be down towards 50% transmission…

Also depends on the filter bandwidth. Seven nanometer filters are less affected but at the expense of lower contrast. Three nanometer filters have better contrast but, as Jon stated above, lose a significant amount of transmission.

Jon Woodhead · Feb 22, 2026, 12:50 AM
you just end up with far less transmission through the filter than it is designed for. There is a detailed explanation in this Baader document:
Baader white paper on narrowband filters

Ok, i will read it and i hope i can understand it

Tommy Mastro · Feb 22, 2026, 01:14 AM
I wouldn’t necessarily say “far less” transmission. It depends greatly on the filter. For instance, the standard Hutech NBZ filter claims to lose only about 5% to 8% transmission when using their standard filter with an f/2 system. Simply put, some filters require more shift than others.
But Jon’s overall statement is correct. Nothing evil will happen, but you will collect varying degrees of less data depending on the filter and the optical system.
Fyi - I do it all the time.

I thought faster optics will gather more lights which means more data, or does it mean that the filter isn’t wider enough to collect that much light? Because the point of shift i can’t understand at all even with the explanation image, but i think it happens whether i understand it or not.

Jon Woodhead · Feb 22, 2026, 02:39 AM
yeh, probably a poor choice of words on my part but, if you decided to use a standard 3nm bandpass H-alpha filter on say a RASA, then you could be down towards 50% transmission…

Not just RASA but also lenses such as 135mm f/2 at f/2 or even 300mm at f/2.8 or 85mm at f/1.8 and such.

Arnie · Feb 22, 2026, 03:55 AM
Also depends on the filter bandwidth. Seven nanometer filters are less affected but at the expense of lower contrast. Three nanometer filters have better contrast but, as Jon stated above, lose a significant amount of transmission.

So the best way or choice is to buy a filter that is precisely designed for fast optics such as from Baader and alike.

Jim the Filter Guy has a four video series on filters - pt 4 has a section on Fast Optics - https://youtu.be/gy_GX6KnQvU?si=AVM9bVdbsPj4yK03 if you want to understand the why better without necessarily taking a course in physics and another in optics.

CS,

Dave

Bottom line is it makes no sense to invest in a super fast system only to throw away a considerable amount of the light being delivered to the sensor.

David Jones · Feb 22, 2026, 01:05 PM
Jim the Filter Guy has a four video series on filters - pt 4 has a section on Fast Optics - https://youtu.be/gy_GX6KnQvU?si=AVM9bVdbsPj4yK03 if you want to understand the why better without necessarily taking a course in physics and another in optics.
CS,
Dave

Ok, thank you

Tony Gondola · Feb 22, 2026, 01:35 PM
Bottom line is it makes no sense to invest in a super fast system only to throw away a considerable amount of the light being delivered to the sensor.

I didn’t buy any fast scope yet except 6” f/4 which is not super fast and it is ok with regular filters, but i do have DSLR lenses which are fast and i have them long time ago, i am mainly would like to use 135mm and 300mm, f/2 and f/2.8, and i don’t want to stop them down, no point to stop them down and make them almost like scopes speed, i want them at wide open for light gathering, but then it means i need filters for it, the other question in my mind is, why is it only with narrowbanding filters and not LRGB, just because they are so broad wide band filters?

Tareq Abdulla · Feb 22, 2026, 04:44 PM
Tony Gondola · Feb 22, 2026, 01:35 PM
Bottom line is it makes no sense to invest in a super fast system only to throw away a considerable amount of the light being delivered to the sensor.
I didn’t buy any fast scope yet except 6” f/4 which is not super fast and it is ok with regular filters, but i do have DSLR lenses which are fast and i have them long time ago, i am mainly would like to use 135mm and 300mm, f/2 and f/2.8, and i don’t want to stop them down, no point to stop them down and make them almost like scopes speed, i want them at wide open for light gathering, but then it means i need filters for it, the other question in my mind is, why is it only with narrowbanding filters and not LRGB, just because they are so broad wide band filters?

That’s right.

Tareq Abdulla · Feb 22, 2026, 12:44 AM
Hi,
What will happen if using not a highspeed designed filter with fast speed optics such as f/2 or f/1.8, f/2.8? What could be the problem if doing that?

Depends on how tight the filter is.

I know that 7nm filters are fine on F2 systems like the RASA and Hyperstar, but any narrower and you need to get pre-shifted F2 filters.

Tareq Abdulla · Feb 22, 2026, 12:01 PM
Arnie · Feb 22, 2026, 03:55 AM
Also depends on the filter bandwidth. Seven nanometer filters are less affected but at the expense of lower contrast. Three nanometer filters have better contrast but, as Jon stated above, lose a significant amount of transmission.
So the best way or choice is to buy a filter that is precisely designed for fast optics such as from Baader and alike.

Ideally, yes.

With camera lenses you might be able to mount one up front, depending on size, etc and avoid the band shift issue.

Kyle Cerniglia · Feb 23, 2026, 04:40 AM
Tareq Abdulla · Feb 22, 2026, 12:44 AM
Hi,
What will happen if using not a highspeed designed filter with fast speed optics such as f/2 or f/1.8, f/2.8? What could be the problem if doing that?
Depends on how tight the filter is.
I know that 7nm filters are fine on F2 systems like the RASA and Hyperstar, but any narrower and you need to get pre-shifted F2 filters.

I use 12nm Ha and SII filters with no problem with my RASA11, 6nm OIII is borderline - it works but not as transparent as my other one with 5nm but designed for fast optics. So as it was stated already before - nothing will happen, you only will get less emission band data and a reduted SNR.

Tony Gondola · Feb 22, 2026, 06:08 PM
Tareq Abdulla · Feb 22, 2026, 04:44 PM
Tony Gondola · Feb 22, 2026, 01:35 PM
Bottom line is it makes no sense to invest in a super fast system only to throw away a considerable amount of the light being delivered to the sensor.
I didn’t buy any fast scope yet except 6” f/4 which is not super fast and it is ok with regular filters, but i do have DSLR lenses which are fast and i have them long time ago, i am mainly would like to use 135mm and 300mm, f/2 and f/2.8, and i don’t want to stop them down, no point to stop them down and make them almost like scopes speed, i want them at wide open for light gathering, but then it means i need filters for it, the other question in my mind is, why is it only with narrowbanding filters and not LRGB, just because they are so broad wide band filters?
That’s right.

Ok, thank you

Kyle Cerniglia · Feb 23, 2026, 04:40 AM
Tareq Abdulla · Feb 22, 2026, 12:44 AM
Hi,
What will happen if using not a highspeed designed filter with fast speed optics such as f/2 or f/1.8, f/2.8? What could be the problem if doing that?
Depends on how tight the filter is.
I know that 7nm filters are fine on F2 systems like the RASA and Hyperstar, but any narrower and you need to get pre-shifted F2 filters.

Ok, it is better to go with a correct filter then, i don’t want to lose something by using not the correct filter, it is clear to me from all answers here.

Arnie · Feb 23, 2026, 05:48 AM
Tareq Abdulla · Feb 22, 2026, 12:01 PM
Arnie · Feb 22, 2026, 03:55 AM
Also depends on the filter bandwidth. Seven nanometer filters are less affected but at the expense of lower contrast. Three nanometer filters have better contrast but, as Jon stated above, lose a significant amount of transmission.
So the best way or choice is to buy a filter that is precisely designed for fast optics such as from Baader and alike.
Ideally, yes.

Ok 👍️

TiffsAndAstro · Feb 23, 2026, 07:59 AM
With camera lenses you might be able to mount one up front, depending on size, etc and avoid the band shift issue.

How? All my filters are either 1.25” or 36mm or 2”, and they are all smaller than lenses frontal except those which has smaller than 2” filter thread such as 42mm maybe.

I wouldn't recommend using very narrowband filters of fast systems. I used to have a C8 with the Hyperstar. I got a 7nm dual and filter and the transmission for Ha was only about 17%.

I was getting brighter images with my 70mm refractor than the Hyperstar setup, however it was still way better than without a filter. Oiii transmission was slightly better at around 39% but that's still quite bad.

I wouldn't recommend anything narrower than 12nm although more modern filters have less of a spike for the bandpass and they have a “flat top” which may get better transmission. Basically be very careful when selecting filters and remember that there is variation between each individual filters so you may not get the same results others get.

Arpad R · Feb 25, 2026, 09:04 PM
I wouldn't recommend using very narrowband filters of fast systems. I used to have a C8 with the Hyperstar. I got a 7nm dual and filter and the transmission for Ha was only about 17%.
I was getting brighter images with my 70mm refractor than the Hyperstar setup, however it was still way better than without a filter. Oiii transmission was slightly better at around 39% but that's still quite bad.
I wouldn't recommend anything narrower than 12nm although more modern filters have less of a spike for the bandpass and they have a “flat top” which may get better transmission. Basically be very careful when selecting filters and remember that there is variation between each individual filters so you may not get the same results others get.

Ok, then because i will use lenses with vary speed from f/1.4 or even f/1.0 if possible up to f/2.8 then i will get a filter for that, i think one filter is enough as i don’t plan to use those lenses to capture all narrowband signals, i only care about Ha, or, if OIII is a weak signal then i buy OIII for this fast optic then i can get better data maybe.

I’ve just had to purchase all new filters for my new scope as the old 3.5nm Ha/SII/OIII set I had was a long way from optimal at f/2.8…. I have a different set one of my filterwheels that is 7nm and that is suitable enough at f/2.8, but its still not optimal… I may have to purchase a second set for each camera that I can swap out depending on what optics I’m using on the scope at that time.

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