3nm vs 7nm - Forums

Topic starter

Feb 19, 2023

I am imaging in the city, Bortle 7.

I am experiencing that 3nm filter (L-utlimate) gives significantly worse results than 7nm filter (L-extreme). Noise and SNR values for 3nm filter are better. But the 3nm filter blocks large portion of usefull signal so end result is significantly worse than for the 7nm filter.

It appears that there is a limit of diminishing returns for how narrow filterband can be in poor light conditions, this is wider than 3nm.

This is a hint for those that are planning to buy extreme narrowband filters.

Janez

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Chung Hyun Lee

Feb 19, 2023

Hi Janez, are you using "fast" optics? For a fast telescope with no-preshift filters, it makes sense that 3nm results can be worse than 7nm due to the blue-shifting.

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Stefan Pfleger

Feb 19, 2023

Hi Janez,

I'm in no way an expert here, but with fast optics, the filters experience bandpass shifts. This means for example, that an F/2 RASA Telescope with a 3nm standard filter would only capture a fraction of the actual O3 bandpass, as the whole curve of the filter gets (iirc) shifted to higher wavelengths.

I'm slightly confused though, you mention that your SNR values are better, yet your signal gets worse. I know that there are two bandpasses for O3, but I find it hard to believe that from B7, the 7nm has more Signal due to capturing both O3 bandpasses? Is that even possible? And if it is indeed just bandpass shift as I assume in beginning, you would simply experience less SNR.

Can you share some pictures, that would be more helpful I bet.

Greetings, Stefan.

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Mau_Bard

Feb 19, 2023 · Edited: Feb 19, 2023

Janez Skubic:
I am imaging in the city, Bortle 7.

I am experiencing that 3nm filter (L-utlimate) gives significantly worse results than 7nm filter (L-extreme). Noise and SNR values for 3nm filter are better. But the 3nm filter blocks large portion of usefull signal so end result is significantly worse than for the 7nm filter.

It appears that there is a limit of diminishing returns for how narrow filterband can be in poor light conditions, this is wider than 3nm.

This is a hint for those that are planning to buy extreme narrowband filters.

Janez

I have seen some time ago a comparison among popular narrowband filters, and one of the outcomes was exactly that L-ultimate was giving a clean background (as expected) but was dimming the stars.
We have to consider that narrowband filters, pass some of the continuum spectrum, that is needed to give the image a natural look. If the band is too narrow, this continuum is by design reduced, irrespective of the focal ratio.
On top, as correctly mentioned here, band shift has much more effect with narrow narrowband. If you have a focal smaller than f4, you should definitively consider this, and buy either a preshifted filter for fast focals, or a wider band filter.

I am even using on one of my scopes an L-eNhance that has 24nm Ha and 10nm in OIII with satisfaction, in Bortle 7/8, and with f/4.2.

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Janez Skubic

Topic starter

Feb 19, 2023

I use APO80 and ASI2600MC, pure vanilla setup. Have been using 3nm filter now for 2 months. I was getting lately very poor images, lower noise but but also less signal. I see now that this is related to my switch from 7nm to 3nm.

Janez

Stefan Pfleger

Feb 19, 2023

Janez Skubic:
I use APO80 and ASI2600MC, pure vanilla setup. Have been using 3nm filter now for 2 months. I was getting lately very poor images, lower noise but but also less signal. I see now that this is related to my switch from 7nm to 3nm.

Janez

Ok, I think at this point we would like to see some prove of such a statement (i.e. Comparison of the two images), as that scope should not be faster than f/4, nor can I believe that 3nm captures less signal. Additionally, i'd like to point out that your noise isn't lower, since you are still using the same camera. Only your signal changes depending on LP and Filter setup.
Of course if you shoot broadband targets, the 3nm filter would cut signal, i.e. stars and reflection nebula. Idk why you would do that though with such a filter, so lets assume you don't. In the case that you are shooting emission nebula as expected for such filter usage, I simply cannot understand why you would gather less signal with 3nm filters. Either the filter is broken and you should replace it, or you are doing something wrong. Idk what it is, we can only guess without seeing the comparison of the data you are claiming to see.

Greetings,
Stefan.

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