5 replies to this topic

[John Berger]

assuming I use an F/6.4 system

 

would it be better if I get the 2.5nm h-a, or should I stay with the 3nm?

 

there doesn't seem to be enough reviews of the 2.5, so I'm a little nervous about purchasing that

 

I'm going mono and choosing my first narrowband filters; looking for the best, highest contrast, H and O filters besides Chroma and Astrodon

 

how much of a benefit would it be, buying the 2.5 over the 3?

[daveco2]

I would venture to say you will not see any difference in the final image with those two bandwidths.  
At my Bortle 8 site, I see no difference between 7 and 3 nm. 

[Avgvstvs]

It depends on your levels of light pollution. With more light pollution you benefit from narrower band widths. There's probably not much in it between 2.5 and 3 nm unless you are bortle 7 or higher. Besides how accurate are the numbers compared to the actual product? Many times batches differ. And it could be marketing to rate filter at 2.5nm to justify the extra cost or make it seem better than a 3nm filter. Overall, there's not much in it

Edited by Avgvstvs, 14 July 2024 - 07:02 AM.

[Drothgeb]

The 2.5nm filters use a better glass, so transmission is higher. They also come with individual test results. My 2.8’s are made with Ohara glass, and both the Ha and SII are above 98%. The OIIII is over 96%. But the OIII does halo with bright stars, whereas the 3nm OIII doesn’t

Edited by Drothgeb, 14 July 2024 - 04:22 PM.

[John Berger]

why does the 2.5nm h-a filter graph show wider transmittance bandpass than the 3nm?

 

I see what you mean about the higher transmittance, and I like that a lot

 

but the apparent wider bandwidth is confusing

[HubSky]

why does the 2.5nm h-a filter graph show wider transmittance bandpass than the 3nm?

 

compare.jpg

I see what you mean about the higher transmittance, and I like that a lot

 

but the apparent wider bandwidth is confusing

 

It's just the scale.  The bandwidth is FWHM, full width half max.  So go half way up the graph and measure there.  On the 3nM graph, each division is 1nM for a total of 3nM (3 divisions).  On the 2.5nM graph, each division is 0.5nM for a total of 2.5nM (5 divisions).