7 replies to this topic

[Ken J Cunningham]

Think about taking the step into SHO narrow band shooting but have a question. I know the conventional wisdom is to buy the narrowest band pass you can but I'm wondering if that applies to shooting on longer focal lengths? I'm currently shooting on scopes with focal lengths of 323mm, 900mm and 1280mm

 

Wondering if getting 3 nm filters will cause problems getting focus etc on the longer focal length scopes?

[gcardona]

The problem is not long focal lengths, but short focal lengths and fast f-ratios. See this post:

 

https://www.cloudyni...with-f2-lenses/

[daveco2]

I have five scopes with focal lengths from 180 mm to 2000 mm, and have used filters with 3 nm and 7 nm bandpass in all of them.  F-numbers vary from 5 to 10.  I see no difference in the focusing or the data for any of those combinations.

I went from 7 nm to 3 nm hoping to eliminate more light pollution at my Bortle 8 site, but no difference was noticeable.  You could save some money by going with 7 nm.

 

Whether 3 nm or 7 nm, the signal for Ha, Oiii, and Sii will be the same; because the atomic linewidth is much smaller than the filter pass-band.  

[smiller]

It’s not the focal length, but the focal ratio that causes the issue.  Small focal ratios are the challenge with narrow filters.

 

This paper which reviews a few filters talks about this phenomena and models the impact of various f-ratios.  Even though these aren’t the specific filters you are looking for I think this would be helpful in educating you:

 

https://www.research...Comparison_Test

 

and this one probably does have filters you are interested in:

https://www.research...Comparison_Test

Edited by smiller, 23 April 2025 - 07:35 PM.

[KGoodwin]

I use 3nm filters with an f/8 scope (2500mm focal length).  Everything is great.

[DirtyRod]

The only challenge I have with 3nm is focusing and platesolving at 2032mm on targets that don’t have lots of brights stars right around them. The ASIAir has a special platesolving mode under Experimental conditions that helps with my shorter refractors but, with the small FOV of my C8 at 2032, about half the time I have to revert back to 7nm especially if the target is lower on the horizon. With Nina, I sometimes have to bump up the exposures for focusing to 40 seconds and bump the gain but that doesn’t work half the time. I’m in Bortle 7-8 so perhaps these are not issues in darker skies.

[KGoodwin]

Don't plate solve with narrowband filters.  The only reason I can think of to plate solve through a narrowband filter is a RASA/Hyperstar where you can't change the filter.  Otherwise, use the L filter for plate solving.  For autofocus I use filter offsets and focus with the L filter also, but I can see reasons why you might want to autofocus on a certain filter.

Edited by KGoodwin, 25 April 2025 - 02:50 PM.

[Ken J Cunningham]

Thanks for everyone's input on this and sorry for the late response. Sounds like I’m probably safe to go 3nm. Unfortunately due to the current economic turbulence and so bad news at work, I'll probably hold off on this purchase for a bit. Clear sky's!