Ha (Narrowband) Filters and Light Pollution

[Farzad_K]

Hello,

I am newer to LRGB imaging than I am DSLR imaging and have done some LRGB imaging already, and occasionally from the carport at my house with limited view to the north and a lot of light pollution. I am considering these to be practice sessions with hardware and software and would be doing serious imaging in darker locations in my area.

I am reading up on Ha filters to improve contrast and to allow in more substance, I am not looking to mitigate light pollution from society or the Moon. In particular when I hear it being very useful for light pollution I can't hep but wonder if we don't need a filter in front of the filter wheel so light pollution doesn't affect the quality of the RGB channels. I don't see how having one filter, Ha, can help eliminate the effect of, say, moon, on all the other three channels when it is not superimposed during imaging.

So that is my main question: how does Ha eliminate light pollution on an LRGB imaging system.

 

Thanks for any insight.

 

Farzad

 

 

 

 

 

 

 

[newbie alert]

Depending on the bandwidth of the HA filter, ie 12nm,7nm etc that's what the bandwidth it will let through..as in its narrowband..rgb is broadband,so the HA filter won't allow it through..

[Farzad_K]

So narrow band doesn't let light pollution in when it is in front of the camera; I get that. How does narrow band keep the light pollution out when the G filter is in front of the camera? The G filter is wide band, so light pollution still gets in, no?

[ollypenrice]

It's very simple. An Ha filter passes a very specific and restricted part of the red spectrum and nothing else. It blocks most of red and all of green and blue. If you put a green or blue filter behind it there is no green or blue light left for those filters to let through so they are effectively useless, or opaque if you prefer.

An Ha filter does beat the light pollution and some of the moonlight but it only passes the light from ionized hydrogen, which is deep red. If the object in your scope is not largely composed of ionized hydrogen then it won't be glowing in the colour that the Ha filter passes so you won't get any light from it.

It happens that most light pollution and much of the light reflected by the moon is not of the same wavelength as the light from ionized hydrogen so it does not spoil the signal from those glowing gasses out in space. So you can photograph those glowing gasses but you cannot photograph anything else. 

Olly

 

[Thalestris24]

Hi

Narrowband still lets lp through. These days a lot of lp is broadband like the Moon. I live in a red zone and, although a 7nm Ha filter helps me image emission nebulae, it doesn't keep the lp out - I still get a lot of skyglow coming through and gradients from streetlights, even with modest exposure lengths e.g. 360s. I've thought about trying a Baader 3.5nm Ha filter but they are quite expensive and I'm not convinced it would make a huge difference. I need persuading. In the meantime, I generally use an IDAS lp filter (for osc imaging) and 'expose to the right' - not ideal, but at least it lets me do some imaging. RGB filters will still let lp through (albeit less than no filters) so a lp filter is still recommended, I believe. You probably wouldn't want to use a R,G,B filter plus a narrowband filter - how would you and what would be the point anyway? I can only imagine that there's nothing like a dark site to get the best out of your imaging equipment.

Louise

[Thalestris24]

Here is why you want to avoid a full Moon!

[Thalestris24]

5 minutes ago, ollypenrice said:

It's very simple. An Ha filter passes a very specific and restricted part of the red spectrum and nothing else. It blocks most of red and all of green and blue. If you put a green or blue filter behind it there is no green or blue light left for those filters to let through so they are effectively useless, or opaque if you prefer.

An Ha filter does beat the light pollution and some of the moonlight but it only passes the light from ionized hydrogen, whidh is deep red. If the object in your scope is not largely composed of ionized hydrogen then it won't be glowing in the colour that the Ha filter passes so you won't get any light from it.

It happens that most light pollution and much of the light reflected by the moon is not of the same wavelength as the light from ionized hydrogen so it does not spoil the signal from those glowing gasses out in space. So you can photograph those glowing gasses but you cannot photograph anything else. 

Olly

 

Not quite true Olly! Any broadband light source can also include the Ha wavelengths. That includes stars, lp, skyglow, and the Moon. 

Louise

[ollypenrice]

11 minutes ago, Thalestris24 said:

Hi

Narrowband still lets lp through. These days a lot of lp is broadband like the Moon. I live in a red zone and, although a 7nm Ha filter helps me image emission nebulae, it doesn't keep the lp out - I still get a lot of skyglow coming through and gradients from streetlights, even with modest exposure lengths e.g. 360s. I've thought about trying a Baader 3.5nm Ha filter but they are quite expensive and I'm not convinced it would make a huge difference. I need persuading. In the meantime, I generally use an IDAS lp filter (for osc imaging) and 'expose to the right' - not ideal, but at least it lets me do some imaging. RGB filters will still let lp through (albeit less than no filters) so a lp filter is still recommended, I believe. You probably wouldn't want to use a R,G,B filter plus a narrowband filter - how would you and what would be the point anyway? I can only imagine that there's nothing like a dark site to get the best out of your imaging equipment.

Louise

I've shot through two parallel and identical scopes in very bright moonlight with 3nm Astrodon Ha in one scope and 7nm Baader in the other. The 7nm data was not worth keeping while the 3nm was not bad. I don't know about LP but I'm sure there would be a difference. Regarding LP, I'm glad that I don't have any (or much) and I know that full spectrum city lighting is changing things but the fact remains that many fine Ha images are being taken from cities.

Olly

[ollypenrice]

5 minutes ago, Thalestris24 said:

Not quite true Olly! Any broadband light source can also include the Ha wavelengths. That includes stars, lp, skyglow, and the Moon. 

Louise

I said it would beat LP and some of the moonlight. This is correct, it will. I should, perhaps, have said, 'It will beat most of the LP and some of the moonlight,'  but given sufficient exposure it will beat the LP. The LP can be considered as noise and the final image quality as being defined by signal to noise, so it is perfectly possible to obtain an excellent Ha image in LP (or moonlight) but it will take longer. Most of us would be happy with the Ha images taken in London by Malcolm Park, for instance.

Olly

[Thalestris24]

I'll let you off, Olly! As you say, a 7nm Ha isn't really much good against the forces of evil. I'm sure a 3nm Astrodon would help but, for me, the price is too high. I'm lucky if I get 6 imaging opportunities a year these days (what have we done to the climate/weather??) so can't justify the expense of one. I'm still tempted by the 3.5nm Ha Baader as a compromise though it's still a lot of money for an OAP. I'll sleep on it for a few months...

Louise

[DaveS]

I image from bad LP with proliferating LED streetlights but find HII still very do-able with 3nm filters even under moonlight. [NII] is often almost as moon-proof. [OIII] still needs moonless nights but is still do-able under LP with 3nm filters. I hardly bother with [SII] so cannot comment.

[TareqPhoto]

I live in LP too, but sounds my house isn't in the center of heavy or bad LP, so sometimes i am able to get pass or through LP with my Ha filter, and i am planning to buy LRGB filters soon but i am not sure how good they are against LP, but i have to give it a try and see.

I will try to avoid the directions where i see more LP, lucky for me that East has least LP, and some targets are there, M45 and M42 are the ones, and they rise higher to above my head when the time is passing, so i will be away from LP dome anyway, and North has partially LP too regardless the streetlights that are almost in distance of my house towards North, but my house's fence can block them, i have to consider all the possibilities, i took some images as tests last few months ago, and i was amazed with what i saw already, which will give me hope anyway.

[wxsatuser]

Loads of LP here, locally compact flourescents but still sodium from further afield as well.

This is California Nebula at 70 degrees with a full Moon at 135 degrees, plus local LP of course.

16x600secs with Atik 383L+ Samyang 135mm @f/2 with an Astrodon 5nm Ha filter.
If I was still doing dslr AP I would not have even ventured outside, narrowband is an AP life saver.

 

[ollypenrice]

16 hours ago, Thalestris24 said:

I'll let you off, Olly! As you say, a 7nm Ha isn't really much good against the forces of evil. I'm sure a 3nm Astrodon would help but, for me, the price is too high. 

Louise

I know, they cost the earth. I've just baulked at the 2 inch Astrodon OIII and settled for a Baader. We don't do enough OIII to justify the Astrodon but the Ha is another matter. It is such a good filter.

Olly