Red shift and narrowband filters

[Kawakaze]

Im planning on trying out some narrowband filters to hopefully dodge the light pollution here and get some nice photos, I was just wondering if red shift ever comes into play, could a distant object be shifted enough to be actually blocked by the filter? I know it depends on the filter and so on, I was hoping for a general idea before I break out the textbooks and calculator! :)

 

[russ_watters]

You mean cosmological redshift due to the expansion of the universe? It doesn't exist on at the level of our local group of galaxies and you won't be imaging much beyond that.

 

[Kawakaze]

I thought as much, better to confirm it before spending hundreds of euros on filters. I guess it goes on how narrow, is a narrowband filter? :)

Thanks Russ!

 

[collinsmark]

Im planning on trying out some narrowband filters to hopefully dodge the light pollution here and get some nice photos, I was just wondering if red shift ever comes into play, could a distant object be shifted enough to be actually blocked by the filter? I know it depends on the filter and so on, I was hoping for a general idea before I break out the textbooks and calculator! :)

Like Russ said.

But let me elaborate a little.

The short answer is that things that might have a significant Doppler shift, such as galaxies, have a spectrum more-or-less like our sun, which is all over the spectrum. (no pun intended). So you don't have to worry about any light from galaxies being shifted out of the passband, because there would be just as much light shifted into it. Now a little more elaboration.

Narrowband filters are most profound on emission nebula. They not only block out nearly all the light pollution, but also a large portion of the surrounding starlight from stars. The end result is that the nebula really stands out a lot. And since emission nebula are all relatively near-by, Doppler shift isn't a big concern.

The bandwidth of the filter is particularly important for the starlight reason. If you have a very narrow narrowband filter (for example 3 or 7 nm bandwidth as opposed to 12 nm bandwidth), it will block out much more starlight from the surrounding stars compared to a narrowband with a larger bandwidth. As an example, take the veil nebula (google an image of it if you're not familiar). It has a lot of background stars around it -- so much so that an astrophotographer might say, "oh my, I wish I could dim the brightness of all those pesky stars, and let the beautiful nebula shine through." Well, narrowband filters with a small bandwidth can do just that. The smaller bandwidth of the filters, the more pronounced the nebula.

But there are a few big downsides to narrowband filters with a very narrow bandwidth.

• Generally speaking, the smaller the bandwidth the higher the price.
• Generally speaking, as the bandwidth gets smaller, some filters will have significant attenuation even in the center of the passband. That means you would have to increase your exposure time, even when photographing emission nebula.
• When photographing broadband objects such as galaxies, very small bandwidth narrowband filters don't have any advantage at all over wider bandwith narrowband filters; and will require you to increase exposure time for no good reason. Both work equally well at blocking out light pollution.

On that last point: Narrowband filters are great at blocking out light pollution. And when it comes to emission nebula you don't even really need to change the exposure time much compared to a regular color filter. As long as the nebula's particular emission frequency is somewhere within the passband, that's about all that matters. That's not the same with galaxies though. The galaxy light that makes it through the filter is proportional to the filter's bandwidth. Narrowband filters block out light from the galaxy just like they block out light from stars.

If I had my choice and the resources (aka $), I would I would get a set of 7 nm or less narrowband filters for emission nebula (mapped to LRGB); but for galaxies a 12 nm Hα for the galaxies' luminance channel and use some other (non-narrowband) filters for the galaxies' color channels. (I live in a very light-polluted area.) Then again, maybe a "light pollution" filter might be better for galaxies, maybe. It wouldn't block out as much light pollution as a 12 nm Hα, but but the decreased exposure time might justify it. Then again, as long as I hypothetically have all the resources (aka $) anyway, I'd get both.