First light pollution filter for a Canon 600D?

[SiD the Turtle]

Hi folks,

After some advice on a light pollution filter. I'm in a Bortle 5 and I'm combining a William Optics ZS73 and a Canon 600D unmodified. This winter I've been focusing on the Andromeda galaxy, which I've gotten some decent subs on, but now we're getting to the point where it sets behind the trees too early to get more than a few images. So I'd like to attack 'beginner' nebula targets like Orion and other large-ish targets that suit my scope.

Initial results are not super great, and I fear I have more of a problem with light pollution with nebulae than I did with the nice, bright Andromeda galaxy. So I'm looking at what filter to buy to fit this telescope and camera pairing. Any ideas?

Some questions to clarify:

• Most narrow band filters are isolating the wavelengths that are common in nebulae, so am I right there'd be no point using them for galaxies?
• The Optolong L-enhance and L-extreme seem popular. The former is tri-band, the latter is dual-band. Why would you go for dual over tri? Is it simply more aggressive for high light pollution areas?
• How do these wavelengths map to a colour camera? Am I going to find, for example, that the images fall mostly to the red end of the spectrum?
• If I move to a dedicated astro-camera in future, would these be useful for the luminance layer? If so I'll put the money up for a 2 inch now, rather than a 1.25 or clip-in, for future proofing.

Cheers!

[rnobleeddy]

I think you'll find a few opinions but....

- I'd be surprised if light pollution is your issue in Bortle 5. More likely, an unmodded DSLR is better suited to galaxies that nebula due to the fact that a lot of red light is being filtered out. 

- The narrow band filters work well in areas of high LP but probably best if you don't consider them as just a LP filter. They fundamentally change what you're imaging as you're only capturing 2 emission lines.

- For general LP you can also consider something like https://www.firstlightoptics.com/light-pollution-reduction/idas-d2-light-pollution-suppression-filter.html which passes much more light, but tries to remove common pollution sources like street lights.

 

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Most narrow band filters are isolating the wavelengths that are common in nebulae, so am I right there'd be no point using them for galaxies?

Mainly. I've seen some people add Ha data from galaxies to good effect but always in addition to LRGB or RGB data.

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The Optolong L-enhance and L-extreme seem popular. The former is tri-band, the latter is dual-band. Why would you go for dual over tri? Is it simply more aggressive for high light pollution areas?

As best I can tell, the tri-band label is just a marketing trick, and the dual band is strictly better, but costs more. I know these filters really come in to their own in areas with more LP, so the cheaper tri band would probably produce good results.

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How do these wavelengths map to a colour camera? Am I going to find, for example, that the images fall mostly to the red end of the spectrum?

Ha maps to red. O maps to G/B. So you can get bi-color images. My experience was that the veil nebula produced stunning dual color images using this type of filter, but that most nebulae just ended up red. But the increase in contrast was very noticeable. The red data was far less noisy and much more detailed for the same exposure times.

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If I move to a dedicated astro-camera in future, would these be useful for the luminance layer? If so I'll put the money up for a 2 inch now, rather than a 1.25 or clip-in, for future proofing.

They'd work well with color cameras. I'm not sure I've seen anyone use them with mono cameras. The concept could work, but if you've got a mono camera, you're almost certainly going to use a filter wheel and it'd be much easier to stick with the usual SHO + LRGB filters.

[rickwayne]

Since  hydrogen is so abundant, hydrogen emission is by far the brightest component in emission nebulae. It's a deep-red frequency that gets knocked down a lot by terrestrial cameras' IR-cut filters.

However, if you image reflection nebulae, you'll get much less signal -- they tend to be blue (like scattering produces a blue sky).

Really, these "light pollution" filters are a poor man's narrowband setup, so poorly adapted for broad-spectrum targets. For some kinds of lighting (e.g. sodium vapor)  you can find filters that knock out just that particular line, but as LEDs proliferate, those become less useful.

It's much more likely that you're losing Ha signal to the IR cut filter in your camera, rather than fighting light pollution per se. I mean, since the signal is dimmer the LP noise does drown it out more. But a better solution IMO would be to either get a modded DSLR, get a dedicated astro cam, or simply swamp your IR cut filter by shooting a crap-ton of integration time. I prefer to gather all the photons and fight LP gradients in software.

Concur that a filter wheel is going to work much better if you wind up  with a mono camera. Unless you have some whopping-huge sensor, you won't need a 2" filter (my IMX183 sensor does fine with 1.25" filters). And for that kind of camera, a luminance filter is best for the luminance layer. I know I sound like a smart-ass there, but it's one of the great advantages of a mono setup that you can use a luminance filter to rapidly gather all the photons onto all the photosites to gain maximum  SNR where it counts most, since detail inheres in the luminance data due to the way our vision works.

Edited February 27, 2021 by rickwayne

[MARS1960]

On 25/02/2021 at 10:41, SiD the Turtle said:

Hi folks,

After some advice on a light pollution filter. I'm in a Bortle 5 and I'm combining a William Optics ZS73 and a Canon 600D unmodified. This winter I've been focusing on the Andromeda galaxy, which I've gotten some decent subs on, but now we're getting to the point where it sets behind the trees too early to get more than a few images. So I'd like to attack 'beginner' nebula targets like Orion and other large-ish targets that suit my scope.

Initial results are not super great, and I fear I have more of a problem with light pollution with nebulae than I did with the nice, bright Andromeda galaxy. So I'm looking at what filter to buy to fit this telescope and camera pairing. Any ideas?

Some questions to clarify:

• Most narrow band filters are isolating the wavelengths that are common in nebulae, so am I right there'd be no point using them for galaxies?
• The Optolong L-enhance and L-extreme seem popular. The former is tri-band, the latter is dual-band. Why would you go for dual over tri? Is it simply more aggressive for high light pollution areas?
• How do these wavelengths map to a colour camera? Am I going to find, for example, that the images fall mostly to the red end of the spectrum?
• If I move to a dedicated astro-camera in future, would these be useful for the luminance layer? If so I'll put the money up for a 2 inch now, rather than a 1.25 or clip-in, for future proofing.

Cheers!

Hi, 

These were take under Bortle 4 skies and a full moon with my ZS73 and Canon 60D (unmodified) using a Skytech quad band clip in filter.

60s subs, heart is 172mins, cigar severe crop is 78mins

 

Edited February 28, 2021 by MARS1960

[SiD the Turtle]

Thanks @rnobleeddy @rickwayne @MARS1960 for the input on this topic, lots of food for thought. I want to keep the DSLR for daytime photography too, so I think I'll continue to save towards a full mono setup rather than invest in a LP filter for now!