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A questions about how bandpass filters work


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Hi

I have an Astro modified 600d dslr, ie allowing h-alpha through now. I’ve been advised that an optolong l extreme filter would be an excellent choice for capturing detail and colour from galaxies and nebula. I’m in a bortle 5 area, and have a 80ed scope with 0.85 reducer. 

I understand the filter works by allowing two different frequencies of light through, while blocking others (presumably light pollution and everything else).

Does this suggest that galaxies only emit light in those two different specific frequencies? Hence why you’re able to capture a complete looking image even with the filter? Or perhaps they are just the two most prominent frequencies emitted by galaxies. 

Related to that, when capturing images would you usually only take images with the filter or would you take both with and without and then stack them all together? 
 

Thanks! 

 

 

 

 

 

 

 

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1 hour ago, AstroLearnerWill said:

Hi

I have an Astro modified 600d dslr, ie allowing h-alpha through now. I’ve been advised that an optolong l extreme filter would be an excellent choice for capturing detail and colour from galaxies and nebula. I’m in a bortle 5 area, and have a 80ed scope with 0.85 reducer. 

I understand the filter works by allowing two different frequencies of light through, while blocking others (presumably light pollution and everything else).

Does this suggest that galaxies only emit light in those two different specific frequencies? Hence why you’re able to capture a complete looking image even with the filter? Or perhaps they are just the two most prominent frequencies emitted by galaxies. 

Related to that, when capturing images would you usually only take images with the filter or would you take both with and without and then stack them all together? 
 

Thanks! 

TL;DR - the L-extreme is designed for emission nebulae and would be of little use for galaxies, the exception being to add some Ha data to a broadband capture.

The longer answer: the L-extreme is a dual narrowband filter with 7nm bandpasses around the Ha and Oiii emission lines (Oiii actually has a couple, but I believe the L-extreme focuses on the 500.7nm emission).

The Ha and Oiii emission lines are caused by ionised gases which emit light at specific wavelengths. The L-extreme blocks out ~95% of visible light to isolate just a small section around those 2 emission lines, thus significantly increasing the contrast when photographing these regions. 

Galaxies, on the other hand, emit light across the whole of the of visible light spectrum, and so are best photographed without the L-extreme. That said, a good number of galaxies do have some quite strong Ha regions, so it can be of benefit to add some narrowband data into a broadband capture to accentuate these areas (most would do this by adding the Ha data to the red channel when processing).

To answer your other question, you can choose to either create a full false-colour narrowband image, or to blend the narrowband data into a true-colour broadband image. When doing this, the images would be stacked separately and the narrowband blended in during processing. As above, Ha would be added to the red channel, and Oiii would be added to green and blue.

Edited by The Lazy Astronomer
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Question for the group:

If I were going to image a galaxy using a osc multi pass filter, would I be better or worse off getting a triband or dual band?

I'm debating about picking a filter up for just general is imaging (nebulas as well as h-alpha on galaxies) but not really sure whether to go for a more narrow dual band or wider Tri or quad band.

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4 hours ago, Ratlet said:

If I were going to image a galaxy using a osc multi pass filter, would I be better or worse off getting a triband or dual band?

Neither for galaxies. These filters are for emission nebulae. You could add Ha using and Ha filte to a RGB image, but galaxies should be imaged in broadband.

Quad band and Tri band filters are very similar, but the quad is used where there is less light pollution as I covers a broader area of the spectrum. The names are a bit misleading really as both are actually dual band. It is just one covers Ha and Oiii plus H beta, the other covers Ha and Oiii, H beta and SII.

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1 hour ago, Clarkey said:

Neither for galaxies. These filters are for emission nebulae. You could add Ha using and Ha filte to a RGB image, but galaxies should be imaged in broadband.

Quad band and Tri band filters are very similar, but the quad is used where there is less light pollution as I covers a broader area of the spectrum. The names are a bit misleading really as both are actually dual band. It is just one covers Ha and Oiii plus H beta, the other covers Ha and Oiii, H beta and SII.

Perfect. I'm relatively light pollution free in bortle 4.  I meant to say for adding h-alpha to a galaxy, but my brain was running on fumes earlier.  I like the bit of extra flavour that the emission spectra gives to galaxy images.  Its very exciting to think that from millions of light years away you are imaging what could be some other lifeforms Orion nebula.

I wasn't sure if you could do it with the the broader pass of these filters.

Need to see what later in the year brings.  Both in terms of budget and releases.  I believe svbony are entering the market with some new filters and I'll be interested to see what their dual band looks like.

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16 hours ago, The Lazy Astronomer said:

TL;DR - the L-extreme is designed for emission nebulae and would be of little use for galaxies, the exception being to add some Ha data to a broadband capture.

The longer answer: the L-extreme is a dual narrowband filter with 7nm bandpasses around the Ha and Oiii emission lines (Oiii actually has a couple, but I believe the L-extreme focuses on the 500.7nm emission).

The Ha and Oiii emission lines are caused by ionised gases which emit light at specific wavelengths. The L-extreme blocks out ~95% of visible light to isolate just a small section around those 2 emission lines, thus significantly increasing the contrast when photographing these regions. 

Galaxies, on the other hand, emit light across the whole of the of visible light spectrum, and so are best photographed without the L-extreme. That said, a good number of galaxies do have some quite strong Ha regions, so it can be of benefit to add some narrowband data into a broadband capture to accentuate these areas (most would do this by adding the Ha data to the red channel when processing).

To answer your other question, you can choose to either create a full false-colour narrowband image, or to blend the narrowband data into a true-colour broadband image. When doing this, the images would be stacked separately and the narrowband blended in during processing. As above, Ha would be added to the red channel, and Oiii would be added to green and blue.

Thank you and everyone for the advice here!

That makes a lot of sense. I've also heard that using something like the l-extreme with a 600d (modified) will require long exposures since its such a narrow pass,and as such the images get quite noisy.

So perhaps sticking to a ha filter will be best, as a starting point as you say. If I'm going for galaxies but also some emission nebula. Then combine with a true-colour broadband image.

Are there any HA filters you or others would recommend on that front? for my setup of astromodified DSLR, 0.85 reducer/flattener, ED80 scope?

Edited by AstroLearnerWill
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6 hours ago, AstroLearnerWill said:

Thank you and everyone for the advice here!

That makes a lot of sense. I've also heard that using something like the l-extreme with a 600d (modified) will require long exposures since its such a narrow pass,and as such the images get quite noisy.

So perhaps sticking to a ha filter will be best, as a starting point as you say. If I'm going for galaxies but also some emission nebula. Then combine with a true-colour broadband image.

Are there any HA filters you or others would recommend on that front? for my setup of astromodified DSLR, 0.85 reducer/flattener, ED80 scope?

Apologies, perhaps I wasn't very clear - I think a dual narrowband filter is the best option for a DSLR (or any OSC colour camera) over a single bandpass filter because it makes use of the whole sensor. If you had a Ha only filter, then only the red pixels would register signal - the green and blue pixels would do virtually nothing. If you only wanted to use the Ha signal in an image, there are ways split the channels in processing.

When doing narrowband imaging, the narrower the bandpass, the better the result. I think the filters with 6 - 7nm bandpasses are the best comprise between performance and cost, but unfortunately the 600d does have a relatively low QE and relatively high read noise, so long exposures would probably be needed (maybe something in the region of 10 - 20 minutes!). I think the 600d is quite a common camera choice, so it would be worth having a hunt around for some images on here or astrobin to see what sort of results others are getting with similar set ups and hopefully they will have included some capture info that will give a better idea of what can be obtained in the real world.

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I imaged the Veil Nebula earlier this year and before I started I tested briefly the Lpro, lenhance, and lextreme with the Canon 600d, they each got progressively clearer (target) going up the range. I ended up sticking with the extreme as it singled out the ha and o3 signal the most whilst keeping background light lower, it's an ideal target to test on. Such a filter would not work on galaxies though as they are a broadband target. They can only be used with certain targets depending on how much hydrogen or oxygen energy they emit. Hydrogen beta is very similar to alpha, and very small signal wise as it's usually swamped by the alpha and few targets have presence of it.

I was glad I got some sort of result with the modded DSLR, but the astro cam I was using at the same time was just better (I shot luminence, then later ha), less noise, cleaner definition and easier to post process. Don't think it was a completely fair test though as I was using a vintage lens with the dslr so it would be poor in comparison.

 

Edited by Elp
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On 25/10/2022 at 13:49, AstroLearnerWill said:

Are there any HA filters you or others would recommend on that front? for my setup of astromodified DSLR, 0.85 reducer/flattener, ED80 scope?

I used a 6nm astronomik clip in filter with my DSLR, seemed to work very well and introduced me into the delights of HA.. just need to have in mind that your exposure length will need to increase

I used it with the same scope/ reducer combo

Edited by newbie alert
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