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Using RGB filters with a DSLR?

blackparticle

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blackparticle

blackparticle

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Before I try this, is there any benefit to be had as the DSLR already has its own RGB colour filters in place in the Bayer matrix over every pixel or are RGB filters best left to monochrome cameras?

I use red filters a lot in photography for B+W images so I know how the process works and what to do with the stacks afterwards.

It's just with filters on I'm going to be reducing light levels even further although I imagine noise levels may be reduced as well if I'm only selecting a single colour channel at the end of it. Is this the trade-off?

Cheers - Alan.

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RobH

RobH

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Hello Alan.

There's no benefit at all to be gained from using RGB filters wityh a DSLR.

As you said, you already have a bayer matrix on the chip, and if you want to look at each colour separately, you can do this easily enough.

You will increase your noise levels, as you will becutting down your light levels, and therefore decreasing your signal to noise ratio.

Cheers

Rob

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blackparticle

blackparticle

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Cheer Rob,

This was my thinking as well although with 'digital' B+W photography, filters make a massive difference. Much more so than just selecting the red channel afterwards for example.

I presume this is because green and blue light is much more intense than red and tends to hit maximum data limits and burnout the pixels way before the red does? I also suspect that no Bayer matrix is perfect and some light does bleed between pixels?

With regards to noise, I find it's mostly in the red-yellow range on my camera although there are a scattering of blue/green random pixels as well. Does not shooting the colours individually help isolate this, even on a DSLR?

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RobH

RobH

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With regards to noise, I find it's mostly in the red-yellow range on my camera although there are a scattering of blue/green random pixels as well. Does not shooting the colours individually help isolate this, even on a DSLR?

What you need to remember is that what you are shooting is generally very faint, and the bulk of astrophotography capture revolves around getting the S/N ratio as good as possible, in order to be able to stretch the data later.

I can't really see any practical advantage in shooting through filters with an OSC camera, as even if there is a slight improvement in S/N (I can't see why this would be), it would be more than offset by the decrease in light levels caused by the filters, and corresponding drop in the S/N ratio.

Cheers

Rob

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roundycat

roundycat

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I agree with Rob. First, you cannot equate daylight photography with astro. Second, your DSLR is hopelessly inefficient at gathering red light, maybe only one fifth as efficient as a mono CCD astro camera.

Dennis

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blackparticle

blackparticle

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Dennis,

Wouldn't that argument be in favour of using filters then? With a red filter for example, I could capture many more subs of red light only to compensate for the inefficiency without totally blowing out the blue and green parts of the spectrum? I agree the DSLR is woeful at red reproduction when pointed upwards. That's part of the reason for asking the question.

I think there is only one way to answer this to my own satisfaction and that's to try it out.

M27 is fairly colourful by all accounts and is within my field of view at home. I got it the other week with a 200mm lens although it ended up totally devoid of colour after processing so will try that.

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Psychobilly

Psychobilly

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In reality except for brighter stars, your unlikely to blow out the green or blue parts either so you might as well grab all you can and "lift" the red in the post processing...

This is where the modified Canon cameras help by greatly improving the Red response for astro use...

Other than LP filters (My preference being the Hutech IDAS P2) the only other filter I use with a DSLR (MOdified) is a Ha filter to grab soem Higher contrast "red stuff" when the moons around...

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blackparticle

blackparticle

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I've seen a few modded Canons appear in the for sale section recently and have been tempted but I'm still researching suitable Mac compatible mono CCD cameras as well.

It's a shame that DSLRs have the bayer matrix filter bonded to the sensor as they are just a mono CCD underneath after all. But, because of this, how effective is the bayer filter? If it's only a few microns thick is it really going to be 100% effective at colour separation?

Also, from what I can understand of the bayer matrix, a 6x6 grid pattern of RGBG sensors is then processed down to a single pixel. Surely cutting out the green and blue by using a red filter prior to the matrix should just leave the 9 red sensors to record the data without the others triggering?

Now if only someone would make a truly mono DSLR. I'm sure they would be onto a winner with both the astro and photo community. :)

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Merlin66

Merlin66

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Re the RGB matrix - the final outcome pixel by pixel is totally dependent on the de-bayering model used by the manufacturer. There is some "leakage" across the various filters. (Only the corner intersections of four pixels is normally used to made a visual pixel on your screen)

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RobH

RobH

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I appreciate I'd have to capture and stack a lot more subs but that's not an issue. :)

Unfortunately, in the UK's climate, it is a big issue due to the scarcity of clear skies....you need to grab high quality data as efficiently as you can when the opportunity arises. :icon_eek:

Rob

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ollypenrice

ollypenrice

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I agree with Rob and Dennis.

You want more red? Shoot more data and ditch the blue and green. And/or mod the camera. Added filters mean less light and it doesn't grow on trees.

It seems to me that you would like a mono CCD and there I would back you all the way. However much you sharpen a screwdriver it still makes a poor chisel. (Not original!)

Olly

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blackparticle

blackparticle

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Olly,

You're probably right about me wanting a mono CCD. As I'm on a Mac things are a little more complicated and I'm currently trawling through lists of IIDC compatible firewire offerings. I'm also debating whether to get something pre-built or just get the sensor/circuit board and build my own housing for it. That way I can add whatever cooling system(s) I like.

I think for tonight though I'm going to try the filter option. Even if its just to satisfy my curiosity about this and also to determine how much light reduction the filters have on the image. This will at least be useful information when I go down the mono CCD route. I'll make sure I get a suitable amount of unfiltered subs as well. :)

As with a lot of things, if you don't try it for yourself, it's very hard to learn from your mistakes. :icon_eek:

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blackparticle

blackparticle

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As a Microsoft Certified Systems Engineer, I absolutely refuse to use a windows based PC for personal use. I'd spend more time having to rebuild the thing than doing anything useful with it. I won't even entertain the dual-boot option and stick with my unix-based fan-boy OS seeing as it works flawlessly. :icon_eek:

As for the filter experiment, the moon has so far scuppered any meaningful tests. I did a couple of frames with each filter on M13 and, surprise surprise.. could see absolutely nada. :)

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roundycat

roundycat

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I think you are missing the point and will end up cutting off your nose to spite your face. Millions of users around the world do not find it necessary to re-build their computers. The strange snob value that people place on their favourite operating systems counts for nothing if you cannot get applications to run on your favourite computer. Home built or not

If you were in the business of writing software to sell on a commercial basis to as many people as possible what system would you write it for;

a Microsoft

b Apple Mac

c A N other

Incidentally, I have an iMac in my observatory now, it is completely useless for astro work so mine runs Win XP.

Dennis

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