Narrowband Imaging

[Daniel]

Hi All, Im looking at getting into a bit of narrowband imaging, every shot I've seen through seperate filters seems to have a lot more contrast than in one shot. I'll still be using a colour camera (the 40D) butim looking at mixing the channels myself in photoshop.

just wondering which filters are used for which channels to get the most faithfull colour representation. I was thinking Ha for red, OIII for green and SII for Blue, is this correct?

Thanks

Daniel
:O)

[Rocket Pooch]

Hi,

Yes sounds about right, but you can move them about, one thing is the image sensor size I don't know if 1.25 filter will cover the ccd, a lot of EOS users seem to use the Baader Ha, OIII and S2 2 inch filters, a because they are cheap and b becuse the Ha is not to narrow and therefore its usable.

Chris

[Daniel]

Ahhh, cool, I've ordered a set of 2" filters since the 40D's chip is pretty big and vignetting is my biggest nightmare, can't wait hopefully have them for DSC

[MarkS]

I'm a bit dubious about the benefits of using narrowband filters with a colour CCD.

If you're taking a picture of, say, the Dumbbell or Veil Nebula then you'll get the same amount of total light from the nebulosity on the CCD but you'll take 3 times as long to do it.  The only advantage I can see is that you'll substantially reduce the light from continuous spectrum sources - stars.  This certainly will have the benefit of reducing star bloat but is it really worth the 3x pain factor?

Why are you considering it?  Maybe you seen results from folk using narrowband filters with a Canon?

[Mike]

I personally don't see how it will work at all unless the CCD was both Ha sensitive and monochrome.

[Daniel]

I've seen some nice narrowband stuff done with DSLR's and I think because it's cutting out emmission inbetween the bands I might be able to use it to boost contrast. Also, If I image narrowband I should be able to split the channels up easyer since all Im doing is collecting photons through one emmision line then splitting them into channels. My 40D's modded now so it'll be more Ha sensitive, though I was going to use this before I got the mod to help me balance out the difference when i wasn't getting enough Ha through the camera.

[Ian]

Doesn't the OIII fall between the spectral response of the blue and green photosites? That's not going to help...

[Daniel]

I've seen some nice stuff done just using Ha and OIII such as this Veil nebula http://www.pbase.com/gbachmayer/image/51520941/original

The SII is more curiosity than anything, though i've seen a few narrowband images done with this aswell

This is a tri colour image with Ha, OIII and SII, admitedly, not particularly faithfull colour representation, it's definetly pretty.

http://www.pbase.com/gbachmayer/image/50812304

Admitedly, this guy is obviously extremely good and probably imaging from a dark site, but im also wondering if cutting back a lot of the other emmision lines would help my light polution problems too.

[RobertM]

You will be able to boost contrast and massively cut LP but unfortunately the cost will be in exposure time.  That link you posted to gbachmayer was 2 hours in Ha plus 3 hours each for SII and OIII, total exposure 8 hours.  Unfortunately he didn't post the ambient temperature which will affect the sensor noise greatly but even so I would expect a big improvement.  It's a shame that nearly all the faint emission nebula are well placed in the summer months when the temperatures are highest...

I'll be greatly interested to see how the narrow band images come out at DSC.

[Mike]

Yep. A cooled monochrome CCD would be a better investment for narrowband.

[Daniel]

Hehe, not sure im ready for the jump into Astro CCD camera's yet, i've got the starlight xpress Mx7, but I don't like the low resolution of the camera, and the Hi res ones cost a bomb!

[RobertM]

You just hit the nail on the head Daniel !

[MarkS]

Daniel,

I've thought about this carefully and I think you're right.  Narrowband imaging will increase contrast for emission nebulae and will tighten the stars - reducing their bloat.

On the other hand you don't want to be forced to do multiple exposures (increasing imagaing times by a factor of 2 or 3) because the Canon already forces the need for  long exposure times.

The ideal compromise would be a single filter that lets through only the emission wavelengths of interest -then you still have the convenience of single shot colour exposures.

Maybe the Astronomik UHC filter is a good compromise.  See it's tranmission spectrum at Christain Buil's site: http://www.astrosurf.com/~buil/filters/curves.htm

I currently use the Astronomik CLS filter with my Canon.  But the UHC filter would let through exactly the same emission lines as the CLS but with a huge reduction (factor of 2?) in the other rubbish.  I think it would therefore give superior results for imaging emission nebula.

Maybe there exist filters even tighter around those emission lines?

Rats, it looks like I have yet another expense coming up 

Mark

[Ian]

Mark, I have one such filter, although it's 1.25"... It'll be at DSC.

It's type one, but that's not going to trouble a serial Canon worrier like yourself.

[Daniel]

Damn, I've gone and bought the filters already, although the UHC looks a little noisy in the 7000 - 9000 range apparently the IR cut filter Im getting aswell is good for bad seeing conditions for planetary work, it'll be interesting how that works out.