Which Ha filter?

[step_hen]

I'm interested in getting a Ha filter for mono imaging on moonlit nights, but am really confused by the different wavelengths available- whats the advantages of each and which brand would you recommend?

Imaging will be with a DSLR on the MN190, ed80 or ed66, though i may upgrade to a 16HR or 314 in the future

Thanks

Stephen

[swag72]

I use the Astronomik 12nm clip filter on my modded DSLR. I am working at around 20 min subs at the moment and seem to be able to catch stuff reasonably well. I'm not sure how effective Ha would be on a normal unmodded camera though.

[johnrt]

All Ha filters are centered on he same wavelength 656nm - the number denotes the bandwidth either side. A 12nm filter lets in 12nm either side, 6 is 6nm (narrower) etc.

The lower the number the longer the exposure required but you cut out unwanted noise.

Do you have a modded cam Stephen?

[riklaunim]

12 nm will pass H-a and [N-II], while 6 nm only part of it (which may give dimmer nebula if it's rich in [N-II] emission).

[step_hen]

Thanks guys

Yes i have a modded 400D. so a 12nm will give me shorter exposure times and pick up some other detail that the narrower 6nm may not?

[FraserClarke]

Saying it will give shorter exposure times is a bit misleading. The amount of H-alpha from the object is fixed, no matter what the width of the filter is (assuming it is greater than the width of the H-alpha emission, which is almost always <1nm). The advantage of *narrower* H-alpha filters is that they cut out more of the background (broad-band) light that you are not interested in. They are also a lot harder to make, which is why they are a lot more expensive. So, a wider H-alpha filter will "give you shorter exposure times" -- but only because you are seeing more of the background light you are not interested in! Taking that to the extreme of course, you could use a 100nm wide H-alpha filter and get really short exposure times -- but that is just called an R filter

Maybe a better way to put it is that the narrower the filter the better the contrast is between the H-alpha image and the broad-band image.

Edit: The point about NII is valid though, but NII is usually a lot weaker than H-alpha, so it's not a massive gain for most objects.

[OzDave]

Sorry if this is considered hi-jacking, but I'm interested in the same thing as the original poster.

I have a modded-with Baader-replacement filter EOS 450D. Will an Ha filter of any description work well? I mean, will the camera be able to pick up the full spectrum segment allowed through by the Ha filter? Or would it be clipped at one end?

Basically I thought Ha filter were really only useful with monochrome CCD cameras. Is that not the case?

David

[fwm891]

Cameras are normally modified so you can gain the red/near IR part of the spectrum which would distort normal daylight imaging. The spectrum passband of an H-alpha filter is narrow whether of 6, 7, 12, or 30 nm compared to the 'red' or IR transmit filters which Ha basically sits between. Your modified camera will pick-up all the light transmitted by the Ha filter. Some Ha filters also have spurious passbands well away from the intended Ha region (some UV/blue some deeper IR) so the way in which a camera has been modified may effect the filter choice. i.e. full spectrum or UV/IR cut mods...

[Grinde]

Saying it will give shorter exposure times is a bit misleading. The amount of H-alpha from the object is fixed, no matter what the width of the filter is (assuming it is greater than the width of the H-alpha emission, which is almost always <1nm).

Maybe a better way to put it is that the narrower the filter the better the contrast is between the H-alpha image and the broad-band image.

That's a good way of putting it§. You will have the same amount of Ha signal, but the background light (lightpollution) will be darker & stars smaller, using a narrower filter, using the same exposurelenght. Which means you can go for longer exposures, without saturating the background, it doesn't take longer to capture the Ha signal

[ollypenrice]

There was a good thread a while back contradicting the 'wider is faster' notion.

I haven't said anything on this thread because I have no idea how Ha works with OSC cameras, whether DSLR or CCD. I have never tried it. But I find better contrast with a 7Nm than with a 13. No doubt about that. Since you already have a 'red' layer in any colour imaging system, you want something else out of your Ha layer. Vive la difference!

Olly