Hydrogen-alpha filter query

[lukebl]

Just a quickie regarding H-a filtered imaging, as I intend having a go sometime soon.

Probably a daft question, but I know that Ha filters can cut through moonlight and light pollution, so can it cut through twilight? Just thinking of the forthcoming short summer nights and maximising imaging time.

[FraserClarke]

Yes, H-alpha (or other narrowband filters) are "less" sensitive to sky background, wherever that sky background comes from.

I put "less" in quotes, because it's not that there is anything special about narrow-band filters that lets them "cut through" sky background**. If the total sky background goes up by 10x, the sky background in the narrow-band filter goes up 10x too. It's just that you will notice it less because the sky background level in the narrow-band filter is so low to start with...

** caveat being, if the LP is coming in discrete lines (such as low pressure sodium lights), then those are completely rejected by the H-alpha filter and it essentially doesn't matter how bright they are.

[ollypenrice]

Well, didn't know that!

Olly

[lukebl]

Thanks for the info.

Well, I've got an Ha filter now, along with an LRGB set. It is very dark. How much longer should exposures be using it, and what objects are best with it, apart from emission nebulae?

[NickK]

One way to look at it - CCD pixels pick up a spectrum and therefore fill according to the area under the spectrum (and can't differentiate between the wavelengths).

A narrowband only has the area in that shorter spectrum hence the sensitivity drop.

(ok.. that's blindingly obvious Nick..)

[Andrew*]

Yes, a Ha will block off ambient light of any sort, but it will only do so much, depending on its passband. Of twilight, moonglow and light pollution, twilight is potentially the brightest, and if it's still too light, your images will still be washed out. Best way is to try it, but I'd be surprised if the Ha filter allowed you to start imaging significantly earlier.

You can expose for longer with an Ha filter as bright areas are less likely to saturate, but it doesn't mean you NEED to. Exposure limits are the same as without a filter, i.e. go for as long as you can before tracking errors come in, the background sky starts to lighten, the dark current becomes too noisy (amp glow, hot pixels etc.), and bright areas saturate.

Andrew

[ollypenrice]

I don't know about DSLR H alpha imaging but with CCD I find I do need to get longer exposures. 15 Minutes, 20 minutes...

Apart from emission nebulae it is the active parts of galaxies that are worth going for. M81/82 have some fireworks. M33. M101.

Olly

[lukebl]

Thanks Olly. it was CCD imaging I was referring to. Not sure if my guiding is up to 20 mins, though.

[AndyUK]

Not sure that this helps at all, but I took some test flats last night at dusk (white t-shirt job) using 2 filters - A UV/IR cut (which I was anticipating using as a temporary Luminence) and a 7nm Ha. I didn't really know what I was doing (or if I was doing it correctly) but both sets were taken within 10 mins of each other and I checked their histograms in CS5 so that they were both pretty much the same.

The UV/IR cut filter exposures were 0.02s, whereas the Ha exposures were 2.0s, which shows exactly how much visible light is actually cut by the Ha...

[PortableAstronomer]

A narrowband only has the area in that shorter spectrum hence the sensitivity drop.

Sensitivity drop, no.

Sensitivity of other wavelengths that you are choosing to discard (by using a narrowband filter), yes.

If you are going to image H-Alpha deep sky sources , then a reasonable H-Alpha filter will let through >90% in the H-Alpha wavelength. I can understand why you would need massively increased exposures to get stars to appear in the H-A filtered frame to give a nice starry background, but surely to capture the nebula itself in HA shouldn't need a significant increase in exposure because of the 90% transmission of that wavelength.

For the same reason, I never understood the argument about narrowband requiring large-aperture scopes. You want to observe a particular wavelength, the filter passes 90% in that wavelength, so why should the nebula itself appear radically dimmer?

Since the Sun emits quite a lot in H-Alpha then I would have expected the H-Alpha filters to let through this from the twilight glow. Or is it that the Earth's atmosphere alters the spectrum of light from the Sun in such a way that the Sun's H-Alpha is somehow absorbed or shifted by the atmosphere so that it moves out of the transmission line of H-A filters? I doubt it. So I'd expect the H-Alpha component of the Sun's radiation to still leak through into the background of any H-Alpha-filtered images you take during twilight or northerly summer nights.

You cannot use the "it dims the view by a certain percentage" analogy. It's what happens to intensity and transmission at particular wavelengths that are important.

If you want to record H-alpha nebulosity , and your H-Alpha filter passes 90% in that wavelength, then the nebula itself should be almost as bright as unfiltered view. If that's what you want to record, then no massive increase in exposure times.

If the total sky background goes up by 10x, the sky background in the narrow-band filter goes up 10x too

... only if the sky background includes wavelengths that are passed by the narrowband filter. Which I guess it does to a small extent.

I think the conclusion here is that using a narrowband in twilight should improve contrast of the DSO (provided it's not extremely faint) and reduce but not eliminate background glow.