UHC filter for AP?

[Ant-33]

Hi Folks,

I've recently been having some success in observing faint fuzzy nebulae from my light-polluted back garden, so I've done some research into suitable filters, and concluded that my first filter should be an Orion Ultrablock. What I can't find (I've searched here and elsewhere) is any insight into whether this filter will also enhance my photography (I use an unmodded EOS 1100D). I'd appreciate any feedback on this from anyone who has experience with this filter.

Thanks - Tony

Clear skies.

[D4N]

Not sure about the Orion version but I used to use a Baader UHC-S filter all the time with my modded DSLR.  I found it was great for eliminating bloat in stars and allowing longer exposures.

UHC filters can have a negative impact on wideband targets like galaxies and reflection nebulae though as they filter out a great deal of the light coming from them.

[DaveS]

I found the Astronomik CLS filter to be better than the Baader UHC, as it let through more wanted light while still keeping the LP at bay. I've used it (2") with a DSLR on the nose of a flattener, and with a CCD in the train before the filter wheel.

[Ant-33]

3 hours ago, D4N said:

Not sure about the Orion version but I used to use a Baader UHC-S filter all the time with my modded DSLR.  I found it was great for eliminating bloat in stars and allowing longer exposures.

UHC filters can have a negative impact on wideband targets like galaxies and reflection nebulae though as they filter out a great deal of the light coming from them.

Thanks for the feedback, D4N. That's reassuring. I can't think of any reason why the filter shouldn't work with dslr photography, since the narrow-band is within the camera's range - it's just always referred to as an observing filter.

1 minute ago, DaveS said:

I found the Astronomik CLS filter to be better than the Baader UHC, as it let through more wanted light while still keeping the LP at bay. I've used it (2") with a DSLR on the nose of a flattener, and with a CCD in the train before the filter wheel.

Interesting - thanks, Dave. The Astronomik CLS is slightly more pricey, and as far as I can tell, it has a slightly bigger band-pass, while still blocking those pesky LP frequencies. It looks like a great filter - and there's a lot of great filters to choose from, so it's pretty mind-boggling for someone like me who's working with a fairly tight budget! 

[D4N]

52 minutes ago, DaveS said:

I found the Astronomik CLS filter to be better than the Baader UHC, as it let through more wanted light while still keeping the LP at bay. I've used it (2") with a DSLR on the nose of a flattener, and with a CCD in the train before the filter wheel.

Indeed this would be better for wideband targets, however the lower light transmission is nice for targets like horsehead where Alnitak tends to explode if not filtered.

[DaveS]

Indeed. I've not got round to the Horsehead yet so I'll bear it in mind, as I have a Baader UHC kicking around, though I may have to go NB.

[rl]

It's worth checking your UHC filter passes the Hydrogen alpha line at 656 nanometres as well as the usual passband at 500 nanometres. They are useful in AP for line emission objects. Some do, some don't.

rl

[Ant-33]

2 hours ago, rl said:

It's worth checking your UHC filter passes the Hydrogen alpha line at 656 nanometres as well as the usual passband at 500 nanometres. They are useful in AP for line emission objects. Some do, some don't.

rl

I take your point. I'm using an un-modded DSLR, so I get precious little H-alpha recorded anyway. I'll take a look at the spec/graph, since any H-alpha is beter than none. 

[Ant-33]

2 hours ago, rl said:

It's worth checking your UHC filter passes the Hydrogen alpha line at 656 nanometres as well as the usual passband at 500 nanometres. They are useful in AP for line emission objects. Some do, some don't.

rl

Ah - a question, though. I know that ir tends to come to focus at a different point than visible light, so an ir cut filter helps to avoid blur with a wideband image.

- does the H-alpha inclusion cause a blur when included with the visible 500nm of the UHC bandpass - all at the same focus?

Hope the question made sense.

Thanks - Tony

Clear skies

[DaveS]

No, it shouldn't as it's not really IR, just deep red. Depending on your 'scope, you're more likely to be fighting "blue bloat".

[Ant-33]

10 hours ago, DaveS said:

No, it shouldn't as it's not really IR, just deep red. Depending on your 'scope, you're more likely to be fighting "blue bloat".

I was hoping you'd say that - well the red bit, anyway. Thanks, Dave.

[Ant-33]

So... If I want a filter for observing and phtotography, it seems there's no way without making compromises. The Orion Ultrablock gets great reviews for observing, but as Dave has pointed out, the reason people focus on observing with this filter is because it blocks H-alpha, which makes a small, but useful contribution even with an unmodded DSLR.

The Astronomik CLS filter also gets great reviews, and looking at the graph for the bandpass shows that it blocks light pollution from mercury and sodium lights as well as some other lp wavelengths. On the surface this looks like great news, since it gives good transmission for H-beta, OIII and other goodies in that region. On top of that it allows H-alpha, and is even useful for (the future) when I get a ccd if I add a uv/ir cut filter to the mix. (Rather than buying the CLS-CCD version now, which is *really* expensive, and a uv/ir cut filter is useful anyway)

So, looking at this, and at the lp wavelengths shown in the plot for the Orion Ultrablock, I wondered about those LED streetlamps they're installing outside my house. It turns out they're a problem:

Not only do they have some significant emmission all around the H-beta and OIII regions, but there's that nasty peak in the blue. The Astronomik CLS, as far as I can see, won't block most of this peak. However, the Astronomik UHC is a narrower filter, and from the look of it, it will block a lot of the LED light, and most of that nasty blue peak:

But... There are limits as to what this UHC filter is good for. The CLS is a more generally useful deep sky filter, while the UHC is better for specefic nebulae.

Sigh.

You can't blame the filter makers - light pollution is a messy problem. I already have a Baader Neodymium filter, and looking at the plot for that, it's hard to tell what it does and doesn't do in the main light-pollution regions between H-alpha and H-beta - I only know I've had pretty good results with it, but it'll be getting worse with these pesky LED streetlamps:

So... Given that I can't just purchase a stack of expensive filters all at once, I'm thinking I might go for the Astronomik CLS first - but that's just my current thought, and I'm still contemplating...

Thanks for listening

Clear skies

Tony