Question about H-alpha filter

[Klape]

Hi everyone !

First off, sorry for my bad english.

So here is my problem :

Winter is coming and so is Orion and the nebulae surrounding it (especially the Barnard's loop) so I decided to buy a H-alpha filter (a Lumicon Night sky Hydrogen - Alpha 72mm) that I can place on the front of my lens. It cost almost 180$ so it's a long pass filter, it mean the filter block everything below 640nm, and let pass 90% of the light at 650nm.

So I have some question :

-Despite the fact that the filter let pass everything above 640nm and not only 4nm or 7nm, can I take photography of nebulae in light polluted area or with the moon ? Just so you know I'm taking photo in a very dark sky so LP is not an issue.

-I want to use this filter on a 85mm F1.4 Samyang lens and I've heard there is some issues with hight aperture and H-alpha filter, would I need to take my picture at F4 or more ?

- I'm going to use this filter on a Nikon D90 and it has a IR blocking filter, is it an issue ?

Thanks in advance  and have a nice week-end !

 

[Knight of Clear Skies]

Yes, an Ha filter will block most Moonlight and LP, making it possible to image from locations and at times when RGB imaging isn't practical. It's even possible to shoot with a full Moon although some reflected moonlight will make it through, impacting results.

An unmodded camera and Ha filter isn't a great combination as something like 2/3 - 3/4 of the Ha signal will be blocked by the IR filter. On top of that, due to the bayer filter, only 1/4 of the pixels on the camera are responsive to Ha. Shooting narrowband on a DSLR can work well but CCDs do have a big advantage in that area.

I'm not sure about how the filter will perform at low focal ratios.

[Klape]

Thanks for your response !

Okay I will try to remove the IR filter, however the bayer filter is great to merge Ha and visible light pictures without using 2 camera.

[D4N]

Yeh I wouldn't advise removing the bayer matrix as a high proportion of cameras break during that procedure. Removing or replacing the IR filter is usually pretty simple though.

Be aware that if you remove it without replacing it you will always need to use some sort of filter or you will get lots of red bloat, if you replace it then you shouldn't have a problem.

I think the problem you are referring to with Ha filters and aperture is actually down to the focal ratio and the angle at which the light cone hits the filters. A very fast focal ratio like f1.4 will produce a very steep light cone and when this hits a narrow band filter the results may be less than satisfactory or even nothing at all. This is why they make high speed narrowband filters for use at f2. However you stated that you bought a long pass filter so this should not be a problem

/Dan

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[Davesellars]

I just had the back filter removed by Astronomiser. I was under the impression that no IR extra filter was required as long as the front filter is kept in I.,e not made full spectrum.

[Knight of Clear Skies]

I just had the back filter removed by Astronomiser. I was under the impression that no IR extra filter was required as long as the front filter is kept in I.,e not made full spectrum.

I think that's correct. The modded cameras I've used have just had a single filter removed and do not require an additional filter when using them with a refractor or camera lenses.

[Davesellars]

I had a chance to image for the first time after getting the camera modded and the results are good... Quite a few targets and the Ha came through with 5 minute exposures although I noticed that the camera without the back filter seems to have more light hitting the sensor in all channels and significantly more red noise coming through from light pollution. I think for nebulae it's a good idea because of the longer exposures needed to have a UHC or equally narrowband fitter covering both OIII and Ha. Now realising how much more difficult Ha signal is with a DSLR..

[Merlin66]

If you are using the camera filter externally fitted, then there's no issue with the f ratio being used (only affects the transmission when it's positioned behind the lens)

[Klape]

thanks for all your reply !

I can't find anything to replace the IR filter, I have a Nikon D90 so it's hard (I can see a ton of thing for EOS camera ..)

[davew]

Hutech have filters for Nikon cameras on their web site. The only retailers I've ever seen who advertised the fact that they existed were in Australia !

I've no idea whether Hutech still do them but they're in their price list,  about half way down the page for the D90

http://www.sciencecenter.net/hutech/prices/filters.htm

Dave.

[Klape]

Hi Davew, thanks for the link !

240$ ! I mean come on that thing is like 2.5cm by 1.6cm

I think I'll start with only the H-alpha filter in front on the lens, at least it's going to be usefull to delete light pollution ...

Thanks everyone for your help !

[Davesellars]

I wouldn't put a h-alpha filter in front of a non-modded DSLR...  you'll get virtually no signal.

[Klape]

I talk about this on a french forum, so my H-alpha filter let 90% of the light after 650nm, and as you can see I get around 20% of the Ha signal with a non modified camera. So I still get some signal, that not big but still better than nothing. To be honest I bought this filter because I'm going in a very dark place in Corsica this winter. And from the 21th of December to the 2nd of January (the holiday basically) The moon is out, it's the worst period .. I get the moon every night .. so this filter can allow me to take some picture of nebulae even though the moon is out.

I mean, I would love to put that IR filter out but if the only option is to pay 240$ I would prefer to take some picture at f1.4

[Davesellars]

$240...  that's for a light suppression filter.  You can have taken out the camera's IR cutoff filter if you can find a company to do it for the Nikon which will make it much more sensitive to Ha but that has nothing to do with that filter...  the internal filter does not need to be replaced.

[Klape]

Apparently it does, there is big issue with focus at infinite if there is no filter.

[Klape]

*to Infinity if there is no filter to replace the IR one.

sorry for the double post

[Knight of Clear Skies]

This does get quite confusing. My camera (Canon 1100D) has just the cutoff filter removed but does reach infinity focus. However, the sensor did need to be re-shimmed for this to work, many modded cameras are only suitable for use with scopes. I had mine modified by Cheap Astrophotography, I don't know what the options are for Nikons.

I'm not sure whether an Ha filter is worth using with an unmodded camera but it's certainly a far from ideal combination.

[Klape]

Yeah that's for sure. And you are right, French astrophotographer told me that modded cameras are only suitable for use with scopes and not lens because you can't turn the focus ring after it reach infinity on a lens and you can on a scope.

[D4N]

Depending on the camera that can be fixed though.

I modded my Fuji DSLR by removing the IR filter then further modified it by grinding down the sensor mounting flange to move the sensor closer to the lens. Infinite focus is now infinite focus again when using a lens.

/Dan

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[Klape]

I would love to be as good as you with my hands, sadly I can't even unscrew without breaking everything ..