Ha filter with modded dslr. 7nm or 12nm?

[StargeezerTim]

Hi Stargazers,

I am thinking of getting an Ha filter so I can image during moonlight and also start producing some HaRGB images. I don't use a laptop and currently use either the dslr live view screen  or plugged in phone screen to focus and frame the image (with bhatinov mask).

My question is, will it be possible to still focus in the same way with an Ha filter attached? Also, I understand that a 7nm filter lets through less light than the 12nm, but is it enough to allow me to manually focus on the same way.

Thanks in advance, Tim.

[geordie85]

Yes you can as long as you using a bright star for focusing. 

I'd advise you get focus as close as you can via live view then take a couple test exposures with the bahtinov mask still in place at a high ISO so you can see the diffraction spikes more clearly. 

This is what I used to do with my old modded 600d and 12nm Ha clip-in filter. Just remember to change ISO settings etc back to what you want to use for imaging 

[gnomus]

I have never tried this, but ....  I use a cooled-CCD camera and my PC has a full size monitor in my darkened warm room.  Even then trying to manually focus using an Ha filter is a challenge.  My guess is that doing it on a DSLR live view screen would be extremely challenging.  However, I have never tried - maybe others have.   I'll be interested to see.

[almcl]

Here's a heavily cropped 10 sec Bahtinov exposure at ISO 1600 using my Astro modded Canon 700d with a 12nm Ha CCD EOS clip filter.  

It's definitely not as easy as normal, but reasonably do-able, I think?

[StargeezerTim]

Thanks for your help... Its looking like 12nm will be difficult but doable... anyone tried using a 6 or 7 nm with a dslr?

[Tim]

As DSLR's, even modded ones, are only slightly responsive to Ha emission anyway, due to the way the bayer matrix works, my gut feeling is that a 6nm filter is going to rein in what you can achieve.

You can get some signal through them though , in fact I am right now imaging with a colour APS-C cmos camera (uses a DSLR chip)

and have tried my 6nm Ha filter on it. In fact, I've just setup up a 10 minute run to try it on a large expanded supernova nebula so will let you know how it goes. Won't be able to post any pics at the moment though...

[StargeezerTim]

8 minutes ago, Tim said:

As DSLR's, even modded ones, are only slightly responsive to Ha emission anyway, due to the way the bayer matrix works, my gut feeling is that a 6nm filter is going to rein in what you can achieve.

You can get some signal through them though , in fact I am right now imaging with a colour APS-C cmos camera (uses a DSLR chip)

and have tried my 6nm Ha filter on it. In fact, I've just setup up a 10 minute run to try it on a large expanded supernova nebula so will let you know how it goes. Won't be able to post any pics at the moment though...

Best of luck and clear skies! By rein in..do you mean the 6nm will achieve more or less than the 12nm (sorry if i didn't get your point).

[Tim]

Just had a couple of 10 minute 6nm Ha subs in on a reasonably bright target, the Veil. Camera is the equivalent of a modded DSLR. If I REAAALLLY stretch the image there is a trace of Ha emission, but not going to get anyone excited.

I would have thought, but might be wrong, that you would get more background emission signal with the 12nm filter, but even then you are going to need long subs, and lots of them.

FWIW, you can split off the Red, Green, and Blue channels from your camera. When you use a Ha filter you will see that neither the green or the blue channel record any useful signal, making your poor camera work at 25% of what it can do

Oiii filters on the other hand do permit green and blue channels to record some signal, but nothing in the red.

[Stub Mandrel]

I found that a bright star like Vega is fine with my 450D and a 7nm Ha filter, but I then take a 10s shot to check it's spot on.

Still learning the ropes of processing DSLR Ha images, but now I can get OSC images when the moon is hidden and Ha when its about. This is 5 minute subs (EDIT - for some reason images processed in PSP come out really bright and look awful...):

[Stub Mandrel]

To be honest, I was surprised to discover that, on my camera at least, the G and B channels pick up a surprising amount of Ha. The camera is quite brutally cooled though. I tried one shot at processing all three and it seemed less noisy but less detailed. All processed using super pixel mode & stretch in DSS, just split to RGB in paint Shop Pro:

Red:

Green:

Blue:

[Merlin66]

You can see from the overlapping transmission curves that the red filter still picks up the green and blue.........