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Old 01-10-2009, 01:16 PM
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telecasterguru (Frank)
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H alpha processing

I am going to give H alpha imaging a go as I live right in the city.

Would someone be so kind as to explain whether there is any special tricks when processing H alpha data.

Are light and dark frames important? Bias frames? Anything else?

Thanks

Frank
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Old 01-10-2009, 01:40 PM
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Octane (Humayun)
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Yes, all of the above are important.

In Photoshop, simply use the red channel as that is where the majority of the data will lie.

Bias is included in dark frames. You don't need them.

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Humayun
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Old 01-10-2009, 02:12 PM
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Humayun,

Thanks for the advice.
Can I ask one more question, I will be using my modded 350D which I think has ICNR so I would like to use that for the darks, but with the lights how long an exposure should they be?

Frank
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Old 01-10-2009, 03:14 PM
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Frank,

I'd advise against using ICNR and to take your own darks. The less you have the camera doing, and the more you do, the more control you have over your images.

The other thing to note is that depending on the bandwidth of the hydrogen alpha filter, you may need to take up to 20+ minute exposures to allow enough light to get through to the sensor. Remember, the red channel only picks up 1/4 of the signal on the Bayer matrix (1 red, 2 green, 1 blue). With ICNR, you'll be exposing for 20 minutes, and then waiting around for 20 minutes for ICNR. I say, grab an hour of darks before you start imaging, spend your night capturing photons and then take an hour or more of darks once you're finished. Median combining the darks should assist in removing the majority of noise.

The other thing to note is that because you're working purely in greyscale, you don't have to worry about chrominance noise, just luminance. These things can be processed out easily enough in Photoshop if you know what you're doing.

I've never imaged in hydrogen alpha before, so can't give you definitive exposure values. I say, get out there, use ISO-400, get your subject roughly in the field of view, take a 1 minute exposure, 2 minute exposure, 4 minute exposure, 8 minute exposure, 16 minute exposure, 32 minute exposure. See which one gives you the best balance between signal, noise and contrast. You might find your optimum exposure may lie somewhere between the values I've mentioned. Once you're happy, frame the subject correctly, then, stick to that exposure value for the rest of the night.

At least, that's what I'd do. And, it is something I'll think about doing in the very near future.

Regards,
Humayun
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Old 01-10-2009, 03:40 PM
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Howdy Frank,
These shots were first go's with my modded 40D and Astronomik 12nm Ha clip in filter. Both iso1600, stacks of 10min subs (can't recall exact numbers...10-18??), ICNR on.
Since doing these I've started to push the exposure times to 15mins.

http://s327.photobucket.com/albums/k...urrent=22d.jpg

http://i327.photobucket.com/albums/k...g?t=1254379219

Doug
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Old 01-10-2009, 04:34 PM
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Doug,
Thanks for the look at the images, what about your light frames?

Frank
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Old 01-10-2009, 05:25 PM
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Quote:
Originally Posted by telecasterguru View Post
Doug,
Thanks for the look at the images, what about your light frames?

Frank
Lobster is 27x10mins, Eta is 6x10mins
Dark subtraction was done in camera and Flats were done too.
Hope that helps.
Are you looking to get a clip in Ha filter?
Doug
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Old 01-10-2009, 08:44 PM
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Yes I am. I want to give it a go. What would be the result of also doing SII and OIII and combining them with the Ha?
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