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Narrow band filter question


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Hello, I just got a set of narrow band filters. I was wondering how to use them. I assume I dont use a Luminence filter. I just take photos with each of the 3 filters   ha  o3 and s2  and still use the dark , flat and bias as usual for processing?

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Hi Carl.

You are correct, you don't need the luminance filter.

Just take images as you would with any other filter, and shoot the appropriate darks and flats. If you already have a master bias, this will be fine for the NB filters too. You will only need the bias for bias subtracting your flats.

What you will findd though is that for NB filters, the longer sub length you can get, the better.

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If you can guide accurately for a long time, then go for a minimum of 10 minute subs for Ha, and longer for OIII and especially SII if possible. I shoot subs of up to an hour duration for NB. With LRGB subs, you are limited by your background sky level....once the left hand side of the histogram starts to move to the right, then you won't get any more depth shooting longer. A light pollution filter helps here. I can typically shoot 6 minutes without, and 8 minutes with, an LP filter when shooting luminance.

With NB subs, background levels aren't really an issue.

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Yes, you can see the histogram in Maxim.

The background levels are simply how bright your actual sky is.

With the skies most of us have to deal with, the sky itself is actually quite bright and will mask very faint data. Obviously, at the point where this happens, it does no good to carry on with longer exposures as you get no more faint data. You can figure out your best exposure by looking at the histogram. The tallest point of the graph ideally should be a little to the right of the bottom ( left hand side) of the graph. It doesn't want to be right at it as there will most likely be faint data just before it. Do some exposures and increase the length of them. At a certain point the peak will start to move over to the right, getting further along as you increase exposure. After more than about an eighth of the way along you will have got to a point where you are starting to fill up the darkest areas with the sky background glow.

Hope that helps :-)

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Hi Carl

I usually use the luminance filter (although strictly, I don't have to w/ a Newtonian) for parfocality (no need to refocus). Also, it's easier to focus with one, as trying to focus through an ha filter is much harder.

Andy

Sorry, I don't understand this. You focus through the Luminance filter then switch to the Ha? Or do you keep the L filter in place as well as the Ha?

If you are dead sure that your filters are parfocal and that your entire optical path is very highly colour corrected then you can focus in L and switch to Ha. How successfully this will work will vary from rig to rig. I wouldn't bank on it working, though.

Olly

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Hi Carl

I usually use the luminance filter (although strictly, I don't have to w/ a Newtonian) for parfocality (no need to refocus). Also, it's easier to focus with one, as trying to focus through an ha filter is much harder.

Andy

I would never focus with a different filter to the one I'm about to image with? I focus using my Ha filter, then image through the Ha filter. If I then want to add SII or OIII data on the same imaging night, I would switch to whichever filter then refocus. It is sometimes more than a mm or two out which makes a big difference. Accurate focus, I think, can make or break a decent image.

Just my two cents.

With regards to exposure times, it will depend on your set up. How long can you accurately image for? Guided or unguided? Focal length? Mount? etc etc. :)

Phil

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