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filters for narrowband imaging?


sgazer

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I'm after some advice please. I have a QHY6 on its way with which I hope to start out in narrowband imaging. Initially I was considering 1.25" narrowband filters due them being cheaper, however I also have a baader mod'd 300d (for sale;)!) and a 2" Skywatcher OIII filter. Baring in mind I have to be very conscious of cost, I am trying to decide which filters to get.

I was hoping to get away with having 2 filters and using Noel's actions in PS to Synthesise the green channel from the red and blue channels.

My question is, could I use a Ha filter (to be purchased) and the OIII (I already have) to image 2 of the channels and synthesis the third? If so, which 2 channels would Ha and OIII be assigned to?

If this is possible, then because the OIII filter is a 2" filter, would it be worth me getting a 2" Ha filter (at much greater expense) which could be used both with the QHY6 and the mod'd 300d (which I would then keep)?

Or would I be better off getting the baader 1.25" Ha and OIII filters and just using the QHY6.

Or.....any other alternative you can suggest.

Thanks

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For DSLR clip filters are better, and CMOS color sensor isn't that sensitive so it would be hard imaging (but doable). For QHY6 you can get 1,25" narrowband filters, which later you will be able to use with cameras like Atik314L+.

As QHY6 is small sensor - you could also add 1,25" focal reducer to reduce the image scale and decrease exposure times ;)

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I've used the 'false' green channel before when I haven't been able to Green data from an RGB session before and it did the trick before I managed to get the real data. On the narrowband side, many people use it to replicate SII data because they either don't have the filter or the time needed to get it (because it's so much fainter in many objects than Ha or OIII).

My question is, could I use a Ha filter (to be purchased) and the OIII (I already have) to image 2 of the channels and synthesis the third? If so, which 2 channels would Ha and OIII be assigned to?

Take your pick. Most people use the Hubble pallette which IIRC is SII=Red, Ha=Green and OIII=Blue. But there's nothing stopping you assigning whatever you want to whatever channel really.

If I was in your shoes, I'd just buy a Ha filter for now (probably 1.25") and see how you go with the QHY. HA is by far the most common element in nebulae so you'd be able to produce some cracking mono shots while you decide what you're going to do with the DSLR.

HTH

Tony..

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i would get O3 and Ha.....

you can use Noel Carbonis synthG method, or you can use a bit of simple science to create an RGB

here is what I would do.....

Go get good colour balance, you need to make an Hbeta channel. Here is where science helps us. It says that the Hbeta is 1/3 as intense as the Ha....

it will have the same distribution as the Ha, so simply Hbeta=1/3rd Halpha...a simple division.

also, the O3 straddles the green blue divide, so we actually assign it to both channels

normally Ha=R

O3=G

O3+ Hbeta=B

this gives an accurate colour balance as the important emission lines have been covered.

Synthetic RGB from Emission Line Data

this gives more info and examples

as for filters, the baader get a good writeup and are pretty affordable

hth

paul

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Go get good colour balance, you need to make an Hbeta channel. Here is where science helps us. It says that the Hbeta is 1/3 as intense as the Ha....

it will have the same distribution as the Ha, so simply Hbeta=1/3rd Halpha...a simple division.

also, the O3 straddles the green blue divide, so we actually assign it to both channels

normally Ha=R

O3=G

O3+ Hbeta=B

Paul,

I like this idea. How are the mathematics done on the image, in PS or something like Pixinsight?

Thanks

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i think any processing software can do....

i use images plus. I open the Ha and quite literally multiply each pixel value by about 0.3.....

and then save a 'Hbeta' or something....

then to make the B channel, just simply add OIII+Hbeta, save as Blue.....

but i think most software can do it.

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thanks Paul. Looking at Pixinsight, there is a maths function, but it doesn't appear to allow division of an image by a numerical constant, only be another image. As for Photoshop, I can't see how to do it there either. Is it a case of making up a second grey image with R,G,B levels of 2,2,2 and dividing the original by this?

Cheers

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hmmm pixinsight should be able to do it...

in imagesplus there are two math functions....pixel math and image math....

pixel math is what we want, where we can multiply/divide add and subtract numbers on a pixel by pixel basis. Where as image math is where you can subtract an image from another image....and divide etc

you idea of a 3,3,3 greyscale should work!...i think

if the new pixel values are 1/3 of the original then WIN

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