I have some L, RGB, and Ha data for a nebula image. Been playing around with the LRGB to at least produce an image from that, but would like to add the Ha data in there. Searching around the forums I've come across many HaRGB posts, and sooner or later one comes across "
Vicent's Method"
I've also come across a
HaRGB script by Silvercup. I posted a question in that thread, but no response so far and it appears development has stalled somewhat with no posts from the author since Sept 2011. I had a bit of a play with the script with less than desirable results, but that's another post.
And I have also seen Harry's video tutorial on adding Ha to galaxy images.
Now I've read through "Vicent's Method" and I think I understand most of the theory behind it, although the tutorial doesn't go into details (ie a full walkthrough for dummies like me) on how to actually achieve the results. So I do have some questions. I've searched through other threads to see if I can find answers, and I turned up many different posts. And these different posts all seem to claim the pixel maths in them is as per "Vicent's Method", but the maths itself seems to vary from one to the other. And not sure of the dates, but it seems that the theory behind the method was in a bit of a fluid state at the time. Not clear on which posts on HaRGB combination and the associated pixel maths is before, during, or after the actual publication.
Adding more confusion for me is that my data is all different exposure times and binning, so the linear Ha image is totally different to the R linear image. In the tutorial it appears that it's based on the nebulosity brightness being very similar in the R and Ha images, just the Ha stars are dimmer. But in my case the R nebulosity is dimmer, but stars brighter.
So now for my questions.
* I assume it's best worked on linear data, before stretching? If so, how does one get the nebulosity the same intensity in both the R and Ha images? My Ha shots were 4 mins each at 1x1, while the the R was 30 secs at 2x2. Is it just done by eye and HistoTrans'n, or is there a definitive way?
* Vicent talks about iterations after combining the narrowband image (N) back with the noise reduced continuum image (Cnr) to get a new broadband R image (Bnew). Not clear, but does he mean using Bnew, divide by orig N to get a new Cnr2? Or use the Bnew as the new N, so C=B/Bnew? Sorry, just read that back and it's a mess. Hope it makes sense.
* All the posts about this HaRGB combination just talk about the Ha and RGB. Haven't seen too much mentioned about L. Does that get added at the end, after Ha introduced into the R? Or when reference is made to the RGB, is that actually an LRGB image being referred to? I don't think so, because as I understand it, L is best combined with RGB after a bit of stretching?
* Some of the formulae talk about the filter bandwidths. I'm using 12nm Astronomik for Ha (at the moment, will shortly be going to Astrodons), but I have Astrodon Gen 2 E LRGB filters. There is heaps of info on narrowband bandpasses, but I had to use this chart to try to guess the Astrodon RGB bandpasses. Not sure whether to use the bandwidth at 50% or 100%? At 50% the R is about 65nm, but at 100% it's around 50nm. Any comments on the importance or accuracy of this? Am I overthinking it?
* Similar to my question above about how to match the star brightness for the second half of Vicent's Method. By eye, or definitive method?
* Vicent's Method seems to work with division with that formula C=B/N, but some of the pixel maths seems to use subtraction instead, including Harry's video. So which is it? Please don't say both, I hate it when people say both
* Harry's video has 2 pixel math process icons with formulae:
((Ha*100)-(R*12))/(100-12) - the 100 is R bandpass value, 12 is Ha bandpass value. This gives a new image, which as mentioned above appears to be the continuum image via subtraction rather than division.
$T+(ha-Med(ha))x4 - ha is the continuum image from above, 4 is the "boosting factor" from Vicent's Method.
Any tips on modifying the above to suit different binning and exposures? I saw one post by Ionnis(?) that did seem to take these into account, but for the binning it wasn't clear on whether it should be multiplied by 2 for 2x2 binning, or by 4 for 2x2 binning.
* Might be getting ahead of myself considering the enormity of above post, but I assume similar theory could be adopted for OIII in the G and B channels?
That'll do for now. I'm sure I'll have more questions. Thankyou so much if you've read this far.
