use hubble palette->grayscale as luminosity in LRGB?

troypiggo (Troy) · #1 14-04-2009, 10:23 AM

Quick background - yesterday I was killing some time reading threads on narrowband filters and it occurred to me that in Brisbane city's light polluted (and probably even moonlit) skies I could use OIII:Ha:SII filters to get some pretty clean "Hubble palette" deep sky images. I had read that some shoot Ha in the city, then when they get to dark sites they shoot the RGB and put it all together at LRGB. Cool idea.

Next thing I know, click click click, I had pulled the trigger at Bintel and bought myself all 3 1.25" Astronomic narrowband filters! Please don't tell my wife.

So back to my question. I note that it's quite common to use Ha as the L channel in LRGB shots. But I got to thinking, what if I shoot a false colour hubble palette type shot, then convert that to grayscale, and use that as the L channel instead of just the Ha? Would this improve things, or better just to stick with Ha? Has anyone tried this?

rat156 · #2 16-04-2009, 11:58 AM

Hi Troy,

Very few objects emit more in the SII or OIII regions than they do in Ha. A few planetary nebulae do, but in those cases you could shoot OIII as luminance. The reason people usually don't do as you suggest is that it takes a lot of time to shoot the OIII and SII, and if it's not going to add much to the image then why bother.

There are some people that have shot Ha, SII, OIII, R, G and B and then combined them as a six colour palette, maybe Fred has done this.

Cheers
Stuart

troypiggo (Troy) · #3 16-04-2009, 12:04 PM

I see. That all makes sense. Thanks Stuart.

multiweb (Marc) · #4 16-04-2009, 06:07 PM

Hey Troy, I'm a noob too in NB but I tried a few things. Using Ha only as luminance gives you very sharp details and tight stars. Not many stars either when you compare to SII or even OIII that have many more. So if you use Ha only as Lum you'll get very few stars. But if you stack your Ha+SII+OIII into one lum or any combination of these I reckon you can get different effects and certainly more stars. I've got 5h of Ha and 4h of OIII on Eta so far and a crap 1h of SII. SII is the hardest to take. Takes ages. OIII was the easiest for me. Once you have you 3 masters nice and clean I reckon you can play to your heart content and shuffle things around for some pretty groovy pictures.

troypiggo (Troy) · #5 16-04-2009, 06:38 PM

Thanks Marc. Can't wait to get out there and go for it!

rat156 · #6 16-04-2009, 06:42 PM

Marc,

Remember that you're using a OSC sensor. This is true mainly because you've got twice as many green pixels. Monochrome CCDs don't suffer from this effect.

Cheers
Stuart