LRGB Exposure times

[Marky1973]

Okay, I haven't asked a stupid question for a while so here goes....

In my forays into LRGB imaging I have been exploring different ways of collecting the data....so straight imaging with, for example, 1 hour of Lum with 1 Hour each of RGB or, as another random example, 6 hours of lum and 1.25 hours each of RGB binned 2x2...all theoretically correct so far...I hope....

But, I've just been blindly imaging DSOs with, say 10 minute exposures for the LUM, and the RGB, without much though about the effects of the filters...a couple of nights ago I started collecting some data on M51 and have about 3 hours of Lum so far. I exposed to 1/3 of the Histogram in Artemis. If I was just following my usual pattern, I would be setting up for RGB and imaging at 10 minute subs again for either the full duration, or 1/4 of the time if binning.

I'm doing this wrong aren't I?

I should be finding the right exposure time for each filter to expose to 1/3 of the histogram and then build that up to the requisite time. So, if the RED Channel needs just 5 minutes to expose to a third, for an Hour of 10 minute Lum subs, I would need 12 Red Subs or 3 or 4 if I was binning 2x2?

Am I thinking this through right?

I haven't been too happy with the colour balance in my images to date and this feels like ti might be contributing to my problems.

My foot is poised to start kicking myself.....

 

[ollypenrice]

Interesting question and far from silly.

You're likely to be skyglow limited so you need to be sure you don't just fill up the chip with LP. I don't have that problem so I can expose for as long as I like, though I've only ever gone to 30 minutes. I base my decisions on guesswork, to be honest. If I'm after the Integrated Flux, or the tidal tail in the Leo Triplet, or anything dusty, I like long L subs. Thirty minutes seems to work. I don't make my colour so long because I want to retain as much colour in the stellar cores as possible but I don't worry about hitting saturation in parts of the brightest cores. It can be dealt with in processing. My colour subs are 10 or 15 minutes.

When you have far more L than RGB you can still coax out the colour but it is far more complicated than working on an equally weighted LRGB set. The L has to be added iteratively in small doses with NR applied and saturation boosted with each iteration prior to the last one.

I don't believe binning gives 4x the signal. Some insist that it does or that it ought to, but I'd give it a try. It doesn't for me. I find it might just make 2x. However, I don't bin because I often try to use 'RGB only' for the stars and I need good stars for that.

I honestly don't believe there is any substitute for experimenting.

Olly

[Marky1973]

Cheers Ollly - definitely been experimenting - even if only accidentally. And I think I definitely need to give more consideration to the histogram when I am imaging - being affected by Light Pollution 4 miles outside of Gloucester is something I have to deal with. Been binning 2x2 up to now, but think I might forego binning in my next effort and see how that compares to my efforts with Binning...It may just take me a few months.... :-)

Does the histogram help track the effects of Skyglow, or is that more of an "expose and check" kind of thing?

 

(although tomorrow and Saturday have clear forecasts....but I am not counting my chicken just yet...)