3 replies to this topic

[John Miele]

I'll be at a bortle 4 site this weekend  with a  ASI533MM pro camera and a FF107 refractor shooting at f7. Want to capture a close in view of the Blue HH nebula. It's a reflection nebula and the most important thing to me is to try and capture that beautiful blue color. With limited imaging time, should I do LRGB or just pure  RGB on this one?

 

Thanks!

 

cs...John

[ngc2218]

I'll be at a bortle 4 site this weekend  with a  ASI533MM pro camera and a FF107 refractor shooting at f7. Want to capture a close in view of the Blue HH nebula. It's a reflection nebula and the most important thing to me is to try and capture that beautiful blue color. With limited imaging time, should I do LRGB or just pure  RGB on this one?

 

Thanks!

 

cs...John

I'm going to quote Juan from Pixinsight from an older post:

 

 

 

 

As for the LRGB vs RGB thing, just to state my opinion clear:

- LRGB: Good to save time. This is true as long as RGB is shoot binned; when shooting unbinned L and RGB, the savings are marginal IMO.

- LRGB: Bad for quality. Assuming unbinned data, an independent L does not provide more resolution. At the contrary, it may provide less resolution since it has been acquired through a much wider band pass filter.

- LRGB: Problems to achieve a good match between luminance and chrominance.

- LRGB: More limitations to work with linear data. LRGB combinations are usually performed in the CIE L*a*b* and CIE L*c*h*, which are nonlinear. It is true that a linear LRGB combination is doable in PixInsight, though, working in the CIE XYZ space.

- RGB: Perfect match between luminance and chrominance, by nature. No worries about luminance structures without chrominance support, and vice-versa.

Curently I am trying it myself on galaxy M106, shooting in RGB with a Moravian C3 Cmos. I chose 30 hours per channel, the red channel is finished and I like what I see. 

I guess with the new high QE sensors, pure RGB is more approachable, eventhough we should keep in mind it does require more imaging time since we limit the bandpass.

 

Since the CCD era, most people take LRGB for granted, but I think Juans thoutghs are worth considering

[whwang]

F7 may be too slow for the blue HH.  You pretty much have to spend a total of more than 10 hr on it, maybe much more.  For example, this is my blue HH:

It was taken with an F2.8 optics, from a world-class dark site.  The exposure time on the main target is 4 hr.  This translates to 24 hr for F7.  If your site is not as dark as this one, the required exposure can go to 40 hr or even 60 hr to match this.  Of course, you don't have to go this deep.  So I suppose 20 hr under F7 can lead to some nice results.

 

I think LRGB is definitely the way to go, provided that you know how to handle LRGB composition.  On this forum and Astrobin, there are many threads about this topic. After seeing many comments from various people, my current feeling is that many people think LRGB is inferior simply because they don't know how to do it in an optimal way.  And to be honest, I don't think any people (including myself) can say with 100% confidence that he/she knows how to properly do LRGB composition.  At least I haven't seen anyone claiming so while having consistent nice results to back up the claim.

 

So, if you are confident about your LRGB processing skill, just do LRGB.  It's much more efficient than RGB on continuum spectrum objects, such as the blue HH.  The example above was taken with a DSLR, so it's an RGB case rather than LRGB.  You may cut down the total integration time to 60% or even lower if you use LRGB.

Edited by whwang, 24 April 2025 - 12:21 AM.

[freestar8n]

Neither.  If you have some light pollution and the main color you are interested in is blue, then do something like RGBBB.

 

Using the blue filter is like using a narrowband filter to isolate the key signal you are after and block out light pollution as much as possible.

 

If you are after faint colors across the board then you will need to gather signal in depth in each channel - and L at least has a chance of being useful.  But blue is special since it contributes so little signal to actual perceived luminance - and it will just get washed out by L, which itself is loaded with light pollution.

 

As long as your stacking software can handle different numbers of exposures in each channel - it makes sense to take many more exposures in the channel you are after - particularly if you have light pollution.  So - lots more time on blue.  It amounts to a light pollution filter for the signal you are after.

 

If you find the resulting stacks have insufficient red or green and the noise shows - then do more for them and less for blue.  But there is no reason not to boost signal in a particular channel that has faint signal you are after.

 

Frank

Edited by freestar8n, 24 April 2025 - 07:38 PM.