Imaging the Crescent in OIII - Do I keep my exposure the same?

[CKemu]

So for my Ha data I imaged at 600s binned 1x1 using my 8" LX90 reduced to f/6.3

NGC6888 - The Crescent in Cygnus by Chris Kennedy, on Flickr

My question is, do I keep my OIII subs the same, or, due to the OIII portion being very faint, shall I increase to 900/1200s? I am probably going to experiment tonight, but not sure how changing exposure lengths between data sets will effect processing and whilst a stretch as it's "artificial colour" to start with, the authenticity of it's appearance.

Opinions welcomed.

[MattJenko]

I would go for as long a sub as you can to get as much data as you can. You can always balance things in processing, but if you don't have the data to start with, then there is not much you can do. Narrowband imaging processing is artificial, no matter how you look at it, and is representational only, so authenticity is not really a consideration in my view.

[Jannis]

I'm new to narrowband, but i think I'd expose it to roughly the same on the histogram - ignoring the exposure time, because like you say Ha and OIII may have significantly different brightness depending on targets.

[ollypenrice]

Depending on what's causing it, the distortions on the left hand side of the image might become worse with longer exposure. (This would be the case if you have field rotation, which might be the cause if PA is out and the guide star is off axis.) Personally I don't worry about using the same exposure lengths in Ha and OIII. If the OIII is very faint it might benefit from longer and this won't, in itself, cause a processing problem.

Olly

[swag72]

I just use 30m subs for all narrowband targets in both scopes and then I can always use the same darks  Personally if you can I'd go longer on OIII as it does tend to be very faint when compared to Ha. As Olly says though if the longer subs are going to cause an issue elsewhere... then you need to decide what is most important. It matters not a jot id you use different length subs.... take for example LRGB imaging. Most people use longer luminance subs than RGB.

[Davey-T]

Is that an LX90 ACF ? if so the normal 6.3 focal reducer isn't designed for it.
The distortion pointed out by Olly looks radial, bottom left, top left and possibly bottom right, the sort of thing that indicates wrong focal reducer spacing if not wrong focal reducer.

Dave

[Scott]

I agree with others that state that this target may benefitfrom longer exp. I did it using the same length subs (15 min), and the OIII was really lacking :(. I'll go longer this season if time allows