MONO: Equal subs or not?

[evltoy]

Hey All,

I've been imaging with a one-shot color (OSC) camera for over three years now, with good success. Recently, though, I’ve gone down the rabbit hole of mono imaging.

I have a basic understanding of how mono imaging works, but I’d really appreciate some clarification from those of you who have more experience with it.

1. I understand that my sub-exposures for each color channel (R, G, and B) should be the same—say, 180 seconds each. But what about Luminance? Should the Luminance subs also be the same exposure length as the RGB subs?

2. When stacking, should I aim to have roughly the same number of subs for each RGB channel? And for Luminance—should I try to match the number of subs there as well, or is it okay to have more or fewer?

Thank you for helping me out.
Wayne

[Startrek]

There’s not one hard and fast rule with Mono and you will get different experiences and opinions from Mono imagers.
I used the 2600MC for just over 3 years and took the plunge into Mono ( 2600MM ) nearly 2 years ago , glad I did , my images have so much more detail and fidelity but you have to work for it.
This is what I do at both imaging sites Sydney B8 skies and South Coast NSW B3 skies.
I use Startools for post processing so my channels can be adjusted for weightings manually , either broadband or narrowband. ST can even provide missing channels like luminance ( Synthetic Luminance from the RGB ) if you fall short capturing it.
Anyway with any session I always capture the same exposure lengths for each channel , broadband LRGB or SHO narrowband.
If you want to add RGB stars to Narrowband images ( I don’t bother ) you can capture short exposures like 15 sec RGB say 20 to 30 mins worth each channel.
For LRGB broadband I use the following rough ratios in relation to total exposure time with all subs the same exposure time.
3:1:1:1 or 2:1:1:1
Both have worked well
For HaLRGB images of emission nebula I just gather my LRGB as usual and capture a few hours of Ha ( 2min to 3min subs )
In simple terms , Luminance is your main detail and RGB is your colour and some detail.
Don’t be too obsessed about gathering exact totals , I often have different RGB totals
If you have a look at some of my posts in Deep Sky Images , I detail my whole capture from A to Z including number and length of subs.
Most importantly don’t think that Mono is a short cut from OSC , you still have to get as much data as possible to improve SNR and get the most out of your camera. The more the better especially under light polluted skies.
Most of my OSC sessions over the years were 1 night to 2 nights max. With Mono it’s taking up to 5 nights to gather my data, sometimes more.

Hope the above is helpful

Other Mono folk will probably have different advice or experiences. Each camera , rig , location and target will require slightly different approaches.

Good luck !!

Cheers
Martin

[evltoy]

Hi Martin,

Thank you for taking the time out to provide your information. Your information was generally what I was thinking or understood.

Please tell me if I have read this right about your LRGB rough ratio

Exposure time
3:1:1:1 = L-60mins : R-20mins : G-20mins : B-20mins
2:1:1:1 = L-2hrs : R-1hr : G-1hr : B-1hr

I have been using my 294MCp for my SCT8 & ED120 and have a 183MCp for my ED80 and SCT8 hyperstar. I have now just bought a 294MMp. I know its old, but it matches my aspect ratio & pixel size of my colour Cameras. I just could not find a new model camera that had large pixels to pair up with my SCT with .63 reduce like the 294MCp.

I'm picking up my Narrow Band filters this weekend... the rabbit hole just got deeper :)

Cheers
Wayne

[Startrek]

Hi Wayne,
Yeh those rough ratios are ok but again let your camera and rig be the final judge after imaging for a few months or so.
There’s no hard and fast rule only that Luminance is your detail so one would expect to gather more luminance than colour filters. Some folk disagree and just go RGB but in my limited experiences Luminance is “king” especially with the 2600MM which is a tremendous camera.
Good luck with your Mono journey

Martin

PS: I see that you image from the South Coast NSW , what rough location ?

[evltoy]

Cheers Martin. I know I will fall/fail, but that is how I learn.

It’s been sometime that I have forgotten to update my location. I did have a place in Merimbula (Mirador) until last year when I sold the place :( I have been going up there for over 30years... miss that beautiful part of NSW.

[ChrisV]

An interesting thread for me, as I've been waiting for the clouds to bugger off so I can have my first go at RGB on a mono camera.

I was going to do just RGB rather than LRGB as I thought the larger bandwidth through the L filter would increase the noise. Plus more star bloat through the L filter. Maybe?

All just conjecture in the absence of any research!

[Startrek]

I image under Sydney B8 with an 8” f5 ( 1050mm fl ) so LP gradients are off the Richter scale
With Luminance for broadband targets I keep my subs short , in the order of 30 sec and try to gather a minimum of 2 to 3 hours on galaxies and emission nebula, the more the better. Luminance provides all your fine detail. I’ve compared RGB to LRGB and LRGB wins hands down with my experience. Others will disagree !!
I also use 30 sec subs for RBG as well
I’ve been getting good results from Ha LRGB too ( Ha are 2 min subs using a 3mn Antlia filter )
My camera is the 2600MM set at low gain or Gain 0 for maximum well depth 50K and Dynamic Range, cooled to -10C.
Here’s a Ha LRGB of M20 captured last year ( 30 sec subs LRGB and 2 min subs Ha , total of 8 hours integration )
Obviously no moon around !!

Cheers
Martin

[The_bluester]

For my mono RGB imaging, I keep the R-G-B exposures the same, but the luminance (If I shoot it) tends to be shorter. I have always worked on the basis of using a sub length which results in a very small percentage of saturated pixels, to preserve star profiles as best I can, and the luminance filter will hit that point earlier than any other.

The only time I go beyond that would be if I am chasing detail in something really faint, in which case I might end up using an exposure time that results in more heavily saturated stars but then I shoot shorter subs to be able to mask in the brighter stars without the saturation problems.

[evltoy]

Why could you not take 2 different exposure times from Luminance?

20-30sec subs to retain star shape
60-180sec subs for detail.

In pixinsight you could just use the stars of the 20-30sec subs and remove the stars from the longer 60-180sec subs and use just for detail.

Could this work?

[Startrek]

The answer to your question in principle is yes depending on the target but ….,
Unless your skies are dark or darkish ( Bortle 1 to 5 ) I doubt whether you can shoot Luminance subs longer than 60 sec before the noise floor becomes too overwhelming and the detail becomes swamped or saturated.
At my B3 South Coast location, 60 sec to 120 sec subs ( target dependant ) are the sweet spot during New Moon.
In Sydney under B8 Skies 20 sec to 30 sec for Luminance are about as long as I can push before struggling with too much gradient or sacrificing detail.

The only way to find out is test and experiment with your gear , imaging location and specific target !!!

Martin

[evltoy]

Hey Guys,

What are your thoughts on taking flats after every night on a multi day session?

I'm doing 3 or 4days straight this week on the same target. Is it required to take flats after every night or do you think you can miss a night or two?

When I do OSC it is very easy to do 30 flats after each night, but mono now means flats for every filter (4) and sometimes I dont have enough time to do flats before the next image session starts.

[Startrek]

I asked the same question back in mid 2023 when starting Mono …..,

Temporary Set up
Flats and Flat Darks for each filter for each night if your optical image plane ( camera sensor to filter , flattener , reducer to focuser ) has moved or rotated etc….

Permanent Set up
Flats and Flat Darks for each filter “once only” for a target over multiple nights session if your optical image train has not moved at all.

I’m lucky as I use a Newt , my camera , coma corrector and filter wheel are permanently fixed and sealed as one unit. So I can remove , re install each night if I want to and only require one set of Flats and Flat Darks for each filter for the whole project. However if I choose to remove it , I do ensure it is re installed in the same orientation ( as close as possible) into the focuser to allow my stacker ASTAP an easy alignment.
My rig in Sydney is permanent with just a cover over it , however I generally bring the optics inside if I’m imaging the same target over weeks. If imaging within same week I leave everything in tact.

My optics suffer from severe Vignetting ( caused by my high quality TS GPU coma corrector) however my calibrated Flats always correct this vignetting problem together with any dust motes.

Never had an issue at all

I do the whole calibration process…,
Darks from Dark Library
Flats
Flat Darks in lieu of Bias ( Flat Darks are the gold standard in astrophotography )

Camera ZWO 2600MM ( All filters )
40 Darks to create Master Dark ( I use Darks as IMX571 sensor still suffers from RTN )
40 Flats
40 Flat Darks with Flats to create calibrated Master Flat
Same Temp
Same Gain and offset

NB: Minor focus changes between filters do not affect my calibrated Flats. I can take my Flats after a session or before the next session or do them between sessions on a cloudy night. I can take one Flat or the whole set at a time. ( LRGB , Ha LRGB , SHO , HOO and so on ….. )

I have re used existing Flats on a couple of occasions with different targets but I normally take new Flats and Flat Darks for each project. They don’t take that much time but are so important for a clean flat evenly illuminated field. Your post processing software will thank you each time.

Hope others chime in with their experiences

Cheers
Martin

[evltoy]

This information is what I was hoping to hear.

I have a permanent rig in my obs and I only swap it out season to season. Camera, filter wheel, reducer and now CAA all stay intact for months on end.

I already do Dark Libraries for all my camera every 6months, so flats for the rig in the obs and be done too provide the image train doesn't move.

This information is now going to make my mono experience a joy. ;)

Thank you for sharing this Martin

[big-blue]

Just be aware that various exposures being quoted should be referenced to the speed of the optical system being used. eg I use a reflector with f4 optics which can capture photons around 3x faster than (say) a refractor at f7 (ignoring secondary effects such as central obstruction).

personally, I am a convert to capturing as much quality Luminance as possible as that provides the fine detail. RGB just colourises it.

enjoy your mono journey.
to me it is mostly about trying to get better, mostly failing, and hopefully learning more from the experience.

cheers

[Startrek]

Excellent point which I failed to mention but I did mention one has to get out there and experiment for a while on different targets to understand your new Mono set up.
I’m still learning after 2 years using Mono.

Clear Skies
M