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Monochrome time factor question

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#1 Aerodave3

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Posted 16 February 2020 - 02:38 PM

So I am about 2 years into this wonderful hobby. I have been shooting with my Nikon D800 dSLR and my 600mm f/4 Nikon telephoto lens. I am ready to move to a dedicated Astronomy camera and am torn between color and mono. I really want mono as I seem to understand that there is more detail in the images (which is why I want to move up in the first place in addition to reducing the noise with chip cooling)  However, I hear all the time how much extra time it takes to shoot in mono. 

My question is, do you have to shoot the same number of subs with each color filter? Seems those of you that shoot mono have come up with a way (through binning?) that you do not need tons of subs for each color. Can someone who knows enlighten me on this? Usually I go out and shoot with my D800 and spend maybe 3 hours shooting light frames. I can not imagine doing that same thing for each color. There are not enough hours in the night. Where am I missing it here?

I would love any information, advice, direction you great folks out there can offer. (smiley face)

 



#2 kathyastro

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Posted 16 February 2020 - 02:49 PM

I usually shoot about 60 minutes of L (12 x 300s), and 24 minutes each of R, G, and B (12 x 120s, binned 2x2).  If I am shooting Ha, I aim for 120 minutes (12 x 600s).  I find that 12 subs gives me a decent SNR.  Shooting a full HaLRGB series gives me 4 hrs 12 min of total exposure time, which makes for a reasonable-length session, typically around 5 - 5.5 hours long.

 

If logistics (such as getting it all in before the clouds move in) require a shorter session, I might reduce the number of frames to 8 per filter.  And if I find I need more frames, I will shoot them on the next clear night.

 

I am imaging at f/4.  At slower speeds, you may want to increase the total time.  I really ought to shoot longer exposures for my narrowband - but increasing to 15 minutes each would be pushing my luck for guiding.


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#3 Aerodave3

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Posted 16 February 2020 - 02:59 PM

Dont your subs all have to be the same exposure time for the darks to cancel out the noise properly? What do you use for stacking?



#4 OhmEye

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Posted 16 February 2020 - 03:09 PM

First of all, imaging mono RGB is more time efficient than OSC, so don't worry about the number of hours in the night. The same total exposure time gets more signal for mono RGB than with OSC.

 

CMOS binning doubles SNR for 2x2 at the expense of resolution, so unless you are undersampled at 1x1 binning is an option to get higher SNR in the same time, or the same SNR in less time or some balance between. By collecting enough luminance for high SNR and detail, you can essentially just use the RGB to color the luminance channel, so you don't really need resolution in the RGB and can sacrifice some for higher SNR to minimize the time it takes to get satisfactory low color noise. This lets you trade some RGB time for more L time in a session. Even without binning, LRGB is more time efficient than OSC, but having the flexibility to maximize time on a per-channel basis gives options to be even more time efficient based on your target and goals.

 

Dont your subs all have to be the same exposure time for the darks to cancel out the noise properly? What do you use for stacking?

Darks should be the same exposure time as the lights, so if you have lights at different exposure times then you simply use darks of the same exposure times. I have a library of master darks for all the exposure times I use for both 1x1 and 2x2 binning. I also have a master flat library for each filter, and I update it with a new set each time I make a change to my optical train.

 

I use APP for stacking and processing, and PS Camera Raw for final noise reduction and tweaking.

 

EDIT: Here's a mono vs. OSC analogy video from Dylan: https://www.youtube....h?v=lD0ZH2doSlk


Edited by OhmEye, 16 February 2020 - 03:14 PM.

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#5 MalVeauX

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Posted 16 February 2020 - 03:11 PM

Heya,

 

There's actually quite a few approaches to this. If time is of the essence, the more important thing is to get as much luminance data as you can. While technically this is usually done with an UV/IR block clear filter, many commonly replace it entirely with a narrowband HA filter (such as an Astronomik HA 12nm filter for example) when the subject matter has HA involved (basically most things as its the most common being literal star light). This luminance data is what is processed and is the detail in your image, and it's B&W. Then, one commonly will grab less data in the color filters, RGB, binning to decrease exposure time, as you're only after color information, not the detail. And then when processing, the color is combined but is mostly blurred to be noise free and avoid exploding chromatic noise, but a gentle blurred RGB image with the luminance B&W detail as the luminosity layer on top. That's just one way, to speed things up. This is not suitable for all subjects. Galaxies for example often are not HA subjects and simply need LRGB in general. But most other DSO targets, nebula, etc, are fairly rich in HA. The idea is that you're not processing the RGB image for the detail, just for the color and that you are putting more effort and subs into your luminance data that will serve as the detail.

 

The other reason mono is important is because of light pollution. That includes full moon times. Mono with narrowband filters (HA, S2, O3) allow you to image through light pollution and a full moon when you otherwise would not be able to image, or would require a ton of integration time to have a chance at getting workable data. You cannot do this with a color sensor as efficiently right now (maybe one day... such as with the triad and quad narrowband filters that work on color sensors but require very long exposure and lots of integration time). This increases how often you can image. You normally would not be able to image DSO when the moon is in the sky, which is quite often! Having a mono sensor and narrowband filters lets you image things that you normally wouldn't be able to simply due to the moon.

 

Under a dark sky, you can absolutely shoot color on all subjects basically. A cooled sensor is important. Nothing wrong with shooting color. It greatly simplifies things and is less expensive.

 

Very best,


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#6 kathyastro

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Posted 16 February 2020 - 03:19 PM

Dont your subs all have to be the same exposure time for the darks to cancel out the noise properly? What do you use for stacking?

I use the BPP script in PixInsight.  It has no trouble matching darks to lights.  I keep a library of darks for my standard temperatures and exposure times.  I can usually match any of the lights with library darks.  And the BPP script will scale the darks by time.  So I could theoretically get away with just using darks of 600s or more.  In practice, I usually use darks specific to the exposure times.


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#7 sg6

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Posted 16 February 2020 - 03:35 PM

Couple of years ago someone somewhere posted an image of The Veil and gave individual images of each collected band with exposure durations. I took a copy for the durations, also the image was good.

 

Theirs was:
Lum   18min

Red    15min

Green  15min

Blue  15min

Ha     45min

OIII    36min

SII     30min

 

So as a very (Very, very) rough guide L,R,G,B = 1 unit, SII, OIII = 2, Ha = 3 units of time.

Total units = 11 so if 3 hours then 1 unit = ~15 minutes (didn't realise it more or less matches the example).

 

So head out, set up expect 3 hours collecting and try L+R+G+B at 15 minutes, SII+OII at 30 minutes and Ha at 45 minutes.

 

The target may well mean subtle alterations, a Ha rich target could mean reduced Ha collection. However only you can determine or guess that.

 

For a first go I would be tempted to opt simply for L,R,G,B at T minutes and Ha,OIII,SIII at 2T minutes. Initially keep it simple.


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#8 SilverLitz

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Posted 16 February 2020 - 06:26 PM

I generally scale my exposure times: Lum x1, RGB x2, NB x4




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