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Old 25-10-2018, 01:34 PM
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Stonius (Markus)
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Grey Card Tests. Is this just silliness?

I did a bad thing as tested my gear out of interest, and now it started me thinking. I should have known better than to go looking for issues where there were none :-D

I did some tests shooting a photographer's neutral grey card with my 1600MM and a set of ZWO RGB and NB filters.

Compared to the Luminance filter, exposure times increased by a factor of 3.25 (R), 3(G) and 3.75 (B) respectively - a range of roughly 1.5-1.8 stops.

I notice most people do sets of RGB subs at the same exposure times for all filters regardless of the chip's QE in that frequency, or the individual filter's ability to transmit light.

Given that the blue channel is noisier anyway, I'm thinking maybe I should compensate exposures for R and G to get the same amount of data density on each sub. That way, the colour balance when RGB channels are combined will be more accurate and per-sub SNR levels should be more similar.

BTW, the NB filter exposures increased by 43.5 (Ha), 32.5(O3) and 50(S2) times relative to Luminance (which from my reckoning is about 5-5.8 stops).

I suppose a OSC would suffer from these inconsistencies too, given the bayering filter and chip would have the same associated problems.

Maybe this is all irrelevant given that some subs will get tossed so you're never going to have exactly the same data for each channel, but surely that is the initial objective?

It seems to me that without compensating I will have a 'sensitivity footprint' across all my images where the Blue channel is consistently poorer than the Red or Green.

I know most people bump the gain up for NB imaging, but even with that in mind, the same thing applies - should I expose S2 for slightly longer than O3 (with Ha somewhere in between the two)?

Am I completely overthinking it? Maybe I'd be better off just getting more Blue subs to compensate, for example.

What do you guys think?

Markus
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Old 25-10-2018, 01:48 PM
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multiweb (Marc)
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I did a grey card test on my QHY8 OSC a long while ago and I was asking the same questions. It came back with RGB ratios to 1:1.14:1.51 for my camera at the time. So I'd have to shoot a lot more B to match the R and G.

It really depends on where your target is in the sky at the time you're imaging. In RGB red will be les affected than green or blue at lower altitude. So you'd shoot blue close to the zenith then red as it goes down.

For NB again Oiii will be turquoise and will more affected than Ha or Sii which are deep brown. But in NB it also depends on your target. Some have a lot of Ha , little Oiii and hardly any Sii. So for each 1h of Ha you might have to shoot x2 Oiii and x4 Sii to get the same SNR.

There's no easy answer to all this. But a grey card will give you info about your camera definitely. It's a good start.
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Old 25-10-2018, 06:04 PM
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... looks like Glen's post evaporated again...
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Old 25-10-2018, 06:56 PM
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Sounds logical to me.

What I aim for in a single colour sub is to get B sufficiently over the read noise, then set each channel to the same exposure time, for simplicity.

OIII happens to be slap bang in the middle of the peak QE for the sensor, whereas Ha and SII aren’t, but it doesn’t really matter. I aim for a given noise reduction factor / SNR in each channel stack, and of course that will vary by object also, especially so in narrowband.

With RGB or narrowband you’ll be wanting to colour balance appropriately, which will scale the intensities of each channel accordingly. That eliminates the sensitivity footprint, although I’m sure if you looked at the noise you’d be able to tell which band the sensor was weakest with.
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Old 26-10-2018, 08:17 AM
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Markus, this makes sense to me. I could not understand how mono camera users always seemed to give RGB filters the same exposure. My first job many years ago was in the printing industry as a Lithographic Camera Operator. This was in the days before digital reproduction had started so all printing plates were produced using photographic film.

My job was to colour separate an original piece of artwork into its primary colour components, ie RGB film separations. I always had to give the Blue filter maybe twice or more than the Red and Green filter exposures. In fact, each filter required its own different exposure.

I put the longer exposures necessary for the Blue filter down to its low transparency characteristics. If you hold a Red and Green filter up to your eye, you can easily see through them but the blue is much more opaque/darker and is difficult to see through.

Last edited by Mickoid; 26-10-2018 at 02:18 PM.
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Old 26-10-2018, 08:38 AM
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I may be right out of the ball park with this however, regardless of the differences in what signal is recorded using different filters cant we make up for it with the digital manipulation we do during processing.....meaning we make the blue channel more pronounced if that is the one lacking?
In my amateur mind, as long as we get a reasonable amount of signal then we can make up for it with those magical sliders.

Analog is one for one so I understand this would apply in that situation....what ever you have is all you got so to speak.

......or am I completely wrong?
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Old 26-10-2018, 09:08 AM
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Yes you can compensate with processing. But isn't the point of different exposures times with each filter to get similar signal levels. Then you swamp read noise by the same amount and avoid too much saturation?

I'm just guessing though as I've never done mono
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Old 26-10-2018, 09:15 AM
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It can be both useful and not so useful, more so with LRGB. The blue subs may need longer exposures to swamp the read noise as much as the red and green frames.

Is it worth while? Maybe. As long as all subs are sufficiently above read noise it doesn’t really matter too much. If your red subs are 15x read noise, your green is 20x read noise and your blue is 8x read noise (just making up a radon figures) then all of your RGB frames are fine anyway and changing the exposure times isn’t going to make any SNR impact.
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Old 26-10-2018, 09:30 AM
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I guess it is also target dependent. If shooting the Pliaedes,, you probably want to make sure your blue channel up to snuff.

It's probably more important (if it's important at all - still making my mind up, given the excellent points raised by others here) if doing RGB as opposed to LRGB.

Markus
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Old 26-10-2018, 09:40 AM
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Quote:
Originally Posted by Stonius View Post
I guess it is also target dependent. If shooting the Pliaedes,, you probably want to make sure your blue channel up to snuff.
It's going to be easier to get good SNR in blue with the Pleiades because there's lots of signal!

My thoughts are:
  • always do sky limited subs for L/R/G/B (may not be feasible for NB)
  • try to avoid a big imbalance between SNR in the colour channels but rough is good enough and if you're doing Lum then it's less important
  • for NB the signal in each filter is much more variable and target dependent - just do the best you can and rely more on processing to make up for poor quality masters (tone mapping is a big help)

Cheers,
Rick.
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Old 30-10-2018, 01:08 PM
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Quote:
Originally Posted by multiweb View Post
It really depends on where your target is in the sky at the time you're imaging. In RGB red will be les affected than green or blue at lower altitude. So you'd shoot blue close to the zenith then red as it goes down.

For NB again Oiii will be turquoise and will more affected than Ha or Sii which are deep brown. But in NB it also depends on your target. Some have a lot of Ha , little Oiii and hardly any Sii. So for each 1h of Ha you might have to shoot x2 Oiii and x4 Sii to get the same SNR.
That's interesting. So you'd get better resolution by capturing subs in order of wavelength. Something like S2, Ha, Red, Green, O3, Blue, Lum, in that order (on a rising target). Of course you may not be wanting to use all filters. You'd pick an choose. But for example, if shooting LRGBHa, you might be best shooting in this order; HaRGBL, correct?

Markus
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Old 30-10-2018, 01:25 PM
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multiweb (Marc)
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Quote:
Originally Posted by Stonius View Post
That's interesting. So you'd get better resolution by capturing subs in order of wavelength. Something like S2, Ha, Red, Green, O3, Blue, Lum, in that order (on a rising target). Of course you may not be wanting to use all filters. You'd pick an choose. But for example, if shooting LRGBHa, you might be best shooting in this order; HaRGBL, correct?

Markus
Better SNR for sure and possibly resolution provided the air is stable where you're pointing. Yeah I'd get the blues at the highest point when possible. So B and Oiii. The rest is more forgiving. IR you can shoot anywhere you want. Even through light clouds or haze.
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