Dual-Narrowband filter+OSC vs. Ha/O3 filters+Mono Comparison

Hi Andre,

I admit, I haven‘t elaborated the numbers in detail for your suggested application but I once did a calculation for LRGB vs. OSC. 

It turned out that for R the SNR for the OSC was about 80% than the SNR if it were shot with mono. For green, the OSC had a slight edge over the mono.

While it may turn out that the SNRs are quite comparable, my point is this:

The OSC data will be interpolated and *might* miss detail due to reduced resolution. If resolution matters depends on the image scale, on the other hand. If you oversample, you can afford the loss of resolution and shoot OSC for sure. 

Another point: if you chose OSC + multi-band, you cannot distinguish Ha from S2 in case your filter transmits both. 

Personally, for NB imaging, I‘m clearly favoring mono and filters for each frequency.

CS!
Björn

In my opinion, the whole mono vs osc debate comes down to weather. Do you live in an area where clouds are not an issue and you can shoot every night? Mono. Do you live in an area with only 14 clear nights per year? Osc. 

Of course you can use mono in the latter, but it could take many weeks or years to capture required data. There’s nothing wrong with that, but not everyone has that kind of dedication or time. 

Where I live, most clear nights are during the summer where we only have about 3 hours of night time at our disposal. During fall and winter, there is a consistent cloud cover that only lets us peek through maybe once or twice per month. These are very difficult circumstances for this hobby throughout the year, but osc can help mitigate a lot of these issues (at the expense of reduced SNR).

If time is your enemy, osc is definitely the more efficient choice.

I think I would put more focus on the quality of filters you can get instead of crunching exposure numbers.

To give you an example, I do own both the L-eXtreme Filter and an Antlia 3nm Filter set.

For targets without too bright stars the L-eXtreme works wonders and given that your OSC Camera has enough megapixels you will be very happy with the results.
I was quite happy with the 16 Megapixels of my old ASI071 Pro but now that I have 26 Megapixels I can easily live with the possibility that I might loose a little of resolution because of the bayer matrix. So for that use-case your calculations are quite right minus the fact that I usually throw away blue as it has a bad Signal to noise ratio compared to the green channel for Oiii

Now on to the things I think that are worth thinking about:

To get true SHO images (which are a lot easier to process to a pleasant form in my opinion) you'd need an extra Sii Filter to have complete signal coverage which overall increases your exposure time because you now need two instead of just one exposure compared to three exposures for mono.

When you are experienced in combining only Ha and Oiii channels then this may not be relevant for you, but for me the struggle of making something pleasant with only H+O on popular targets was too much, with the added Sii things suddenly became a lot easier. But this of course has a lot to do with personal taste and experience in processing.

If you are a fan of RGB stars then the OSC camera helps a lot, you get them with just one more exposure compared to 6 (or 7 with lum)(or 5 with only RGB+HO) exposures that you would need with a mono camera. So here the time savings are more relevant.

Things get a little messy when you have bright stars in the picture, the Halos that the L-eXtreme produces (at least mine) in Oiii are quite visible.
For the bright targets I at some point decided to get the Antlia Filters, I still use my OSC Camera to shoot but am a lot more pleased with the very reduced halos. 

Again here, when you are a good at Post-Processing then you will be able to get rid of the Halos and the limitation will not apply.

There is also a lot of development going on, Antlia has released the 5nm ALP-T filter which is said to be much better at halos and Optolong has spread a few samples of a new Ultimate filter that is supposed to be 3nm.

I have the ALP-T on order and when it holds it's promise then my prefered way of shooting will likely be to do

1xRGB for the Stars, 1xALP-T for Ha and Oiii and 1xAntlia Sii which will be a very relevant time saver over having to use 6-7 filters on a mono camera.

And yes, a big plus to the thought that when you have a limited number of clear skies you likely make the best out of that with an OSC Camera.

Andre Vilhena:
Hello everyone,

Lately I have been thinking on which of the following - Dual-Narrowband filter+OSC and Ha/O3+Mono - is most time efficient for shooting and I'm starting to reach the conclusion that Dual-Narrowband filter+OSC may be a better option. This is a bit against astrophotography common sense but still I'd like to share my thougts for your appreciation.

Let's suppose you have 10 hours to shoot a given target and you want to capture Ha and O3. For that you have:

i) an OSC camera and a dual-narrowband filter (DNB)
ii) a mono camera and an Ha and O3 filter

The sensors of each camera are the same and the transmitance of each bandpass is equivalent between each filter and the DNB filter.

In the OSC camera you'll have:

1) 10 hours in Ha, collected in 1/4 of the sensor
2) 10 hours of O3, collected in 3/4 of the sensor
When debayering, software will interpolate the missing pixels. Hence, in the end, you'll have two masters, 10 hours each.

In the mono camera, you'll have:
3) 5 hours in Ha, collected in 100% of the sensor
4) 5 hours in O3, collected in 100% of the sensor
There's no debayering and in the end you'll have two masters, 5 hours each.

In this situation, having only half of the exposure will yield a 33% more noisier image, which makes me think the best approach to follow would be to shot OSC+DNB filters.

Would you agree with this? I would really like to have your opinion to confirm/dismiss my thinking.

Cheers,
André

I am afraid  it is not as simple as that.  Just try to think about the total number of photons you will really collect/register . If the same number of let say "red" photons hits the area of let say 2 x 2 pixels on OSC  within a given time and they are randomly spread over the surface.  An OSC camera will collect only 25% of them . Mono camera with the same pixel size will collect them all. For OSC camera the debayering  process will try to estimate how many photons other 3 pixels should have  collected and what should have been their color. Therefore Debayered image is somewhat "smoothened " already , when you get it as "raw" .

Lynn K:
I don't agree that exposure time equates into photos gathered (sigal) equally in a Bayer array and a non-Bayer array. The Red filtered pixels on a Bayer array will only gather 1/3 of the photons. I understand that most of the photons from an emission nebula are in the Red spectrum, but a mono chip will gather all of them. Not just 1/3. So, to assume that 10 hours of exposure renders 10 hours of Ha segnal, I think is incorrect. It's more like 3.33 hours of signal. Where the mono Ha & OIII gets a full 5 hours each.

Signal is intensive not extensive. Claiming otherwise would be ridiculous.  Each pixel "sees" a fraction of sky and that's that. How many are there is pointless and  as long as you satisfy the Nyquist sampling criterion you are in, the original signal can be reconstructed.

Guillermo de Miranda:
In my opinion, the whole mono vs osc debate comes down to weather. Do you live in an area where clouds are not an issue and you can shoot every night? Mono. Do you live in an area with only 14 clear nights per year? Osc. 

Of course you can use mono in the latter, but it could take many weeks or years to capture required data. There’s nothing wrong with that, but not everyone has that kind of dedication or time. 

Where I live, most clear nights are during the summer where we only have about 3 hours of night time at our disposal. During fall and winter, there is a consistent cloud cover that only lets us peek through maybe once or twice per month. These are very difficult circumstances for this hobby throughout the year, but osc can help mitigate a lot of these issues (at the expense of reduced SNR).

If time is your enemy, osc is definitely the more efficient choice.

Makes sense to me. I do OSC, and have been considering to try Mono, but when I factor in the hours needed and the particulars of where I live, I keep thinking that it's not worth the investment and additional effort. As it is, I am getting results I would not have dreamed of years ago with OSC!

One other issue (that may be mentioned above but I didn' see it?) is that O3 images are more sensitive to light pollution and moonlight than are HA.  Therefore I like to reserve those rare clear and moonless hours for O3 or RGB and do HA and SII even when the moon is up.  This tailoring to conditions isn't really an option for an OSC plus L extreme.  Also you don't necessarily want to be restricted to equal HA and O3 acquisition times?  i.e. O3 SNR is usually lower than HA  (except for planetary nebulae)  and so it often makes most sense to  image O3 for longer than HA

So overall I tend to use mono camera for narrowband beacuse it is more flexible– but prefer OSC for RGB because of  time constraints and the simplicity,

With (presumably) halo-free Antlia dual-band filter OSC imaging with 2600MC is very close to basic mono usage.

What you give up is mainly:

flexibility - how long to expose each line, what to expose with the moon up.
SII - very inefficient until an OIII/SII dual band filter comes out (it will eventually be produced).

With such OIII/SII filter on the market I'd say OSC will have almost caught up with mono on narrowband other than flexibility and moonlight issues.

The advantage would be painless and efficient gathering of RGB data.

BUT

So what does mono still have over OSC? Luminance, if you at all care about faint reflection objects and galaxies, mono is still king.  Its advantage is tremendous and won't be soon overcome, if ever.

Writing as an owner of both 2600MC and 2600MM.

Thank you for the comparison picture!
I the MC version drizzeled or is it simply the debayered version? If drizzeled, would you mind adding a 3rd picture with OSC debayered only?

Another relevant information would be if you need to increase exposure time with the color cam to get the (roughly) same result. Quantum Efficiency of the Mono Camera seems to be (a little) higher, but both are very close so I am asking myself if this results in a relevant difference.

I see a lot of people bringing up over and over the bayer matrix (25% yo!) and how much more resolution mono has -- it's not really that important.  With CFA drizzle you recover pretty much all of it.  Bayer matrix is a red herring in 2022.

Here's a link to L-Extreme Red Channel OSC vs Mono H 7nm comparison.  The resolution advantage of mono isn't too pronounced.

gallery_352013_17466_369009.png (1490×954) (cloudynights.com)

Hi,

The Nyquist-Shannon theorem is fundamental in digital signal processing and  gives you the maximum possible resolution of a continuous signal that you can reconstruct from discrete samples. In our case, the signal is the image and the samples are the pixels. As a theorem, it is not something that can be overcome algorithmically. Instead, it is what the best algorithm can possibly hope for. Regardless of year. 

To make it absolutely clear: that 25% resolution you get in R with a Bayer device is not something you can work your way up from, reconstructing better detail. It is, as a matter of fact, the BEST (accurate) result any conceivable reconstruction will ever be able to come up with, starting from way worse. 

Using CFA drizzle does not change that. For CFA drizzle to work you must have dithered, which means at least 2X exposures (so that each red pixel "falls" on different places) and typically more. So you are not really sampling with superpixels 4x4 large, you are sampling twice and thrice more fine grained, consequently spending twice and thrice more time imaging. CFA drizzle simply makes sure the extra data is geometrically arranged and combined in a way that tends to approach what is theoretically possible (and avoids the small "leakage" of information between colour channels inherent in Bayer demosaicing algorithms, which is a good thing but irrelevant to narrowband). 

Now, I don't think anybody said 1000 well dithered subs from a Bayer device cannot produce something as good as 250 not-so-well dithered subs from a mono device, or even better. But this is not really apples to apples, is it? And note: you can take 1000 well dithered subs with the mono and drizzle those. I guarantee you the result will absolutely blow away that from the 1000 Bayer subs. 

So, the simple fact remains you cannot get the same result using just 1/4th of the samples. 

The only exception that I can think of is when your incoming signal is inherently low resolution. Say you have bad seeing or you are using a small telescope or you have tracking errors or whatever, in that case it truly does not matter if you are bayering or not because stuff is too low res to tell the difference. Your example seems to fall under that category. Nevertheless, one cannot help noticing that the subs in the OSC image are 180 seconds long while the mono image is only 120. 

Cheers,
Dimitris

andrea tasselli:
The recovery of resolution by drizzling does not fall foul of the N-S theorem. Since the sampling occurs at spatially diffrent locations (and mind you, they don't need to be unit-pixel or more apart) at different times, it is irrelevant whether that is from a Bayer pattern, X-Trans (forgot those?) or mono. Is the mono resolution better? Sure, even because it is going to have a better SNR than equally exposed CFA. It is worth it? Maybe or maybe not. I have done LRGBHaOIIIHbetaUVIR compositions for more than I care to remember but now I chose to go OSC and all too happy about it. Will I go back to mono shooting as mainstay of my imaging even if the sky conditions were better than currently are here? I doubt it.

That's the right path to follow honestly unless one wants to go all-in.  Having used both up to now I can say that the big showstopper for OSC was halos that came with the L-Extreme filter.  Barring that it's very viable way to go shooting narrowband.  Antlia's 5nm filter is supposed to make that issue go away.

For galaxies and other faint L stuff, mono still can't be beat.

This topic goes back years and years, it's the eternal mono vs osc debate...bottom line, I own both so I don't have a horse in this race, and mono is my main tool.  I do recognize that OSC is closest it's ever been.

Yes for people who do NB with achromats or the ubiquitous budget "apo" doublets the ability of mono to focus separately oxygen and hydrogen lines is an advantage.