Mono or Colour?

[rsarwar]

13 minutes ago, ollypenrice said:

Hi Rashed, I'm not sure what the two contradictory points are...? (I'm not being evasive, After re-reading the thread I just don't know.)

I never said that mono is four times faster, however. (At least I hope I didn't! Because it isn't.) The broadband equation is roughly thus: Luminance: All object photons. Each colour, 1/3 of all object photons. So in 4 hours LRGB you have 3+1+1+1 = 6. In RGB/OSC you have 1+1+1+1 = 4.  That makes the mono advantage 6 to 4, not 4 to 1. It is not that simple, though. The OSC filters don't, in fact, cut off sharply between colours so each one does pass more than 1/3 of the signal. What adds complexity is that some targets, with extremely faint parts, are not going to yield any colour with present technology but may yield a bit of signal - which they will do best in luminance, not through colour filters. The LRGB speed advantage is not going to go away but it is target-variable.

Meanwhile, back in the real world, there are stunning new OSC CMOS cameras and ingenious dual band filters for OSC which have changed the OSC/mono debate since I made the earlier points in this thread. Would I buy a modern OSC CMOS?  You bet I would.  I had the good fortune to be invited to post-process Yves Van den Broek's mega-mosaic of the galactic equator. Even without a dual band filter his CMOS/OSC data went deeper than my CCD in HaLRGB, though he did not find any significant OIII. (The Squid in what is otherwise his image came from my CCD data.) With a dual band filter he would have found the Squid for sure. See Gorann's RASA images on here.

Yves' image with my scrap of OIII thrown in:  https://www.astrobin.com/g82xf7/B/?nc=user

I'm happy to clarify further any earlier points I made if I can.

Olly

Thanks Olly, Much appreciated. That is a brilliant 32 frame picture. 

I may have misread you on 4:1 ratio, i did not realize when you said "6 to 4" you meant it like a ratio - makes more sense now. So what you are suggesting is a mono image with 1 hr worth of data in LRBG will be theoratically equal 1.5 hrs of OSC? That is not much but still quite tangible.

I see you comment on the latest CMOS sensors - in fact one of my more experienced friend is of the same mind as you - that he is considering getting a qhy268c after discounting CMOS/OSC for similar reasons for a while.

I live in a suburban area with moderate to high LP. I feel i can get decent images with IDAS V4 on its 50+nm Ha bandpass. and takes a lot of work at post to get the details when using a standard IDAS P2 LP filter.

So i think i need to go narrowband really. With that in mind, I am seeing two options for myself:

1. get a qhy268M with SHO filters (and LRGB to get the star colour)

2. get a qhy268C with Dualband + SII filter and create SHO images from there. (thats two filters for SHO - not sure if that is even possible as the O will get split between B and G 😜)

if you were in my shoes what would you prefer if you were primarily interested in nabulas? i.e. would you recommend using a QHY268C for SHO? Would the individual exposure time and total integration time be comparable between OSC/Mono?

[ollypenrice]

50 minutes ago, rsarwar said:

Thanks Olly, Much appreciated. That is a brilliant 32 frame picture. 

I may have misread you on 4:1 ratio, i did not realize when you said "6 to 4" you meant it like a ratio - makes more sense now. So what you are suggesting is a mono image with 1 hr worth of data in LRBG will be theoratically equal 1.5 hrs of OSC? That is not much but still quite tangible.

I see you comment on the latest CMOS sensors - in fact one of my more experienced friend is of the same mind as you - that he is considering getting a qhy268c after discounting CMOS/OSC for similar reasons for a while.

I live in a suburban area with moderate to high LP. I feel i can get decent images with IDAS V4 on its 50+nm Ha bandpass. and takes a lot of work at post to get the details when using a standard IDAS P2 LP filter.

So i think i need to go narrowband really. With that in mind, I am seeing two options for myself:

1. get a qhy268M with SHO filters (and LRGB to get the star colour)

2. get a qhy268C with Dualband + SII filter and create SHO images from there. (thats two filters for SHO - not sure if that is even possible as the O will get split between B and G 😜)

if you were in my shoes what would you prefer if you were primarily interested in nabulas? i.e. would you recommend using a QHY268C for SHO? Would the individual exposure time and total integration time be comparable between OSC/Mono?

I have no experience of shooting in LP and wouldn't want to advise you on that. I began imaging at our dark site and that's all I've ever done. For serious narrowband, though, I think I would go for mono and individual filters. From what I have seen OSC/dual band is essentially modified OSC, not pure narrowband.

Olly

[Stub Mandrel]

Now I have just over a year's experience of using a mono camera, I have to conclude that @ollypenrice is mistaken about the efficiency of Mono versus one-shot colour.

He is significantly underestimating the relative efficiency of mono cameras!

Despite my initial scepticism I've found my ASI1600MM requires much shorter overall exposure times than my DSLR. They are both cooled and the DLR is astro modded so two sources of 'inefficiency' in the OSC are effectively eliminated or at least greatly reduced. But I find that 45-60 minutes of RGB data knocks the spots off an hour's OSC data by far more than the 6:4 ratio Olly suggests.

I think this is because my RGB filters have  'square' passbands while OSC cameras more closely mimic the eye's response - sacrificing efficiency for more accurate colour rendition.

Here's the curves for my filters (actually the 2"v versions I have the 1.25" ones) you have to imagine these normalised to the QE curve to be strictly comparable, so efficiency drops slightly at far blue and far red) :

This is the ASI1600MC Pro OSC version (curiously FLO have this graph on their page for the mono version):

The key points are:

Both the green and blue RGB filters are highly sensitive to an Oiii signal, giving a stronger response to that band of nebulosity and causing it to render as turquoise (aqua as we now call it...).

The RGB filters have a gap at the sodium lines, increasing contract under many light-polluted skies.

The RGB filters have more sensitivity into the near infra-red including at Ha and Sii wavelengths.

My estimate of the 'area under the curves' even allowing for the QE of the sensor suggest only marginally more sensitivity for the RGB filters than the OSC, but the shape of the curves gives stronger colour contrasts with our typical targets which probably means we can get expected results with gentler processing  =  better s/n ratio and smoother results.

[ollypenrice]

10 hours ago, Stub Mandrel said:

Now I have just over a year's experience of using a mono camera, I have to conclude that @ollypenrice is mistaken about the efficiency of Mono versus one-shot colour.

He is significantly underestimating the relative efficiency of mono cameras!

Despite my initial scepticism I've found my ASI1600MM requires much shorter overall exposure times than my DSLR. They are both cooled and the DLR is astro modded so two sources of 'inefficiency' in the OSC are effectively eliminated or at least greatly reduced. But I find that 45-60 minutes of RGB data knocks the spots off an hour's OSC data by far more than the 6:4 ratio Olly suggests.

I think this is because my RGB filters have  'square' passbands while OSC cameras more closely mimic the eye's response - sacrificing efficiency for more accurate colour rendition.

Here's the curves for my filters (actually the 2"v versions I have the 1.25" ones) you have to imagine these normalised to the QE curve to be strictly comparable, so efficiency drops slightly at far blue and far red) :

This is the ASI1600MC Pro OSC version (curiously FLO have this graph on their page for the mono version):

The key points are:

Both the green and blue RGB filters are highly sensitive to an Oiii signal, giving a stronger response to that band of nebulosity and causing it to render as turquoise (aqua as we now call it...).

The RGB filters have a gap at the sodium lines, increasing contract under many light-polluted skies.

The RGB filters have more sensitivity into the near infra-red including at Ha and Sii wavelengths.

My estimate of the 'area under the curves' even allowing for the QE of the sensor suggest only marginally more sensitivity for the RGB filters than the OSC, but the shape of the curves gives stronger colour contrasts with our typical targets which probably means we can get expected results with gentler processing  =  better s/n ratio and smoother results.

 I did say the advantage is very target-specific and would stand by that.

But, as ever, I think it's even more complicated than that. The thing about catching, say, OIII is that your filters have to pass it and your chip has to record it. That much is obvious. But what is less obvious is that, for the imager to exploit what is passed, it may also have to be isolated.  A common or garden red filter will pass more or less the same amount of Ha as an Astrodon 3nm Ha filter. It isn't what the Ha filter passes which is the key to its success, it's what it blocks.

So I'd throw this into the conversation: what do we do when we add Ha or OIII to RGB? I think we do this: we take an image in which the NB filter isolates particular gas emissions already present in the RGB, but we take a deeper image by exposing for longer. This allows us to stretch it harder, and LP noise is also held down by the filter to help us. (If we don't it won't show when added to the red.) The NB image has high contrasts due to its isolation of the gasses and we find ways of incorporating those high contrasts into the image. The strong signal we can add to the appropriate colour channel to brighten it but the 'exclusion zone' from the NB filter will only appear fully once the dark parts are as bright as the bright parts of the colour channel. At this point all the contrasts can be incorporated into the colour channel, though we are unlikely to achieve this. The dark parts of the NB contrasts can be inserted into the final image by being applied very slightly in luminance, for nebulae, though personally I'm reluctant to do this because it's false.

Olly

[Loki1978]

Just thought I would give my experience as I've just started in imaging in the last few months and I had this exact question. 

I understand the benefits of Mono, and this is a direction I really want to go in. 'But' I decided to go with colour at the moment while starting out. I found on a typical night I'm trying to get all my equipment set up. Get aligned, get guiding working, find my target. The learning curve is quite steep and while I'm sure this process will become quicker each time I do it I feel I would have got frustrated with running out of time and the extra complication using a mono camera. I didn't want an 'all the gear and no idea' type of scenario.

As I say though I do want to head over to mono when I have everything else set up and running correctly. The equipment is always upgradable.

If I had a permanent setup I would possibly go mono straight away.

Edited March 21, 2021 by Loki1978

[Stub Mandrel]

On 20/03/2021 at 07:35, ollypenrice said:

But, as ever, I think it's even more complicated than that.

Indubitably!

[Stub Mandrel]

5 hours ago, Loki1978 said:

Just thought I would give my experience as I've just started in imaging in the last few months and I had this exact question. 

I understand the benefits of Mono, and this is a direction I really want to go in. 'But' I decided to go with colour at the moment while starting out. I found on a typical night I'm trying to get all my equipment set up. Get aligned, get guiding working, find my target. The learning curve is quite steep and while I'm sure this process will become quicker each time I do it I feel I would have got frustrated with running out of time and the extra complication using a mono camera. I didn't want an 'all the gear and no idea' type of scenario.

As I say though I do want to head over to mono when I have everything else set up and running correctly. The equipment is always upgradable.

If I had a permanent setup I would possibly go mono straight away.

I agree with this and am glad that I progress very much incrementally,m ironing out the snags at each stage.

My next leaps in the dark (arf!) are autofocus and platesolving...

[Jessun]

Approaching 4 years from first asking a question, poor Jez is quite possibly none the wiser...

 

 

[Loki1978]

lol, I didn't even notice that, how did this thread get resurrected. Jez is probably on his third or fourth setup by now.

[DaveS]

I'll chuck this idea into the mix.

If you're running mono then you can cycle your filters RGB, RGB, etc so you will have a reasonable chance of getting a complete data set during your night.

Or....

If  you have a moon that's going to rise at some point in the night, or is going to set, then you can schedule your exposurs so your red data is captured with the moon as it's  more likely to record without too much gradient, then get your green and blue once the moon is out of the way. Either RRRR, GGGG, BBBB if the moon is setting, or BBBB, GGGG, RRRR if the moon is rising.

Of course, if the moon is going to be up all night then shoot H-alpha in as narrow a bandwidth as you can manage, 3 nm ideally.

 

Edit: forgot to add, that if you're running OSC then you'll be stuffed, as all your exposures will have moon affected B and G, and even H-alpha will be less than ideal.

Edited March 21, 2021 by DaveS

[Stub Mandrel]

On 21/03/2021 at 19:04, DaveS said:

BBBB, GGGG, RRRR

I think that sums up my typical imaging session quite well, I always end up chanting "BBBB, GGGG, RRRR! BBBB, GGGG, RRRR! BBBB, GGGG, RRRR!"

[DaveS]

Sounds familiar!