good beginner astrophotography camera

[Gina]

May I make a small correction to your diagram please Olly?  The bottom row should be "B" rather than "R"  ie. RGB.

[ollypenrice]

55 minutes ago, Gina said:

May I make a small correction to your diagram please Olly?  The bottom row should be "B" rather than "R"  ie. RGB.

Oh bother! You may indeed, Gina. Thanks for pointing it out.

Olly

Edit. Voilà

[Alien 13]

20 minutes ago, ollypenrice said:

Oh bother! You may indeed, Gina. Thanks for pointing it out.

Olly

Edit. Voilà

You could expose twice times as fast if the mono camera used CMY instead of RGB filters 

Alan

[Stub Mandrel]

3 hours ago, ollypenrice said:

I've made a simple graph to show the 6 to 4 minimum speed advantage of the LRGB system over the OSC

Olly, dare I say this?  I have read ythis argument several times now and think you are right, but I think that the detail is wrong.

Your approach assumes that:

(a) The debayering algorithm is totally useless, when actually even bilinear debayering is reasonably good at reconstructing the missing data.

(b) the OSC filters stop 100% of the colours they are supposed to stop, which they don't, they are actually fairly leaky, especially with a stage 1 mod allowing extra UV and IR through.

This is what i think the the real reason mono + filters is more sensitive:

Compare the areas under the astro filters (solid lines). The red and blue are at least twice as sensitive (although a stage 1 astro mod partly addresses this).

They will, because of the lack of graded overlap, produce a very different colour balance which is probably easier to process to give clean colours than OSC data, but they will be less effective at distinguishing subtle colour differences pure orange and deep red light will both give pretty much the same result with RGB but look very different with OSC.

I sit back and await the debunking with interest!

[Stub Mandrel]

2 hours ago, Alien 13 said:

You could expose twice times as fast if the mono camera used CMY instead of RGB filters 

Alan

True and LRGB OSC cameras would be better too!

[Some Dude With A Mak- Cass]

well, my head now has fallen off my shoulders from all this new info I'm cramming in, but i think i understand what i need now. thanks, everybody!!

[Gina]

Why would CMY filters be better?  I would have thought RGB would be better for light.  I know CMY secondary colours are used in printing.  Additive v subtractive.  Eyes have RGB receptors.

[Alien 13]

35 minutes ago, Gina said:

Why would CMY filters be better?  I would have thought RGB would be better for light.  I know CMY secondary colours are used in printing.  Additive v subtractive.  Eyes have RGB receptors.

If for example you used the magenta filter you would collect RED and BLUE information at the same time, after using all three you would have twice the data than you would get from a set of RGB filters. The downside is that they are not so common and you need a very well corrected scope because you have to focus two colours at once.

Alan

[Gina]

I have two APO telescopes and colour correction is excellent so no problem there.    I wonder though if the colour saturation would suffer but I guess you could always increase the saturation in the processing.

[Gina]

Of course the RGB can be obtained using PixelMath in PixInsight  viz.

R = Y + M - C
G = Y + C - M
B = C + M - Y

[ollypenrice]

On 22/10/2017 at 14:06, Stub Mandrel said:

Olly, dare I say this?  I have read ythis argument several times now and think you are right, but I think that the detail is wrong.

Your approach assumes that:

(a) The debayering algorithm is totally useless, when actually even bilinear debayering is reasonably good at reconstructing the missing data.

(b) the OSC filters stop 100% of the colours they are supposed to stop, which they don't, they are actually fairly leaky, especially with a stage 1 mod allowing extra UV and IR through.

This is what i think the the real reason mono + filters is more sensitive:

Compare the areas under the astro filters (solid lines). The red and blue are at least twice as sensitive (although a stage 1 astro mod partly addresses this).

They will, because of the lack of graded overlap, produce a very different colour balance which is probably easier to process to give clean colours than OSC data, but they will be less effective at distinguishing subtle colour differences pure orange and deep red light will both give pretty much the same result with RGB but look very different with OSC.

I sit back and await the debunking with interest!

I mostly agree with you. My explanation is a simplification and I'd wondered about the relative passbands of the OSC and interferomtric filters. Your graph is excellent. However, how much do we know about the transparency of the relative filters? I'm sure the tiny absorption filters of OSC are less efficient than the dedicated astro filters. Another variable, alas.

I'm a great admirer of the debayering algorithms and always say, in these discussions, that they effectively eliminate loss of resolution in OSC. I never insist that mono out reolves OSC because I have never found that it did so in broadband DS imaging. I used to have OSC and mono Atik 4000s together for several years. However, the debayering routine is making an educated guess about missing pixel values which is not the same as recording real photons, so I'd expect more real signal to be recorded by the mono. (I'm distinguishing between real signal and reconstructed resolution. A valid distinction? Dunno!)

The real purpose of my graph was to show the role of luminance as a time saver.

Olly

[Gina]

I'd love to try CMY filters (if not too expensive) but I can't find any that would be suitable for astro use

[Stub Mandrel]

2 hours ago, ollypenrice said:

Your graph is excellent. However, how much do we know about the transparency of the relative filters? I'm sure the tiny absorption filters of OSC are less efficient than the dedicated astro filters. Another variable, alas.

I wish it was my  graph, internet linked, I'm afraid...

Yes lots of known unknowns, far apart from the unknown unkowns...

[Adam J]

On 10/22/2017 at 09:23, ollypenrice said:

Because the OSC/RGB speed question comes up so often I've made a simple graph to show the 6 to 4 minimum speed advantage of the LRGB system over the OSC. Time is constant in both cases. An OSC camera has a single colour filter over every pixel, usually 2xG, 1xB and 1xR. The vertical* lines represent the filters. The horizontal lines represent the incoming beam of full spectrum light and the same beam after arriving at the filter. The luminance filter passes all three colours.

If we think of each horizontal line as a unit of light we see that more of them reach the chip in LRGB than in OSC and in a better proprtion for astronomy.

However, please don't take this as an attack on OSC. There is a lot to be said for OSC, especially under frustrating skies.  The only point I want to make is that, contrary to poplular belief, they do not confer a speed advantage at capture.

Olly

* Well, they were meant to be vertical! 

 

It depends on how you define speed. In terms of pure photons captured the you are correct. But I think that another way of looking at it is that a beginner is more likely to turn there camera to relatively bright targets like m42 and so I would pose you the question - is a beginner more likely to produce a image of such a target that they will be happy with in a single night with minimal processing skills. I think the answer is that they are and so for that beginner the OSC was in practical terms faster than the Momo.

[ollypenrice]

20 minutes ago, Adam J said:

It depends on how you define speed. In terms of pure photons captured the you are correct. But I think that another way of looking at it is that a beginner is more likely to turn there camera to relatively bright targets like m42 and so I would pose you the question - is a beginner more likely to produce a image of such a target that they will be happy with in a single night with minimal processing skills. I think the answer is that they are and so for that beginner the OSC was in practical terms faster than the Momo.

There has to be some merit in this argument, I agree. But then again, how long does someone remain a beginner? How long, in other words, does it take for them to out-grow their beginner choice of camera?

Besides, I'm not all that persuaded that OSC is easier in every respect. For example, anyone working in light pollution will find that they can easily take a good emission nebula image in Ha. The filter will do much of the 'processing' for them. It will block gradients and skyglow. It will hold down star sizes. It will provide high levels of local contrast. And it can be shot in a fair amount of moonlight. Getting a decent colour balance and freedom from gradients is certainly not easier from an OSC capture than from a mono RGB capture. I think they are equivalent.

Following advice from Ian King I went straight into mono imaging. This may be unusual but I think it was good advice whcih is why I make the case for mono as I tend to do. I stress again that I'm not anti-OSC. I only urge people to choose it for the right reasons (which do exist) rather than for the wrong ones.

Olly

[John78]

On 23/10/2017 at 17:34, Gina said:

I'd love to try CMY filters (if not too expensive) but I can't find any that would be suitable for astro use

CYGM bayered chips are imminent from Sony - because its "faster" (more photons per photo-site) than RGB and image processors are now becoming fast enough to deal with the calculations to present the user with a final image live.

[Stub Mandrel]

2 hours ago, John78 said:

CYGM bayered chips are imminent from Sony - because its "faster" (more photons per photo-site) than RGB and image processors are now becoming fast enough to deal with the calculations to present the user with a final image live.

They have been done before.

https://en.wikipedia.org/wiki/CYGM_filter

Would be interesting to try a modern one

[Shibby]

On 23/10/2017 at 14:26, Some Dude With A Mak- Cass said:

well, my head now has fallen off my shoulders from all this new info I'm cramming in, but i think i understand what i need now. thanks, everybody!!

Poor Some Dude!

Don't worry too much about the way bayer matrices work right now... The point is, you already have a camera that you can use for astrophotography so, with a limited budget, I would invest in the mount or optics first. I had a great time imaging with the DSLR I already owned for several years.

In the meantime, you could have a go with what you've got - just find the right adapter to connect the camera directly to your mount, set it tracking and focus manually.