Colour v Mono

[Pete D]

I have a Atik 314lL mono, my question is if I take 1 hour of Luminous and an hour each of RGB, 4 hours in total how much better would the image be than 1 hour with a  colour Atik 414EX, or what would be the difference if I took 4 hours worth of images with the Atik 414EX, would I achieve a better result. Thank you.

[ollypenrice]

If you replaced the term One Shot Colour with the far more accurate, Half a shot green, quarter of a shot red, quarter of a shot blue, you'd get a more accurate idea of what's going on.

If we slightly over-simplify, we can say that a colour filter blocks 2/3 of the light. Red blocks green and blue, green blocks blue and red, etc. It doesn't matter whether these filters are in front of a mono camera or placed over each pixel of an OSC camera: that's what they do. This means that an hour's red, an hour's green and an hour's blue in a mono camera is equivalent to three hours in an OSC. (I'll give a couple of caveats at the end but that's the gist of it.) So, at this stage, the mono and OSC are passing the same amount of light in the same time and are equivalent.

Enter the luminance filter: this is not available to an OSC because its colour filters are fixed.  In just one hour the luminance takes an hour's red, an hour's green and an hour's blue. It can't distinguish between them but it doesn't need to, that distinction being provided by the colour layer. So...  RGB and OSC are equivalent in light per unit time but LRGB is faster than OSC by at least 20%.  If shooting Ha, which you can do in both, the mono camera is four times faster because only the OSC's one-in-four pixels can pass Ha (the ones under the red filters.)

Caveats:

- OSC green is actually wider than just green and takes light from a wider spectrum because it serves a 'luminance' function in daytime photography. However, we are rarely fond of green in AP and might like less rather than more.

- Good quality RGB filters are more efficient than the tiny filters placed over OSC.

- A naive view of OSC will suggest it has 1/4 the resolution of mono because of the 4 pixel array (RGGB.) In reality, subtle de-bayering algorithms replace most of this 'lost' resolution by interpolation. There's little real-world difference.

- My purely subjective impression, having tried both, is that CMOS OSC is better than CCD OSC. I don't know why or whether this would stand up to real scrutiny.

- Modern Dual and Tri-band filters make OSC much more attractive.

Olly

Edited November 4, 2022 by ollypenrice
Typo

[Pete D]

Thank your informative response, it certainly gives me plenty to digest. The OSC would certainly be more attractive for the poor weather we have here, thank you again.

[ollypenrice]

6 hours ago, Pete D said:

Thank your informative response, it certainly gives me plenty to digest. The OSC would certainly be more attractive for the poor weather we have here, thank you again.

Attractive in that it won't leave you short of a colour, but less attractive because it is slower and because it is inefficient in Ha, which can be shot in moderately bright moonlight...

Olly

[Pete D]

Think I will stick with my mono camera, especially with being able to image in Ha. There is so much to learn!

Pete

[tomato]

I think mono CCD cameras outsold their OSC equivalents by quite a margin for the reasons outlined above, but (and this is purely anecdotal based on the posts here on SGL) I think the reverse is true for the latest mid format CMOS cameras based on the IMX571 sensor. I can only put this down to the perceived  convenience of using a OSC camera in the UK and the savings made on filters etc. Also the results obtained with fast optics and the latest OSC CMOS cameras have also made quite an impression.

The mono 383 is a fine camera btw, and has delivered many award winning astro images both in LRGB and NB.

[Ouroboros]

The more I read threads on colour versus mono I conclude that we’re not really comparing like with like.  There are reasons to use a mono camera for imaging and similarly for colour. But the one is not in all respects a straight substitute for the other.  

[Xilman]

5 minutes ago, Ouroboros said:

The more I read threads on colour versus mono I conclude that we’re not really comparing like with like.  There are reasons to use a mono camera for imaging and similarly for colour. But the one is not in all respects a straight substitute for the other.  

Whole-heartedly agree!

Another "caveat". If you wish to do precision photometry, and some do as well as wanting to take pretty pictures, you have essentially two choices. You either don't put a filter in front of a OSC camera and then spend a lot of time and effort measuring how to convert to standard photometry bands, or you put a filter in front of your mono or OSC camera, with the light loss that entails.

Horses for courses.  My interest is primarily photometry and imaging exceedingly faint objects, so I use a mono camera almost all of the time. The OSC is for planetary work where things are so bright they tend to swamp the CCD.

[Ouroboros]

Good points.  In the past I’ve shied away from mono because I set up and tear down each time I do some imaging. Mono just seemed like an extra set of layers of complexity on something that was already somewhat hit and miss in  terms of success. But over the past year or more I’ve been using an ASIair which just seems to work. Plus I started keeping my refractor, flattener, guide scope, camera, ASIair pre-assembled. So all I have to do is pick up the telescope rig, plonk it in the mount, balance, polar align and I’m ready.

So I’m now feeling more confident that I could use a mono assembly in the same way - scope, filter wheel, camera etc. - and do some mono work.  Not simply as a substitute route to colour imaging but rather to try the sort of astrophotography possible with narrow band filters. 

[Xilman]

Also a good point about setting up and tearing down. Although I have the convenience of permanently set up kit in an observatory, swapping cameras is a real pain, not least because it is then necessary to re-take flats at all binning levels in every filter.

[Ouroboros]

Have two interchangeable rigs.