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Mono vs One Shot Colour (OSC) CCD's
Submitted: Tuesday, 19th February 2008 by Dr Dietmar Hager

I have noticed a lot of discussions about this topic in several specific web-forums and yahoo groups (SXV, OSC). I am an enthusiastic Starlight Xpress user, being experienced with OSC CCDs like SXV H9C, SXV M25C and monochromatic CCD like SXVF H16. I have also had the chance to become familiar with and process SBIG CCD data coming from ST-2000XMC and ST 1301E cameras.

For better understanding of my point of view, here is what I use for deep sky imaging (DSI). My current equipment used for DSI consists of:

  • 9" f/9 TMB Apo and a
  • Starlight Xpress SXVF M25C one shot color camera.  (M25C review)

After a couple of years using OSC I wanted to explore the claimed difference in luminance resolution power between an OSC and a monochromatic CCD and see if it is really that dramatic as so many people claimed it would (or should...?) be...

Quite frankly - it is not.

Terry Platt was kind enough to loan me his demo SXVF H16 monochromatic CCD (that has really a very low dark signal) for this purpose (H16 review). In my humble experience (I use the 9" f/9 TMB Apo mostly at around f/7) the difference in luminance resolution (pseudo-luminance in case of an OSC that is extracted in the channel mixer in PS CS2) is almost negligible, as I am working at an image scale of some 0.8 - 0.9 arc seconds per pixel under a night sky with average seeing and good transparency.

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NGC4631 with both the H16 and M25C

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NGC4631 with the H16 and M25C

Above is a comparison (click the link to see full size for better evaluation) I made in April 2007 (the info is on the image). The more over-sampling the less the difference – as a very rough rule of thumb.  Naturally this has certain limits – as you will require a very stable and reliable mount that allows guiding at this rate of resolution.

Well, what I missed in many threads was the discussion of the importance of the conditions of the observing site.  In order to take full advantage of a monochromatic CCD, which in my eyes lies most decidedly in the power of acquiring real deep color data, you need to have very good conditions.  When over-sampling comes into play, the OSC will easily catch up with the monochromatic CCD in terms of (pseudo)luminance signal, as long as you have average seeing conditions like I have most of the time at my observatory’s place (northern regions of Austria). 

But it is harder to equalize the color data.  In case of very good seeing a monochromatic CCD will also perhaps be able to outgun the OSC in luminance signal and therefore resolution, but never as much as so many people believe (in cases of severe under-sampling (more than 2 arc seconds per pixel I have to admit, a mono will always do a better job in terms of resolution, since the information of an object (say a star) being focused onto a couple pixels will be summed up by 4 pixels when creating the color in an OSC – thus you loose resolution as a consequence and the stars will look less smooth).

At my place I can not be certain to find a couple of nights for imaging an object - so in case of an OSC I can kind of "make every single frame count" for the final result, as it holds both pseudo luminance and color.

Another point is the dark signal of the chip. In case of severe noise (like in ST 1301 e) faint detail will simply drown in the dark signal of the chip and/or in the read out noise as well.  This detail will therefore be lost (or at least it takes much more effort and tricks to work this issue out with heavy pre-processing) in comparison to a low noise chip (like the M25C), even if this is carrying a Bayer matrix. 

So there is nothing about the proclaimed "big advantage" of a mono CCD in terms of better resolution.  At least this applies to the comparision between the monochromatic CCDs I used for the tests (SBIG 1301e, SXVF H16) and the OSC I described.  The pseudo luminance (that e.g. can be extracted in Photoshop by using the channel mixer and choosing the best s/n composition of all pseudo r,g,b) will surely be superior to any noisy (almost "loud" (-: ) luminance image taken by a monochromatic chip.

So just to have a mono CCD is no guarantee for anything! Make sure to pick a CCD with decent numbers concerning S/N.  One of the real big advantages of a monochromatic CCD in my eyes is the better color data that can be achieved.  In the option to use different color filters for all kinds of purposes, an OSC will always stay (a little) behind.   OSC suffers (a bit) in color depth and color data of the stars. The only chance to work this problem is to go for real long total exposure time with long sub frames. In my case (f/7 and M25C) I achieve the best results with 10 minute subs (it was trial and error stuff to figure out the appropriate value) and at least some 3 hours total exposure time (the more the better). 

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NGC7331 medium size

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Above is an example (please click the link above the image to see at full size) that shall demonstrate how nicely star color comes up with these parameters. (The second big advantage is, full scale narrow band imaging is only reasonable with a monochromatic CCD, though you can use h-alpha filters in very fast optical systems with an OSC. But then, in case of an h-alpha filter, only every 4th pixel (those behind the red matrix element) will be active, as all others do not get any data since it is swallowed by the narrwobandfilter and the bayer matrix as well...this will cause loss of effective sensitivity and resolution.)

I have just started to go for real long total exposure time - that is over 6 hours. OSC will do much better with more total exp. time and will come rather close to an RGB, but never be equal or beat it; at least this is my personal experience.


An alternative technique for combining monochromatic luminance and color data from one shot color CCD to save time.

OSC and monochromatic CCD do not necessarily have to be competitors - you can combine the data of both.  I tried this and found the result of such a combination most exciting.  Luminance data was taken with a H16 mono (to be "faster" and color data taken with M25C, to make sure to finish this object in one single night (as I indicated, I can not be certain to have another clear night with good conditions).

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NGC6207 full size

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This is the galaxy I imaged for this purpose  -- NGC 6207 (please click link above image to see at full size) - a most beautiful object, I think (a bit "under-appreciated" as she is located so close to her "famous and well renowned competitor" M13).

The reason why I did this? Monochromatic H16 is much more sensitive and OSC gave me the chance to finish this object in the same night - I saved time! Another experiment was conducted with a different demo CCD: SXVF H36. Unfortunately I failed to take maxmimum advantage of this huge chip (36x24mm) since the flattener I would have needed was not availabe at the time when the demo CCD was in my observatory. Nevertheless I took some images of rather tiny planetary nebulas and combined this luminance data with color frames I had acquired with SXVF M25C.

Why combine Luminance with OSC color data in the first place? Well, I think it is a good compromise for all those imagers who like me have to operate in locations that usually do not provide you with a row of nights granting decents imaging conditions. Unfortunately in my case there are "gaps" between 2 good nights up to a couple of months! In LRGB however (and even more in narrowband imaging) much time is consumed until a reasonable complete data set is acquired that can be processed to obtain a nice DSI. One single night is hardly enough time when you operate at a focal length that is considered to be "slow" (say f/9 like my optical system). In order to finish the desired object as fast as anyhow possible taking L frames with a monochromatic CCD which is much more sensitive to the luminance signal than OSC and taking color with an OSC that makes every single frame count for the final (color) result might enable the imager to get it done in one single night of good sky-conditions. In case of planetary nebula this works nicely.

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NGC1514 full size

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This is an example that is supposed to demonstate the efficiency of such method. NGC 1514 (please click link above image to see all data on the image and the full size frame)
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NGC2372 full size

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Above is another image taken with the same strategy. NGC 2371/72 (please click link above image to see the full size frame and all essential data).

To kind of sum up what I'd recommend if someone is looking for a CCD -- it would be wise to deal with some certain considerations before picking a CCD, mono or OSC.  Some of them are listed below:

  • Conditions of weather and climate at the observing site (steady skies with good seeing and transparency - is the weather a reliable partner...? Will you be able to take full advantage of a monochromatic CCD)
  • Photographic equipment (mount, focuser) stable enough for "heavy" CCDs like SBIGs?  S/N ratio of the chip used in the (monochromatic) CCD?  In cases of monochromatic chips with moderate s/n you might have to operate with dithered guiding mode -- does your mount support this?
  • Focal length mostly used in DSI --  under/over sampling? Bayer-matrix -- when going for an OSC, make sure you work with an image-scale better than at least 1 arc second per pixel. The lower the value the better the results in terms of resolution and smooth stars.
  • Strong nerves and endurance for highly demanding pre/post processing in monochromatic CCD, since developing an LRGB image takes more time, particularly if the s/n is not so hot.
  • Budget...

Feel free to browse through my images and do not hesitate to contact me when you have some remarks or questions. Any comments are appreciated.

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Review by Dr Dietmar Hager (Dietmar). Discuss this review on the IceInSpace Forum.


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