Monochrome vs Color CCD

cloud_cover · Post by **cloud_cover** » Mon Feb 21, 2011 2:15 pm

I never originally considered getting a CCD since I'm very much a daytime photography person as well but increasingly I'm beginning to see the limitations of even the mighty D700 where AP is concerned - amp glow is going to limit my practical shots to 5mins max with fairly significant glow.
So having started looking at CCDs, any opinions on one-shot color (OSC) vs monochrome?
My initial impression of OSCs is that they require longer exposure times due to the bayer matrix resulting in lower quantum efficiency of each pixel as compared to monochrome and are supposedly less useful in narrowband imaging because of this.
Is this just a matter of total exposure time or are there other factors involved as well?
Also, to compare relative exposure times with my DSLR, what is the effective "ISO" of most CCDs (maybe mid range, e.g. Orion Starshoot Pro v2 to SBIG 8300)?
I also get the idea that other than the camera, purchasing a monochrome will also incur the very significant cost of filter wheel and filters?
Thanks for everyone's advice!
P.S. Not thinking of buying now, of course, maybe later but thought I'd find out the basics of CCD-ing

Post by **weixing** » Mon Feb 21, 2011 2:51 pm

Hi,
Instead of getting a CCD, why don't you just get a used entry level Canon DSLR and mod it. It's a lot cheaper and just look at the images cataclysm took using a mod Canon 350D... IMHO, it's excellent!

Anyway, I think you already have some very good equipment and what you need to do now is to shoot. As you start shooting, then you'll know what's the limitation of your equipment and what to upgrade next.

Happy imaging and have a nice day.

orly_andico · Post by **orly_andico** » Mon Feb 21, 2011 3:03 pm

OSC's also have lesser resolution, because of the color interpolation.

The main draw of the mid-size mid-price CCD's like the Starshoot Pro and QHY8 is that they are cooled. The QHY8 actually uses a Sony chip (ICX413AQ, ICX453 in later models).

The ICX413 is the sensor in the Nikon D100.

So in terms of ISO it's a wash. Cooling is the key to killing noise and amp glow. So as Weixing says, a modded DSLR (and put it in a small refrigerated box) will give the same results albeit in an ugly manner.

andeelym · Post by **andeelym** » Mon Feb 21, 2011 3:09 pm

Is a modded DSLR equivalent to a one shot colour CCD? Is exposure time needed still lower in the CCD?

orly_andico · Post by **orly_andico** » Mon Feb 21, 2011 3:17 pm

I have no way of knowing, most folks say an OSC is "better" but given that they use the same sensors, I'd say it's all down to the cooling. When you can hit 5* Centigrade, of course you'll have a better noise profile than an uncooled DSLR which also has other hot parts in it aside from the CCD or CMOS.

Canon cameras 20D and up also turn off the amp during long exposures, which pretty much eliminates amp glow. Of all the DSLR makers, Canon has made the most effort to accommodate astrophotographers.

That said.. CCD's are more convenient (have only been using a lowly Meade DSI) but a USB cable and Nebulosity will turn your Canon DSLR into a fair approximation of a dedicated astro-camera (except for the cooling).

cloud_cover · Post by **cloud_cover** » Mon Feb 21, 2011 3:22 pm

Orly,
I think I might just settle for the icebox first less the modding - I don't think I'll want to mod my D700, not unless I get a successor camera.
I must say I'm really liking the large field of the FX sensor, not to mention I've taken the low noise performance for granted (had to shoot with a D300 few weeks ago - yucks)
I saw it once quoted that astro CCDs effectively work at an ISO equivalent of 160. If that's true, then I'm operating at at least 5-10x the sensitivity for the same noise at 30deg C
How do you keep the chilled sensor from fogging though?

orly_andico · Post by **orly_andico** » Mon Feb 21, 2011 3:37 pm

I don't know where that ISO 160 figure came from.

My ancient Meade DSI can resolve nebulosity in M42 in a 2-second exposure, which my DSLR can't do even at 800. And note Christian Buil's measurements on the Nikon cameras which achieve their low noise through over-aggressive processing.

Here's a more correct piece of data I found

..the image sensor is the same on both QHY8 and D40 (SONY ICX453), so the sensor contribution is the same amount. The ADC are both using Analog Device chip (D40 uses its 12-bit version while QHY8 uses 16-bit version)

So I find it impossible to believe that a QHY8 would only have "ISO 160" sensitivty if it's the same sensor with a superior ADC and cooling to boot. Or maybe the DSLR makers ramp up the gain (meaning increase the ISO) because they need it, and this causes lots of noise.

The Canon DSLR's mostly work their best at ISO 400 to 800. You never see Canon APS-C chips in astro-cameras because Canon uses their own chips, they don't buy off-the-shelf from Sony.

Fogging: you have to keep everything sealed up so that the sensor doesn't fog.

cloud_cover · Post by **cloud_cover** » Mon Feb 21, 2011 4:03 pm

erm, how do you seal up an open tube? Even then, how do you dry the air in the camera chamber?

orly_andico · Post by **orly_andico** » Mon Feb 21, 2011 4:31 pm

The refractor isn't an open tube.. it's closed at the front by the lens, and at the back by the camera. Of course it will leak (slowly). Some folks even with the dedicated astro-cameras put some dessicant in the camera chamber.

EDIT: oh yeah I forgot. If you're using a VC200L then it's not closed. But if it's a refractor, Schmidt, or Mak, then it's closed (more or less).

orly_andico · Post by **orly_andico** » Mon Feb 21, 2011 4:44 pm

found this graph from our friend Christian Buil:

Look at that line up there (the Kodak KAF-3200 used in some SBIG cameras)

The QE of a Bayer sensor is 1/3rd of a mono sensor, because you've got 4 pixels (2x2 matrix) 2 green, 1 red, 1 blue, then you interpolate that into 4 pixels each with RGB values. The intrinsic QE of the DSLR sensor is about the same as the mono sensor, but it gets flattened by the Bayer filter.