ADU + Well Depth ?

[mickw]

OK I'm half way through making a light box, similar to the one Fay made last year, but much nicer of course 
5mm foam board, 4 x "bright white" LEDs, Opal acrylic for the diffuser.
Even light over the opal diffuser.

I made the mistake of reading Fays light box post and questions were being asked about well depth and ADU 

I think that well depth is the saturation point of a CCD and I found out what ADU stands for but have no idea what it means, so..........
How do I find out what the well depth of a CCD is ?
What is ADU and how does it apply to a light box - or anything ?

[Fay]

http://www.posterpoweruk.co.uk/page12.htm

funny enough, I was just looking at this

[mickw]

I'd heard of tghat but not seen it in this country - bit pricey though, in fact very pricey 

[Fay]

Yes, but not bulky

[Daniel]

I use one of those in my lightbox, only way I can get a flat field across 14 inches. one great thing about using them is that my light box is 14 inches across but only about 4 inches deep.

[Fay]

Daniel, isit floppy or rigid? Is there a wire going around the outside?

[Daniel]

Hi Fay,the panel itself is floppy, but i've put it into a light box I made with foam floor tiles which keep the weight down whilst keeping everything relatively rigid.

There's a wire that comes out of the light panel, but that's only about 2 inches long with a connector to connect to the power supply, so it's all very compact to install.

Daniel
:O)

[Fay]

Oh that's interesting Daniel

[JohnP]

Mick,

How do I find out what the well depth of a CCD is ?
What is ADU and how does it apply to a light box - or anything ?

Easiest way to find out well depth (normally specified in electrons e-) is to look at manufacturers spec sheets. Sometimes you'll find it on Camera manufacturers webpage e.g. Starlight Xpress always seems to quote them & other times you may need to go to spec sheet for the particular CCD in camera. Obviously depth varies depending on number of factors including of course the physical depth of the pixel well.. Typically camera's have from around 20,000e- or so up to 100,000e-or more. I think most astronomical CCD cameras are around the 20-45,000e- mark.

ADU - A 16 bit camera has 65,535 digital bits to encode (digitize) the available data. So if for example a particular CCD has a well depth ot 65,000e- then each digital bit (or grey level) represents 1e- In this instance the gain of the camera is 1e-/ADU. For a camera with a well depth of 20,000e- the gain is approximately 0.3e-/ADU. Note relationship is probably not as simple as I explained here as noise, conversion factors, quantum efficencies etc. all play a part. Normally imagers refer to how full a well is by its ADU count so if for example a star is saturated you know its at 100% ADU (65535) if its half exposed ADU count is around 33,000ADU. When using a light box you normally try & expose the flat for around 1/3 well capacity so if you take an image & then use some software to look pixel values (ADU's) you aim to get most of them around the 20,000 mark. It's not critical...

HTH,  John

[mickw]

 

Thanks John,  I think I'll need to read this a few times though