From: "milton_aupperle" <milton@outcastsoft.com>

Date: February 5, 2011 4:09:18 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Astro IIDC 4.06.05 Testing version available


Hi Folks;


A testing version (1.1 megabytes) of Astro IIDC is now available at:


http://www.outcastsoft.com/Prototype/AstroIIDC40605Test20110205.zip


This new 4.06.05 test version (no PDF Help manual to save ~4 megabytes of download) will enable hardware Gamma correction and set it to a Gamma of 1.0. This will only be applied for cameras that support the floating point CSR Absolute Gamma register (mainly PGR cameras). The Gamma control in the Camera Control window is still available and works just like it did before. All this changes is forcing some cameras to a Gamma of 1.0 as a starting point.


If it is applied, then the phrase:


Gamma has been set to 1.0 and enabled


will appear in the console.log window of the "Console" application. That way you know it has been set when the camera is first accessed.


Here is the why this was changed.


I "logically assumed" that all camera manufacturers would configure the CCDs and electronics to produce a "linear" output as a default. Basically a linear output means that as you increase or decrease the exposure time, the change in brightness output from the camera changes in an linear manner. So if I was to double the exposure time, then the brightness will double too. In a non linear output situation (for example Gamma not 1), a doubling of exposure time may produce more or less than doubling of the exposure time. When doing photometry, flats  frames or dark frame images, we depend on the CCD output being linear.


See this link for images showing linear and non linear output:


http://www.astro-imaging.de/astro/gamma.html


TIS and other manufacturers do this already and output is essentially linear, but PGR does not and leaves the Cameras output uncorrected. The only time PGR says it will produce linear output is if the Gamma control is turned on and the Gamma control register is set to 1.0. Previous versions of Astro IIDC did not enable the Hardware Gamma control and left it off.


Secondly, this appears to only affect some PGR cameras and not all of them. For example when I characterized the maximum saturation and linearity for my EXHAD Grasshopper camera output, I came out with a very linear exposure output until I crossed about a brightness value of 60,000. However the Flea 3 cameras have a uncorrected output curve of  a Gamma of about 0.70. So this version will force some cameras to start off with linear output.


Thirdly, the Gamma control in the Camera Controls window is done in software by Astro IIDC for both 8 and 16 bit modes. This was done because not all cameras have hardware Gamma control, many have no gamma control except in 8 bit modes and many cameras have a limited range of supported values. Doing it in software means I can offer a uniform settings regardless of what camera you have.


Lastly, a word of warning. If you took Flats and Darks with a PGR camera that has non linear output and with an earlier version of Astro IIDC, you will have to re-shoot them for any new images you capture with Astro IIDC 4.06.05. They may not have the same response.


I'd like to thank Bob Majewski for his work on measuring the output curves for his TIS and Flea 3 cameras and then contacting me with his results. That helped a great deal in figuring out what is going on.


Have a great weekend..


Milton Aupperle