it is an image of a uniformly illuminated background.
the purpose is to be able to "calibrate" out the defects in the sensor and
various aberrations such as dust motes on filters and vignetting due to the
optical train.
since the image data is actually just digital numbers in an array, it is
straightforward to divide the data by the flat field to perform that
calibration step I mentioned.
Someone once said the only truly uniform ccd sensor is one that is dead.
----- Original Message -----
From: "Randy Muller" <randygmuller@No-Spam>
To: "Richard Crisp" <rdcrisp@No-Spam>; "The Astronomy Connection"
<sf-bay-tac@No-Spam>
Sent: Wednesday, March 09, 2005 8:26 AM
Subject: Re: [TAC] Thor's Helmet from Castro Valley
> On Wed, 9 Mar 2005 08:22:17 -0800, Richard Crisp <rdcrisp@No-Spam>
> wrote:
>> the lower left hand side of the image shows a hard to see green circle. I
>> had a dust mote in my [OIII] data that my flatfield failed to remove.
>
> What's a flat field? An image taken with a cover over the primary,
> which is then subtracted from all the other images?
>
Received on Wed Mar 9 12:54:05 2005