<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type">
</head>
<body text="#000000" bgcolor="#ffffff">
On 13-11-30 8:28, David Galbraith wrote:
<blockquote cite="mid:BLU402-EAS12252E35C019A117E14BC4FEDE80@phx.gbl"
type="cite">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta name="Generator" content="Microsoft Exchange Server">
<!-- converted from text -->
<style><!-- .EmailQuote { margin-left: 1pt; padding-left: 4pt; border-left: #800000 2px solid; } --></style>
<div class="PlainText">There's compression and then there's
compression. "lossless compression" changes file size without altering
pixel-by-pixel raster data. Lossy compression (like JPG) alters the
bitmap data to save file space. Some cameras can store data straight to
TIFF files. No loss.<br>
</div>
</blockquote>
<br>
I do not declare to be an expert on digital imaging. I can only
guess that the way a dark raw is compressed and data frames are
compressed is enough different that an external dark subtraction is not
quite as good as an internal. I am only the messenger here. I concede
that doing your own dark subtraction is attractive given that we have
limited actual dark sky time and that doing internal darks cuts one's
imaging time in half.<br>
It could also be possibly because the dark frame done in the camera
is done at pretty much exactly the same conditions, while a dark
library is slightly different. As originally stated, both Terry and
Alan conclude that in camera dark subtraction is marginally better.
They do have the experience in this sort of thing to be taken
seriously. But imaging a moving object like a comet, where you have a
very limited window for imaging, you are going to be better served by
using a dark library.<br>
<pre class="moz-signature" cols="72">--
Clear Skies!
MK
<a class="moz-txt-link-freetext" href="http://www3.sympatico.ca/mark.kaye/index.htm">http://www3.sympatico.ca/mark.kaye/index.htm</a></pre>
</body>
</html>