Re: Dark frames
ajeans_at_hpl.hp.com
Date: 09/21/04
- Next message: Clif: "Re: An Antenna for Visible Light"
- Previous message: Zulfy: "Re: What makes an ideal Moon base?"
- In reply to: Thomas Womack: "Dark frames"
- Next in thread: Jon Isaacs: "Re: Dark frames"
- Messages sorted by: [ date ] [ thread ]
Date: 21 Sep 2004 15:10:24 GMT
I recently found that the hot pixels seen in a frame with the lens
completely covered were different from the ones which appeared in
a picture of the open sky; there were far fewer in the completely
dark case. By taking a few frames of the sky and not tracking, you
can see the stars move while other dots stay still. The fainter dots
tend not to show up in the usual dark frame and hence remain in
stacked images. I'm guessing that pixels have different "turn on"
thresholds. Registax has a way of making dark frames from multiple
images (it just stacks them with no alignment) so maybe the thing to
do is to take lots of pictures with no tracking so that all the
stars average out.
Albert
Thomas Womack (twomack@chiark.greenend.org.uk) wrote:
: I've been quite impressed by the results of simply pointing my Olympus
: E10 at the sky and opening the shutter for eight seconds; it's got a
: good f/2.4 lens, and I get images down to about magnitude 8.5 through
: dire suburban skies.
: On the other hand, it seems to have hot pixels. I thought hot pixels
: were essentially a *manufacturing* fault in the CCD, and so a single
: dark frame (leave aperture and speed settings fixed, just also leave
: the lens-cap on) would characterise them adequately for all time. But
: in a set of 100 photos taken last Tuesday, I'm still seeing hot pixels
: after subtracting a dark-frame taken two weeks ago.
: My temptation is to compute a minimum for each pixel position across
: all the photos and use that as a dark frame; the photometry is
: dreadful in any case, even after I've summed across 2x2 pixel groups -
: not sure if that's a matter of the CCD response and the V filter being
: vastly different, I should probably plot brightness-residual against
: spectral type and see if there's a correlation. Is this a sensible
: way to proceed, given that most of the photos are star fields?
: Should I in fact be taking a dark frame for each observing session,
: and if so does this indicate there's something wrong with the camera?
: Tom
- Next message: Clif: "Re: An Antenna for Visible Light"
- Previous message: Zulfy: "Re: What makes an ideal Moon base?"
- In reply to: Thomas Womack: "Dark frames"
- Next in thread: Jon Isaacs: "Re: Dark frames"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
