RE: [APML] Black Holes around stars

["Richard Klappal" <klappal@xnet.com>]

Resurrecting an old thread ...

I found an article in one of the photo trade newspapers that talked about
ghosting in DSLRs. They talked of the higher reflectivity of the CCD sensors
compared to film and of resulting reflections from the back of the lense.
Canon, Nikon, ... have been "digitally optimizing" lenses (at least some of
them) for several years, but rarely identify which models are optimized.
The "optimization" is improvement in the anti-reflection coatings to the
back elements of the lenses.

I don't how much this applies here, or if it may help explain 'intermittent'
problems people may be having.

Richard

-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Ray Butler
Sent: Tuesday, 07 September, 2004 04:33
To: Discussion of Film Astrophotography
Subject: Re: [APML] Black Holes around stars



Hi Vic,

Dust shadows and digital/CCD imagers go hand in hand, because unlike
film, the same sensing surface is used over and over and over....

In CCDs, the dust is not actually on the sensor chip itself - it is on
the protective optical window slightly in front of the chip. I think the
same would be true of digital cameras. The separation between the window
and the chip causes the shadows to be out of focus on the chip, so in a
regular lens or refractor they look like dark disks, while with a
reflecting telescope they look like dark doughnuts (because of the
central obstruction).

Since the shadows are a form of vignetting, you can correct them if you
divide by a flat-field. The grey card you mentioned would be suitable
for this. Ideally, record the flat-field on the same day as you take the
image you are correcting, since dust particles can come and go and move
around.

Ray "who notes that with film, dust only becomes a problem _after_
you've taken and processed the image" Butler


Vic Levis wrote:

> Hi Guys
> I was just reading the threads about the Canon MkII and the incidents of
> black holes.
> Both my son and I have Canon cameras. I have a Canon 10D and my son a
Canon
> MkII.
> We have had  similar problems with these irregular black blotches showing
up
> on our images both on astrophoto's and shooting fashion shows.
> We have sent the cameras in for a check-up and the news was that the
> problems is related to tiny specks of dust on the sensor. After I cleaning
> by the Canon people the problem has disappeared. I  am just thinking after
> looking at the images that it could be dust. On way we test to find if
dust
> is on the sensor is to take a photo of a white card - set the camera to
> overexpose 2 stops to compensate the whiteness of the card otherwise all
you
> will get is grey. If dust is present you will be able to see it even on
the
> LCD.
> Hope this helps
> Cheers from down under
> Vic
>
>
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--
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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