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Re: (ATMoB:Discuss) Question for the CCD'ers (off on a tangent, maybe)

Here's my first experience with astrophotography using a digital camera:

	I used my Nikon Coolpix 885 on a tripod to take images of the Leonids.
This camera has a 60s max bulb setting and I made a little bracket for it
so that a cable release holds the shutter down instead of my shaky finger.
I used Nikon's wide-angle lens adapter and took mostly 60s grayscale
exposures with a gain setting of ISO400.  The camera has a Noise Reduction
(NR) setting that automatically subtracts a dark frame, but I of course
forgot to turn this feature on so that the images came out pretty noisy,
though I did catch a few meteors.   In addiition to randomly-distributed
hot pixels, electroluminescence due to the amplifier chip in one corner of
the CCD was very visible.  
	So I attempted to make some dark frames myself and subtract them using
Photoshop as well as Richard Berry's et al. AIP software.  It appears that
the noise is very repeatable from frame to frame so the process worked
beautifully, removing nearly all the hot pixels as well the
electroluminescence.  However, becasuse there were so many hot pixels a few
of them coincided with the image of the meteor trail, so that after dark
frame subtractionthe trail appears a bit patchy.  
Furthermore, if I adjust the levels in photoshop, little donuts are visible
around the pixels that were dark-subtracted.  I think this may be due to
the lossy JPEG encoding of the images and the dark frames, but I am not sure.
	I also found that the "dust and scratches" filter in Photoshop with the
default radius 1 and threshold 0 removes nearly all hot pixels (for a long
exposure real stars span more than one pixel and are not removed) and is
not as detrimental to the image quality -- of course this method does
nothing for the electroluminescence.   
	Though they are nothing to be proud of, I'll post my pre and
post-processed images as well as the processing methods I used on ATMoB's
CCD page later tonight.

Dan

At 01:48 PM 12/4/01 -0500, you wrote:
>Folks,
>
>We could try cooling the camera to reduce noise, huh?  ;-)
>
>How are such cameras focused precisely?  
>Seems like a calibrated knife edge focuser would be the most accurate
>method.
>
>Is the chip sensitive to IR or UV (wavelengths that don't converge even
>in APOs)?
>
>Matt
>
>"Mario E. Motta" wrote:
>> 
>> Interesting, ... this is the first report i have heard of anyone
>> actually trying dark subtracting on a small digital camera, and you
>> say it works. Good idea.
>> 
>> Mario Motta
>> 
>> George Roberts wrote:
>> 
>> > ----- Original Message -----
>> > From: "Bruce Berger" <berger (a) mediaone dawt net>
>> >
>> >> The question, and I hope the answer is positive, is of course, can
>> >> I use it
>> >> for any kind of astrophotography?
>> >>
>> > Of course.  I have a standard middle of the line Sony DSCS-50 2.1
>> > MPixel camera that takes
>> > exposures up to 8 seconds maximum.  I experimented with tripod night
>> > photography and found
>> > that if you take a second picture with the lens cover on immediately
>> > after (before it changes
>> > temperature much) you can subtract the "black" picture from the star
>> > picture and it removes
>> > all those fake stars.  This is about equivalent to the exposure one
>> > would expect with using
>> > ASA 400 print film with a 30 second exposure - enough to capture get
>> > down to about mag 5.
>> > Not real impressive but good enough to see orion.  Of course you can
>> > take pictures through
>> > the eyepiece of a telescope of the moon, and planets also.
>> > - George Roberts
>> > mailto:gr (a) pobox dot com
>> > http://www.pobox dot com/~gr
>> >
>
>-- 
>Matt BenDaniel
>http://starmatt dot com
>
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