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Subject: [M]: Flat field frames and dust
From: Dr. Michael Blaber
Reply To: mapug@shore.net
Date: Sat Jun 09 08:52:06 2001
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This is a multi-part message in MIME format.
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Dear Colleagues
Being a moron, I have struggled with an imaging issue and would like to =
pass on what I have found (which, no doubt, will be equivalent to =
rediscovering the wheel). I had taken prime focus CCD images with =
reasonable results and wanted to try imaging through a barlow. When I =
did this, to my dismay I noticed numerous dark spots and blotches. =
Blaming the barlow (i.e. more specifically the Barlow manufacturer) I =
purchased another Barlow, but had the same results. I then decided to =
try taking flat-field images (something I have never bothered with =
before) and with any Barlow I tried I could see the offending spots. I =
cleaned the corrector plate on the scope (SC type), blew off any dust on =
the secondary, and tried again. No change. I rotated the CCD camera =
(something I should have done before performing the above cleaning) and =
the spots on the flat field were exactly the same. So, the problem was =
with the CCD. I cleaned the outside of the CCD optical window and =
notice some improvement, but many spots remained. I decided to =
disassemble the CCD optical window and clean the inside surface, as well =
as the CCD chip itself (probably a terribly stupid thing to do). After =
reassembly, the flat field images were outstandingly uniform. However, =
further experimentation with different Barlows indicated that some give =
a visibly dark central region, others do not (don't understand why) - =
although the spots were gone. Also, I installed a round fluorescent =
light ($50) from home depot in the observatory and it gives flat-field =
images equivalent to twilight. =20
The thing I don't understand is why the dust on the CCD optical window =
is in sharper focus when using a barlow, but is out of focus (and =
therefore not a problem) with prime focus photography. Anyway, I have a =
much deeper appreciation of flat field imaging.
Mike Blaber
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charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
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<HTML>
<HEAD>
<META content=3Dtext/html;charset=3Diso-8859-1 =
http-equiv=3DContent-Type>
<META content=3D'"MSHTML 4.72.3110.7"' name=3DGENERATOR>
</HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT color=3D#000000 face=3DArial size=3D2>Dear =
Colleagues</FONT></DIV>
<DIV><FONT color=3D#000000 face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Being a moron, I have struggled with an =
imaging=20
issue and would like to pass on what I have found (which, no doubt, will =
be=20
equivalent to rediscovering the wheel). I had taken prime focus CCD =
images with=20
reasonable results and wanted to try imaging through a barlow. =
When I did=20
this, to my dismay I noticed numerous dark spots and blotches. =
Blaming the=20
barlow (i.e. more specifically the Barlow manufacturer) I purchased =
another=20
Barlow, but had the same results. I then decided to try taking =
flat-field=20
images (something I have never bothered with before) and with any Barlow =
I tried=20
I could see the offending spots. </FONT><FONT face=3DArial =
size=3D2>I cleaned=20
the corrector plate on the scope (SC type), blew off any dust on the =
secondary,=20
and tried again. No change. I rotated the CCD camera =
(something I=20
should have done before performing the above cleaning) and the spots on =
the flat=20
field were exactly the same. So, the problem was with the =
CCD. I=20
cleaned the outside of the CCD optical window and notice some =
improvement, but=20
many spots remained. I decided to disassemble the CCD optical =
window and=20
clean the inside surface, as well as the CCD chip itself (probably a =
terribly=20
stupid thing to do). After reassembly, the flat field images were=20
outstandingly uniform. However, further experimentation with =
different=20
Barlows indicated that some give a visibly dark central region, =
others do=20
not (don't understand why) - although the spots were gone. Also, I =
installed a round fluorescent light ($50) from home depot in the =
observatory and=20
it gives flat-field images equivalent to twilight. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>The thing I don't understand is why the =
dust on the=20
CCD optical window is in sharper focus when using a barlow, but is out =
of focus=20
(and therefore not a problem) with prime focus photography. =
Anyway, I have=20
a much deeper appreciation of flat field imaging.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Mike Blaber</FONT></DIV></BODY></HTML>
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