Drift Scan
James Kessel May 29, 2000
Hi Bill-
I have a new CCD camera installed and working and I could send you a
couple of images attached to a message. I like to ask. Most people,
including myself, don't like to get unannounced email with large
attachments and I use Eudora and have settings to reject email larger than
40k. I am probably going to load all my raw images up on my web site as
fast as I can after I take them. So that might be another place to get them
(but not to view them). To view them you will need some program like
IMDISP, Guide, Charon, or Astrometrica. Mira doesn't seem to work. Perhaps
Max Entropy program would work, I don't have it yet. I would think that
there would be some IRAF imaging program but I have never been able to
figure out IRAF even though I had it installed once.
Anyway, the new camera is custom made and uses the Site 1k x 1k chip. It
is a custom made camera and the software for it is a little rough. But I
can take drift scan as well as regular images.
These cameras are made specifically for a SC. They are attached to the back
of the telescope and they are so heavy it would not be practical to place a
focuser between the telescope and the camera. In fact they attach very
firmly to the back of the telescope and I just leave the camera attached
to the telescope all the time. The C14's internal focus mechanism that
moves the primary is stable enough (so far, anyway) so that I only have to
refocus occasionally as the temperature changes. In fact I use a 'focus
equation' which is a linear equation that gives the focus position as a
function of the temperature of the tube. You just measure the temperature
of the telescope and set the focus.
This is an evacuated camera and you have to get a vacuum pump and pump it
down every week or so, Evacuated is good. I typically am able to operate
the camera at -50 C even in 80 F degree weather in the room. You should
see this noise level or lack of it!
There is also the lack of concern about dew with an evacuated camera. I
mostly use the camera in the drift scan mode and that is interesting. I
could take pictures unattended all night but I am afraid to go off and
sleep with the roof open.
I am going to try to sell my ST-8 and build a second one of these. However,
the going rate for a used ST-8 seems to be so low that I will probably just
keep it on the shelf. It is not that it is a dog.
With the new camera on a C14 and my fairly dark site I can see down to mag
19.7 on my drift scans. The images are about 24 arc minutes square. To do
drift scan you first move the telescope to the area of the sky you want to
start from and then you turn the telescope drive off and start the camera
in drift scan mode. As the earth turns the camera reads out a line and
shifts the lower lines up one. Every x milliseconds it reads another line
and shifts the remaining lines up one. X is a function of the declination
and you set it before you start a scan. Its about 380 milliseconds near the
ecliptic.
I have use Charon to view the scan images and I love it. I need to study
it more but at this point I don't think it will handle the time in reports
properly You could say that each line in the image has its own time. The
top line would be the same as the start time listed in the fits header (I
think), and each subsequent line would start x milliseconds later. Can
Charon take this into consideration?