Using a Webam
Using a webcam is a good way to start with astrophotography. Here are some
introductory comments.
Which Webcam?
Two things to consider in selecting a webcam are:
-
It should have a CCD chip rather than CMOS.
CCD is generally more sensitive.
-
It must be possible to remove (unscrew) the
lens so that you can attach an adapter to fit the camera in place of an
eyepiece.
I have used two webcams - the Philips Toucam Pro and the Philips SPC900N.
The two cameras are technically very similar. The old Toucam did have quite
a few hot pixels while the fairly-new SPC900N has a good CCD.
I have seen other cameras recommended including the Logitech QuickCam in
various models, but I have not tried them.
Prime Focus
The camera needs to be mounted at prime focus
which means that the image from the telescope objective is projected
directly onto the CCD chip. To do this you must remove the lens and fit an
adapter.
The photos below show my SPC900:
Many astro-equipment suppliers can provide the adapter & filter but make
sure that the screw thread on the adapter is correct for your camera.
The IR filter is not mandatory but will improve the images. CCD chips are
sensitive into the IR range and IR light will not focus at the same point as
visual wavelengths. So without the filter it will be more difficult to
obtain a sharp focus for the image. Also, the filter will help keep dust
away from the CCD.
Camera Control Software.
The camera will come with software but I tend not to use it. There are
various pieces of software around designed for astronomical use. I use K3CCD
Tools 1 (Lite) which provides the ability to capture AVI video sequences at
a variety of speeds and lengths and is available here
http://www.pk3.org/Astro/. This is a
free version with limited function.
However, you can use any camera control software you find convenient.
Telescope
You will need a telescope with at least a RA drive that can keep a target
near the centre of the field of view for a reasonable amount of time.
Set up and align the telescope as well as possible. The alignment does not
need to be good enough for long-exposure unguided images but you will still
need to be able to keep the target near the centre of the field of view
(FOV) for a few minutes at a time.
Focus a target object using an eyepiece. Make sure the target is exactly in
the centre of the FOV because the FOV of the CCD chip will be very small in
comparison to the eyepiece.
Replace the eyepiece with the camera and view the target on the computer
screen.
Focussing
After fitting the camera the target will probably be way out of focus.
Focussing is a very delicate task. It is worth taking time to get a good
focus as that will pay dividends later when you come to process the images.
I find it is best to focus on a star (or for example a satellite of Saturn,
or Jupiter) because it is much easier than trying to judge a good focus on a
large object.
Adjust the focus a little at a time and let the image settle before deciding
whether more adjustment is needed. The image will probably be jumping around
due to 'seeing' and will go in and out of focus. You will need to adjust the
exposure and gain of the webcam as the target comes into good focus.
Exposure
Once you have the target well focussed set the webcam to 5 frames-per-second
and the gain so that the target is clearly seen but not overexposed. Maximum
luminance should be around 75 - 80%. Do not leave the camera on
'automatic' as that will overexpose most targets.
Depending on the size of your 'scope and the brightness of the object you
may be able to use 10fps. The main consideration is to have the webcam
'gain' as low as possible because gain introduces noise.
You may need to adjust the colour balance if you don't like the way the
target looks. Sometimes automatic 'white balance' will work OK, sometimes it
will not.
Start recording a video. I normally take a 120 second video (which is 600
frames at 5fps).
Processing
The best software for processing webcam images is Registax5 which can be
found here
http://www.astronomie.be/registax/. This is quite a complex piece of
software and I am not going to try and explain it here. It goes through
through the steps of:
-
Initial alignment and quality assessment of
all the frames.
-
Optimising the alignment of the selected high
quality frames.
-
Stacking the selected frames.
-
'Wavelet' sharpening filter.
-
Various adjustments such a
contrast/gamma/colour balance etc.
At the end of the day, processing is as much an art as it is a science and I
can only suggest you experiment.
Other stuff
The notes above should get you started but there are other things to think
about and
I have made a few notes about astrophotography HERE
that may be of interest.
Any questions? Want to talk about it? Want to show your first attempts? Why
not use the forum at
http://astronomy.fm/.
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