SBIG ST-402 Operating Manual
Page 6
3.
Using the Camera Inside
When you first receive the ST-402 it’s wise to get acquainted with its
operation inside the house where it’s warm and not to try to fight the
telescope operation in the dark. We’ve included a brief tutorial below to get
you acquainted with the ST-402 and the
CCDOps
software.
Running CCDOps
Once installed, it’s easy to run CCDOps from the Windows
Start
menu.
Click the
Start Menu
, then find the
SBIG Folder
, which under Windows
XP is under the
All Programs
section. Finally click the
CCDOps Icon
to
start the program. You’ll be presented with the startup
About Dialog
that
shows the SBIG logo and the version of CCDOps. To dispense with it click
anywhere in the dialog other than the two blue fields or simply wait and the
dialog will close itself.
Getting Help
CCDOps includes an extensive help file that can be accessed through the
Help Topics
item in the
Help
menu. In addition many of the
dialogs have an embedded
Help button
that will take you to the
Help Topic
for that specific command. Just remember, help is
always just a click away.
Opening and Inspecting Dark Frames
We’ve included some sample ST-402 images on the
SBIG Software and
Catalog
CD-ROM to get you acquainted with what you should expect from
your camera.
First, let’s look at what is called a
dark frame
. In the
File
menu use
the
Open
command and then navigate to the CD-ROM’s
Images
directory.
Double-click on the
ST-402 Tutorial
folder to show its contents. Finally
double-click on the
Image 1 – Sample Dark Frame
icon to open the image.
CCDOps will present you with a dialog listing all the parameters
associated with the image such as the
Exposure Time
, the
Date
and
Time
the image was acquired, etc. Note that the
Exposure Time
was 1.0 second.
Spend some time inspecting the data then click in the dialog to close it.
Data like this is always attached to saved images and you can show it
using the
Parameters
command in the
Display
menu. After the
Image
Parameters
dialog has gone you’ll see the image displayed in its own
window and the
Contrast
dialog will appear. The image is shown below:
SBIG ST-402 Operating Manual
Page 7
This is what’s referred to as a
dark frame
and it’s what you get when you
take a picture with the nosepiece covered and the shutter closed. It’s a
picture in the dark. Although there’s not much interesting to look at in this
image there are some very important features that you need to understand.
The first thing you’ll see is that while the image looks mostly like salt
and pepper there are hundreds of bright specks. These are called
hot pixels
.
They are present in every CCD image to one extent or another, even in
images from our most expensive cameras. If your digital snappy camera
didn’t automatically remove them for you, they would be seen in those
images too.
What causes hot pixels? It’s an attribute of CCD sensors called
dark
current
. A pixel in an ideal CCD, in the absence of light, would maintain a
steady value. When exposed to light the pixel’s value would increase in
response to the light but then as soon as the light went away the pixel would
maintain its value again.
In the real world CCD pixels suffer from the affects of dark current
whereby the pixel’s value slowly increases (brightens) over time. All the
pixels in the image shown above have some component of their signal due
to the build up of dark current.
Dark current builds up over time in a linear fashion but
cooling
the
CCD can reduced the rate at which it accumulates. For example, cooling
the CCD by 25°C reduces the dark current 16-fold. That’s why
Astronomical CCD cameras are cooled – to reduce the dark current when
imaging very faint objects.
