We are going to capture our images to the folder 'Demo' which will reside on the desktop, and which does not yet exist.
Launch AstroVideo Click on Driver, and select the driver for the camera or capture card in use.
An image should appear in the left hand, Input Window.
Click on Video and adjust the brightness etc and set the Format to 320 x 240. (In practice, you can set the format to whatever the capture device supports. We shall use 320 x 240 for the purpose of this tutorial.
Click on Capture Setup
Click on the Image directory button, Navigate to the Desktop and Create a new folder called 'Demo'
Click on Open
Click on Save
The Image directory button will now show the path to the folder 'Demo'
Make sure that Auto save images, Colour images and Display images during capture are all checked.
As this is a tutorial, we shall do things on a small scale, but Video Integration comes into its own when we work on a large scale.
Set Frames per image to 20 (although we would normally set this to at least 100. I normally use 300. Any more and the poor tracking in my system can lead to some trailing. The rule is, set it to as many as you can, without getting star trailing) ( NB. If your camera was set to take true long exposures, then you would set the Frames per image to 1)
Set Total images to 5 (although normally I would set this to several hundred) For this tutorial, I am using a web-cam in a dimly lit room.
We will first of all capture 5 dark-frames each of 20 integrated (summed) video frames.
Append the Subject ID with a filename, I shall use 'dark' (Plate 1)
Cover the camera to completely darken it.
Click on the Capture start button and the captures will begin.
The progress can be followed in the bottom frame of the AstroVideo window. The result of summing 20 dark video frames can be seen in the right hand results window (Plate 2)
We now have 5 dark-frames, each comprising 20 summed video frames stored as Fits files in the folder 'Demo'.
Each dark-frame comprises 3 Fits files, one for each colour channel (RGB). (Plate 3)
We can construct an even better dark-frame by summing these 5 dark-frames.
Click on Tools, Combine images, Fixed sum.
You will be presented with the folder containing the dark-frames and will be invited to select the colour images to combine. Hold down Ctrl and select just the red channel (as Colour images is checked in setup, AstroVideo knows that for every red file, there will also be a blue and a green file) of each file (Plate 4)
Click 'Open'
The images are summed and you are then invited to enter a filename (Plate 5)
Enter 'sum5x20dark' for example.
The folder 'Demo' now contains the 3 colour channels for this dark-frame comprising 5 x 20 dark video frames (i.e. 100 dark video frames) (Plate 6)
We are now going to capture 10 images of our subject, each comprising 20 video frames and we are going to dark-frame correct each image as it is captured by subtracting the dark-frame we have just created, scaled to 0.2 (as it comprises 5 times as many video frames as each of the images we are about to capture)
Click on Dark and Use dark-frame from file.
Select the red channel of the dark-frame. (Plate 7)
Click 'Open'
Click on Dark and Auto subtract for saving
Click on Dark and Scale dark-frame
Set the scale parameter to 0.2
Click on Capture set-up
Set Total images to 10
Append the Subject ID with the subject name (Plate 8)
Click OK
Make sure that the camera is uncovered and point it at the subject.
Click on View and Video preview and the Input window will display the live image.
Click on the Capture start button and the captures will begin. As each image is captured, it will be displayed in the right hand results window.
The progress can be followed in the bottom frame of the AstroVideo window. The result in the right hand window can be seen to be of much higher quality, having a much higher signal to noise ratio.(Plate 9)
However, we are only part way there, for we have 10 such images (in normal practice, we will have hundreds of such images)
We have to register and combine these images into a single final result image.
We shall illustrate this by using the Auto sum selected function (which automatically registers and sums a set of files, but allows each one to be viewed in turn and be either accepted or rejected.
Click on Tools, Combine images, Options
The parameters of the search area to be used for registering the images are user defined. Set them as shown in Plate 10.
Click on Tools, Combine images, Selective auto sum
You will be presented with the contents of the folder 'Demo' Hold down Ctrl and select just the red channel of each image file (make sure that you only select the image files and not the dark-frames by mistake) (Plate 11)
Click on 'Open' You will be asked to define an alignment region by placing a square over a part of the image with lots of structure (Plate 12)
You will be presented with each image in turn and be given the chance to accept the image by clicking anywhere in the image (Plate 13)
Alternatively, you can reject the image by clicking on the bottom bar of the window where it says 'skip' (Plate 14)
The images will be summed and you will be invited to give the final result (which is the sum of 10 x 20 dark-corrected video frames) a filename. Call it 'Final'
The folder 'Demo' now contains three more Fits files which are the three colour channels of the image 'Final' (Plate 15).
If you so wish, you may then click on View and Contrast adjust to manipulate the final image in the right hand results window, and the result saved as a 16 bit Fits file, a BMP or a JPG (Plate 16)
However, the power of Video integration can only be seen fully by combining the colour channels in a program such as FitsX, The Gimp or Stellar Magic and then adjusting the levels.
Plate 17 Shows a single video frame captured in the darkened room
Plate 18 shows the sum of 200 such video frames captured as described above and having combined the colour channels into a single colour image.
The image looks as though it was captured in a well illuminated room.
This is the difference between stacking (averaging) 200 frames which would produce an image no brighter than Plate 17, but with an improved signal/noise ratio, and integration (summation), which accumulates photons from the fainter parts of the image and synthesises a 'longer' exposure. This is why Video Integration allows the imaging of Deep sky objects even with low light surveillance cameras. Think what this technique can do with medium length exposures of say 30s duration with a VestaSC or a SAC7.
Go though this tutorial for yourself indoors and practice ready for an astronomical imaging session. It will not take long to master. However, remember that when capturing astronomical images, you should work with far more images than we have used in this tutorial. The more the better.
Copyright © S.J. Wainwright 2002