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Live Stacking

SharpCap now has the ability to perform live stacking of multiple frames from all types of camera. This also includes the ability to subtract dark frames and perform flat frame correction as part of the stacking process.

What is Live Stacking?

Live stacking - a video astronomy technique - is a great way to observe deep sky objects without needing cameras capable of very long exposures, highly accurate mounts or cooled cameras. Instead of taking a relatively small number of exposures of several minutes each to image a galaxy or nebula, SharpCap will take hundreds of images, most likely with an exposure of a few seconds each. You don't need to have a separate stacking program either as SharpCap will automatically add each new frame to the stack and display the stacked image for you. Images will appear before your eyes as the number of frames in the stack grows and noise levels will drop as the final image is averaged over more and more frames.

What Cameras support Live Stacking?

In SharpCap 3.0, all cameras support Live Stacking.

How do I use Live Stacking?

Select a supported camera and then press the 'Live Stack' button on the toolbar. Mono, Raw and RGB modes are all supported by live stacking, but you are likely to get the best results (and the best performance) from either a Mono or Raw mode. Once the Live Stack button is pressed, SharpCap will immediately begin stacking frames and will display the stacked image instead of individual frames.

It's a good idea to get your camera settings configured before beginning a live stack (or restart the stack by pressing 'Clear' after changing them). Having a high gain is often a good idea, as this gives a better range of values on each individual frame and the noise that a high gain generates is removed when many frames are stacked together.

While live stacking is active, you will see some information at the bottom of the screen, including a histogram of the stacked image and details on the number of frames stacked and total exposure. You can adjust the rendering of the stack by using the sliders above and below the histogram to control the black and white levels.

If your frame rate is very high (several frames per second) you may see a warning that some frames are being dropped. This occurs when the stacking calculations can't keep up with the rate at which frames are arriving. As you'd expect the limit for this depends on the speed of your PC.

Saving a Stacked Image

To save a stacked image, simply press the 'Save' button. The image will be saved as a 'fits' file and will be named according to the same rules used for naming other SharpCap capture files.

Using the dropdown options on the Save button you can save the stack either as

  • A 16 bit image with the histogram stretch applied - this will appear roughly as viewed onscreen
  • A 16 bit image with no adjustments applied for later processing
  • A 32 bit FITS image containing the raw stack values for later processing

It's also possible to save the individual frames that go into the stack  - this can be handy to do a full re-process of the imaging session later.

What else should I know?

Sharpcap automatically corrects for drift and rotation while live stacking (as long as at least 3 stars - ideally about 15) can be found in each frame. You can also setup filters on both star FWHM and image brightness, both of which can help give a better final stacked image by rejecting frames that are less sharp than usual or those that are dimmed (perhaps by passing cloud).

There is essentially no limit to the number of frames that can be stacked as the results of the stack are stored as 32 bit data, so it would take over 16 million 8 bit images or 65000 16 bit images to reach the limit of the stack!