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.
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.
In SharpCap 3.0 and above, all cameras support Live Stacking. In practice, you will require a camera that can set an exposure of about 1 second or more to have a chance of picking up enough stars for alignment to work.
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 medium to 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 moving the three level lines (black, mid, white) on the histogram. Moving the ‘Mid Level’ line to the left is usually a good starting point as it stretches the fainter parts of the image, bringing out detail in galaxies and nebulosity. If you need very fine control of their position then hold down <SHIFT> while dragging sideways or move the mouse above the graph while dragging sideways – both will cause the bar to move more slowly and give finer control.
SharpCap Pro users also have the ability to adjust colour balance using the colour balance adjustment sliders if you are imaging with a colour camera, and have the use of the auto stretch (lightning bolt) button.
Note that the histogram always shows the levels of the stack held in memory, so adjustments to the colour sliders and the level stretch do not change this histogram (they do change the way the image looks on screen, how it is saved when using ‘save as seen’ and the mini histogram graph).
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.
Live Stack Controls
In addition to being able to stretcht the histogram to brighten faint objects there are a variety of controls that can be adjusted to control the stacking process.
- Stacking Algorithm – Default or Sigma Clipping (Sigma Clipping requires a SharpCap Pro license). Sigma clipping will help exclude unwanted artefacts like satellite or aeroplane trails from the stacked image.
- FWHM Filter – discard frames where the average star size (FWHM) is above a threshold – this lets you keep the frames where the seeing is best
- Brightness Filter – discard frames where the average star brightness drops below a threshold – great for stopping stacking when thin clouds come across the field of view
- Star detection settings (in the Alignment section) – adjust these if you have problems with alignment due to too few (or too many) stars being detected
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.
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).