FITS Viewer Live Stacker¶
The FITS Viewer Live Stacker (Live Stacker) stacks and displays subframes. It monitors a directory so can be used to stack the existing contents of the directory and / or to add new subframes as they are taken during a live session.
Multiple Live Stackers can be run to monitor different directories. These Live Stackers can be run as separate FitsViewer windows or as multiple tabs within a single FitsViewer tab. In addition, it is possible to run Live Stacker as a standalone process which is still under the control of KStars (so it is stopped when KStars itself is stopped) but has its own process space within the operating system.
Live Stacker follows the FitsViewer architectural approach of displaying the result (in this case the stacked image) and allowing existing functionality such as rotation, object highlighting, histogram manipulation, etc. to be run on the image. The stacked image can be saved by the user using the existing Save functionality.
Live Stacker only operates on .fits files. They can be either mono or color.
Dark and Flat master files may optionally be specified to improve image quality.
Each section of the Live Stacker input / output widgets are enclosed within a minimize widget that allows the block to be either expanded or minimized.
Following is a description of the UI widgets:
Stack Directory: This is the directory that Live Stacker will watch. Any files in the directory will be stacked when the Live Stacking process is started, and as new files (subs) are added to the directory these will be added to the existing stack.
Stack Directory Button: Launches a directory dialog box to choose a directory.
Proc / Fail / Total: Output statistics of the number of subs Processed successfully, Failed and the Total. The Total is the number of subs in the directory.
Start Button: This button has 3 states:
“Start”: Pressing the button runs the stacking process on any subs in Stack Directory and watches the directory for new subs. Button text changes to “Stop”.
“Stop”: Pressing the button stops watching the directory and cancels any in-flight stacking operation. Button text changes to “Cancelling…”.
“Cancelling…”: Button cannot be pressed. When the cancel operation completes, the button text resets to “Start” and the user can start another stacking process.
SNR Av / Min / Max: Output statistics of the subs processed so far. Average SNR / Minimum SNR / Maximum SNR.
Memory Widget: Displays the amount of memory the current process is using as a percentage of the available memory. Try to keep memory usage in the Green (<50% of available) rather than Amber (<70%) or Red (>=70%). Double click the widget to hide text and just display the progress bar.
Image SNR: This is the SNR of the stacked image at that time.
Save Settings Button. This saves the configuration of the Live Stacker to disk.
Calibration Settings:
Master Dark: Blank or pathname to a Master Dark .fits file to be used to calibrate subs.
Master Dark Button: Launches a file picker dialog box to choose a Master Dark.
Master Flat: Blank or pathname to a Master Flat .fits file to be used to calibrate subs.
Master Flat Button: Launches a file picker dialog box to choose a Master Flat.
Alignment Settings:
Master Sub: Blank or pathname to the sub to be used to align all other subs to. If blank, the first sub is used as alignment master.
Master Sub Button: Launches a file picker dialog box to choose the alignment master sub.
Method: Plate Solve. Use plate solving as the method of aligning subs.
Stacking Settings:
In Memory Subs. This specifies the maximum number of subs that will be held in-memory at any one time and specifies the number of subs that cause a transition from initial stack to running stack.
The stacking process will accumulate this number of subs (if available) before stacking them and will continue in batches of this number before stacking. If the number of subs available is less than this then the stacking process runs in order to display the stacked image to the user.
Once this number of subs is reached, the initial stacking phase completes and intermediate results are then held in order to more rapidly add new, incoming subs to the stack.
Set this number low to be more efficient with memory usage. However, if you use sigma clipping (or winsorized sigma clipping) setting this too low can compromise results.
Downscale: Options of None, 2x2, 3x3 and 4x4 are available. This options downscales the subs by the selected factor. For example, if 2x2 is selected than each group of 2x2 pixels is averaged to a single value per color, reducing both the image width and height by a factor of 2. So the number of pixels in each sub is reduced by a factor of 4 which will significantly speed up processing
Rejection Method:
None: No pixels are rejected.
Sigma Clipping: Sigma Clipping is applied to the first “In Memory Subs” to be processed. Once accumulated, a pixel stack is built for each pixel in the subs that contains the value of the pixel across all subs. The median and standard deviation of pixel values is calculated any values lying outside the thresholds of Low Sigma and High Sigma are excluded as outliers. The inliers are then used to calculate the stacked image. As new subs arrive they are checked against the saved pixel statistics to determine whether they are inliers or outliers and are processed accordingly.
Winsorized SC: Similar to Sigma Clipping except where outliers are excluded in Sigma Clipping, they are set to Low Sigma / High Sigma and included in the calculation.
Frame Weighting:
Equal: All subs are given an equal weight (=1.0)
HFR: Subs are weighted by 1 / (Av HFR). So the better the sub, the lower the HFR and the higher the weighting.
Num Stars: Subs are weighted by Num Stars. So the better the sub, the higher than number of stars detected and the higher the weighting.
Low Sigma. This is the number of Standard Deviations on the low side of the median to clip pixels out. Used by the Sigma Clipping and Winsor SC Rejection Methods.
High Sigma. This is the number of Standard Deviations on the high side of the median to clip pixels out. Used by the Sigma Clipping and Winsor SC Rejection Methods.
Winsor Cutoff. This is the number of Standard Deviations away from the median to use as cutoff to adjust outlying values before running Sigma Clipping.
Post Processing Settings:
Post processing options can be turned on / off with the associated minimize widget checkbox. The system temporarily turns off post processing during the initial stacking phase to speed up processing. Thereafter, control is with the user.
These settings are applied to the stacked image to improve its appearance. There are 3 tools: Deconvolution, Denoising and Sharpening. Typically the parameters to use must be found through experimentation by the user. By changing values and hitting the Reprocess Button, the user can adjust values and rerun the Post Processing without re-running the stacking process.
Deconvolution Settings:
Wiener Deconvolution can be applied to the image. The PSF sigma to use must be specified along with an amount.
Deconv Amount: This is the amount of the Deconvolved image to blend with the original stacked image. The range is between 0 (no deconvolution) to 1 (use only the deconvolved image). Set to 0 to disable Deconvolution.
PSF Sigma. The PSF Sigma to use in the deconvolution process.
Denoising Settings:
Denoise Amount: The amount of denoising to apply. Higher values denoise more. Set to 0 to disable denoising.
Sharpening Settings:
A basic Unsharp Mask sharpening algorithm is used.
Sharpen Amount: The amount of sharpening to apply. Higher values denoise more. Set to 0 to disable sharpening.
Sharpen Kernal. The size of the kernal used for sharpening in pixels (usually 5 or 7 works well). Choose an odd integer.
Sharpen Sigma. The sigma used in the sharpening process.
Reprocess:
Reprocess Button. Pressing this button reruns the Post Processing functionality (Deconvolution, Denoising and Sharpening) on the stacked image using the current values of the Post Processing settings. This means that on a stacked image the user can adjust settings and re-run, adjust again and re-run, etc.
Using Live Stacker for the First Time¶
Here are some suggestions for using Live Stacker for the first time:
Start in daylight on your imaging computer / setup with some subs you have already taken during a previous imaging session. If you have say 30 subs this will be enough to see how Live Stacker performs.
Select the directory of the subs, and press Start. The default parameters should be relatively memory efficient but may need tweaking.
See how the Memory Widget performs; you want to keep memory usage Green or at worst Amber intermittently. If you are having issues with performance and / or memory usage then here are the things to speed things up (start at the top and work down the list):
Downscale: Usually 2x2 is enough to significantly increase performance with mono subs. Color subs may need more downscaling.
In Memory Subs: Decrease this number. It will then hold less subs in memory at the same time. Note that for Sigma Clipping and Winsorised SC this will potentially compromise image quality.
Calibration Files: Adding a dark and or a flat master will improve image quality, potentially significantly, but at the cost of extra processing.
Frame Weighting: Equal is the most efficient option. Num Stars requires an extra step during Plate Solving so will always be slower. HFR requires the same extra step as Num Stars but also needs more computation so is the slowest option. HFR and Num Stars should, however, yield a better result than Equal weighting.
Rejection Method: None is much more efficient than the other options (at the cost of a potentially better result).
You can rerun the stacking process from scratch by just pressing Start, so its easy to adjust parameters and rerun.
It’s recommended to switch off Post Processing during early stages and come back to it later (see below). To do this make sure the Deconv Amount, Denoise Amount and Sharpen Amount are all set to 0.
When you have a configuration you are happy with press Save Settings and this will be the default configuration for subsequent settings.
Post Processing¶
When you have the settings for stacking sorted out for your equipment you may wish to try the Post Processing options. Note that the settings you choose will be applied every time Live Stacker displays a stack, e.g. when a new sub is added to the stack. For this reason when you are setting up the stack it is advisable to turn off Post Processing and then when stacking has finished, to experiment with it.
To experiment, set one of the options and press Reprocess. You can then adjust the parameter and press Reprocess again until you have the optimum settings.
Running Live Stackers¶
It’s possible to run more than 1 Live Stacker to monitor multiple directories. This would be useful if you are imaging multiple targets or filters and want to keep an eye on everything without having to reset the Live Stacker to a new directory periodically.
Live Stacker is started by selecting “Live Stacker…” from the KStars Skymap. Selecting “Live Stacker…” a second time will launch another Live Stacker. Depending on how FITS Viewer is configured will determine whether multiple Live Stacker windows are started (“Single Window Capture” unchecked) or whether multiple tabs within a single Live Stacker window are started (“Single Window Capture” checked).
By default the tab name for Live Stacker is “(n) DirectoryPath” where n is the number of subs in DirectoryPath. Right click on tabname to change the name to whatever you like. This is particularly useful when runing multiple instances of Live Stacker as separate tabs.
The “Live Stacker own process” option allows Live Stacker to be started as a separate process. When “Live Stacker own process” is unchecked, Live Stacker resides in the KStars / Ekos process space. The advantage of running Live Stacker as its own process is that if it crashes it will not disturb other KStars / Ekos functionality.
Using FITS Viewer functionality with Live Stacker¶
Live Stacker is integrated into FITSViewer. This means that existing FITSViewer functionality can be applied to stacked images displayed in Live Stacker. For example:
Saving image stacks to disk.
Re-opening a saved stacked image.
Using blink functionality alongside stacking.
Zooming in / out.
Rotating the image
Marking stars / highlighting clipped stars
Plate solving the stacked image, although by default the stacking process will have already plate solved it.
Stretch / Histogram adjustments
Viewing image statistics / FITS Header keywords
Viewing Catalog objects (e.g. Simbad objects)
Trouble Shooting Issues¶
The pre-requisites for Live Stacker are that KStars is built with WCS, CFITIO and OpenCV. If any of these components is not present, the Live Stacker menu item in the SkyMap will not be visible and Live Stacker will be disabled.
Subframes need to have WCS FITS Keywords. These are added by the Indi driver when WCS is enabled. Also, on the FITS options tab in Ekos, AutoWCS should be enabled.
Live Stacker makes use of KStars internal Plate Solving so it is crucial that this is setup to work correctly. Plate Solving in KStars is flexible and functions like Align and Guide have their own Plate Solving SEP profiles. Live Stacker uses the FITSViewer Plate Solving functionality and default profile, so this is the profile to check if you have issues. Go to the Plate Solve tab in FITSViewer and see which default SEP profile you are using. Choose a different one if that works for you. Make sure you can reliably plate solve subframes with your chosen SEP profile. See the Fits Viewer Solver section of this manual for more information.