Quality deep-sky astrophotographs are achieved by digitally combining the signal present in multiple subexposures (subs). In this manner, inherent noise and other artifacts are attenuated, so that even the weak signal of dim regions becomes strong enough to be displayed and even enhanced. To that end, preprocessing, which can include image calibration, cosmetic correction, debayering, alignment, and image stacking, must be performed accurately to ensure best results. In a perfect world, you could forge ahead with these tasks. In the real world, however, it’s good practice to evaluate the quality of your exposures before proceeding. Although some imperfect subs may contribute to a good result, those with particularly poor attributes, such as bad focus or tracking, weak contrast, or excessive artifacts such as airplane or satellite trails, may best be discarded rather than permitted to adversely affect the stacked master file. PixInsight (PI) offers several resources for image evaluation.
Quality deep-sky astrophotographs are achieved by digitally combining the signal present in multiple subexposures (subs). In this manner, inherent noise and other artifacts are attenuated, so that even the weak signal of dim regions becomes strong enough to be displayed and even enhanced. To that end, preprocessing, which can include image calibration, cosmetic correction, debayering, alignment, and image stacking must be performed accurately to ensure best results. In a perfect world, you could forge ahead with these tasks. In the real world however, it’s good practice to evaluate the quality of your exposures before proceeding. While some imperfect subs may contribute to a good result, those with particularly poor attributes such as bad focus or tracking, weak contrast, or artifacts such as excessive airplane and satellite trails, may best be discarded rather than permitted to adversely affect the stacked master file. PixInsight (PI) offers several resources for image evaluation.
Processes, Scripts, and Explorer Windows
By the way, ‘ImageInspection’ isn’t a typo – most of PI’s functions are named with this conjoined convention (e.g., ImageInspection, AutoHistogram, etc.).
You can increase the delay in milliseconds of the Process Console’s appearance via Edit/Global Preferences/Miscellaneous Process Settings. In many situations, the console will not be displayed if a large enough delay is set.
If the contrast levels of individual images vary, double-click Blink’s Compute AutoSTF button, second from the top of the dialog, to reset the visualization of the stack.
An image that initially appears to be very dark generally has excellent contrast and will likely have a good signal-to-noise ratio (SNR) . While you can brighten up the image by resetting the temporary stretch with the Compute AutoSTF button, be aware that images with weaker contrast will then appear washed out.
You can alternatively apply a nonpermanent, automatic histogram transformation to all of the images by clicking the button at the very top. With the cursor placed in the image window, you can use the mouse’s scroll wheel to zoom in for a closer examination. Hovering the cursor over an area of interest, hold down the Ctrl key while zooming to keep the area centered.
At this point, you’d be inspecting the field of view for overall quality, and the stars for sharp focus and roundness. With the first image clicked and highlighted, you can blink manually through the file stack, using the mouse’s scroll wheel, the up and down arrow keys of your keyboard, by clicking on file names, or by using the arrows in the gray column at left. These arrows are part of the automated blinker. You can begin automated blinking by clicking the Play Animation button or stop it with the pause button. Set the desired interval between images (between 0 and 5 sec) in the drop-down menu.
Removing the default checkmark from a file name will exclude it from auto-blink. You can close a highlighted file with the Close Selected Images icon at the bottom of the dialog. Use Ctrl or Shift with a left-click of the mouse (or Ctrl+A) to highlight multiple files beforehand. Use the Close All Images icon to do just that. Blink also has the ability to make a movie of say, a comet (Chap. 22).
As you determine a file unworthy of further consideration, remove the check mark to identify it as bad. Don’t reject an otherwise good-looking image due to an airplane or satellite trail unless you have a more than adequate number of ‘perfect’ images. Should you decide to use it, make a note of the issue for later use.
When finished identifying bad files, highlight them in orange (Ctrl or Shift+Click), then click the fifth icon from the bottom-left called Move Selected Files.
Select or create a new folder, perhaps naming it ‘Junk.’ Blink will then move the unwanted files to it. Due to safety protocol, you can’t delete files directly from Blink, but you can easily delete the junk folder after the fact.
Now, only good looking subframes would remain for further analysis. It’s also good practice, especially for imagers who acquire their data remotely, to evaluate calibration frames as well as light frames. This could help avoid using bias or dark frames with light leaks or unevenly illuminated flat frames.
Before leaving Blink and its BlinkScreen, let’s take a moment to learn a few key features of the PI user interface (UI).
The Image and Process Windows
The BlinkScreen window offers several controls: the View Identifier tab at top-left, fitting options at lower-left, and the icons at top-right. When an image window is clicked on, it becomes active, with the color of its border changing from gray to blue.
Double-clicking the View Identifier tab allows you to change a file’s name. It should be noted that the word View is a term that generally refers to an image open in PI.
When an Auto Stretch is active, a green vertical line appears in the Identifier tab.
While you learned that zooming in and out of an image can be accomplished with the mouse’s scroll wheel, the icons at lower-left, offer window fitting options such as Fit View. Zoom also appears in a context menu with other commands, available by right-clicking inside an image window. Very useful is the Alt+P command, which enables the cursor to pan inside an image window. This can also be accomplished by holding down your keyboard’s space bar while dragging with the left mouse button depressed.
In addition to the standard ‘X’ icon to close an image window, to its left is a plus sign that represents Maximize. The dash and arrow icon to its left is called Shade, and minimizes an image window into a small blue bar. Though that bar can be dragged anywhere in the workspace, ‘unshading’ the process will return the window to its prior, open position. Finally, at far-left is Iconize, and unlike Shade, an icon will sit over-top of another image window and as you’ll see later, can be dragged from one workspace to another (Chap. 14).
Turning our attention to the Blink process itself, we again find Shade available, as well as Close. In this instance, once automated blinking had begun, shading the process would be ideal for getting it out of the way for a better look at a full-screen image.
Begin by loading ‘Target’ Subframes – clicking the Add Files button, navigating to, selecting, and opening all of the files you wish to analyze . Although raw files will be used for this example, as you gain experience, using calibrated and cosmetically corrected files will yield a more accurate result.
Under System Parameters, enter the image scale of your telescope and camera in arcseconds per pixel (206 × pixel size in μm ÷ focal length in mm) and other pertinent information.
- 3.If the default Star Detection and Fitting settings are inadequate, the console will report failures. These settings can be altered in the section of the same name, allowing the script to run successfully. Note too, that a region of the images may be subframed to expedite the result.
If the stars are faint or the background is bright, decrease the Log (detection sensitivity) slider to between –1.5 and –2.
You can also increase the Star Peak Response to 0.9.
If the stars are out of round, try decreasing Maximum Star Distortion to 0.3.
Click Measure, and the Process Console will appear as the script does its calculations.
In the Table section at the bottom of the dialog, the script reports its findings in tabular form. Note under Sort Table By, you can change the field of interest, and whether results are displayed in ascending or descending order. If you wanted to prioritize criteria based on the images’ SNR , choose SNRWeight, and Descending. The file at the top of the list would have the highest SNR reading – a good thing. Say you were prioritizing based on the Full Width at Half Maximum (FWHM) of the stars in the images. Choosing FWHM and Descending, the file at the top of the list would have the highest FWHM value – a bad thing. You might therefore switch to ascending order, placing the file with the smallest/best FWHM on top to make that category easier to read.
Below the table is the Plots section – a graphical representation of the measurements. When either the ‘X’ to the left of an image file in the table, or that image’s corresponding dot on the plot is clicked, a dash appears to the left of the file name. These images would then be candidates for moving.
As you did with Blink, you can use the script to manually move unwanted files that don’t pass muster. Highlighting the undesired files in the Table, drop to the Output section, and specify the parameters for the move. In this case, choose the Rejected Action of Move, and create a Rejected Directory named ‘Junk.’ Clicking Output Subframes at the bottom of the dialog, the script will, by default, add a postfix (suffix) of ‘_x’ to the rejected file names, moving them to the junk folder for subsequent deletion.
Advanced users can enter a formula into Expressions/Approval, with conditions that will either approve or reject subframes automatically (click the Edit button for assistance). With an expression in place, specify an Approved Directory, and choose Move as the Approved Action (or Copy). Now, do the same for Rejected files. When ready, click Output Subframes.
Although factors such as star roundness and FWHM are important, when star quality looks reasonable, an image’s SNR may be the most critical factor for selecting premium subframes.
Record the names of one or two of the images with the strongest SNRWeight for each monochromatic filter (n/a for one-shot color) . The best of them can be used later as the Reference Image during a manual ImageIntegration (Chap. 6). You could alternatively rename the file something like ‘ReferenceImage’ to make it easier to locate.
Visit https://www.springer.com/us/book/9783319976884 (‘Free Download: Extra Material’ link) to download the SubframeSelector Weighting Expressions.xlsx Excel spreadsheet by David Ault.
Enter the Min and Max measurements for the three given parameters as measured by the script, into the blue boxes at the top of the spreadsheet. Set the Range between 40 and 60. The Weight assigned to each of the parameters is up to you, though their total should equal 50. This step can be made easier using a variation of David’s spreadsheet by Brian Valente. Check the PI forum’s Software Development/New Scripts and Modules section to download it.
Copy (Ctrl+C) the expression below it in green to the computer’s clipboard.
Paste (Ctrl+V) the expression into the Expressions/Weighting field. Useful, less complex expressions are available via the PI Forum.
In the Output section, choose the Approved Directory where you want the images to be written to. Make sure that the Approved Action is the default Copy. ‘Move’ won’t work.
Near the very bottom is the Weight Keyword field. Type in SFSWEIGHT (SubframeSelector Weight).
Click the Output Subframes button.
The script will write each image’s weight to the SFSWEIGHT field of its XISF (Extensible Image Serialization Format) or FITS (Flexible Image Transport System) header, for later use. We’ll return to harvest the fruits of these labors in Chap. 6.
Although other image analysis tools exist within the program (Statistics process, etc.), Blink and SFS are the most efficient means of choosing the best images for subsequent processing. For additional excellent information on the SFS script and other topics, visit Kayron Mercieca’s Light Vortex Astronomy tutorials on the World Wide Web.