The FITS Image Blinker is a dedicated tool for the visual detection of transient phenomena. By rapidly alternating — or blinking — between two FITS images of the same star field, any object that has changed between the two exposures is immediately revealed to the eye, while the static background of stars remains motionless. This makes the FITS Image Blinker ideally suited for the discovery and confirmation of novae, supernovae, dwarf nova outbursts, moving objects such as asteroids and comets, and any source that appears, disappears, or significantly changes in brightness.


The technique requires the two images to be perfectly registered, so that every star occupies exactly the same pixel position in both frames. Even a slight offset, rotation or scale difference would cause every star in the field to jump during blinking, masking the real transient. Phoranso solves this automatically: it extracts the stars from both images, builds a catalog of their positions, and then computes and applies the geometric transformation needed to warp the Science image onto the Reference image. Once the two frames are aligned, only genuine changes in the field stand out when the blinking starts.


The FITS Image Blinker is purely a visual inspection tool. It does not perform photometry and does not modify the original FITS files.


Opening the FITS Image Blinker


The FITS Image Blinker is accessible from the Discovery menu. Selecting it opens the FITS Image Blinker window, where you can load your image pair, control the alignment, and start blinking. When the window first appears, the status bar at the bottom reads Ready. Load two FITS files to begin.




Loading the images


The top of the window contains two input areas:


    • Reference image — the frame that serves as the baseline for comparison. This is typically an earlier, well-known image of the field, or a reference frame from a survey or your own archive. All alignment is performed relative to this image, which itself is never altered or warped.

    • Science image — the new frame you wish to inspect for transients. Phoranso aligns this image onto the Reference image.

For each image, type the full path into the text field or click the ... button to browse to the FITS file. When both paths have been entered, click Load images. This will display the Science image.



Alignment options


The checkboxes, located to the right of the image display, govern how the two images are registered before blinking begins.


    • Images pre-aligned: Enable this option when the two images are already registered — for example, when they originate from the same calibrated, plate-solved sequence and share an identical orientation, scale and pixel grid. When checked, Phoranso skips the star extraction and warping step and loads the two frames as they are. This is faster, but should only be used when you are confident the images are genuinely aligned.

    • Save aligned image: When checked, the warped (aligned) Science image is written to disk as a new FITS file, alongside the original. This is useful when you wish to keep the registered frame for further inspection or processing outside the Blinker. The original Science image is left untouched.

    • Fine alignment: Performs an additional refinement pass after the initial registration, improving the accuracy of the alignment down to the sub-pixel level. Enable this option for the most demanding fields, or whenever residual star movement is still visible during blinking. The refinement adds a small amount of processing time.

    • Remove hot pixels: Removes hot pixels from both images before the blinking starts. 


How the alignment works


When the images are not pre-aligned, Phoranso uses a star-pattern matching approach similar to that of the well-known astroalign1 technique. It first detects the stars in both the Reference and the Science image and records their positions in a catalog. It then searches for matching geometric patterns of stars (asterisms) that are common to both frames, and from these correspondences derives the transformation — translation, rotation and scaling — that maps one frame onto the other. Finally, the Science image is resampled so that its stars coincide precisely with those of the Reference image. Because the method relies on the relative arrangement of stars rather than on coordinate headers, it works even when the images come from different instruments or have no WCS information.


Important

🡺 The aligned Science image is intended for visual inspection only and should not be used for accurate photometry. The warping process resamples the image onto a new pixel grid, which slightly redistributes the flux of each star and subtly distorts the stellar profiles. This is perfectly adequate for revealing transients by eye, but it alters the point-spread function enough to compromise precise brightness measurements. For photometric work, always measure the original, un-warped frames. (This is an inherent property of image registration in general; the astroalign algorithm itself was designed for applications where precise astrometry is not required).

🡺 For the alignment to succeed, both images must share a sufficient number of common stars. If Phoranso reports that registration failed, verify that the two frames cover overlapping areas of sky, that they are not too dissimilar in depth, and that they contain enough detectable stars. Adjusting the Contrast beforehand has no effect on alignment, which always operates on the underlying pixel data.


Blinking the images


Once both images are loaded, the Blink panel on the right controls the alternating display:


    • Start — Phoranso begins the star extraction, builds the position catalog, and registers the Science image onto the Reference image. Then the blinking starts, alternating the display between the Reference image and the aligned Science image at the chosen rate.

    • Stop — halts the blinking and freezes the currently displayed frame.

    • Interval — this slider sets the blink rate, i.e. the length of time each image is shown before switching to the other. Move the slider towards one end for a faster blink and towards the other for a slower, more deliberate comparison. A moderate rate is usually the most comfortable for detecting subtle changes.

You can also start and stop the blinking at any time by pressing the Space bar, without having to move the cursor back to the buttons.



Display controls


The image is shown in the large display area in the centre of the window. Several controls let you optimise the view for transient detection.


    • Zoom and pan: scroll the mouse wheel to zoom in and out, centred on the cursor. Zooming in is often essential for confirming faint transients or separating closely spaced objects.

    • Drag with the mouse to pan across the image when zoomed in.

    • The Contrast slider adjusts the display stretch applied to both images. Increasing the contrast brings out faint objects against the background, while reducing it can help when the field is dominated by bright stars. The contrast setting affects only the on-screen presentation and is applied equally to both frames, so it does not interfere with the comparison.

    • Toggle background switches the display between a black and a white background. A black background is the traditional choice for astronomical images, while a white (inverted) background can make faint stars and low-contrast transients easier to perceive. Click the toggle to find the presentation that best suits the field you are inspecting.

    • The Status bar and shortcuts at the bottom of the window reports the current state of the tool and the outcome of each operation, from the initial Ready. Load two FITS files to begin. through loading, alignment and blinking. 




Tutorial: Detecting an artificial nova in M31


This brief tutorial illustrates how to use the FITS Image Blinker to discover an artificial nova added to a FITS image of the Andromeda galaxy M31.  



  1. Open the FITS Image Blinker from the Discovery menu.
  2. Browse to the Reference image by clicking the ... button. Navigate to Documents > Phoranso > Tutorials > ImageBlinker and select the file Reference_M31_A0.fit
  3. Select the Science image by clicking the ... button in the Science image box. Navigate to the same folder and select Science_nova_M31_A0.fit.
  4. Leave Images pre-aligned, Save aligned image and Fine alignment unchecked, and check the Remove hot pixels option.
  5. Click Load images.  
  6. Adjust the Contrast and, if desired, the background.
  7. Press Start to begin alignment and blinking. Adjust the Interval slider to increase or decrease the blinking speed.
  8. Watch for any object that flickers against the static star field.
  9. Zoom in on any candidates to confirm them and pan across the field to inspect the entire frame.
  10. Can you locate the artificial nova at position X=129, Y=330 ?


    1 Beroiz, M., Cabral, J. B., & Sanchez, B. (2020). Astroalign: A Python module for astronomical image registration. Astronomy and Computing, 32, 100384. https://doi.org/10.1016/j.ascom.2020.100384