Siril Photometry: Building a Light Curve for a Variable Star Most of what we photograph in the night sky is effectively frozen. Galaxies and nebulae look tonight the way they looked last year. Variable stars are the exception. Algol fades by more than a full magnitude in a few hours, Delta Cephei pulses with clockwork regularity, and Mira swells from invisible to one of the brightest stars in its corner of the sky over months. Measuring those changes used to belong to professional observatories. With the photometry tools built into Siril, a normal backyard rig can produce a genuine light curve, and in this tutorial we will walk the full path from calibrated frames to a plotted curve. Why Variable Stars Variable stars change brightness for fascinating reasons. In an eclipsing binary like Algol, the Demon Star of Perseus, a dimmer companion passes in front of the primary every 2.87 days and the combined light drops from magnitude 2.1 to 3.4. A pulsating star like Delta Cephei breathes, swelling and shrinking over a 5.37 day cycle that carries it between magnitudes 3.5 and 4.4. Mira in Cetus, the prototype of the long period variables, swings between roughly magnitude 3 and magnitude 9 over about eleven months, by turns a naked eye star and a telescope object. These rhythms are real science. Cepheid periods revealed the true scale of the universe a century ago, and eclipsing binaries still yield stellar masses and radii today. The American Association of Variable Star Observers, the AAVSO, has coordinated amateur measurements since 1911, and backyard photometry still feeds professional research. What You Need The requirements are modest. You need a run of light frames on the same star field, taken with identical exposure and gain, ideally spanning a few hours so a fade or a dip has time to show itself. Calibrate the frames first with darks and flats; our guide to calibration frames covers the details, and photometry on raw frames will bake sensor artifacts straight into your measurements. You also want a current version of Siril, 1.2 or newer, which introduced the dedicated photometry mode and light curve plotting. No special camera, no special filter, just consistent data. Step 1: Plate Solve and Identify the Star Before measuring anything you need to know exactly which star is the variable, and in an unfamiliar star field that is harder than it sounds. Siril’s Image Plate Solver settles the question. Open the first frame of your sequence, launch the plate solver, and enter your best estimates for focal length and pixel size. If you are unsure of the numbers, iterate: hold the pixel size fixed and nudge the focal length until the solve locks and the console reports your image scale and field center. With the field solved, right click the image and choose Search Object, type the variable’s name, Algol for example, and Siril annotates it on the frame. No more squinting at a chart wondering which point of light is the one that changes. Step 2: Align the Sequence Load the full light sequence and register it with global star alignment, the deep sky algorithm that records the alignment solution in the sequence file itself. For photometry this is a real advantage: no resampled copies are written, so the pixel values you measure are the pixels your camera recorded, and small tracking drifts between frames disappear while the star you selected stays put across the whole run. Step 3: Cull the Bad Frames Open the Graphics tab and plot the weighted FWHM of each image against star roundness. Passing clouds, wind gusts, and focus drift betray themselves as a cloud of points far from the healthy cluster. Click a suspicious point and Siril jumps to that frame so you can deselect it. Discarding a handful of frames costs you nothing, while letting them through corrupts the very measurement you set out to make. Step 4: Measure the Variable Toggle the photometry mode button, right click the variable star, and Siril draws the measurement apertures and fits a PSF to that star in every image of the sequence. The Plot tab then offers curves for FWHM, roundness, background level, and magnitude. The background curve alone is worth a look, because it tells the story of your session: moonrise, thin haze, or a neighbor’s porch light creeping into the sky. The magnitude at this stage is relative. Only the variations mean anything until reference stars put them on a scale. Step 5: Add Reference Stars Right click three or four comparison stars in the same field and run the sequence photometry on each one. Choose stars of similar brightness to the variable that sit nearby in the frame and well away from the edges, and prefer stars known to be steady; charts from the AAVSO mark comparison stars with their magnitudes, which takes the guesswork out. References are the anchor: when sky transparency drifts during the night, they drift along with it, and the difference between variable and references cancels the noise. Step 6: Plot the Light Curve With references in place the Light Curve button activates. Switch the horizontal axis to Julian date, the standard timekeeping of variable star work, and click. Siril computes the calibrated magnitudes and saves a data file, and if gnuplot is installed it renders the plot on the spot. A clean Algol run looks like a flat line with a bowl carved into it: a drop of a bit over a magnitude as the eclipse begins, a flat bottom, then recovery, the whole event wrapped up in about ten hours. Reading Your Curve The shape is the science. An eclipsing binary gives sharp, flat bottomed dips on a strict schedule. A Cepheid rises quickly and fades slowly, a sawtooth rather than a bowl. Mira traces a slow swell and collapse measured in months. Read the period from the spacing of the dips, the depth from the vertical drop, and the character from the transitions, and you are doing the same analysis that taught astronomy the distance to the galaxies. From Here The identical workflow measures exoplanet transits and asteroid occultations, and the official Siril photometry tutorial walks the process on a hot Jupiter transit if you want a second opinion. If your curiosity wants consequences, the AAVSO welcomes observations from amateur equipment and their comparison charts slot straight into step five. And if you enjoy pulling science from your own frames, our earlier piece on Hertzsprung Russell diagrams is the natural companion to this one. https://www.backyardastronomy.net/2026/11/01/siril-photometry-building-a-light-curve-for-a-variable-star/ #rss #feeds #linksharing #qwibbit

The Science of Mosaics: When One Frame Is Not Enough The Problem: Some Objects Are Simply Too Big Every telescope and camera combination paints a fixed window on the sky, and some of the best deep sky objects refuse to fit inside it. That is exactly what happened with the Heart and Soul Nebulae in Cassiopeia. Both were captured here with an Orion 8 inch Newtonian reflector at 1000mm f4.9 and a Canon T3i camera, a combination that frames roughly 1.3 by 0.85 degrees of sky. The Soul Nebula complex covers an area equal to about four full moons. One frame simply cannot hold it. The fix is a mosaic: shoot several overlapping frames that tile the target, then stitch them into a single image during processing. The idea is simple. The execution has real science behind it, and the difference between a sloppy mosaic and a clean one is measured in the details. How Big Is Your Window on the Sky? Field of view follows a simple relationship: divide the sensor dimension in millimeters by the focal length in millimeters, then multiply by 57.3 to get degrees. The Canon T3i sensor measures about 22.3 by 14.9mm. At 1000mm that works out to about 1.3 degrees along the long side and 0.85 degrees on the short side. Now compare that to real targets. The full moon spans about half a degree, so the frame comfortably holds it. The Andromeda Galaxy stretches about three degrees along its long axis, which overflows this frame even in landscape orientation. The Soul Nebula complex is similar. When your target is bigger than your window, you have two options: shorten the focal length, or start planning panels. Overlap Is Everything A mosaic is only as good as its seams. Neighboring panels must share enough sky for software to find matching stars and blend the pieces, and the standard rule is 20 to 30 percent overlap between adjacent frames. Less than that and star matching gets fragile. More than that and you waste precious imaging time on sky you already captured. The numbers from this gallery show the principle at work. The Heart Nebula mosaic below is 4973 by 5757 pixels from two vertically stacked panels, with about a third of each panel height shared between the two. The Soul Nebula mosaic is 9039 by 5976 pixels from four panels arranged in a two by two grid, with roughly a quarter of each panel overlapping its neighbors. Overlap also saves you at the edges. Optical vignetting and field curvature degrade star shapes near frame borders. The overlap zone gives you the option to crop away the worst of it before the panels meet. Planning the Grid in NINA NINA makes mosaic planning painless. The Framing Assistant lets you define a grid of panels over any target, set the overlap percentage, and rotate the entire grid to match the natural axis of the object. That last part matters more than beginners expect. Rotate the grid so the nebula flows across panel boundaries through dense regions rather than slicing a faint outer edge in half, because faint edges are where gradient mismatches show up worst. If you want the full walkthrough, the Framing Assistant basics post covers the fundamentals, and the deep dive on rotating, cropping, and planning mosaics goes into grid strategy in detail. Capture Discipline Consistency is the rule during capture. Every panel should use identical exposure length, ISO, and white balance, because any mismatch becomes a visible brightness or color step at the seams after stitching. Refocus on each panel. Star sizes that differ between panels are nearly impossible to blend invisibly. Keep dithering enabled throughout so fixed pattern noise averages out evenly across the whole mosaic. Be honest about the time budget before you start. Each panel of the Heart Nebula mosaic in this gallery received 85 exposures of 180 seconds, which is about four and a quarter hours per panel, and the full two panel image took five nights. The Soul Nebula mosaic needed the same per panel investment across four panels. Mosaics multiply total shutter time by the panel count, so a target that needs one great night as a single frame needs two nights as a two panel mosaic and four as a grid. Stack First, Stitch Second The processing order matters. Stack each panel independently in Siril first, exactly as you would a normal image, including calibration, registration, and integration. Run background extraction on each stacked panel before any stitching happens, because light pollution gradients differ between panels captured on different nights, and mismatched gradients are the number one giveaway of a mosaic. Only then load the stacked panels into Siril mosaic registration, which finds matching stars in the overlap zones, computes the offsets, and blends the panels into one image. Finish with a crop to remove the ragged outer edge where panels do not fully cover. The general Siril workflow is covered in the stacking to final image guide. The Science Payoff Why go to all this trouble instead of just using a shorter telescope? Resolution. Halving focal length doubles the field of view, but it also spreads the same pixels across four times the sky area, so every detail shrinks to half size. A mosaic keeps the image scale of the long focal length while expanding the field of view to whatever the target demands. The Soul Nebula mosaic in this gallery is 9039 pixels wide. Captured at half the focal length in a single frame, the nebula would occupy barely half that pixel count across. The trade is time rather than equipment. You are choosing to spend four clear nights instead of one to hold onto fine structure. For emission nebulae with intricate filament detail, that trade is usually worth it. Two Mosaics from This Gallery The Heart Nebula, also designated IC 1805, is an emission nebula about 7500 light years away in the Perseus Arm of our galaxy. This two panel mosaic took five nights with the Orion 8 inch Newtonian at 1000mm, guided with a Meade 800mm guide scope on an Orion Atlas EQ-G mount. It first appeared on the blog in the Heart Nebula feature post. The Soul Nebula, designated IC 1848 and known to radio astronomers as Westerhout 5, sits just east of the Heart and hosts embedded clusters including Collinder 33 and Collinder 34. It spans an area of sky equivalent to four full moons and sits about 6500 light years away. Four panels of 85 by 180 second exposures were stitched in Siril to hold it all. Both images, along with the full capture details, live in the astrophotography gallery. Zoom Out or Stitch? So how do you decide? A quick guide: Stitch a mosaic when the target overflows your frame and you want to keep fine resolution, especially emission nebulae with filament structure. Use a shorter focal length when the target fits comfortably and you value speed, or when seeing and guiding limit your real resolution anyway. Use a focal reducer as the middle path, buying field of view at a modest cost in image scale. Skip mosaics on marginal nights. Consistent conditions between panels produce the cleanest seams, so a single excellent night beats two mediocre ones. Try It This Season Late October is prime time for this. Cassiopeia rides high in the evening sky, putting the Heart and Soul region near the zenith for northern observers, right where the atmosphere is thinnest and guiding is most stable. Start modest with two panels of the Heart, keep your overlap near a third, and hold every capture setting constant between panels. When the seams disappear and the whole nebula lands in one frame, you will understand why mosaic shooters accept the extra nights. The sky’s biggest objects give no other way to have them whole. https://www.backyardastronomy.net/2026/10/25/the-science-of-mosaics-when-one-frame-is-not-enough/ #soulnebula #frame #science #sky #grid #bigevery #toobig #overlap #simplytoo #window #rss #feeds #linksharing #qwibbit

Siril Scripting for Batch Processing: Automate Your Workflow If you have been using Siril for a while, you have probably noticed that your preprocessing steps are the same every night. Convert your raw frames, stack them, stretch them. Clicking through the same sequence night after night is not just tedious, it also invites mistakes at two in the morning. Siril has a built in answer: its scripting language. In this tutorial we will turn a normal processing session into a repeatable script you can run with a single command. Why Script Your Processing Consistency is the real payoff. When every image goes through the same preprocessing chain, your results become comparable across nights and seasons. If you discover that a slightly different background extraction setting works better, you change one line in the script and rerun it. Compare that with trying to remember exactly which buttons you clicked three weeks ago. Scripts also protect you against fatigue. The most common processing errors, forgetting a calibration step or stacking the wrong frames, disappear when the sequence is written down once and executed the same way every time. Where Scripts Live Siril scripts are plain text files with one command per line, usually saved with a .ses extension. Anything you can do from the graphical interface has a matching command. The easiest way to learn the syntax is to open the Siril command line tab and process one session manually while noting the commands echoed in the log. That log is essentially a draft of your script. A Complete Preprocessing Script Here is a working example for a one shot color session with light, dark, flat, and bias frames in separate subdirectories. cd ~/astrophotography/session1 convert light -out=./light convert dark -out=./dark convert flat -out=./flat convert offset -out=./bias preprocess light -dark=dark -flat=flat -bias=bias -cc=dark -equalize_cfa stack light/redux_light_ -rej 3 3 -norm addscale stack bias/redux_bias_ stack dark/redux_dark_ -sum stack flat/redux_flat_ -norm mul preprocess flat -bias=bias load light/stacked_light.fit bg_extraction -samples=20 -type=1 -degree=2 photometric_color_calibration asinh -stretch 0.1 0.02 save result Every line is a command you could type interactively. The script version just runs them in order without waiting for you. Batch Mode The real power arrives with the command line. From a terminal you can run a script against a directory like this. siril-cli -s pipeline.ses -d ~/astrophotography/session1 Now imagine three folders from three nights. A small shell loop runs the same pipeline over all of them while you are at work. That is batch processing: not one image automated, but the whole backlog. Tips That Save Headaches Always test a new script on a single session before pointing it at your entire archive. Check that the stacking rejection settings suit your frame counts, because a threshold tuned for twenty frames will throw away good data on two hundred. Keep scripts versioned alongside your images, and when a script produces a noticeably better result than your old manual workflow, promote it to your default template. Note that command names and options shift slightly between Siril versions, so if a script from an older release complains, check the release notes for renamed options rather than assuming your data is the problem. From Here Start small. Script just the conversion and stacking steps first, then add background extraction and calibration once you trust the pipeline. Before long you will find yourself copying a template script into every new session folder as a matter of habit, and your evenings will be spent on the creative parts of processing rather than the repetitive ones. If you missed our earlier guides on background extraction, color calibration, and deconvolution, they pair well with this one. https://www.backyardastronomy.net/2026/10/11/siril-scripting-for-batch-processing-automate-your-workflow/