AkastroidImage Doctor
Find out what isactually wrong with your subs.
Before anything is stacked, every frame is measured - star size, star shape, signal to noise, sky gradient, cloud, trails, clipping - and every rejection comes with the reason beside it. Point Akastroid at a folder and read the diagnosis, whether or not you go on to stack it.
Part of the free download. Nothing to enable, nothing uploaded, and your files are never written to.
The problem with blinking through 400 subs
The standard advice for culling frames is to look at them and delete the bad ones. Try it once. By frame 60 you cannot remember what the good ones looked like; by 150 you are rejecting frames because you are tired. And the faults that matter most - a four per cent drop in sharpness, a background creeping up as the target sinks towards the horizon - are invisible at any size a screen can show you.
Culling is a measurement problem wearing the costume of a judgement call. So Akastroid measures, and then shows you the measurements rather than acting on them silently. That second part is the one that matters: software that quietly threw away a third of your night and told you nothing would be worse than software that threw away none of it.
What gets measured, and what each number lets you conclude
Every one of these is measured from the frame itself. Nothing is inferred from your equipment list, and nothing is a preset.
Star size (FWHM)
How sharp the frame is, in pixels across a star's profile. Rising through a session means focus drifting as the tube cools; high on every frame means the seeing, the focus or the sampling. It is measured from your own stars, so it is in your pixels rather than in somebody else's arcseconds.
Star shape (eccentricity and roundness)
Whether stars are round or stretched. Stretched means something moved - a tracking error, a gust, a cable snag - or the optics are at fault. Which of those it is has its own answer below, and it is the difference between buying a guide camera and collimating what you already own.
Signal to noise
How far the subject stands above the noise in this frame. It is what decides whether more frames or longer frames is the better use of your next clear night.
Sky brightness and gradient
How bright the background is and how steeply it varies across the frame. A bright but flat sky is recoverable; a steep gradient is much less so, and both are measured relative to the rest of your session rather than to a constant.
Cloud and haze
Called only when two things agree: the sky is brighter than the session's median and the star count has dropped. Either signal alone is a false positive - a rising target brightens the sky without cloud, and a passing focus wobble drops the count.
Satellite and aircraft trails
Counted, with the longest measured. A trail is not usually a reason to discard a frame, because outlier rejection removes it during integration - but it is worth knowing how many are in there, and a frame with several is a frame worth a second look.
Overexposure
What fraction of the frame is clipped. Clipped stars cannot be colour-calibrated and a clipped core cannot be recovered by anything downstream, so this is one of the few faults that is genuinely unfixable in processing.
Star count
How many point sources were detected. It is the sanity check on everything else: a frame with almost none is a frame where cloud, condensation or a lens cap is the real story, whatever the other numbers say.
Your optics, or your tracking?
This is the question that costs people the most money, and it has a clean answer that almost nothing tells you.
Elongated stars have two very different causes. If the mount drifted, some frames are stretched and others are not, and the direction changes through the night as the guiding corrects and overshoots. If the optics are at fault - coma, field curvature, camera tilt, a pinched mirror, a sagging focuser - then every frame of the night is stretched the same way, because the optics did not move.
So Akastroid looks at the elongation across the whole session rather than frame by frame. When it is present in all of them, it says so in as many words, tells you it is the optics rather than the tracking, and rejects nothing for it. That last part is not a detail: a rule that discards frames for a fault every frame shares leaves you with an empty session and no explanation.
The practical difference is whether your next purchase is a guide scope or an hour with a collimation tool. Optics or tracking covers the one-look test that separates them.
Why the thresholds come from your session
Nothing here is judged against a fixed number, and that is deliberate. A 3-pixel FWHM is superb at one focal length and soft at another. A background at 0.05 is a dark site from a city and a bright one from a desert. A star count of forty is a rich field through a fast lens and a failure through a narrowband filter.
Judging each frame against the rest of its own session asks the only question with a useful answer: is this frame worse than the others I took tonight? It has two consequences worth stating. A uniformly excellent session is not culled, because nothing in it is unusually bad. And a poor session still produces an image, because the cap on rejections holds - at most forty per cent of a session can be thrown away, however bad the numbers look, and the reason is that half a bad picture beats none.
The faults it names
A score on its own is not a diagnosis. Where a measurement crosses into a fault, the fault gets a name and appears beside the frame: soft focus, star trailing, strong gradient, cloud or haze, satellite or aircraft trails, overexposed, or too few stars to register on. No frame is ever rejected without one - a row marked rejected with an empty reason is a bug, not a judgement, and it is treated as one.
Cloud is worth singling out, because it is the one most often called wrongly. Akastroid raises it only when two independent signals agree: the sky is brighter than the session's median and the star count has fallen. A rising target brightens the sky on its own; a focus wobble drops the star count on its own. Requiring both is what stops half a good session being labelled cloudy.
Using it as a diagnosis and nothing else
You do not have to stack anything. Point Akastroid at last night's folder, read the table, and close it. Plenty of people use it exactly that way - to find out whether the focus held, whether the guiding held, and whether tonight is worth another try - and it is free to do so.
Nothing is deleted, moved or altered. Frames are marked for the stack or against it, and any row can be clicked to overrule the decision. Your files are read and never written to, and none of this leaves your computer.
Read further
Reading a quality score goes through the numbers one at a time. A symptom-by-symptom guide works the other way round, from what you can see to what caused it. Optics or tracking takes the most commonly misdiagnosed fault in detail.
What is Image Doctor?
It is the measurement pass Akastroid runs on every frame before it stacks anything, shown to you rather than kept internal. Twelve things are measured per frame - star size, star shape, signal to noise, sky brightness, sky gradient, noise, clipping, satellite trails, star count and more - and each frame is given a score and, where something is wrong, a named fault. You get a table of your whole session with the reason beside every rejection.
Is it a separate product or an extra cost?
Neither. It is part of the free download and it runs on every session, because the same measurements are what the automatic processing decides from. There is nothing to enable and nothing to buy - a frame cannot be graded privately and stacked publicly.
How can it tell an optics problem from a tracking problem?
By whether the fault is in every frame. A tracking error stretches stars in some frames and not others, and the direction changes as the mount fights back. An optical fault - coma, field curvature, camera tilt, a pinched mirror - stretches them in the same way in every frame of the night, because the optics did not move. So when the elongation is present across the whole session Akastroid says so, and says it is the optics rather than the tracking, and does not reject a single frame for it. Rejecting frames for a fault that is in all of them leaves you with nothing.
Why are the thresholds relative to my session rather than fixed?
Because a fixed threshold is wrong for almost everybody. A 3-pixel FWHM is superb at one focal length and soft at another; a sky at 0.05 is dark from a city and bright from a desert. Judging a frame against the rest of its own session asks the only question that has a useful answer - is this frame worse than the others I took that night - and it means a uniformly excellent session is not culled and a poor one still produces an image.
Does it delete anything?
No. Nothing is deleted, moved or modified. Frames are marked selected or rejected for the stack, and you can click any row to overrule it. Your files are read and never written to.
Can I use it just to check my frames, without stacking?
Yes. Point it at a folder and read the table. Plenty of people use it exactly this way at the end of a session - to see whether the focus held, whether the guiding held, and whether it is worth going back out tomorrow.
Does it work on single frames?
The measurements do, and they are shown. The scoring does not mean much, because a score is a comparison and one frame has nothing to be compared to - so faults that are relationships between frames, like too few stars to register on, are not raised against a lone image.
What does it not tell me?
It measures the frame, not the sky above it or the equipment under it. It cannot tell you your polar alignment error in arcminutes, it cannot see a dew heater that is off until the dew arrives, and it will not diagnose a fault that leaves no trace in the pixels. Everything it reports is measured; nothing is inferred from your equipment list.
Diagnose last night's session.
Free, no account, nothing uploaded. Point it at the folder and read the table.