LEARNED IT · 11/26
One star is not an alignment
Aligning on the target alone tells the mount where it is pointing, not how far it is from level. The tracking rates that come out of a one-point model are wrong by a steady amount, and the planet…
STARLOG · 23 LEARNED IT
Learned it
One idea at a time, explained the way it finally made sense.
LEARNED IT · 10/26
The detail was in the recording — the stack was hiding it
The first Saturn stacks from the ZWO came out as white, colour-fringed blobs. The same recordings, put through a proper planetary chain, showed the ring gap, the ring shadow and a belt. Selection at…
LEARNED IT · 09/26
A sync fixes the mount's model, not the mount's encoders
Syncing on the Moon in the SynScan app corrects the firmware's pointing model, which is what GOTOs use. A script that reads raw axis counts never consults that model, so the sync is invisible to it.…
LEARNED IT · 09/26
Bounded by construction: why an offset goto is safe and a rate command is not
An offset goto carries its own stopping condition — a step count the motor controller counts down to zero. A slew-rate command carries none: it runs until something else stops it. That difference…
LEARNED IT · 09/26
Seventeen milliseconds per pixel: untracked panels can never line up
Every goto stops firmware tracking and nothing restarts it, so each mosaic panel is exposed with the mount standing still while the Moon runs at 13 arcseconds a second. At 0.222 arcsec per pixel…
LEARNED IT · 09/26
Compass north is eight degrees off, and the mount has no way to know
A phone compass points at magnetic north. At Goolwa that is about 8 degrees east of true north, and an alt-az goto inherits the error whole. Measured it at 7.4 degrees by centring Venus by hand and…
LEARNED IT · 09/26
A lower resolution is not a region of interest — the SV205 scales, it doesn't crop
Dropping AstroDMx from 3264x2160 to 800x600 shrank the files fifteen-fold, shrank Venus threefold, and left the frame rate exactly where it was. The camera downscales the whole field instead of…
LEARNED IT · 09/26
A raw frame always looks soft — stacking is selection, then noise, then sharpening
Scrolling through a SER in Siril shows single raw frames, and at 0.22"/px a single raw frame of a planet can never look crisp. Stacking picks the lucky frames, averages the noise out, and only then…
LEARNED IT · 09/26
More magnification past the sampling limit makes the image worse, not bigger
At f/18.4 diffraction alone smears an edge over ~10 px. Barlowing the 76/700 spreads the same detail over 2.5x more pixels and adds nothing — while costing brightness and frame rate.
LEARNED IT · 09/26
Daytime focus tests need far more focuser travel than a star does
A target at 50 m needs about 10 mm more outward focuser travel than infinity. Run out of travel on a streetlight and you'll wrongly conclude the camera can't reach focus at all.
LEARNED IT · 09/26
The capture app's output path is the app's state, not yours — I lost nine runs by looking in the wrong folder
Set the output path from a script, changed it later in the GUI, and forgot. Nine SER runs went to ~/Pictures under the prefix "testing" while I searched an empty AstroDMx_DATA and concluded nothing…
LEARNED IT · 09/26
Declination sets the ceiling, rotation sets the hour
"Airmass 8" is not eight atmospheres. It is one atmosphere crossed at a slant. Two separate things decide how much of it you look through: the Moon's declination caps how high it can ever get from…
LEARNED IT · 08/26
A sharpness metric on a stretched image measures noise, not sharpness — twice now
Laplacian variance rewards high-frequency content, and noise IS high-frequency content. Stretch a dim frame hard and it becomes pure amplified noise, which the metric scores as spectacular detail.
LEARNED IT · 08/26
Three sharpness metrics, three different wrong answers — and Brendan's eye beat all of them
Every automatic sharpness measure found a different way to reward something other than sharpness. Judging by whether crater rims are actually resolved beat all three.
LEARNED IT · 08/26
Altitude beats every setting, and dispersion is why
A low target is looked at through more air. Beyond blurring, the atmosphere acts as a prism and smears bright objects into a spectrum — a defect no focusing can reach.
LEARNED IT · 08/26
A clipped lunar disc silently breaks every geometric measurement
The Moon overflows the sensor by 28 px. Area-to-radius, centroid alignment and radial profiles all assume a whole circle, and all return plausible wrong numbers instead of failing.
LEARNED IT · 08/26
2.8 km is the floor, and no amount of processing gets under it
Dawes limit at 76 mm is 1.53\", which is 2.8 km on the Moon. Focus, stacking and deconvolution all work below that ceiling, never through it.
LEARNED IT · 08/26
Measure, don't derive — I had the wrong telescope for a whole session
Every calculation from the first session was wrong because the scope was recorded as a 114/500 and it's actually a 76/350.
LEARNED IT · 08/26
The Moon is an hour later every night — ask if it's up before debugging anything
Moonrise slips ~50 minutes per night. A black frame and a flat histogram look exactly like a broken rig, and the check that rules it out is free.
LEARNED IT · 08/26
AstroDMx's Object Name decides the folder, not what you pointed at
Captures file into ~/AstroDMx_DATA/<Object Name>/<date>/. Swing to a new target without changing that field and the archive quietly mislabels everything.
LEARNED IT · 08/26
The UVC driver accepts writes to inactive controls and throws them away
exposure_time_absolute is flagged inactive while auto_exposure is on. Writing to it succeeds and does nothing. No error, no warning.
LEARNED IT · 08/26
This camera has no gain control at all
Exposure time is the only brightness lever the SV205 exposes. Don't go looking for a gain slider.
LEARNED IT · 08/26
USB 3.0 isn't a performance choice — without it the mode doesn't exist
On a USB 2.0 link the SV205 doesn't advertise its max-res YUYV mode at all. There's no error and no dropped-frame warning, so it reads as a camera limitation.