SESSION 4 · 08/26 · DIDN'T WORK
First planet — Venus, low in the west, and nothing to see on it
Pointed at Venus instead of the Moon on the rebuilt mount. Three short runs so far. Learned that most of what limits a low planet has nothing to do with the telescope.
The runs
All three on the rebuilt friction swivel, with the new focus lever.
| run | frames | duration | exposure | WB | fps |
|---|---|---|---|---|---|
| 000071 | 50 | 3 s | 1 ms | AUTO | 15.07 |
| 000072 | 100 | 7 s | 1 ms | AUTO | 15.11 |
| 000073 | 100 | 7 s | 1 ms | AUTO | 15.12 |
Frame rate is rock steady at ~15.1 fps, so the USB 3.0 link is healthy and the camera is running at full 3264x2160.
What I got wrong before I started
Run length. I'd carried over a rule of thumb from the Moon — park at the leading edge, run about 60 seconds before it drifts out. That number came from the Moon's 2188 px disc leaving only ~1076 px of clear space on a 3264 px axis. Venus is a tiny dot at this scale, so it has nearly the whole frame to cross: about 190 seconds of drift budget. These 7-second runs are leaving an enormous number of frames on the table for nothing.
The object name. AstroDMx still had Object Name set to Moon, so every Venus run
filed itself into ~/AstroDMx_DATA/Moon/2026-08-30/. Three runs and 5 GB of Venus sitting
in a folder that says Moon.
What actually limits a low planet
This is the real lesson of the session, and none of it is about the telescope.
There is no surface detail on Venus to find. It is wrapped in an unbroken cloud deck. In visible light you get the bright phase shape and essentially nothing inside it — cloud structure only appears through a UV filter. So the shot is a clean, crisp crescent, not surface markings. Worth knowing before spending an evening hunting for something that isn't there and blaming focus for the absence.
Altitude, again. Venus was low in the west just after sunset — the maximum amount of atmosphere to look through, and the same lesson the cheatsheet already gives for the Moon.
Atmospheric dispersion. At low altitude the air behaves as a weak prism and smears a bright object into a tiny spectrum: a red fringe on one edge, blue on the other. This is not chromatic aberration in the scope, and no amount of focusing touches it. The instrument that corrects it is an ADC (atmospheric dispersion corrector), which I don't have. On a low target it dominates everything else.
Under-sampling hurts far more here than on the Moon. At the measured 0.853"/px, Venus spans only a few dozen pixels — a coarse ruler on a tiny target. On the Moon, under-sampling cost fine detail on a huge object; on a planet it costs the object. This moves the 2x Barlow from "the obvious next purchase" to the single thing that would most change my planetary images.
To fix
- [ ] Object Name →
Venusso the archive stops lying. - [ ] Auto WB off — it re-decides colour every frame, which fights the stacker.
- [ ] Much longer runs. 190 s of drift budget, not 7 s.
- [ ] Check the histogram isn't clipping. Venus is dazzlingly bright and 1 ms may be blowing out the very terminator edge that defines the phase.
Results
Abandoned after three test runs. 50, 100 and 100 frames at 1 ms, all with auto white balance still on, all filed into the Moon folder because the Object Name was never changed.
Nothing worth keeping. Venus was low in the west and dropping, and once the Moon rose at 20:10 it was the better target by a distance — bigger, brighter, and with terrain that actually shows shadows.
The session earned its place anyway: it produced the two learnings above, and it is where the 2x Barlow first got fitted. The under-sampling maths that came out of pointing at a tiny planet is what made the Barlow obviously the right move — and it went on to define the whole rest of the night on the Moon.
Venus deserves a proper attempt when it is higher and with the phase worth photographing. Not this time.