GEAR · ORDERED
Bahtinov mask
A slotted cap for the front of the Mak that turns focusing from a hunt into a single unambiguous adjustment.
Ordered 13/09/2026, awaiting delivery.
What it does. Three sets of angled slots cut into a cap over the front of the tube. A bright star seen through it throws a diffraction pattern of two crossed spikes plus a third spike through the middle. When the middle spike sits exactly centred between the other two, the telescope is in focus. Out of focus, it drifts to one side, and which side tells you which way to turn.
That last part is the whole value. Judging a blurry disc tells you that you are wrong but not which direction, which is why focusing turns into a bracket.
Why it is on the list. On 12 September finding focus took eight runs and most of the usable sky. The crescent measured 63.5 arcseconds across at the start of the night against a true 35.6, which is 78 per cent of pure bloom, and it took until run 000318 to get to 36.9. The night before went the same way with a formal focus bracket that cost altitude while it ran.
A mask makes that one adjustment on a star before the target is even acquired.
How it will be used here. Cap on, point at something bright, focus until the centre spike is centred, cap off, then go to the target. It wants a star rather than a planet, since the spikes need a point source. Venus at 35 arcseconds is too big to give a clean pattern.
Worth pairing with the [[focus-lever]], which solves the other half of the problem: not knowing where focus is, versus shaking the whole telescope while getting there.