Field guide
Measuring focuser backlash
Backlash is the slack a focuser takes up before the drawtube starts to move. It only costs you anything when the focuser reverses - which an autofocus sweep does at least once.
01 What backlash is
Backlash is like the play in a steering wheel - you can turn it a little before the wheels respond. A focuser has the same slack. When the motor reverses, its first few steps only take up that slack, and the drawtube does not move at all. Watch: the motor moves on every press, but just after you reverse, the drawtube lags behind.
Move one way, then reverse - and watch the drawtube lag.
Press a button to move the focuser, then reverse direction.
Those wasted steps after a reversal are your backlash. It costs you nothing until the focuser changes direction - but an autofocus V-curve reverses at least once, so it pays to handle it.
02 Try this before measuring anything
Backlash only bites on reversal. If your imaging software always finishes focus from the same direction, the exact number barely matters. Most packages call this a final inward move or overshoot compensation. Turn it on first.
Only bother measuring if you still see lopsided V-curves, or your software asks for a backlash figure it can compensate with. Then use one of the two methods below.
03 Two ways to measure it
With a dial indicator - precise
About twenty minutes, and it doubles as a check on the microns-per-step figure the calculator uses. In short: load the tube fully one way and zero the dial, then reverse and count the steps until the needle first twitches. Do it with the camera attached and the tube at a real imaging angle.
Full step-by-step
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Set up against the drawtube
Magnetic base on the focuser body or OTA, indicator tip square to the flat face of the drawtube and pre-loaded about half a millimetre.
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Drive out 500 steps, then zero the dial
This takes up all the slack in one direction so the teeth are hard against each other. Zeroing here is what makes the next reading mean something.
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Drive out 200 more and check the dial
The needle should move by roughly
200 × your microns-per-step. If it doesn't, your calibration is off - fix that before going on. -
Reverse, and drive in five steps at a time, counting
The needle sits dead still while the slack takes up. The moment it first moves, stop counting.
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Steps counted before it moved = your backlash
Repeat twice more and use the largest of the three, not the average. You want the compensation to clear the slack every time, not most of the time.
On a star - no tools
Less precise, and the atmosphere adds some scatter, but it needs nothing you don't already own and it measures the system exactly as it images. In short: defocus outward on a bright star until HFD is climbing steadily, then reverse and count inward steps until HFD starts to fall.
Full step-by-step
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Loop short exposures on a moderately bright star
Bright enough for a steady HFD reading, faint enough not to saturate. Two or three second subs are plenty.
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Drive out until HFD is climbing steadily
You want to be well outside the focus zone, on a clean slope, then stop.
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Reverse, and drive in ten steps at a time, counting
HFD holds flat while the slack takes up. When it begins to fall, stop - your count is the backlash.
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Run it three times
Seeing moves HFD around on its own. Three runs tells you whether you measured backlash or measured the atmosphere.
04 Worth knowing
- Measure loaded, and pointed high. A bare drawtube on the bench gives a number that won't survive a heavy camera at the zenith.
- It depends on direction. Moving against gravity isn't the same as moving with it. Measure the way your software will actually approach focus.
- Cold grease is stiff grease. A summer figure isn't a winter figure. Re-check when the season turns.
- A huge reading is usually a loose coupler, not a bad gearbox. If an EAF or FocusCube reports much north of 200 steps, check the grub screws on the shaft coupler before you believe it - Pegasus notes this in their own manual.
- Zero is a valid answer. Direct-drive and worm-drive units - NightCrawler, Esatto and the like - genuinely add none.