16.12.19 Shorter timing belts finally arrive!

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Monday 16th. Three weeks and three days after posting, my shorter timing belts from the UK have finally arrived. Avoid Standard International Letter Post if you can. £2.80 worth of postage for nothing at all. Let's call it Substandard and be done with it.

My DC, focuser motor driven, PST etalon tuning has been on hold all of this time. I couldn't get enough tension with the 280mm without adding a foolishly long, extension bracket for the motor. I'll leave the clumsy motor arrangements to the ridiculously expensive, commercial, motor units. I much prefer the most compact motor siting with nothing sticking out. Like I did for my big, Feather Touch FT3545 focuser.

Hopefully, I will now be able to complete the project in a short time. The new belts are 250mm 2GT with another of 240 mm still waiting for stock to arrive at a Danish dealer. I shall have spares for other projects.

The belt drive with 16T pulley and 250mm 2GT belt. The angle of the motor bracket needs to be reduced slightly. It is more than 90° as supplied. The bracket is only soft mild steel. So a vice, adjustable spanner or pliers will bend it to 90°. I don't recommend trying to bend the bracket by pressing down on the motor!

I want to drive the helical focuser as well on the H-a telescope. Though that presents even more problems because both ends of the focuser move in opposite directions. The belt will fall off either end of the focusing ring during full travel. At least the ring rotates smoothly and easily. I shall just have to support the motor with sky hooks. Or off a separate bracket not directly attached to the H-alpha OTA.

I have also been stalled on finding a suitable replacement for the original and far too tight, PST etalon band packing O-rings. Several sizes of O-rings were tried but all were too small in diameter. Leaving a lot of play in the outer ring of the etalon tuning band. I tried various strimmer or [US bushwhacker ?] monofilament cord. That was either too tight 1.6mm or too loose in 1.3mm. Most electrical flex had 1.3mm cores and was therefore much too loose. 1.5mm Ø would do but nothing in everyday materials presents itself.

The new 250mm 2GT belt fitted nicely with a 16T pulley on the motor shaft. The 20T needed a curve filed in the original motor bracket to allow the motor body to sink into the bracket. Without the filed curve the 20T made the belt far too tight. I am still trying to lower the motor even more by rotating it by 90° to the bracket. It might then be possible to use the 240mm belt I have been waiting for. We shall have to see if/when it ever turns up.

I swapped original PST O-rings for 1.3mm monofilament strimmer line. When motor driven a bit of play in the tuning band is irrelevant. The motor and belt tension will always hold the etalon in the chosen position Allowing fine tuning without touching the telescope.

Pictures and quite possibly, a YT action video of my motor driven, remote etalon tuning in daylight tomorrow. If it ever gets light! Heavy overcast and fine drizzle. You'll have to make do with a flash image, or two.

The motor happily drove the etalon form end to end of its travel in both directions. See the very dark video at the top of the page. Despite being taken at lunch time it never became fully light today.

I had another thought after carefully centring the belt on the tuning band. The PST O-rings are heavily offset to one side. So the drive belt really ought to be centred over the O-rings. Even if it will look all wrong.

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