Focuser mod to improve Sharpcap Autofocus

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Location: Gloucestershire

Focuser mod to improve Sharpcap Autofocus

Post by nexusjeep » Sat May 04, 2019 10:32 pm

I have always had an issue with the 10:1 reducers on both refractors that I have owned with the ball and cup arrangement that gives the 10:1 reduction on the rack and pinion. I have found that the friction is not perfect even after adjustment and during the course of an evening there is always a small amount of slippage within the mechanism. This is compounded when using the autofocus in Sharpcap with multi star FWHM in my case as the calculated position may not be correct when Sharpcap moves to it due to this random slippage.

To create a more absolute repeatable process for the focusing I have made a ball screw focuser drive, this is controlled by my original sesto senso focus motor. The ball screw + nut is from SKF and is 8mm with a 2.5mm pitch, the sesto is 3200 steps per revolution which gives me a precision of 0.78 microns per step and as it is a ball screw there is zero backlash in the actual nut. I have removed the rack that the original pinion ran on and replaced this with a tighter tolerance PTFE strip to guide the focuser tube and made a PTFE block to apply pressure to this to keep the tube aligned/ slightly tensioned.

After recalibrating the focuser movement range for the sesto I have ran movement tests and the repeatability of positioning is excellent from tests done with digital dial gauges on the focuser tube + camera / filter wheel. There is still a small amount of backlash in the focuser tube due to small amounts of play of the original focuser tube PTFE bearing blocks + bearings but this backlash is constant so easily calibrated out by always finishing focuser movement in the same direction repeatability when approaching from the same direction is 1 - 2 microns so perfectly adequate.

I machined the ball screw so that the shaft runs through a bearing block which the sesto clamps to between the sesto shaft and the ball screw shaft I have a flexible coupling to deal with any slight misalignment. The actual bearing block onto which the sesto clamps has a needle bearing + a thrust bearing at each end to guarantee the ball screw runs true and that there is no backlash at the sesto connection. The brass thumb wheel allows manual movement when the sesto is not powered

Tonight we tried it for the first time on actual stars using the autofocus routine and I had a nice focus curve from out of focus through focus and back out of focus and the move to best position command worked flawlessly so I will now be using this method of focusing in future, and also plan to use it to work out the offset positions between filters and also assess thermal movement in relation to atmospheric temperature change to get an idea of refocusing intervals.

I have included an image below + a video on flicker of it moving if you are really bored :D
Ballscrew Focuser_proc.jpg
Ballscrew Focuser_proc.jpg (193.82 KiB) Viewed 533 times
Video of it in action sorry for the pregnant pause between moving out to moving in but I was holding the phone in one hand whilst trying to operate the laptop balanced on my knee with the other.

ImageBallscrew Focuser by Nick Davis, on Flickr

Just need some clear skies to try it in anger doing an imaging session to see how it performs.


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