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Skywatcher focuser (DS) - Cigarette packet mod


Uranium235

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You know how some great ideas start off on the back of a fag packet? Well, this literally is the back of one :D

I have no pics (as I didnt know if it was going to work), but I'll get it down now while its still fresh.

Problem:

Increasing the tension on Skywatcher DS crayfords for heavy loads (ie: those found on the newts) causes the drawtube to be deflected upwards within the focuser housing. If you have a DS newt that has been tweaked, check for yourself.... is the gap around the drawtube concentric? And if your drawtube is tilted, then what hope do you have of ever getting a perfect field?

Well, I initially counteracted my tilt by adjusting the focuser tilt using the push/pull screws. But I thought it might be worth tackling the source of the problem.

Solution:

Find a way of first increasing the amount of friction available, secondly - reduce play in the focuer without drawtube deflection.

1) Not difficult,  remove the focuser knobs by unscrewing the four corner screw underneath - it should just pop right off.

important:  Those four corner screws have associated rubber spacers - do not lose these! I dropped mine and spent 2 hours looking for them.

2) Remove your drawtube (it will come right out), then check your flat (underside) - if you see tramlines... its not flat. Get a wetstone (or something else thats fine) and sand it down flat... clean it off - thats your friction sorted. You dont want a completely smooth finish... remember, this is a friction based system.

3) Now, time to find a way to apply some downward pressure on the focuser drawtube. Take a look inside the focuser drawtube housing, you will see a plate with four little rollers on it. Remove this plate by unscrewing the two screws on the topside of the drawtube housing. Be careful, you cannot extract this plate via the top end of the tube it has to go out via the secondary (so watch your mirror!).

4) Find a square of thin card, I used the back of a cut up cigarette box - cut it to size so it matches the topside of the plate.

5) Get a gluestick, and stick the card to the plate - give it a moment to set, then poke two holes in the card where the screw holes are.

6) Put the plate back into the focuser and screw it into position (nice & tight).

7) Put the dawtube back in, screw the focuser knobs back on (you may need to back off the tension to start) - dont forget the rubber spacers!  Then re-tune your tension so its nice & grippy.

You will now notice you have *loads* more tension/grip (no need to lock focuser), it feels more solid, and more importantly - you now have a nice concentric gap around your drawtube :)

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:grin: Fag packet - encouraging a bad habit. I smoke a few roll ups as I can't inhale them.

When I read that I thought why not use shim - various thinknesses of either brass,steel,stainless or even plastic these days. Then looked on ebay - ouch price wise much to my amazement but I suspect real suppliers would be cheaper. On the other hand I found ebay 201230882069. I needed shim some time ago so have plenty and it was cheap. Brass shim can be cut with scissors up to maybe 1mm but will bend. Steel can be too but much thinner, B&Q do some fairly cheap metal shears.

:laugh: Never used plastic shim so no warrantee claims but given where it's sold from that pack will very probably be ok.

John

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I did try another idea first of adding runners to the drawtube using gaffer tape - but that didnt work because it deformed quite easily.

So it was one of those moments where you look around the room trying to find something that will do the job for nothing. I should think the card will be ok, its quite well sealed in there and wont be exposed to the elements - it'll probably last longer than the telescope will!

I could have used a metal/brass shim, but that would have cost money and I wanted something I could put in place as a permanent solution using the bits & bobs I had to hand.... all part of the fun :)

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FESAH1BGQTM76TC.jpgThere is always this sort of option - with a

smile on my face of course but they can be made. Many of the designs out there are too complicated. They can be made completely adjustable.

John

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