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Hi all,

Im after a bit of advice about upgrading my rack & pinion focuser to a 2 speed crayford one.

I am thinking of getting a Revelation dual speed for my 6" Orion Dob which at the moment has the standard plastic 1.25" only focuser fitted. The standard one also attaches with 3 bolts, not 4.

Is it going to be possible do you think or not worth the hassle? I think I can buy a focuser base from TS Services to fit them together but Im a bit worried about cutting the OTA and getting the 2" drawtube centered as I dont have existing pre drilled holes to use.

Thanks in advance,

Rob.

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It's worth it to get a bigger FOV. Say you use a 32mm Plossl (if I am not mistaken your scope is f8 = fl 1,200), this yields 37.5 x magnification, 4mm exit pupil and 1.33 true field of view.

Suppose you shift to 2" focuser and use a TS 40mm 68 degrees AFOV. Suddenly you have 30x magnification, 5mm exit pupil (nice and more bright) and most importantly 2.25 degrees true field of view.

Your scope suddenly shows a massively larger field of view and at f8, it should handle the TS 49mm eyepiece really well. That's the benefit of using 2" eyepieces, a larger true field of view.

Word of warning: measure like 10 times and cut once! Best of luck with this project.

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The base of the Revelation DS focuser can be removed by loosening off some little grub screws to give you the dimension of the enlarged hole you will need.

You will need to remove the secondary spider from the tube - but remember to mark down the orientation and measure the length of each vane, they should be equal but there may be a mm or 2 difference - this makes it easier with collimation as you can get the secondary as close to where it was before.

The base can be placed centrally over the existing hole then the new hole size can be marked ready to cut. If you are not confident with placing by eye, tape a sheet of paper over the old hole then cut out the existing hole size in the paper with a craft knife. You can then measure and mark with a pencil from the edge of the old hole to the internal size of the new focuser base to get it central. Once you are happy with where the new hole will be, the tricky part is cutting the tube. Try to position the tube with the primary mirror HIGHER than the focuser end, that way any metal cuttings wont fall onto the mirror, better still if you could block off the tube halfway to keep the primary clean.

I dont know what tools you have but I'd probably drill LOTS of 1mm holes just inside the line in a circle, then a larger hole big enough to get a junior hacksaw blade into it then carefully saw between the small holes. Once the larger hole is cut out, use a round/ half round file to smooth off the rough edges. Now you can mark the base fixing holes, drill the correct size and attach the base to the tube. 

The spider can then be refitted and the focuser drawtube assembly reattached to the base with the little grubscrews. If you get your measurements right, the collimation wont be TOO far off hopefully.

Hope this helps :)

L.

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I was always sceptical about dual speed / Crayford focussers until I bought a scope with one on!!! Its a delight to use, and as above this version allows the use of 2" ep's. This upgrade is worth doing.

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Absolutely nothing wrong with what you propose. I would question the fact that moving from a R&P focuser to a Crayford is an upgrade. It is an opinion that lots of people hold. I think a R & P focuser is generally the better of the two - more positive tuning, takes the weight of FW and cameras better etc.

Ian

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Absolutely nothing wrong with what you propose. I would question the fact that moving from a R&P focuser to a Crayford is an upgrade. It is an opinion that lots of people hold. I think a R & P focuser is generally the better of the two - more positive tuning, takes the weight of FW and cameras better etc.

Ian

This is true, however

Upgrading a RnP from a Pre crayford era is what has given this impression, as quiet often cheaper or old RnP focusers are somewhat naff, The crayford era provided a better focuser overall but without the reliability and weight bearing capacity that a RnP has, now RnPs in the budget end of focusers have caught up with the game and are better than crayfords on the whole, bar crayfords like the 2" feathertouch range as they have extra bars acting as tracks which solve any sag issues and also make it far more robust than the standard designs. (at a price of course)

Overall RnP is the better choice how its got to where it needs to be.

Edited by Earl

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