Revelation superfocus rack and pinion for sct (self inflicted) issue

[Longinthetooth]

Hi everyone,

I have posted here before and was delighted with the help people gave - so here I am again in the hope of sound advice!

I bought the above focuser from TH and was very happy with the quality and price. My problems began when I attempted to motorise it. This, of course, involved disassembly to attach the motor to the single speed side of the focuser. Well - suffice to say my ambitions were somewhat in advance of my abilities! Unable to get the set up working satisfactorily I detached the motor and ......

on reassembling the focuser (without the motor) I was unable to replicate the smooth operation it had when I got it first. I wondered if anyone here had experience of disassembly/reassembly of  said focuser and how to set it up properly.

In my botched attempts so far I have only made things worse! Currently I have broken it down into its component parts (to see if anything was obviously wrong) apart from the dual speed side which I am reluctant to tackle without knowing how to dismantle it, and the fine adjustment knob is moving, but not altering the focus - I saw a tutorial breaking down and reassembling a dual speed crayford

http://www.opticstar.com/Run/Astronomy/Astro-Editorial-Articles-General.asp?p=0_10_19_1_6_200_12

but couldn't find anything specific to my focuser which differs somewhat from this model.

I would greatly appreciate any advice.

John B

[simmo39]

I know this is a different focuser but it might help.

http://polluxchung.com/gso_crayford_fix/

[Longinthetooth]

Thank you Simmo39 - I had seen this - unfortunately there is no (evident) grub screw on the larger of the 2 knobs in my case and the adjustable nut on the inside is a brass collar (I don't know the proper name for this part!)

There could well be a grub screw under the pretty patterned black stuff?

John B

[Longinthetooth]

Yep..... and this is what's inside. But what do I do with it?

I can pack it full with lithium grease (got some). But what next?

John B

[Longinthetooth]

I've packed the ball bearing cavity with lithium grease but would like to adjust the brass collar seen in this photo - I'm assuming it to be analogous to the nut in step 7 of the link simmo39 posted. I've tried some gentle persuasion on it and it won't move either way. It's knurled as though it should move without too much effort but stubbornly refuses to do so. It also has slots 180 degrees apart. To slacken it which way should it turn. Anticlockwise as we look at it?

[JamesF]

I have one of these R&P focusers and intend to fit a motor to it, but without taking it apart.  My plan is to do what Chris has done here: http://stargazerslounge.com/topic/211224-revelation-focuser-minor-disassembly/?p=2256122 but putting the drive belt around the fine focus knob (as Chris said he'd done with his 'frac, I think) rather than replacing it with a second gear.  I'm using the standard Skywatcher motor focuser kit for this.

First of all however, because it's easier to fit a bracket to than the R&P version as much as anything, I'm doing a trial run on the Revelation dual-speed Crayford focuser that I fitted to my PST.  I'm waiting for the gears and belts to arrive at the moment.

So at the moment the situation is that you can reassemble the focuser and the coarse focus knobs work, but the fine focus one spins without turning the shaft at all?

James

[Longinthetooth]

Hi James - and thanks for your reply. Currently the coarse focus works but "lumpily" - I can feel the individual teeth as the tube moves back and forward. If I tension it up any more the barrel won't move at all.

As you turn the fine focus knob the silver part behind the knurled brass collar (in the picture above) rotates freely but does not move the focuser tube unless I take the tension off to such an extent that the tube would slip back under gravity alone.

John B

[ChrisLX200]

I never took my focuser apart but from your description (rough/lumpy and you can "feel the individual teeth") it sounds like the engagement between gear and rack is too tight. You should not be able to feel the teeth at all because the pinion is of helical form. The 10:1 is just an epicyclic gear although I'm not even sure if it uses toothed gears here - could just be ball bearings alone as there isn't much torque required. If the latter then yes, it will slip if there is too much resistance. So get the rack and pinion adjustment correct first (by feel using the coarse knob). I would not have put any extra grease in there because it will likely just increase the tendency to slip if its a friction drive.

ChrisH

[JamesF]

It sounds to me as though something isn't set up right somewhere.  If the fine focus knob worked before and you've not changed it I'd personally be tempted to leave that alone, thinking that the problem must lie elsewhere.  If the coarse focus feels "notchy" I'd suspect that the rack and pinion are too tightly meshed.  Of course now you've had it open that may not be the case

Here's a photo of my own focuser:

I'm assuming the four screws marked in green hold the spindle unit to the focuser body.  I'm really not sure what the one marked in red does.  The one marked in blue I'd assume to be for adjusting the backlash in the rack and pinion.

Have I understood correctly that if you tighten the green screws up fully and leave the blue one reasonably slack then that's when the fine focus knob can work, but the draw tube can slide through the focuser body too easily?

James

[Longinthetooth]

Thanks to ChrisH and James for your replies. Yes indeed James you are correct. The blue is a tension screw.

John B

[JamesF]

Hmmm.  Did you remove the entire spindle unit, or just undo the knobs and slide the spindle out?  If the former I wonder if something isn't properly seated.

James

[Longinthetooth]

Yes James - I took it to bits except for the innards on the dual side - I wondered if turning the knurled brass collar shown in my photo would act as an adjustment (as in the link posted by simmo39) - this would be analogous to the more obvious nut shown in stage 7 of the link.

John B

[JamesF]

If as Chris suggests the fine focus knob uses a friction drive then I'd guess that tightening up that collar would increase the friction and give a more positive drive, but I'm not convinced that's a good idea without getting the coarse drive working properly first as whatever adjustments are necessary to get a smooth and backlash-free (or nearly so) drive there may alter the required adjustment of the fine control.

James

[Longinthetooth]

Thanks for that James - probably best to leave the brass collar alone then - I'll let you know how I get on with it.

JB

[JamesF]

For what it's worth at this point, here's my motor conversion for the Crayford version of this focuser:

http://stargazerslounge.com/topic/212551-revelationskywatcher-sct-dual-speed-crayford-motor-drive-conversion/

I think I've worked out how to fit a bracket for the R&P focuser now, so I shall get on with that as soon as I can.  I need some more brass to make the output shaft extension and locking knob, unless I can find any aluminium I have lying about that will do the job.

James

[Longinthetooth]

Just had a look at your conversion - superb!

JB

[JamesF]

John,

I've just been messing about with my focuser and found that there's a little lateral play in the spindle.  By misaligning the spindle so it was off to one side I could make the mechanism hideously "grabby" and nasty whereas with a 1mm to 1.5mm gap between the inner face of each coarse focus knob and the end face of the spindle housing it runs very smoothly.  Might be worth checking that?

James

[JamesF]

I've decided what I'm going to do about fitting a motor to the R&P focuser, btw.

I've bought some 35mm M3 button-head hex screws to replace the four existing ones that hold the spindle housing to the focuser body.  I'm going to cut some suitable length spacers from aluminium and make up a plate with a flange on the end for holding the motor similar to the mod I've done for the crayford version.  The plate will be fixed on using the 35mm screws through the spacers and into the existing mounting points.  From there on in it should be exactly the same as the crayford.

James

[JamesF]

Just to round this off, here's my motor mod for the R&P focuser:

http://stargazerslounge.com/topic/213068-revelationskywatcher-sct-dual-speed-rp-motor-drive-conversion/

James