Revelation/Skywatcher SCT dual speed Crayford motor drive conversion

[JamesF]

Last year I modded my PST, replacing the "black box" with a slightly modded Revelation dual-speed Crayford that was originally intended for SCTs.  I've been meaning to do a motor drive conversion for some time and had bought a second-hand Skywatcher motor kit to use but hadn't really decided how I was going to go about it.  Then last week ChrisH posted some photos of a conversion he'd done so I shamelessly robbed a few of his ideas and finished it today (sort of).

The first obstacle is that neither of the brackets supplied with the motor drive kit actually fit the focuser and don't really lend themselves to just having a few more holes drilled in them, so I started by making my own bracket from a piece of 2mm steel plate:

The four smaller holes matched the mounting holes for the spindle assembly on the focuser, the larger one in the middle gives access to the tensioning adjustment and the notch in the edge leaves room for the locking knob.

The existing screws fixing the spindle assembly weren't long enough to attach the bracket as well, so I bought some stainless 12mm M3 dome-headed hex bolts from that ebay and fitted the bracket to the focuser:

The two remaining holes in the flange I'd threaded to take the standard Skywatcher mounting screws for the motor, so that went on easily:

This is the point where I stole the really clever idea  Chris had his motor driving the fine focus knob using a clutch mechanism that could be released just by turning a knob on the end of the motor spindle.  The Skywatcher motor output shaft clearly wasn't going to be long enough for that, so I turned an extension piece from brass that would fit over the 6mm output shaft and then drop back down to 6mm.  That carries a 24 tooth timing pulley driving a 90mm belt that loops over the fine focus wheel.  The end of the brass shaft is threaded to accept an M6 nut.  I was going to make a brass thumbscrew to screw onto that and clamp the pulley in place, but didn't have enough brass, so temporarily I've used an M6 wingnut instead.  Here's all the bits:

The timing belt and pulley came from motionco.  And here it all is assembled:

The grubscrew holds the brass extension onto the motor output shaft as normal.  The grub screw from the timing pulley has been removed and it is free to spin on the extension shaft.  With the wingnut done up the pulley is clamped in place and turns the fine focus knob when the motor turns.  By releasing the wingnut half a turn or so the pulley spins freely again and allows the focuser to be used manually.

Part of the motivation for doing this now was as a test run for motorising the Revelation dual-speed R&P focuser I have on my C9.25 which is very similar but without the convenient mounting points for the motor bracket.  I think I see a way to do that now though, so as soon as I can get some more brass I'll get on with it.  However, I'm also now having thoughts about motorising the etalon tuner on the PST, perhaps modding the motor drive handset to drive either motor by adding a selector switch, for completely "hands-free" imaging.

James

[knobby]

Looks very neat James , nice job

[oldpink]

nice conversion
my standard focuser isn't very good (SW) so I plan on a few mods inc a dual setup and motor drive

I need something more stable and accurate, the standard one has no grip when you get into the last 12mm it just falls into the tube
maybe try the astronomy shed mod before hand to improve on the standard setup (honing down the flat spot on the focus tube)

as its really annoying as the slip point is close to my prime focus point for my DSLR

[JamesF]

Having a motorised focuser really helps for imaging because it allows the focus to be changed in tiny amounts without any wobbling of the OTA.  I'd definitely recommend it.  I first did it on my 127 Mak a couple of years ago and the improvement was amazing.  I'm half-tempted to go back and add a clutch mechanism to that now.

James

[oldpink]

motorizing is definitely on my list of to do's
esp as I do a lot of remote imaging from my computer room, its the only function I can't do remotely just now
but a motor won't help unless I improve the basic SW focus tube which leaves a lot to be desired in the quality dept.

[allcart]

Neat job James. Its so much easier to just press a button and watch the image come into focus without any vibration. It really comes into its own when remote focusing on the comp screen.

[JamesF]

Neat job James. Its so much easier to just press a button and watch the image come into focus without any vibration. It really comes into its own when remote focusing on the comp screen.

Certainly does.  I'm really looking forward to getting the SCT focuser done as I really miss it for planetary imaging having got so used to it on the Mak.

James

[nightfisher]

Nice job james, i have one of these motor units but cant decide which scope to fit it to, more likely my 150 mak

[JamesF]

Nice job james, i have one of these motor units but cant decide which scope to fit it to, more likely my 150 mak

The thing you have to be aware of when adding them to Maks and SCTs is that because it effectively moves the focal plane further back and requires the primary mirror moving relative to its position without a secondary focuser, the effective focal length of the entire optical train is slightly longer.

Because I use my 127 Mak for full disc solar and lunar imaging with a DSLR and the image only just fits on the sensor, the additional focal length resulting from adding a secondary focuser would mean I'd have to do mosaics.  For that reason I've not added the focuser on the Mak.  Of course I could just get a full frame DSLR.  That would solve the problem too

James

[JamesF]

Clearly I've not been as bad a person as I thought as having got this sorted yesterday today has been relatively clear and I've had a chance to try it out.  It seems to work very nicely.  I might be tempted to try a slightly larger timing pulley, so I'll do that next time I order anything from motionco.  There's enough adjustment in the system that I think I should be OK with the same belt.  Overall I'm very pleased.

James

[rob0971]

Shamelessly copied this idea for my Revelation sct focuser & I am chuffed to bits with it. Got the belt & pulley from Motionco who delivered next day! Fantastic service.

[JamesF]

Excellent

James

[Joseki]

Another happy motionco customer here. I'm doing my 90mm short tube frac in anticipation of the Quarks arrival. I got a aluminium timing belt pulley that fit the shaft of the focuser but I got a setvo motor for the drive - and a compatible timing belt pulley attachment that fits the servo head.

Sent from my GT-I9300 using Tapatalk

[Joseki]

Setvo = servo

Sent from my GT-I9300 using Tapatalk

[ChrisLX200]

I  also performed the mod on a focuser for a friend, it seems to be working just fine. The good thing about the Revelation R&P focuser is there is no slipping even with a fairly heavy camera payload. Autofocus is so wonderful

ChrisH

[twintin]

Chris can I ask what software that is?

Mark

[ChrisLX200]

Chris can I ask what software that is?

Mark

That is Sequence Generator Pro (SGPro), it has a built-in autofocus routine which you see running there but will also use FocusMax if you prefer it.

ChrisH