My 400mm fork mounted reflector

[tilbrook@rbe.ne]

Hi,
Thought I'd share some ATM.
This is my 400mm F/5.5 fork mounted reflector. I made this scope nearly 10 years ago. The fork is made from 20mm thick box sectioned plywood, and the R.A bearing is a trailer stuble axle bearing kit. I used the axle becuase it's well machined, strong and not expensive.
There's no use reinventing the wheel when something is available off the shelf.
The plywood fork has been very succesful, no sag or warping and vibrations settle out very quickly. At the top end of the scope I made a rotaing head for ease of observing position. The only downside, is it's difficult to collimate.
The drive system is a direct drive onto the R.A. wheel wtih a cable release when moving to other parts of the sky. I recently changed the counter balancing, with a rotaing head it's been a nightmare to balance. It's not bad now, I only have to move one sliding weight on the bottom of the mirror box.
It would be interesting to hear from others with similar designs.
Cheers,

Justin Tilbrook

[Rod]

Hi Justin,

What a great looking mount! I've always liked fork mounts and would love to make one. The metalwork in most designs is what puts me off so a Plywood fork is very appealing. How much does it weigh? Can you tell us anything about how the plywood fork is constructed?

Thanks,

Rod

[tilbrook@rbe.ne]

Hi, Rod.

I'll take some more photo's and supply more information and post it on friday.

As far as the weight, I don't know. I use my ride on lawn mower to shift up to my observatory. You may notice in the images it has a small draw bar for this purpose.

See image of my observatory.

Cheers,

Justin.

[tilbrook@rbe.ne]

Hi, Rod.
Here's some more detail on the fork and drive for the 400mm reflector, and a basic diagam
which you may be able to scale for your scope.
Constuction of the fork was done in 20mm thick plywood, in butt joined box section. Once all sections were cut out I glued and screwed the fork together, making sure to pre drill holes for the screws which are 90mm apart.
Although I did some basic plans, to work off, I constructed a 3D card board mock up of the fork to make sure all the clearances were right. As you may notice this is very compact mount, to keep a low centre of grvaity, so clearances are tight. Better to make it out of card board first than make mistake you can't undo.
When complete make sure you undercoat and top coat thoroughly.
Of course the base is made from metal, I used 50 x50 x6mm square tube for most of it. It needs to be rigid to stop twisting. The R.A bearing as described before is a trailer stub axle and bearings. This can take a 1/2 ton of weight, more than you probably need. But I find it's better to over engineer in this department.
The drive system is 240v A/C synchronise motor, Crouzet 3.5w with a One revolution per hour gearbox. I purchased this from ITC products Dandenong Victoria. It's a little lengthy to describe the construction, but the image I've attached will show you.
One last thing, because of the compact design, the R.A. wheel is quite small so this makes it a challenge to get drive, the R.A.wheel has stiff dimpled rubber on it. The drive motor shaft is grooved along it's length, when the two are meshed together with the use of a compression spring it drives well. The trick is to set the compression on the spring so you get drive, but not enough to break the gearbox if you accidently bump it.
It should be noted, this drive is for visual work on the 400mm, because of the torque created by a larger longer scope along the R.A. axis. I have the same setup on my 200mm observatory scope, and it works well enough for CCD imaging. This is because of shorter length, less weight which means less torque.
Hope this covers most things.
Cheers,
Justin.

[alistairsam]

Hi Justin,

that looks pretty impressive, very happy to see a fork mount, I'm a big fan.
i've been building one myself, although at this stage, its only for an 8".
Hope you won't mind if I share my build so far.

Since i'm into electronics as well, am going all the way and am incorporating autoguiding for astrophotography, optical encoders on both axis, 32bit microstepping, friction clutch, wireless control and so on.
I got the mount working a year ago, but have been constantly improving its stiffness, electronics, drive, clutches and so on, so still not finished.
am redoing pretty much everything.

I initially started off with a 20mm RA steel shaft on standard self aligning bearings, but then have just now moved to 50mm bearings and a delrin shaft as that would allow more contact with the fork RA base and provide better stiffness.
Delrin is a plastic derivative, very very hard but very easy to machine, and very stiff, very light, temperature tolerant and quite cheap. available in any bearing shop.
I'm using timing pulleys, so i got the shaft machined down to 25mm at one end in a machine shop.
my fork is made from a pine / ply sandwich, the ply cladding adds to the stiffness and stops the arms from flexing.

i love DIY and try and make use of stuff around me, so my clutch plates are from hard disk platters and ikea cutting boards.
i just bought a geared stepper, so should have it done in a month with autoguiding. I'm using picaxe microcontrollers to emulate the lx200 protocol so will have full goto from CDC or stellarium scope. i've partially tested this.
Will be mounting it on a custom tripod with a latitude adjustment.

I managed to take an image of eta carina just as a test with no guiding on the 20mm shaft a few months ago, but hope to improve it. need an MPCC as the scope is an F4.
I'm also going wireless with a DSC readout on a wireless hand controller. so very excited if I can say so myself.
will start a thread once I complete it.
its been a lot of fun and learning but does need a lot of patience and perseverance.

[tilbrook@rbe.ne]

Hi,
I knew there had to be someone else making a plywood fork!
I posted my images on several other forums, and was surprised no others replied with similar setups.
I love what you've made, and wish I had your knowledge of electronics. Your setup sounds like it's going to be very efficient.
I too had a clutch drive on the 400mm. The same motor was used with a 10 to 1 reduction box onto the friction cluch, which in turn drove a chain onto a wheel behind the R.A wheel.
The clutch worked ok, but the reduction box wasn't up the job. The drive would take a while to take up, the torsion forces would build up in the reduction box making the scope spring forward, and so the cycle would go on.
Everything worked well in bench tests, but I failed to take into account the torque forces involved with a long heavy scope, even when balanced well.
I'ts certainly one step forward, two steps back when designing and building scopes like these!
My 200mm obsrvatory scope has the same friction drive as the 400mm, and works well for CCD imaging, less forces involved.
Look forward to your completed project.
Thanks,
Justin.

[GrahamL]

Great stuff Guys ,Justin I think you need another scope , theres just not enough of them there ;):thumbsup:

[alistairsam]

Hi Justin,

would you have any pics of your 200mm obs scope?
can you also post some ccd pics you've taken with this scope? would love to see them.
had a few questions.
with the ac synchronous motor for your 400mm scope, how do you vary the tracking speed as the rate varies a bit depending on where the object is, and the ac motors use the mains freq that cannot be varied?

are you using stepper motors for your obs mount, do you have autoguiding on that, how long can you track without stars trailing and how do you vary speed?
can you also post some pics of the dec drive and clutch?
those are two things I've still not gotten further with.
would also be good to see your rotating upper cage.

i found balancing tricky as well. what i use is a few of the two hole clamps from bunnings, on an elastic band at the top and bottom of the scope.
since its on an elastic, i can easily rotate it to where i want, and i can add or remove clamps easily, just takes a while to get your head around how the fork behaves as its sensitive to balancing errors.
pic attached was my first mount with an F6 where you can see the adjustable weights. have since moved to an F4 and am moving to a tripod design to make it stiffer.

I've also been searching for others with ply forks, found a few of them but not many on this forum.
Ken has an 18.5" F4.5 as he's explained on this thread
http://www.iceinspace.com.au/forum/showthread.php?t=71519

do you attend any camps? I go to the snake valley camp near ballarat. would be good to see these DIY mounts in person.

[tilbrook@rbe.ne]

Hi,

I'll put together some images and info on the 200mm observatory scope, and post it sunday.

I've actually been out working on this scope today, general maintenance and installing a sky vector ( frustrating ). One of the communication cables had a break in it which gave encoder errors. Took a couple of hours to fix this.

Hope to get out tonight and do some observing and imaging.

Cheers,

Justin.