DIY fork mount

[Rusty Strings]

Has anybody 'round these parts ever built an equatorial fork mount. Are they always driven or can they be manual? How do they compare with a GEM?

[callump]

The first telescope i built was an eq-fork mount - sorry no pictures of it - it was in 1983!

It was undriven, though I had plans to add a sector ra drive.

Main advantages of a fork are:

1. all the weight of the telescope is inboard (ie. inside the fork) (actually it is singly inboard).

2. no need for counterweights

3. easier to diy

4. don't require the GEM's meridian flip when tracking

For a permanent mount also consider the Yoke mount, for many of the same reasons (but its doubly inboard).

/callump

[Rusty Strings]

Thanks for that. What does inboard mean?

[callump]

Thanks for that. What does inboard mean?

It means that the weight is taken by the bearings inside the structure, ie. inside the fork. In a GEM the it's outboard, meaning that the weight is on the outside of the bearings.

In a fork its singly inboard - the dec axis is inboard, and the ra axis is outboard.

In a yoke its doubly inboard - both of the axes are supported at each end by bearings.

Here is quite a good page that shows some pics of different mounts.

Telescope Mountings

/callump

[Merlin66]

I've built and used telescopes from 10' to 29" and they all were mounted on fork mountings!

Infact in 30 years of doing these things, the HEQ5/ NEQ6 mountings I now use are the first GEM mounts I've used!!

IMHO the fork mounts have a lot going for them.

Ken

[Rusty Strings]

Thanks for the info chaps.

My obsy builds begins this year and I'm considering my options reguarding the mount as I may upgrade to a 12" newt and I not sure wether I want the necessary EQ6.

Hey Merlin66 you DO mean 10" don't you?

[Merlin66]

Typo..10" it was.

[FraserClarke]

I have used a 12-foot fork mounted telescope... The Shane 3-meter Telescope: non-technical description

Forks have a lot going for them.

+ No pier flip (for the best conditions, you should be observing objects at the meridian, so this is a significant disadvantage of GEMs in my option)

+ no need for counter weights (I'd wager a well build fork could be more compact than a GEM)

+ Similarly, smaller sweep of the OTA than a GEM (as it is on the polar axis)

+ nice space under the mirror for instruments (in a cassegrain)

+ no/little pointing restrictions (problem with Yokes generally)

+ easy path to bring out a coude beam (only important for pro telescopes)

+ large surfaces for bearing mounts...

lots of positives. You just have to make sure you make the forks stiff enough not flex/vibrate with the relatively long lever arm.

If you're building a Newtonian, you might also want to consider a split-ring mount... Just to through another spanner into the works

PS -- haven't built one myself, but would definitely consider it.

PPS -- the obvious question from this is why are GEMs so popular? The big disadvantage of forks (and most mounts, to be honest) is that they basically have to be custom built to the telescope OTA -- and that is an absolute killer for commercial manufacture. GEMs are much more adaptable, and effectively independent of the telescope type.

[Peter Drew]

It depends on what type of scope is to be mounted. The compact SCT's and Maks are well suited to forks but it would be structurally challenging to construct a rigid fork for a long focus refractor or Newtonian. Alt-az forks commonly found at present are fine but as soon as you tip them equatorially it's a different matter. GEMs seem to be inherently more stable pound for pound (and £ for £). The one thing that bothers me about mounting SCT's on GEMs is that when you do the zenith "flip", the tube orientation turns through 180 degrees. From my experiences when I had Meade/Celestron agencies and needed to check the collimation prior to collection by the customer, I found an apparent perfect collimation and star image on the bench collimator was anything but if the tube was rotated. Alt-az mounts preserve the same set orientation. An important aspect of fork design is to have as much bearing surface between the fork and the RA axis. If you just bolt a fork to the top of the shaft you will get vibration.

[Merlin66]

DIY forks can be made very rigid and stable.

I've used everthing from 30mm MDF through to 25mm steel plate and schedule #40 steam pipe.

The last one I did for an 18" f4.5 was a "polar disk" fork design; the edge of the disk being driven from a 35mm hardened roller using the Bartel stepper motor circuits and DSC 4000 tick encoders for positional feedback.

Ken

[Moonshane]

I have used a 12-foot fork mounted telescope... The Shane 3-meter Telescope: non-technical description

Forks have a lot going for them.

If only I was THAT Shane.

[Rusty Strings]

This is all interesting stuff, thanks chaps. Does the fork mount onto the RA shaft via a bearing or is the bearing inside the base?

[Peter Drew]

Give some serious thought to using a Polar disc arrangement as suggested by Ken. This item if large enough and fitted to the underside of the fork will make it very rigid, you need only three relatively inexpensive bearings for it to run on, two for the disc and one for the end of the polar axis and they will give the same mechanical advantage as a very large single expensive bearing. This design also lends itself to friction drive potential

[Rusty Strings]

Any ideas where I could see a diagram/photo of such a beast

[callump]

Try

Newton Telescope

and

Picasa Web Albums - Richard - 22" reflector

There are a lot of examples in old issues of Sky & Telescope, but i don't have specific issues in mind at the moment.

Hope these help.

Callum

[DIYASTRO]

John,

Always an option of utilizing Meade / Celestron forks which is what I did with a multi-refractor project.

Hope to be building a larger version from 10" LX50 soon. See my post in this section..Multi refractor.. page 2 for pics.

Forks have some good advantages especially the reduced contorsions required to see through viewfinders in camera on EQ mount or German equatorials.

Boyd

[Rusty Strings]

Once again chaps, thanks for the input. This is giving me more designs options. Just to clarify, the polar disk is essentially a wedge mounted dob with two additional bearings on the lower edge to stablise the whole thing, yes?

[FraserClarke]

Once again chaps, thanks for the input. This is giving me more designs options. Just to clarify, the polar disk is essentially a wedge mounted dob with two additional bearings on the lower edge to stablise the whole thing, yes?

I guess you could look at it like that I suppose. The bearings are different though; the altitude bearings in a dob wouldn't work as declination bearings in an equatorial, as they wouldn't handle the varying gravity vector.

The difference between a polar disc and a classical fork mount is in the support to the RA axis. The polar disc is a much larger bearing surface, and is more amenable to friction driving. The classical fork has an RA shaft in bearings and is usually driven by a worm gear on the shaft itself.

Lots of mount designs have a lot of similarities -- there are only two axes to move after-all

[LeoLion]

Hi, Have a look at Ron Arbours scope and mount here for a large polar disk/fork

Ron Arbour - Supernova hunter and astrophotographer . Hope this helps.

Len E

[sbooder]

My Uncle has built an eq fork mount for his 16" Newt, I am going to put up some photos for him in the not too distant future.

[Rusty Strings]

Simon

Be good to see pics of your uncle's mount.

[philj]

Hi John, back in the 70s I built a mount for a 6" Cass. Id seen the new fangled Celestrons and really liked the idea of a fork, all the advantages have been raised by the other chaps. Heres a pic of it in its 1st early form. This had so many revamps, then I got the scope and mount just right in the early 80s and sold it, typical:D

Construction was ali channel and plate to form box sections, light rigid and strong. Dec bearings were the old Picador Lathe bearings you could but from Exchange & Mart, remember them. The RA shaft was Picador again but it was their captive shaft in cast iron body with ball races at each end, lovely bit of kit that.

The scope was all home made except optics which were Fullerscopes Dall Kirkham.

In its final guize I made it into a Cass / Newt using all Fullerscopes scope kit but kept it on the mount a super job it was too.

The RA motor was just a synchronous motor to drive the RA via a worm and wheel donated by a dead Tasco. Dec drive was a tangent arm driven by a mechano geared electric motor. No goto back then you know

Ahhhh those were the days:D

Philj

PS its pictured sitting on a stool in my bedroom, I had a 6" pier in an observatory for it as well as a home made monster tripod.

[Rusty Strings]

Hi Phil

That looks very nice, if I were you I'd be regretting losing that one.

Just to get some idea of scale is that the cass/newt on there and if so, how big is it.

Are there any limitations with the fork design with say, a 12" F5 newt (next scope) any inaccessible sky for example?

[Merlin66]

I used a 12" f5 on a polar disk fork mounting for many years doing SuperNova searchs. Never had any problems with sky coverage; the stability and rigidity were excellent.

[philj]

Hi John

The version shown is the 6" Cassegrain.

The mount at this early stage did not have long enough arms for me to reach the pole area because the focuser was a long rack and pinion jobby and it fouled on the crotch of the fork. A later mod extended the arms to clear the focuser for use in Newt mode and it worked well.

With a newt only there should be no problems if your fork arms are long enough, do a search on the net, some of the Beacon Hill mounts are forks and are made specifically for big newts.

Philj