Why no generic fork mounts?

[ollypenrice]

I'm no fan of the fork equatorial-wedge often seen on SCTs but I was thinking about big imaging Newts. A fork mount designed to take, say, 8 to 12 inch imaging Newts, might well find a market in observatory based systems, might it not? Beacon hill used to make such things. A large imaging Newt appeals to me but not on a GEM.

The whole assembly just carves out too great a volume of space.

Olly

[Merlin66]

Very good question!

Until I bought the HE5pro GEM a few years ago ALL my previous mounts had been fork mounts.

I had an excellent 12" f5/f20 Newt on a fork/ disk mounting for many years - great to use and never gave any problems...

[brianb]

A fork mount for a Newtonian either needs long, long arms (potential rigidity issue) or blocks access to the polar region. I'm not sure it's really practical for DSO imaging, though where there is only need for access to the equatorial sky i.e. solar system stuff it should be very good.

What is practical, for a fixed observatory mount, is the English type "yoke" mount - true this is awkward to set up & align, needing two pillars, but the rigidity is superb, you don't need a RA counterweight & like a fork mount, there's no meridian flip. I know someone who had a home made yoke mount using old wooden railway sleepers to support a heavy 16" Newtonian tube, it worked very well indeed.

[harry page]

Hi

Here's mine big brute of a thing , but it don't move

http://www.harrysastroshed.com/telescope.html

Beacon hill still make them

I love it can stick anything I like on it

Harry

[Merlin66]

Don't JMI make a split ring Porter type mount???

[SteveP]

Yes, it's a bit of a surprise. I don't know whether the cost of manufacturing makes GEMs more affordable.

But if you really want one, have a look at Mathis Instruments. They make both GEMs and fork mounts ... particularly for larger instruments!

Steve

[ollypenrice]

A fork mount for a Newtonian either needs long, long arms (potential rigidity issue) or blocks access to the polar region. I'm not sure it's really practical for DSO imaging, though where there is only need for access to the equatorial sky i.e. solar system stuff it should be very good.

What is practical, for a fixed observatory mount, is the English type "yoke" mount - true this is awkward to set up & align, needing two pillars, but the rigidity is superb, you don't need a RA counterweight & like a fork mount, there's no meridian flip. I know someone who had a home made yoke mount using old wooden railway sleepers to support a heavy 16" Newtonian tube, it worked very well indeed.

Surely it's the English mount that can't get to the poles? (I had a retired professional here who had used one all his life and he loved the rigidity. Since he was into the Zodiacal Light the poles were no loss.)

Olly

[Merlin66]

Yes you're correct Olly, that's why the 200" Hale has a Horseshoe design.

To answer Brian's comment on the length of the fork arms... you can re-balance the OTA to move the dec axis closer to the main mirror. this reduces the fork length. You can also have a shorter off set fork (with a secondary balance weight) which is even more stable and probably more portable.

I've also seen some very nice designs for portable Porter ring mounts which collapse "flat pack" for moving around.

[brianb]

Surely it's the English mount that can't get to the poles?

The English yoke mount can't get right to the pole but in my experience they usually get a lot closer than long tubes on fork mounts.

[FraserClarke]

Because a GEM is entirely OTA independent (other than mass), therefore practical to market separately from the OTA. A fork has to be tuned to the OTA diameter/length, so it's a lot less practical to sell the mount separately from the OTA.

Brian, I'd suggest that if a telescope on a fork can't get to the pole, then it has been designed wrong... Of course it may be easier to design a yoke than a fork with polar access.

[martin_h]

Seen these?

RCOS: Sidereal and Tracking Mounts

[barkis]

The Border Astronomical Society's 16"f6 Newtonian is mounted in a fork mount Olly. The forks are positioned on a 32" diameter steel disc. The forks are solid steel too, and each is surmounted with a 2" Taper roller bearing to support the declination stub axles that are attached to Aluminium side plates of the cradle that carries the OTA. Fillets are welded to the forks, and the disc for added strength.

The cradle allows the OTA to be rotated, to allow access to the focuser in more covenient positions when pointing to areas of sky that demand it..

The polar disc has a 3 inch diameter shaft, which also has a 10 inch disc near the end of the shaft. This disc carries struts radially up to the polar disc, to ensure stability and resist flexure.

The main shaft sit in a self aligning bearing bolted to an enclosure fixed to the concrete pier that the whole assembly rests on.

The steel framework which supports this monster, has adjustable feet for levelling.

The polar disc periphery rests on two taper bearing set on a 60 degree segment of the disc. the bearing housings are adjustable, both to raise the altitude of the disc, and the angle to allow full contact with the discs face. When accurately aligned in all planes,

and balanced, the fully laden mount can be moved with little effort.

The scope has A friction drive , on the RA, and also on the Declination. I know it is still in use, but It is 8 years since I left the Society. The domed observatory and the mount, was built by Students at the local Technical college.

The drive system was quite adequate for observational use, but deep sky Imaging was out of the question.

The 16 Inch mirror suffered an accident whilst being transported by a carrier, and piece broke off the edge, and encloached into the surface. Fortunately, the Insurance company paid up, and the mirror was re ground, polished and figured by the late David Sinden.

It gets used a lot on open nights, and special times such as National Astronomy Weeks, when the public get a chance to look at the sights through the scope.

I need to pay a return visit, to see what might have changed, if anything.

Ron.

[ollypenrice]

That sounds magnificent, Ron, though perhaps you have answered my question on why generic ones are not widely available!!

Dozing in bed this morning I did think, though, that Brian might have a point about the English mount. The design is good from the POV of suiting a wide range of latitudes and it coould be engineered more lightly and yet cope with many scope sizes. There is not too much to image around the NCP so, hmmm, quite a nice idea.

Olly

[beetlejuice]

How about this? It's the 20" at Kielder observatory - a split ring design based on two rolled steel tubes. It was custom built by a British company that I understand is no longer around.

I think the reason why there are no 'generic' fork mounts is that the fork bit has to be individually tailored to suit the OTA, difficult for a volume manufacturer, such as Synta, as there are so many variations. So the production volumes would be less and therefore costs would be much higher. You can plonk anything on a GEM relatively easily within reason.

[ollypenrice]

How about this? It's the 20" at Kielder observatory - a split ring design based on two rolled steel tubes. It was custom built by a British company that I understand is no longer around.

I think the reason why there are no 'generic' fork mounts is that the fork bit has to be individually tailored to suit the OTA, difficult for a volume manufacturer, such as Synta, as there are so many variations. So the production volumes would be less and therefore costs would be much higher. You can plonk anything on a GEM relatively easily within reason.

I remember those scopes. I saw one at AstroFest on the Pulsar stand when Gary Walker was still involved with them. They looked really convincing. Gary was a great bloke but I think the scope was designed by a friend of his. It is a really nice idea but it was not imaging accurate, I thought? Maybe I'm wrong. However, JMI did a good imaging horseshoe at a price. Maybe they still do?

Olly

[beetlejuice]

I remember those scopes. I saw one at AstroFest on the Pulsar stand when Gary Walker was still involved with them. They looked really convincing. Gary was a great bloke but I think the scope was designed by a friend of his. It is a really nice idea but it was not imaging accurate, I thought? Maybe I'm wrong. However, JMI did a good imaging horseshoe at a price. Maybe they still do?

Olly

This one was the prototype apparently. Kielder mainly use it in undriven form as a big dob. For a 20" it's very compact. I took lots of photos of it if anyone is interested in cloning it.

[ollypenrice]

Beyond my skills but what a neat idea.

BTW I love your Ben Franklin quote! Hope that doesn't put me in the gun lobby...

Olly

[beetlejuice]

Beyond my skills but what a neat idea.

BTW I love your Ben Franklin quote! Hope that doesn't put me in the gun lobby...

Olly

No, but it does me as target shooting is my other passion in life. Has the advantage of not requiring clear skies, which is why I do a lot more shooting than astronomy. It has at least one thing in common though, it's not cheap!

[Rusty Strings]

I'm still looking at mount options for a 12" build, some piccies of the Kielder scope would be very useful.

[beetlejuice]

I'm still looking at mount options for a 12" build, some piccies of the Kielder scope would be very useful.

Sure. They are on my other computer which is in bits at the moment. I'll post them when I've got it back together tomorrow.