Giant Fork Mount?

[ballaratdragons (Ken)]

Hi all,

I have been lucky enough to find plans etc. to build a GEM with motor drives for around $100 - $200 (cost of buiding it, not the cost of the plans).

Does anyone know where I can find plans to build an Equatorial Fork mount with motor drives. I tried googling, but no luck.

I want to compare the two and see which one will be easier for me to build and/or is more practical. It will be for Astrophotography.

Whichever mount I end up building it will be permanently housed and it must be able to carry my GS 12".

Thanks.

[RAJAH235]

Hi Ken. You may get some ideas from Mr. Arnett. You probably have this site, but......
Lotsa links.
http://obs.nineplanets.org/obs/obslist.html
Couldn't find anything re; forks tho. Plenty about knives.
Happy hunting. L.

[mch62 (Mark)]

What's your interpretation of giant?


This is my own Newtonian friction drive fork mount.

It will take up to an 20" although it will be only taking my 12.5" f6 at the moment.

The fork is made from 19mm ply and several layers of fibre glass with a steel frame in the base.
It is very very stiff and easily takes my weight of 110kgs with minimal flexure.

The RA shaft is 90mm x 500mm long with 2 pillow block bearings and the DEC are 1.5" x 2 with 4 flange mounted bearings.

I am using for the RA a 650mm friction roller drive discs made from 3/4" plate steel with SS rims .
The Dec is the same size but made from 30mm ply (to keep fork weight down) with a SS rim .

The drive gear is Mel Bartel's Go-To servo system and has been used on 40" scopes.

http://www.bbastrodesigns.com/#limitedwarranty

This friction drive system is what is used in most research observatories as large gears are to expensive.

Ever priced getting a couple of 12" gears into OZ.

It offers 0 PE and back lash but the controller has electronic correction for this for any type of mount and can be used ALT Az for dobs or any mount design.

I hope to have it all up and running with in the next month.

Could all be down sizeed for a 12" scope only.

Mark

[Orion]

Hey Mark, Thats coming along realy well the last pic you showed of it you just finnished the construction with plywood, is that undercoat you have on it now or the finished paint job?
Is this mount your own design or did you get the plans from somewere?

[mch62 (Mark)]

That's a white primer for painting fibre glass and difficult surfaces.
I like the rough chop mat look of the fibre glass, it hides a few blemishes in the ply wood.
I have started painting it a grey hammer tone finish which gives it that metallic look .

The design is my own but I took about 12 months of procrastination and research on the web of others done a similar way with ply not necessarily fibre glass coated , metal and aluminum .

I had the resources to do it in metal myself but was not confident of getting it square.
I have a machine shop that could have done it but the expense was going to be over my budget .
I found working with the ply easy and cheap to get every thing square my self.
I used fibre glass to ensure stiffness and seal the ply to stop warpage ect.
I found designs done in ply with out fibre glass so thought this is going to make sure of it's stiffness.

This is one thing I found people made comments about a fork mount , is getting the forks stiff so as not to get the tuning fork problem inherent in too light a fork.

I had a SCT a while back which had problems and was in pieces for repairs and the one thing that stuck in my mind was how much the fork arms vibrated and flexed with out a scope attached. Quite scarry! But that's the price you pay for portability, somthing i am not worried about.

The actual shape and dimensions are to accommodate a larger scope for the future .
I used as large a triangular shape in the fork arms as a 1200x1200 piece of ply will allow , and large enough to allow full swing of the scope.
I wanted it made from one single piece for the front and one for the back.

It is made from basically 2 1200x1200 fork pieces of 19mm ply and is as accurate as my drawing the shape on the ply for cutting.
To that are the various side panels screw and glue together into one piece then coated in 2 layers of chop mate and 2 of 75mm bi-direction tape for all the joins.

The base has a 75x35 rectangular steel tube H frame fibre glassed to the bottom panel on the inside.

This gives a non compressible inner surface for the 8 drive disc bolts to seat against and make the base rigid.
The 650mm RA drive disc is directly mounted to the fork base and you can see the bulge on the base where it will sit.

I will do a full article with lotsa pics of the mount in stages when it's finished.

Mark

[ballaratdragons (Ken)]

Thanks Mark.

The fork idea does sound more complicated compared to a GEM. I made my first GEM but it was only for a 4.25" reflector, and had no tracking drive. I will spend a bit of time listening and reading before I decide. I am not in a hurry as I have to build my Observatory first. I will collect parts for the mount (whichever one I decide on) as I'm building the ob.

[gaa_ian (Ian)]

Excellent fork mount Mark, I look forward to seeing it soon!
So I guess the 20" scope will be the next project
I hope the weather in Brisbane is better that here !
It is Raining as I leave Gove this morning

[Orion]

By the effort you have gone to Mark it sounds like you won't have any vibration problems with the mount. I cant wait to see the finished product and hopfully a write up about it's construction. All the best.

[mch62 (Mark)]

I wouldn't say that a fork is necessarily more complicated than a GEM , I just like to over engineer and get it right the first time with out compromises.

I did consider a GEM for a while but soon realised that when you start talking larger scopes a friction drive is by far cheaper than a worm wheel drive to make.
I also don't like the meridian flip that is required with a GEM.

You can use a friction a drive for a GEM but general rule of thumb is to try to make the friction disc 1.5 - 2x the size of the objective so you can imagine how big and cumbersome disc like that would be on a large GEM scope.

There are ways around it by using a smaller disc but then things get more complicated in getting traction.
A fork lends itself very well to a friction drive with large discs.

You use largish discs for getting a good primary reduction and to allow for more traction torque to be used on the bigger rim.

With a worm wheel set it is common on well constructed mounts to use a gear the size of the objective or a bit smaller depending on scope design , and of coarse a GEM is more suited to some scope designs than others.

So don't discount a fork , you just don't have to go quite as big as mine.

My next scope will more than likely be a 16" f6 as I like the Royce Conical Cross Section mirrors and they only come in 16" size at the moment but am hoping for larger in the future.

Are you listening Rob

Mark

[mch62 (Mark)]

Ian as much as I would like a clear night for viewing we really need some rain out here so if you bring some with you i won't be angry.
Sorry guys.

We can always meet up again on another of you trips down .

[Starkler (Geoff)]

Another alternative that I havent seen discussed on this site is the split ring configuration.

http://www.focuser.com/atm/spltring/stellafane1.jpg

[mch62 (Mark)]

Yep toyed with that one as well.
A very good alternative and relativly cheap.
You do need to get that ring accurate though for photograpy but quite doable.
They lend themselves very well to cheap friction drive using the ring rim and the weight of the scope for the engagment force required . Good for portability as can be seen in the pic.
Seen in big observatoies and JIMS Mobile scopes that use a bent tube for the ring.
Notice the little one in the back ground.

[mch62 (Mark)]

Here are few to get ideas from.
if you need more Ihave collected heaps to get ides from.

[mch62 (Mark)]

light weight fork

[mch62 (Mark)]

getting big