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daveyfitz
member


Reged: 01/02/13

Loc: Utah
Equatorial Dobsonian new
      #5631858 - 01/19/13 04:53 PM Attachment (117 downloads)

I'm wondering about the feasibility of building an equatorial mount for a dobsonian telescope.

I have an idea, here's a pic that shows it, though I'm uncertain about the drive motor application.

Anybody seen anything like this? It seems like the ideal solution for a tracking dobsonian.

I would just put a guide laser on the dob support arm, aligned vertically, use it to point the axis at polaris, and turn on the motor.


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fmhill
sage


Reged: 07/17/12

Loc: Cape Cod, Massachusetts, USA
Re: Equatorial Dobsonian new [Re: daveyfitz]
      #5631873 - 01/19/13 05:03 PM

There is a fellow, "CamelHat" on astronomyforum.net who has built an interesting Dobsonion mount he calls the barndoor tracker box. He runs it with an Arduino controller and is making images up to 5 minute exposures with it using a 12 inch Dob.

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kfiscus
Carpal Tunnel
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Reged: 07/09/12

Loc: Albert Lea, MN, USA
Re: Equatorial Dobsonian new [Re: fmhill]
      #5632040 - 01/19/13 07:03 PM

It has been done. If you already have a dob, consider an EQ platform. Platforms offer you tracking without having to modify the rocker board.

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cloudmagnet
sage


Reged: 12/04/08

Loc: Georgetown, Texas
Re: Equatorial Dobsonian new [Re: daveyfitz]
      #5632343 - 01/19/13 10:26 PM

Quote:

"I'm wondering about the feasibility of building an equatorial mount for a dobsonian telescope."




I'd like to attempt to offer a few points of clarification regarding the statement above. To begin, there really is no such thing as a "dobsonian" telescope. What is commonly called a "DOB" is really a Newtonian OTA, that is, telescope that is mounted on a type of mount invented by John Dobson. The Dobsonian mount in turn, is really simply one (very popular)form of the Alt/Az mount. Until Mr. Dobson's mount became popular, most Newtonian OTA's were mounted on GEM's- German Equatorial Mounts. Besides the GEM, there are other types of equatorial mounts, in particular, the fork equatorial. This is essentially what is pictured in the OP.

Edited by Jarad (01/19/13 10:57 PM)


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Eddgie
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Reged: 02/01/06

Re: Equatorial Dobsonian new [Re: daveyfitz]
      #5632956 - 01/20/13 10:52 AM

When a Newtonian telescope is mounted on an alt-az mount, it is called a Dobsonian after the person (John Dobson) that popularized the design.

The configuration you show in your drawing is most commonly called a "Polar Fork" mounting.

This kind of mounting has been used for SCTs for decades.

You can do it, but there are some issues.

The most serious is that unless you provide for a tube that rotates, you will very quickly find that the eyepeice will get into positions that will be almost impossible to use.

Another issue is that the bearings will not work properly. The Dob works by distributing the weight evenly over the bearings. In the design you are proposing, the bottom bearings would be "Pinched" because all of the weight would be directed downward. You would need to use some kind of tapered thrust bearing (Like a car axle bearing) to take the asymetric load.

A better option is almost always a GEM.

A GEM can be made to be very low to the ground and this is what you need to make the telescope stable and the eyepiece accessible.

In fact, with a GEM, it can be possible to get the tube lower to the ground than even a Dob mount.

Here is a link that shows a great example... Notice how short the legs are in the picture.

This kind of mounting would make it possible to use even a CG5 for an 8" reflector because the legs are generally the weakest part of the mount.

And you could use simple wood legs made from 2x4s so you don't have to cut up your tripod legs.

And with this kind of arrangment, you don't have to worry about fabricating some kind of drive system.

Picture of Newtonian optimized GEM mount

Edited by Eddgie (01/20/13 10:59 AM)


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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Equatorial Dobsonian new [Re: daveyfitz]
      #5632979 - 01/20/13 11:04 AM Attachment (77 downloads)

About 15 or 20 years ago I became obsessed with the idea of converting a Dobsonian to an equatorial by tipping it over so the azimuth axis points at the pole. I purchased a 10" Dob and built my equatorial mount. At first it was a disappointment. It vibrated for 8 seconds after a thump on the tube. I made the mount stronger and added a drive similar to what you show in your model. Eventually it worked to my satisfaction and I never used the original Dob base again.

Since that time I have built compact equatorial mounts for 14" and 20" scopes. A recent photo of my 14" is shown below. My 20" is shown in the link 20" Equatorial. I am currently rebuilding the 20" out of aluminum to make it lighter and make the entire tube rotate.

I believe you will find it almost mandatory to incorporate a rotating tube, or at least a rotating eyepiece section. I've tried both approaches and would recommend making the entire tube rotate to avoid possible problems with optical alignment. It also looks like your base will be too flimsy and the scope will vibrate more than you want.

Lightweight portable equatorial Newtonians are definitely feasible and I greatly prefer mine over a Dob that is either computer driven or on a platform. They are a lot more challenging to build than a Dob. Be prepared to spend a lot of time tinkering with it.


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TheBigEye
member
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Reged: 12/08/06

Loc: New Mexico, USA
Re: Equatorial Dobsonian new [Re: Dick Jacobson]
      #5665005 - 02/06/13 11:10 AM Attachment (59 downloads)

I'm with you - whether you refer to these scopes as a "Dob" or "Just Another Alt-Az Newtonian reflector," the simplicity of the design and ease of construction is offset by difficulties with tracking, stability in the wind, etc. Eddgie is correct about the great attributes of the GEM in overcoming those difficulties. However, once you get into the 16"+ range, setup and teardown may require an assistant or special apparatus. The finest design of an "equatorially mounted Dob" I've ever seen is JMI's NGT-18 telescope. I used to own one. It's portable, one-man setup, as stable as any alt-az scope, and has true equatorial movement. The one drawback it has is the upper cage rotates to put the eyepiece in comfortable viewing position. As other have stated in this thread, a method of rotating the eyepiece is an essential function with the equatorial mount. I found that to maintain perfect collimation with the NGT-18, I would have to recollimate after rotating the upper tube assembly. Not very much, but enough to annoy. A split-ring mounted scope with a fully rotating tube might be answer. Here's a pic of a JMI-NGT-style split-ring mount (built by someone else). This one is in a permanent installation, but JMI's rendering can be portable or permanently mounted.

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Mittag56
sage


Reged: 07/08/09

Loc: West Amboy, N.Y.
Re: Equatorial Dobsonian new [Re: daveyfitz]
      #5665035 - 02/06/13 11:29 AM

My project for next summer is to take my 48 pound 10" Dob and rebuild it lighter and slim it down some so as to fit my now unused LXD 75 mount....even thinkin about permanently mounting a camera at focus point.....At least i love to dream

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Eddgie
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Reged: 02/01/06

Re: Equatorial Dobsonian new [Re: TheBigEye]
      #5665648 - 02/06/13 04:53 PM

Once you take it off of a Dob base, it is no longer a Dob.

A Dobsonian is named after the man that popularized the design.

But when you take it off of the Dob base and mount it some other way, it is no longer a Dobsonian.

In your case, your scope is a Newtonian on a Horeshoe mount. This is the technical classification on the mount pictured, and the Horsehoe mount is a equitorial mount.


The most common mount is a "German Equitoral Mount" but there are many other equitorial mounts.

The Horsehoe has been used for large reflectors for over 100 years. The Hale telescope is on the same kind of equitorial mount (a Horseshoe mount).

So it is only a Dob if it is on a Dob mount.

If you remove the Dob base and place it on any kid of mount that rotates on a polar axis, it is then an equitorially mounted Newtonian.

There are 6 kinds of equitorial mounting for a Newtonain (or any other kind of telescope):

German equitorial (GEM, most common)

Polar fork (often seen on large reflectors)

English mount (rarely seen, but actually an easy mount to make and offers very low mounting because nothing is under the end of the telescope).

Horeshoe mount, sometimes seen on large reflectors

Cross Axis mount... Again, allows for very low mounting of the tube because nothing goes under the tube

And the equitorial platfrom. Common on Dobs, but rasies the scope even more, meaing more ladder perching.

With the last exception (where the dob base is still used), all of the other mounts chage it from a Dob to a _________ (fill in the blank with one of the first five) mounted Newtonian.


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Eddgie
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Reged: 02/01/06

Re: Equatorial Dobsonian new [Re: Eddgie]
      #5665658 - 02/06/13 04:59 PM

And by the way.. Regarding the previous post, I am surprised that more people don't investigate the Cross Mount and the Yoke designs for amateur use.

These can be very simple to build and offer many advantages over the Horseshoe.

I like the polar fork mount for a large reflector, but to get them strong enough they can get pretty big/bulky.

But again, the Yoke and Cross mount can be super-easy to build and work quite well. The designs are so elemental that simple materials can be used and no complex construction or machining is needed.

Or.. Just buy an Oroin Go-To and be done with it.. LOL.

Edited by Eddgie (02/06/13 05:02 PM)


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cheapersleeper
Carpal Tunnel
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Reged: 01/22/10

Loc: Sachse TX
Re: Equatorial Dobsonian [Re: Eddgie]
      #5665832 - 02/06/13 06:48 PM

If I ever own property to build on, I have always dreamed of a big Newt on a yoke mount dedicated to imaging. Seems like a good match.

B


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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Equatorial Dobsonian new [Re: cheapersleeper]
      #5666698 - 02/07/13 09:28 AM

I see two problems with the split-ring or horseshoe mount. The first is size. On the JMI split-ring scopes, the outer diameter of the ring is twice the mirror diameter and it's hard to see how it could be made smaller. This impairs portability and the ring can get in the way when observing near the zenith. The second problem is that the ring restricts the length of the mirror box. This can make it difficult to balance the scope without using counterweights behind the mirror. Ball scopes have the same problem. The 200-inch Hale telescope gets around this problem by suspending the tube from an additional pair of arms, making it even larger (500 tons of steel, not a problem if you don't have to load it into your vehicle!)

For these reasons, I've chosen to use fork mounts for my big Newtonians, like the 14" pictured above. The outside width of the fork is about 50% greater than the mirror width. Within limits, you can make the fork as long as needed to balance the tube and provide enough range below the equator. They probably don't have as much stability as a split-ring or Dob, but vibration is low enough that it is not a significant problem.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Equatorial Dobsonian new [Re: daveyfitz]
      #5666888 - 02/07/13 11:16 AM

Quote:

I'm wondering about the feasibility of building an equatorial mount for a dobsonian telescope.




The most popular solution is to build an equatorial platform. Equatorial platforms retain the low profile and ergonomic advantages of the Dobsonian design while allowing continuous tracking for an hour or more. It does need to be reset after an hour or so but the advantage of this is that you do not need to otherwise rotate the tube to maintain a comfortable eyepiece position.

With an Equatorially mounted Newtonian, the position of the eyepiece changes as you move about the sky, requiring rotation of the tube in the rings. Typically an equatorially mounted Newtonian will rings that allow easy rotation.

I would suggest looking at your 3-D drawing and then asking yourself "Where will I be standing?" and "What will be the orientation of the eyepiece?" with the scope pointed in a variety of directions.

A major reason that Alt-AZ mounts have become so popular for all types of scopes over the last 30 years is because they are more comfortable to use, eyepiece orientation does not change, the observer does not get tangled up in the mount the way can with a GEM or fork mount.

Jon


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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Equatorial Dobsonian new [Re: Jon Isaacs]
      #5667134 - 02/07/13 01:53 PM

Quote:

A major reason that Alt-AZ mounts have become so popular for all types of scopes over the last 30 years is because they are more comfortable to use, eyepiece orientation does not change, the observer does not get tangled up in the mount the way can with a GEM or fork mount.



Agreed that alt-az mounts are more comfortable than an equatorial without a rotating tube. But once you have a rotating tube, it's a tremendous convenience. On my 14" Newtonian pictured above, observing positions are comfortable no matter where it is pointed. If the eyepiece is a little low or high, just rotate the tube a little (obviously this doesn't work at the zenith, but the zenith height happens to match my standing height closely).


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Equatorial Dobsonian new [Re: Dick Jacobson]
      #5667154 - 02/07/13 02:01 PM

Quote:

Quote:

A major reason that Alt-AZ mounts have become so popular for all types of scopes over the last 30 years is because they are more comfortable to use, eyepiece orientation does not change, the observer does not get tangled up in the mount the way can with a GEM or fork mount.



Agreed that alt-az mounts are more comfortable than an equatorial without a rotating tube. But once you have a rotating tube, it's a tremendous convenience. On my 14" Newtonian pictured above, observing positions are comfortable no matter where it is pointed. If the eyepiece is a little low or high, just rotate the tube a little (obviously this doesn't work at the zenith, but the zenith height happens to match my standing height closely).




Rotating tubes certainly are great but I think there are also other factors like the larger footprint any sort of GEM requires that still get in the way of comfortable viewing.

Jon


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TheBigEye
member
*****

Reged: 12/08/06

Loc: New Mexico, USA
Re: Equatorial Dobsonian new [Re: Dick Jacobson]
      #5667927 - 02/07/13 09:54 PM Attachment (30 downloads)

Quote:

I see two problems with the split-ring or horseshoe mount. The first is size. On the JMI split-ring scopes, the outer diameter of the ring is twice the mirror diameter and it's hard to see how it could be made smaller. This impairs portability and the ring can get in the way when observing near the zenith. The second problem is that the ring restricts the length of the mirror box. This can make it difficult to balance the scope without using counterweights behind the mirror. Ball scopes have the same problem. The 200-inch Hale telescope gets around this problem by suspending the tube from an additional pair of arms, making it even larger (500 tons of steel, not a problem if you don't have to load it into your vehicle!)

For these reasons, I've chosen to use fork mounts for my big Newtonians, like the 14" pictured above. The outside width of the fork is about 50% greater than the mirror width. Within limits, you can make the fork as long as needed to balance the tube and provide enough range below the equator. They probably don't have as much stability as a split-ring or Dob, but vibration is low enough that it is not a significant problem.




JMI's NGT-18 won't fit into a Fiat, but it does fit into a minvan. See the picture below. The scope breaks down into very small components. For my purposes, I hauled the scope fully assembled in a 6X10 cargo trailer. Since the mirror is mounted in a conventional-type cell instead of a sling, mirror movement was not a problem. I just rolled the scope out onto a blanket, removed the handles and mirror cover, and I was ready to observe. (Yes, I had to tweak the collimation also). Of all the portable scopes I've had, I liked the design of this one best of all.

Actually, I quite like the design of your 20", a photo of which you linked into your earlier post. Another poster likes equatorial platforms, and they are certainly another viable option to equatorializing a Dob (oops, er, I mean an "alt-az mounted Newtonian )

It's been really great reading all the comments and ideas presented in this thread because I am evaluating my options for "equatorializing" my 20" f/4 Obsession. Thanks!


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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Equatorial Dobsonian new [Re: TheBigEye]
      #5668509 - 02/08/13 09:02 AM

Quote:

It's been really great reading all the comments and ideas presented in this thread because I am evaluating my options for "equatorializing" my 20" f/4 Obsession. Thanks!



If I owned a 20" Obsession I think I would put it on a platform. I've never owned a platform but I did look through a 20" on a platform once and it seemed very solid.

On the other hand, if you really enjoy designing and building telescopes, and don't mind destroying the resale value of the Obsession, a fork-mounted equatorial similar to mine would make a very interesting project. I was very pleased with the performance of the 20" that you referred to. The mirror edge was supported by four overlapping wire slings so it could tip in all directions. I am currently rebuilding the scope out of aluminum to make it lighter and make the entire tube rotate. The rotating secondary cage of the previous version was not compatible with a periscope accessory that I built a few years ago.


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TheBigEye
member
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Reged: 12/08/06

Loc: New Mexico, USA
Re: Equatorial Dobsonian new [Re: Dick Jacobson]
      #5669737 - 02/08/13 08:40 PM Attachment (36 downloads)

Thanks for your thoughts. I'm sure there are people out there cringing at the thought of me cannibalizing a beautiful 20" Obsession Classic. Actually, I am one of them! It's really too nice a scope to do that to, but then again, that Galaxy glass is pretty spectacular. If it comes to that, I'll probably sell it and start the project from scratch.

I've been a "GEM" guy for decades - and I'm talking about the old school heavy metal stuff from the 70's. (See pic of my last attempt to do a "big glass/portable GEM" in 2012.) There was never a problem with the wind blowing the scope around, or having to carefully rebalance the scope after changing eyepieces. But this 18-incher is now consigned to my observatory - it's just too heavy to transport and set up. I'm trying hard to come up with a workable, transportable, big scope for doing public viewing. Even with an equatorial platform, the issue of altitude movement remains. I'd like to come up with a "friction clutch" for the Obsession, to reduce the tendency of the scope to drift in a wind gust, or when changing eyepieces.


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Thomas Betts
newbie


Reged: 09/03/12

Loc: Hood River, OR
Re: Equatorial Dobsonian new [Re: TheBigEye]
      #6386829 - 02/20/14 01:33 PM

Hi I am building 16 inch carbon fiber dob and I like the horse shoe mount. Looking for plans to build one I have access to cnc mill to make any part. Also to make the equatorial mount to work, what controller do you use? Is there one that can retro fit to a horse shoe equatorial type dob mount?

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Geo31
Pooh-Bah
*****

Reged: 01/28/13

Loc: Kingwood, TX
Re: Equatorial Dobsonian new [Re: Thomas Betts]
      #6387375 - 02/20/14 06:47 PM

I suggest going to the Stellafane web site and look at the photo galleries from the various conventions for inspiration. It's the premier ATM gathering in the world. Lot's of clever people bring well made scopes there.

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