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freesaxon
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Reged: 12/25/10

Loc: Great ? Britain ( North Wales ...
Dobsonian Telescope or Newtonian telescope ? new
      #4275063 - 12/27/10 07:01 PM

A very basic question to start off with.

As I live in a fairly good location its unlikely that I will have a need to transport any telescope I might build. I do however have a limited budget, but don’t intend to skimp on the essentials ( optics )
Reading up I think I would like to build a telescope with a min 8” mirror
?
Dobsonian Telescope or Newtonian telescope ?

Reading up on these can be a little confusing, I can see myself using many of the cost cutting ideas used in a Dobsonian , sonotubes, d.i.y mounts, etc in either one.
Is a Dobsonian merely a more portable, lightweight version of a Newtonian, minus the equatorial mount ?


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Achernar
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: freesaxon]
      #4275104 - 12/27/10 07:20 PM

The main distinction between a Newtonian and a Dobsonian is not the optics, it's the mounting. Newtonians are in general mounted on equatorial mountings which track objects with a single motion around the polar axis. Dobsonians are mounted on much simpler alt-azimuth mountings that are usually made made of plywood or particle board and use two motions to follow an object. Another key difference is the optics in a Newtonian are subject the force of gravity from various directions, whereas in a Dobsonian, gravity acts on them from more or less the same direction at all times. In other words, the tube can end up in any orientation with an equatorial mounting, but stays in the same orientation regardless of where it's pointed. That is why in a large Dobosonian the primary mirror can be supported in a simple sling, whereas it cannot in a Newtonian. With either telescope, the optics are the same. Building a Dob is much easier than an equatorially mounted telescope, but believe it or not that can be accomplished with things such as U-bolts and common plumbing pipe and fittings if you can do without a clock drive. You can however make a Dobsonian able to track objects for an hour or so at a time just like an equatorial mounting with a special platform. Dobsonians are also easy to build, and when care is taken with their construction they will be both steady and will allow you to track objects manually with ease. It is also possible to add digital setting circles, which makes locating the fainter objects a lot easier when you have light pollution to contend with, as most of us do. An 8-inch Dobsonian is portable enough for you to carry out and set up in minutes, and it will fit in many cars for trips to darker sites. If you have the wood and metal working skills, the money and the time, you could build a much larger Dobsonian than that for a lot less money than an equal sized Newtonian. That is why I am building a 15-inch Dobsonian, an equatorially mounted telescope of the same aperture would have to be permanently mounted in an observatory.

Taras


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Roy McCoy
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Achernar]
      #4275134 - 12/27/10 07:39 PM

Nice answer.

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Mirzam
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Roy McCoy]
      #4275222 - 12/27/10 08:42 PM

There is also a nice chapter in the book "The Dobsonian Telescope" by Dave Kriege and Richard Berry, which describes the construction of an 8-inch dob-style newtonian.

The rest of the book offers considerable food for thought concerning the next steps beyond an 8-inch.

JimC


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Mirzam
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Mirzam]
      #4275285 - 12/27/10 09:34 PM

Another excellent resource:

http://stellafane.org/tm/atm/index.html

JimC


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danmdak
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Mirzam]
      #4275314 - 12/27/10 09:58 PM

I am in the minority here (as always on most issues it seems!!!), so decided to chime in just so you have both sides. Have used both type of telescopes but will admit up front I never personally owned a dob, however, if portability is not an issue I would definitely go newtonian, especially if you build it yourself. A mount for an 8" or 10" newt is not that difficult to build. If you observe from your own property you could even sink a shaft in concrete with the mount attached to it, then simply cover the mount with a weatherproof cover (vinyl, etc). So all you have to take out is the OTA and simply bolt it onto the mount. I used this setup the first 15 years and it worked well.
Achernar is correct about the size tho, my 16" is in a permanent roll off roof observatory wheras if built as a dob it wouldnt necessarily have to be. However, I have had on hold (due to family matters) an 8" f/4 and that when finished, will be on an equatorial mount, so even for the smaller scopes I go newtonian. After 35+ years the motion of an equatorial is very familiar to me, while with a dob I stumble somewhat. If this is your first scope you of course wont have that issue. It is pricier but not greatly so, the main thing is smoothness in the RA axis. A drive isnt really needed unless you need high power and even then, within limits, if the ra axis is smooth a nudge is all thats needed. What you need to do before investing money and time is to find observers in your area and compare both scopes...see what you like and dislike about both. A telescope hard to use or hard to setup winds up being used infrequently after the inital thrill is worn off. You need to explore what works for YOU and the only way to do that is to examine each type and then decide accordingly.


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ad701xx
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Achernar]
      #4275504 - 12/27/10 11:56 PM

Quote:

The main distinction between a Newtonian and a Dobsonian is not the optics, it's the mounting.




On the contrary, being a Newtonian has absolutely nothing to do with the way it's mounted. Newtonian describes an optical system using a concave primary mirror and a flat, diagonally mounted secondary mirror.

A Newtonian can be mounted on an equatorial mount or an alt-azimuth mount. A Dobsonian is a Newt on an alt-azimuth mount.

Dobsonian Definition


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Mirzam
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: danmdak]
      #4275823 - 12/28/10 08:04 AM

Having used both types of mounts for many years, I agree that an equatorial mount, even if undriven, has advantages for finding objects manually using sky charts. Using a chart one can star hop by moving in N-S E-W increments, which is difficult to do with an alt-az mount. Alt-az does work well with digital setting circles, but then you have increased the technology level quite a bit.

JimC


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txmnjim
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Reged: 03/31/09

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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Mirzam]
      #4275958 - 12/28/10 09:46 AM

I think the dob is far more user friendly and easier to
build the base. Unless your going have a motor to track
with I don't see the use of the equitorial. Call me stupid,
but I actually converted an 8" dob to a GEM and have had nothing but problems. Mount is too shaky and its much harder to learn how to navigate!
Jim


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Jon Isaacs
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: txmnjim]
      #4276135 - 12/28/10 11:35 AM

A couple of comments...

- we should be clear, a Dobsonian is a type of mount. It is most commonly used to mount a Newtonian telescope but one can use it With refractors and cats.

- I own both equatorilly mounted Newtonians and dobsonian mounted newtonians, the GEMs range from 4.5 inches to 12.5 inches. The Dobs from 10 to 16 inches but have owned several 6 and 8 inches as well.

My suggestion is to build a dob, the tube can be used with a gem but the dob mount is simpler and easier to build.

Newtonians mounted on a Gem are more awkward to use, often the eyepiece is difficult to get to. Since the tube rotates as you move about the sky, it is necessary to rotate the tube in the rings. Even then, it can be awkward to get to the eyepiece.

30 years ago, newtonians were just about all mounted on gems and an 8 inch was a big scope. The dobsonian changed all that, the simple but rock solid mount made the dob mounted Newtonian into a compact and easy to use package so that 16 and 24 inch Scopes are large Scopes and and 8 inch is just a nice, easy to use scope.

Jon


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freesaxon
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Reged: 12/25/10

Loc: Great ? Britain ( North Wales ...
Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Jon Isaacs]
      #4276366 - 12/28/10 01:51 PM

Thanks to everyone for their replies
errmmm.
It seems the term “Dobsonian” can mean different things to different people.
I have just read up on Dobsons life, and its certainly unusual, seems fitting that the telescope is also `unconventional ‘
All seem to agree with its portability, ease of use, and mounting.
Can I focus on its optics ………… ( sorry couldn’t resist that )
How exactly DO they differ, focal length, Prim. Sec. mirrors etc. ?
Which is better ?

Edited by freesaxon (12/28/10 02:04 PM)


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Beri
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Reged: 06/29/05

Loc: Croatia
Re: Dobsonian Telescope or Newtonian telescope ? new [Re: freesaxon]
      #4276384 - 12/28/10 02:03 PM

I look at it differently
Besides beeing a alt az mounted Newtonian, a Dobsonian is also a telescope (mount and optics) made from scrapped or scrounged parts at a minimum cost.
A kevlar titanium telescope, costing many thousands in materials, then many thousands in electronics and servos, then some more thousands in optics, should not, IMHO be called a Dobsonian, although it is (technically) one.

The main thing John Dobson achieved with his telescope building style is to make a (big) telescope affordable.


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freesaxon
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Reged: 12/25/10

Loc: Great ? Britain ( North Wales ...
Re: Dobsonian Telescope or Newtonian telescope ? [Re: Beri]
      #4276392 - 12/28/10 02:07 PM

Beri,
Yes I read he used ships window glass for the mirror !


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hbanich
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Reged: 06/17/05

Loc: Portland, Oregon
Re: Dobsonian Telescope or Newtonian telescope ? new [Re: freesaxon]
      #4276565 - 12/28/10 03:44 PM

Quote:

Thanks to everyone for their replies
errmmm.
Can I focus on its optics ………… ( sorry couldn’t resist that )
How exactly DO they differ, focal length, Prim. Sec. mirrors etc. ?
Which is better ?




As has been mentioned, a Dobsonian almost always uses Newtonian optics. The original Dobs used mirrors made from porthole glass scrounged from marine salvage, but using any other type of mirror substrate has always been considered fine as long as its characteristics are taken into account.

The original Dobs also had focal lengths that were in the f/6 to f/8 range, making them much longer than today's Dobs which are mainly inthe f/5 to f/4 range (with some now going even faster at f/3). This makes for shorter scopes needing shorter ladders or none at all.

So functionally, a Dobsonian is a Newtonian reflector of any focal ratio on a simple alt-az mount that uses Teflon and laminate bearings. As an example, I have a 28 inch f/4 that most people consider a Dob but I think of it as an alt-az Newtonian because it doesn't use Teflon and laminate bearings - it uses ball bearings on both axis (which makes it easier and smoother for servo motors to drive it).

"Dobsonian" encompasses a fairly broad range of price/quality, but at their heart they're almost always a regular Newtonian optical system on a alt-az mount that uses Teflon and laminate for it's bearing surfaces.


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Scott Watson
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: ad701xx]
      #4277488 - 12/29/10 12:55 AM

Quote:

Quote:

The main distinction between a Newtonian and a Dobsonian is not the optics, it's the mounting.




On the contrary, being a Newtonian has absolutely nothing to do with the way it's mounted. Newtonian describes an optical system using a concave primary mirror and a flat, diagonally mounted secondary mirror.

A Newtonian can be mounted on an equatorial mount or an alt-azimuth mount. A Dobsonian is a Newt on an alt-azimuth mount.

Dobsonian Definition




Yes! ALL so-called Dobsonians are Newtonians. It is perhaps unfortunate that the former nomenclature has replaced the latter in many astronomy circles.


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Jon Isaacs
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Scott Watson]
      #4277582 - 12/29/10 03:05 AM

I hope we can get back on track and discuss the original question which was whether to build a Newtonian that uses a Dobsonian mount or one that uses a German Equatorial Mount.

I suggest start with the simplest, the Dob and if that is not satisfactory, consider more complex solutions.

Jon


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Darenwh
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Jon Isaacs]
      #4277855 - 12/29/10 09:46 AM

The optics are the exact same in either case. A dobsonian is just a newtonian telescope mounted on a simple altaz mount. The mount can vary quite a bit in design but over all it is a rocker box on a ground board. The OTA (Optical Tube Assembly) is the exact same. You are confusing mounting of the scope vs. the type of optics configuration used.

The newtonian telescope is any telescope with a concave primary mirror that focuses light up to a flat secondary mirror that is held at an angle to direct the light out to the side to a focuser. Any scope with that configuration of optics is a newtonian telescope.

There are design changes that can occur in the OTA depending on if it is equatorially mounted or altaz mounted. Lets discuss the different mounts and how they work.

In Common:
Both mounts have two primary axis that are used to point the scope in any direction in the sky.

Equatorial mount: The two axis are called Right Ascension and Declination. The declination axis is tilted so that it is (ideally) parrellel to earths axis of rotation. By motorizing this axis it is possible to turn it the opposite direction to earths rotation at the same speed to keep objects centered in the field of view. Because only this axis needs to move to keep objects centered it is easy to setup a single motor to control guiding. The second axis is used to point the telescope at the right angle to center objects in Right Ascension. Because the mount is holding the scope in relation to earths axis of rotation rather than sitting flat on the ground it becomes necessary to use counterweights to properly ballance the scope. This creates issues with vibration as well as more substantial mounting is necessary to hold the weight of the scope well in the unballanced angles that these are used in. This creates very heavy mounts. These are generally more expensive to build and much more complex.

AltAz mount: This mount has one axis (Azimuth) that is generally level to the ground at the location it is used in and another that is used to point the telescope up or down in Azimuth. By turning the scope on these two axis's it becomes easy to point the scope at any location in the sky. Because the force of gravity naturally pushes directly down on this mount no counterweights are needed as long as the mount is designed properly. The dobsonian mount is one of these have the benefit of being very easy to build and offering very stable support of the optical tube assembly making them much less expensive to build.

Mounting design considerations for the differing mounts: An equatorial mounted newtonian telescope creates many challenges for the OTA builder. First, because the OTA turns as the telescope tracks across the sky it is necessary to create a good mirror cell that will hold the primary mirror still no matter which direction the optical tube assembly turns. This has to be done in such a way that allows the mirror to be held very securely with no slop while insuring that the mirror is not pinched in the cell. The position of the focuser in relations to the observer on the ground also changes as the scope moves across the sky. This can result in the focuser pointing out of the OTA at any angle, straight up, down, to the left, or to the right. To be used visually the focuser needs to be turned so the eyepiece will be in a convienient location for the observer to use it. This requires either a rotating tube or a rotating focuser. This adds to the expense and complexity of building the telescope. These same forces also act on the finderscopes and can make it difficult to use these depending on the scope orientation.

An alt Az mount, such as a dobsonian mount, creates less demands on the design of the OTA because the orientation of the scope does not change beyond pointing up and down. The primary mirror cell can use a simple sling design with just a few clips to protect the mirror in case the ota is tilted down too far or to hold the mirror in place during transport. This allows the use of a much simpler mirror cell. As the orientation of the scope never changes the position of the focuser stays the same resulting in no need to rotate the tube or focuser. Finderscopes also remain easy to use.

Overall, the classic dobsonian altaz mount is the easiest to build and also the least expensive. Many people can build one of these in a single day and even new builders can often complete it in under two. If you are questioning which type of mount to build for your newtonian OTA then I suggest that you build one of these initially and then over time you can build the equatorial mount while still being able to enjoy the scope until the new mount is complete. If you plan to mount the OTA in an EQ mount then you should design the mirror cell appropriately from the start so you do not have to rebuild this following completion of the EQ mount.


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David Knisely
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Jon Isaacs]
      #4286209 - 01/02/11 05:20 AM

Jon Issacs wrote:

Quote:

- we should be clear, a Dobsonian is a type of mount. It is most commonly used to mount a Newtonian telescope but one can use it With refractors and cats.




Well Jon, the truth is that a Dobsonian has never been fully defined formally (the current definition is a convention only). To be perfectly correct, the mount is an altazimuth, and in the Dobsonian design, is usually an open-faced one that relies on gravity for its primary altitude bearing constraint. As stated earlier, by current convention, the Dobsonian is considered to be a Newtonian mounted on an altazimuth mount. The mount was originally called a "Dobson-style mount" in the late 1970's or early 1980's, which, after some time, became known as a Dobsonian mount. However, over the years, the term evolved somewhat once again to the point of commonly referring to the entire telescope+mount system as a Dobsonian. This was probably first done by Coulter around 1980 when they introduced their first line of the Odyssey telescopes (which they referred to as "Dobsonians"). Indeed, in its original form, the "Dobsonian" was a simple solid-tube Newtonian with a Sonotube tube, fixed trunion-style bearings, a friction-bearing open-faced altazimuth mount, a primitive "sling" mirror cell, and not a lot else. Even that has changed to where some "Dobs" are rather sophisticated truss-tubed monsters that bear little resemblance to John Dobson's original telescopes. I saw one of Richard Berry's telescopes in his book which was a 6 inch f/15 refractor and was also mounted on the same open-faced altazimuth mount idea that is used for the Dobsonian. Almost no one in this forum refers to that as a "Dobsonian", as most would still call it a refractor. Similarly, to be excessively correct, what is marketed now are Newtonians on simple altazimuth mounts. However, if someone wishes to refer to this entire telescope/mount configuration as a "Dobsonian" or even a "Dob", I don't feel there is any reason to correct that person. I know pretty much exactly what they mean. Clear skies to you.


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Jon Isaacs
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: David Knisely]
      #4286233 - 01/02/11 06:06 AM

David

The important point is that " Dobsonian" refers to a type of mount, Newtonian to an optical system. The rest is unclear. Some refractor owners build a wooden mount that is identical to a Dobsonian except that it sits a tripod. It is fommn to refer to this as a Dob....

The mwinhijg, this thread is about gems vs dobs...

Jon


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operascope
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Re: Dobsonian Telescope or Newtonian telescope ? new [Re: Jon Isaacs]
      #4288375 - 01/03/11 08:27 AM

Build the Dob mount. If you have no pressing need for an equitorial, the dob is cheaper, easier, and very, very stable.
But the great thing about ATM, is that you can always rebuild! Build the Dob, and then later on build the Equitorial, and move the mirror to which ever one you need for that particular appilication or preference.

And don't forget to post the pictures for us all to see!!!


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