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pointing at something near the zenith with a dobsonian base

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#26 stargazer193857

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Posted 24 November 2020 - 12:40 PM

thats unfortunate as I'll also be missing the best possible views when its near zenith


Maybe put a block under the leg opposite the north star, but not too high to destabilize it. That should buy you some easier aiming.

#27 stargazer193857

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Posted 24 November 2020 - 12:44 PM

Would a tracking platform solve the problem?


Once you have it in sight, yes.

#28 Jon Isaacs

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Posted 24 November 2020 - 01:02 PM

It turns easy using that handle down there, hardly any effort at all, the big deal is not hitting it by accident, or walking around the scope and hitting it, if I had it in a park where others were around or if I let my neighbors look thru it I will take them handles off so no one trips over them, all I really do is put my shin against it and lift my heal a little and move it without even taking my eye away from the eyepiece.

 

I always take the handles off my Dobs when I am observing.  I learned that lesson the hard way.

 

I had left the handles in place on a second Dob and had forgotten about them.  I tripped on one and went down hard.  I bent the 3/8" steel eye bolts but I only suffered bangs and bruises.  

 

They are too easy to take off and too risky to leave on.  You do this astronomy thing for enough years, if it can happen, it will happen. 

 

Jon


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#29 Jon Isaacs

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Posted 24 November 2020 - 01:04 PM

My big problem with zenith objects is finding them with a reflex sight when the tube is short. I'm getting too old for the acrobatic contorsions required with an 8 inch f/5 newtonian when the finder is only 4 feet off the ground.

 

 

That's why I use a reflex finder plus a 50mm RACI finder.  

 

Jon


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#30 Chucky

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Posted 24 November 2020 - 01:39 PM

<<  I'm getting too old for the acrobatic contorsions  >>

 

My doctor wrote me a prescription for a Nexus DSC.


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#31 KLWalsh

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Posted 24 November 2020 - 02:06 PM

If the Dobson ‘base’ was mounted horizontally, the ‘hole’ would be at the horizon.

Obviously this would require a beefy support system, and probably a lot more trouble to set up than most Dob owners would bother with.

 

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#32 Pierre Lemay

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Posted 24 November 2020 - 06:47 PM

If the Dobson ‘base’ was mounted horizontally, the ‘hole’ would be at the horizon.

Obviously this would require a beefy support system, and probably a lot more trouble to set up than most Dob owners would bother with.

There's a name for this type of mounting: it's called and "alt-alt" mount (as opposed to an "alt-azimuth" one). I remember someone (Russ Genet?) mentioning it as a "trend of the future" in, if I'm not mistaken, a Sky and Telescope article about the future of amateur telescopes. The trend never materialized.

 

Obviously such a mount would cause awkward eyepiece positions similar to a Newtonian on an equatorial mounting (in fact it is an equatorial mounting for an observer on the equator). The Dobson mounting is far simpler and superior.


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#33 John Fitzgerald

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Posted 24 November 2020 - 07:17 PM

There's a name for this type of mounting: it's called and "alt-alt" mount (as opposed to an "alt-azimuth" one). I remember someone (Russ Genet?) mentioning it as a "trend of the future" in, if I'm not mistaken, a Sky and Telescope article about the future of amateur telescopes. The trend never materialized.

 

Obviously such a mount would cause awkward eyepiece positions similar to a Newtonian on an equatorial mounting (in fact it is an equatorial mounting for an observer on the equator). The Dobson mounting is far simpler and superior.

The standard Dobsonian mount is an equatorial mounting for an observer on the pole, as are most alt-az mounts. tongue2.gif lol.gif   The mount in the drawing would not be so hard to make work if it had a large counterbalance.  It would only be an equatorial mount at the equator if pointed at one of the poles, and allowed to rotate on the first horizontal axis.  Rotating as a standard Dob would not make it so.


Edited by John Fitzgerald, 24 November 2020 - 07:22 PM.


#34 Jon Isaacs

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Posted 25 November 2020 - 06:50 AM

There's a name for this type of mounting: it's called and "alt-alt" mount (as opposed to an "alt-azimuth" one). I remember someone (Russ Genet?) mentioning it as a "trend of the future" in, if I'm not mistaken, a Sky and Telescope article about the future of amateur telescopes. The trend never materialized.

 

Obviously such a mount would cause awkward eyepiece positions similar to a Newtonian on an equatorial mounting (in fact it is an equatorial mounting for an observer on the equator). The Dobson mounting is far simpler and superior.

:waytogo:

 

The beauty of the Dobsonian is that it's incredibly simple.  The mount is robust and sits on the ground, no tripods or cantilevers, it doesn't get any better than that.  That makes it possible to mount incredibly large telescopes that are rock solid stable.  

 

When you start adding the complexity to try to track more easily near the zenith, everything else is compromised... 

 

Doing a little math:

 

The area of the sky that is within 10 degrees of the zenith is 1.5% of the total area and 3.0% of the area above 30 degrees.

 

The area of the sky that is within 15 degrees of the zenith is 3.5% of the total area and 7% of the area above 30 degrees.

 

The area of the sky that is within 20 degrees of the zenith is 6% of the sky and 12% of the area above 30 degrees. 

 

At the equator, it takes 40 minutes for the zenith to move 10 degrees. 

 

Either get there early or wait.. 

 

Jon



#35 stargazer193857

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Posted 25 November 2020 - 05:04 PM

Tipping the scope 10 degrees would need a 3" brick, for an 8" f6 dob. I wonder how much that would eliminate dobsons hole.

#36 Pierre Lemay

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Posted 25 November 2020 - 07:52 PM

Tipping the scope 10 degrees would need a 3" brick, for an 8" f6 dob. I wonder how much that would eliminate dobsons hole.

It would create stargazer193857’s hole . 


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#37 Pinbout

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Posted 25 November 2020 - 08:13 PM

I shoot my green laser thru a 35mm eyepiece and where ever it points the object is in the eyepiece. So if I get the laser on the object I don’t worry about it. That way I don’t have to collimate the laser to the scope. Just point and view.



#38 Pinbout

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Posted 26 November 2020 - 12:02 PM

525AFABF-F938-4B13-8C39-10A59D00B5C8.jpeg

 

FB81A138-A99F-4301-9C11-561E135FF663.jpeg

 

Just remember this 

 

at zenith I’d much rather have a dob 


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#39 izar187

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Posted 26 November 2020 - 12:11 PM

Hey Burt!


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#40 Jon Isaacs

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Posted 26 November 2020 - 12:47 PM

attachicon.gif525AFABF-F938-4B13-8C39-10A59D00B5C8.jpeg

 

attachicon.gifFB81A138-A99F-4301-9C11-561E135FF663.jpeg

 

Just remember this 

 

at zenith I’d much rather have a dob 

 

And at the horizon, with a focuser mounted at an angle, a Dob works reasonably well..

 

The Newtonian focuser is at the sky end of the scope rather than the dirt end like Cats and refractors.

 

Jon


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#41 nof

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Posted 26 November 2020 - 01:53 PM

<< I'm getting too old for the acrobatic contorsions >>

My doctor wrote me a prescription for a Nexus DSC.



#42 nof

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Posted 26 November 2020 - 01:55 PM

+1 - it works to point at the zenith


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