Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

pointing at something near the zenith with a dobsonian base

  • Please log in to reply
41 replies to this topic

#1 joelin

joelin

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,600
  • Joined: 14 Jan 2008
  • Loc: Saratoga, CA

Posted 22 November 2020 - 02:01 PM

Yesterday I was trying to get to M31 and it was really really hard because from where I'm at its close to the zenith most of the evening. The dob is great for objects below 60 degrees, but above that it gets harder and harder to get to a particular point due to the way the azimuth base moves. What would you do?



#2 sg6

sg6

    Cosmos

  • *****
  • Posts: 9,604
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 22 November 2020 - 02:05 PM

You cannot do anything, well wait a few months is I suppose one.

It was, or is, often referred to as "The Hole".

 

In a way ironically not a problem for the scope, it will go wherever you point/aim it, it is a problem for the operator.


  • SteveV, John Lightholder and Augustus like this

#3 ShaulaB

ShaulaB

    Mercury-Atlas

  • *****
  • Posts: 2,650
  • Joined: 11 Oct 2012
  • Loc: Missouri

Posted 22 November 2020 - 02:07 PM

It looks like you discovered what is called "Dobson's Hole." Near zenith, the design of a Dobsonian mount has difficulty moving small amounts to point precisely at an object.

 

Solutions: 1) wait for the object to get past zenith. It will eventually. 2) save up for a different mount for carrying the optical tube.


  • vdog likes this

#4 joelin

joelin

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,600
  • Joined: 14 Jan 2008
  • Loc: Saratoga, CA

Posted 22 November 2020 - 02:31 PM

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


  • brentknight likes this

#5 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 13,599
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 22 November 2020 - 02:34 PM

Hmm. I never have any particular difficulties around the zenith with my 12" Meade Lightbridge, except my chair is just a little bit too low, so I have to stand, rather than sit. 

 

The dob needs to be very smooth in azimuth.

 

 

Clear skies!
Thomas, Denmark


  • izar187, havasman and Voyager 3 like this

#6 ngc7319_20

ngc7319_20

    Surveyor 1

  • *****
  • Posts: 1,596
  • Joined: 25 Oct 2015
  • Loc: MD

Posted 22 November 2020 - 02:46 PM

You can temporarily put a brick or similar under one of the feet of the Dob to get the object off the azimuth axis.


  • KerryR and stargazer193857 like this

#7 Sincos

Sincos

    Explorer 1

  • -----
  • Posts: 59
  • Joined: 21 Jan 2020
  • Loc: 100ft from the Atlantic Ocean in Newfoundland

Posted 22 November 2020 - 03:19 PM

C5F6E87B-777E-44EF-9595-971ED66CACE1.jpeg Dobson’s Hole  does exist  joelin , but there are ways. When you are pointing at zenith the tube of your scope has less area out from axis of rotation, centre of gravity. You need something away from the centre of gravity that will make it easier to rotate small amounts accurately when near zenith. I drilled a hole in the base to accommodate what is in the attached picture. Now when using near zenith I just have to push/lean into the dowel that protrudes, with my leg. If you do not want to drill into your base maybe some sort of clamp can be affixed to hold a dowel extending out from the base. It is not permanently attached only slide it into position when working near zenith.


Edited by Sincos, 22 November 2020 - 03:30 PM.

  • rowdy388 and Waynosworld like this

#8 25585

25585

    Voyager 1

  • *****
  • Posts: 11,367
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the UK. Dark end of the street.

Posted 22 November 2020 - 03:42 PM

Would a tracking platform solve the problem?



#9 ngc7319_20

ngc7319_20

    Surveyor 1

  • *****
  • Posts: 1,596
  • Joined: 25 Oct 2015
  • Loc: MD

Posted 22 November 2020 - 04:03 PM

The platform will let you move the azimuth axis around a bit on the sky.  So yes, it will help.


  • Augustus likes this

#10 KerryR

KerryR

    Soyuz

  • *****
  • Posts: 3,991
  • Joined: 05 Dec 2007
  • Loc: West Michigan

Posted 22 November 2020 - 04:42 PM

You can temporarily put a brick or similar under one of the feet of the Dob to get the object off the azimuth axis.

This.^^

Whatever you have on hand.
1 or 2 volumes of Bernham's Celestial Handbook.

Typically, I just try to get at targets before or after they pass through the hole.



#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 89,193
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 22 November 2020 - 05:07 PM

It is doable but somewhat difficult.. practice and smooth motions are part of the equation. The other night I was tracking an object less than 4 degrees from the zenith at over 200x. 

 

I use two hands, one on the tube for altitude and one on the rocker box or truss tube to roate the base.

 

It's not necessary to wait a month, the sky moves at 15 degrees an hour..

 

An equatorial platform tracks across the zenith but it isn't much easier to acquire the object.

 

Jon


  • rowdy388, havasman, stargazer193857 and 1 other like this

#12 John Fitzgerald

John Fitzgerald

    In Focus

  • *****
  • Posts: 8,976
  • Joined: 04 Jan 2004
  • Loc: ROR Obs. near Pettigrew, Arkansas

Posted 22 November 2020 - 05:12 PM

Use a couple of small pipe straps and fasten a piece of wooden broom handle or thick dowel to the side of the rocker box, slip fit tightness.  Problem solved.



#13 airbleeder

airbleeder

    Vanguard

  • *****
  • Posts: 2,197
  • Joined: 04 Jan 2014
  • Loc: Powder Springs, Georgia

Posted 22 November 2020 - 06:30 PM

    As Jon says, it is a bit difficult, but doable. I use both hands, one on the upper tube and one one the lower of my 8" f6 dob. Because of the tight axis at zenith, I seem to do a better job when I make a conscious effort to rotate instead of pushing or pulling, if that makes sense. 

    My biggest problem with dobson's hole is my neck and back.


  • rowdy388 likes this

#14 brentknight

brentknight

    Vanguard

  • *****
  • Posts: 2,396
  • Joined: 29 Dec 2014
  • Loc: Foley, Alabama

Posted 22 November 2020 - 06:50 PM

Using a brick or something sounds like a decent solution - I'll have to try that.  With my 10" though, I'm able to push the telescope is AZ with my eye on the eyepiece and use my hand to move the tube in ALT.



#15 OIC

OIC

    Vostok 1

  • -----
  • Posts: 166
  • Joined: 05 May 2020
  • Loc: Suburbs of Denver, USA

Posted 22 November 2020 - 10:04 PM

One thing I liked about the Sky Watcher 8" I used to have was the handlebar sticking out. There are two and they hold the OTA on the base. Using that made it easier to move around the zenith.



#16 coopman

coopman

    Fly Me to the Moon

  • *****
  • Posts: 6,755
  • Joined: 23 Apr 2006
  • Loc: South Louisiana

Posted 22 November 2020 - 10:11 PM

Yea, it's a bit of a PITA and applicable to all alt-az mounts.



#17 Don H

Don H

    Apollo

  • -----
  • Posts: 1,293
  • Joined: 28 Dec 2006
  • Loc: Near Tucson, AZ

Posted 23 November 2020 - 02:09 PM

I have 2 scopes that work well for me when viewing near the zenith, my 10" f/6.3 and my 4.5" f/4 on a Versago mount. I actually enjoy zenith views, as it allows me to stand up straight and there is no need for any neck bend or twist. It also puts objects far away from any horizon light and looking through the least amount of atmosphere. Having the rdf on the 4.5" up at the tube end, or the finder by the eyepiece on the 10", makes locating objects up high very easy. Like Jon said, smooth motions are very important, as is good balance. When examining an object near the zenith, I will check my charts for anything nearby, as short adjustments in any direction provide close to the same benefits as straight up. With the Versago II, I like to pull the tripod legs in up to the round eyepiece tray I made. It makes walking around it in a circle effortless, and changing eyepieces is a breeze. With that mount, the eyepiece position does not change much from zenith to horizon. The 10" Dob is another story, as viewing close to the horizon puts the eyepiece uncomfortably low. Typically, I don't view down low too much, due to excess atmospheric interference. 

 

You might try putting your Dob on a small, short and sturdy table to raise your eyepiece height when you plan to view a number of things near the zenith. This is provided you could still keep your feet on the ground. Here is one I made for my 8" travel scope. I actually made the table for the Starblast before I got the tripod mount. But it worked well to get my 8" travel Dob off the ground for more comfortable seated or standing viewing. It also lets me check underneath it at the end of a night to be sure no scorpions or such have crawled under it B^) ...

 

 

 

 

 

Attached Thumbnails

  • 4.5 + 10.jpg
  • 4.5 and 8.jpg

  • Kipper-Feet likes this

#18 rowdy388

rowdy388

    Skylab

  • *****
  • Posts: 4,132
  • Joined: 09 Apr 2013
  • Loc: Saratoga County, NY

Posted 23 November 2020 - 03:22 PM

attachicon.gifC5F6E87B-777E-44EF-9595-971ED66CACE1.jpegDobson’s Hole  does exist  joelin , but there are ways. When you are pointing at zenith the tube of your scope has less area out from axis of rotation, centre of gravity. You need something away from the centre of gravity that will make it easier to rotate small amounts accurately when near zenith. I drilled a hole in the base to accommodate what is in the attached picture. Now when using near zenith I just have to push/lean into the dowel that protrudes, with my leg. If you do not want to drill into your base maybe some sort of clamp can be affixed to hold a dowel extending out from the base. It is not permanently attached only slide it into position when working near zenith.

What a clever idea. I use two hands when operating near the "hole". One hand at the top and the other around the corner on the dob base which gives

me the turning torque I need.


  • airbleeder likes this

#19 MitchAlsup

MitchAlsup

    Aurora

  • -----
  • Posts: 4,957
  • Joined: 31 Aug 2009

Posted 23 November 2020 - 03:33 PM

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

While technically the atmosphere is thinnest at zenith, 

in practice, the atmosphere remains pretty thin up to 30º away from zenith.


  • Jon Isaacs likes this

#20 Pierre Lemay

Pierre Lemay

    Surveyor 1

  • *****
  • Posts: 1,539
  • Joined: 30 Jan 2008
  • Loc: Montréal, Canada

Posted 23 November 2020 - 05:16 PM

I observe with ball scopes so gimble lock (Dobson's hole) is not a problem. Mel Bartels' modified dobson mount with a new third axis to observe objects near the zenith is also really neat: it mimicks the ball scope's smooth motion near the zenith.

 

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.


  • Jon Isaacs, Don H and Dana in Philly like this

#21 Waynosworld

Waynosworld

    Messenger

  • *****
  • Posts: 497
  • Joined: 22 Oct 2020
  • Loc: Vancouver Washington

Posted 23 November 2020 - 06:14 PM

My 14.5" Starsplitter has wheel barrow wheels/handles to move it around, I use my shin against one of the handles with my foot on the ground to move it small amounts.



#22 Sheol

Sheol

    Viking 1

  • -----
  • Posts: 690
  • Joined: 20 Mar 2010

Posted 23 November 2020 - 06:53 PM

          Waynosworld, that sounds like a very good way to get bruised shins. LOL    How about an EQ table for your Dobs? I dunno, as I have long suffered from the dreaded Dobson's Hole problem. Its very frustrating. And can lead to a literal pain in the neck.

 

     Clear Skies,

        Matt.



#23 Waynosworld

Waynosworld

    Messenger

  • *****
  • Posts: 497
  • Joined: 22 Oct 2020
  • Loc: Vancouver Washington

Posted 23 November 2020 - 09:16 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.


  • airbleeder likes this

#24 izar187

izar187

    Skylab

  • *****
  • Posts: 4,418
  • Joined: 02 Sep 2006
  • Loc: 43N

Posted 24 November 2020 - 12:29 AM

Another vote for two hands, one high, the other low, and twist the scope thru zenith.

Another vote for raising the scope so you can stand and look straight into the focuser at zenith.

Another vote for smooth movements, without backlash.


  • rowdy388 likes this

#25 airbleeder

airbleeder

    Vanguard

  • *****
  • Posts: 2,197
  • Joined: 04 Jan 2014
  • Loc: Powder Springs, Georgia

Posted 24 November 2020 - 09:58 AM

One thing I liked about the Sky Watcher 8" I used to have was the handlebar sticking out. There are two and they hold the OTA on the base. Using that made it easier to move around the zenith.

  I've thought about trying the handlebar.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics