41 replies to this topic

[joelin]

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?

[sg6]

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.

[ShaulaB]

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.

[joelin]

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

[Astrojensen]

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

[ngc7319_20]

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

[Sincos]

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.

[25585]

Would a tracking platform solve the problem?

[ngc7319_20]

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

[KerryR]

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.

[Jon Isaacs]

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

[John Fitzgerald]

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.

[airbleeder]

    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.

[brentknight]

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.

[OIC]

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.

[coopman]

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

[Don H]

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^) ...

 

 

 

 

 

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[rowdy388]

C5F6E87B-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.

[MitchAlsup]

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.

[Pierre Lemay]

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.

[Waynosworld]

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.

[Sheol]

          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.

[Waynosworld]

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.

[izar187]

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.

[airbleeder]

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.