Dalmatian observing chair

a fairly simple project for the amateur astronomer

by Berislav Bracun

Dalmatian dog A good observing chair makes an all night session much more comfortable than standing up, or when using an inappropriate chair.

Even if its hard to believe, you can actually see more while seated than standing up. The reason is very simple.

When standing up, your body always moves back n fort a little bit, a few millimeters only, keeping balance. And your eye pupil needs to be centered over the exit pupil of the eyepiece.
chair from side
But, since you are moving around, it seldom is.
When the observer is seated, the amount of balance corrections your body needs to do drops dramatically.
As a result, your eye pupil moves much less, and is therefore nicely centered over the eyepiece exit pupil most of the time.
Practically, you gain aperture, and you are able to see fainter or finer details.

 

Denver observing chair It seems that the most popular homebuilt observing chair is the "Denver", originally developed by Dave Trott, member of the Denver astronomical society.

It is a practical, stable, cheap and easy to make chair.
Here is a beautiful example, built by David Martinez from Southern Chile.

So, what is wrong with this chair? Why did I have to take a different approach?

There is absolutely nothing wrong with this chair, but look at the ground underneath it. It is soft, level soil grown with grass.

We don't have that over here. The area where I go observing are the Dalmatian hills.
Mostly uninhabited , unaffected by light pollution, but the ground is rocks.... grey, sharp, uneven rocks as far the eye can see.

Look at the image on top of the page. That is a Dalmatian dog standing on these rocks.

The Denver chair is not suitable  for such  terrain.
 

 

So, the basic idea is to make some kind of hybrid between the Denver observing chair and a tripod.

The seat is more or less the same, with the difference that it is locked via slits in the seat column, instead with a antislip surface like on the original design. Somehow it feels safer when everything (including the chair) dews up.

The two rear legs are the same as on any other regular tripod. This provides a stable, 3 point support, and since the rear legs are also adjustable in height , it also enables the user to compensate for the uneven terrain.
This also means that you can use an existing tripod (that is strong enough) and modify it to become an observing chair.
Because there is no wide, horizontal bar,  this chair can be placed with the observer very close to the telescope. This might not be an issue if your scope is mounted on a tripod, but with a Dobsonian it certainly might get in the way.
 
central column observing chair observing chair front view tripod frame observing chair observing chair side view
seat supporting column
1100x100x40 mm wood
front view side view without seat side view with seat
 
locking mechanism
The chair tripod is supported with a metal "T", and together with the legs makes a stiff and stable structure. The "T" can be completely disassembled and removed via unscrewing the brass thumbscrews. The flat metal pieces are 30 mm wide and 4 mm thick.
 
chair folded
On the picture on the left it is visible how the seat locks into the seat column slits. The larger the load, the stronger it locks in.

 Chair folded for transport

The part that holds everything together is the Syntha tripod head, available separately as a spare part.
You can also make your own tripod head, or use another type.
 
chair view 2
chair view 1
observing with telescope observing with binoculars
This chair can be used for observing with a telescope.... ..and, since it is basically a modified tripod, it can accept an observing instruments itself, like for example a binocular parallelogram
   

The chair on rocky ground

   

Basic measurements of the wooden parts for the chair

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