Need help with eyepiece height

Considering a dobsonian such as an Obsession or similar construction, what MAXIMUM size mirror (aperture?) would permit comfortable viewing at zenith for a person 6'3" tall, with both feet firmly on the ground. Thanks

The focal ratio of the mirror is something else you'll have to consider.

Dave Mitsky

Hi David,

Most of the websites for the Dob makers include the info in the specs for each scope.
They usually refer to it as " eyepiece height at zenith ". The focal ratio of the mirror is the most determining factor.
The faster the mirror, the shorter the scope for a particular diameter of mirror.
Happy hunting,
Brad

david roberts said:

Considering a dobsonian such as an Obsession or similar construction, what MAXIMUM size mirror (aperture?) would permit comfortable viewing at zenith for a person 6'3" tall, with both feet firmly on the ground. Thanks

David: I am just about 6feet tall, my buddy is about your height. I have a 16 inch F/4.42 Dob (Focal length 1800mm) that right at the limit for me and easily doable for my friend.

It's the focal length that is most important in determining the eyepiece height. You could go to an 17.5 inch F/4 and probably be OK, but as others have said, it is important to contact the manufacturer because a difference of an inch or two can be important.

Also, as you probably know, Newtionians that are F/4-F/5 require precise collimation and Paracorrs and Naglers/Ethos's/Panoptics are a big help in getting the good views.

Jon

Thanks all. I was considering a FL of 4.5 or so. That's about as fast as I want to go. I want to gaze at the occasional planet but more interested in DSOs. My hobby is woodworking so I want to try my hand at building as much of the scope as I can, except grinding the mirror, which I will leave to the pros. Is thre a book someone could recommend to build an open truss dobsonian, especially showing plans for a mirror cell. Thanks again.

In adddition to the other advice (basically that focal ratio has as much as anything to do with it)--

THe design of the rocker box, bearings, and baseboard will also affect the eyepiece height.

The design of the tube, in combination with the size of the secondary/height of the focuser, will also have a slight effect.

Together these can make a difference of as much as a foot in eyepiece height from one set of design criteria to another.

And while we are at it, remember that you may want to optimize for just below straight up and down--very rarely do you look into "Dobson's hole" anyway. And stepping on a two step ladder, or up on a block may not be all that big a deal.

Alex

I am 6'1" and was surprised to find that I could reach the eyepiece flat-footed on a Meade 16" f/4.5 LightBridge telescope they had at Scope City. I'm not sure if it was a trick they did with the floor model, or if all of them are that height.

One variable not mentioned is the height of the focuser above the secondary. One can shorten the zenith eyepiece height a couple inches by extending the focuser out a couple inches. That is not a desirable solution, however, because you need a larger secondary to get a fully illuminated field of any decent size.

Previously, I had thought a 16" f/4 would be needed for me to see the zenith flat-footed, but I guess it depends.

I think the Obsession 15" f/4.5 or the Starmaster 14.5' f/4.3 would safely get you to Dobson's hole with your feet on the ground.

As someone else mentioned, you may want to add a tracking platform at some point, which adds 5 +/- inches to the height calculation.