Focussers

[cyborg421]

Can anyone explain the difference between a 2, 2.5 and 3" focuser in terns of how they would be used and why you would choose one over the other. I need to choose one potentially with binoviewers.

Simon

[Oddsocks]

Hi Simon,

It's purely down to geometry.....

In very general terms, the bigger the primary mirror or objective and the faster the scopes focal ratio then the bigger the bore of the focuser needs to be.

Imagine the cone of light diminishing in width as it leaves the primary mirror (or objective) as it travels to the eyepiece, the bigger the mirror and the faster the ratio then the more steeply the shape of the cone is angled.

If you chose a small bore focuser and fitted it on a fast ratio scope with a big mirror or objective then the outermost parts of the cone would be cut off by the barrel of the focuser, only a fraction of the light would reach the eyepiece.

Since the bigger bore focusers cost more the trick is to choose the minimum bore size for objective/mirror size and ratio of the telescope.

Binoviewers have quite a long light path outside the focuser so imagine that light cone is going to be quite wide by the time it reaches the furthest, innermost edge of the focuser draw tube.

The other point you haven't mentioned is the difference in "profile" or the height of the focuser and the "racked length"

This is another variable in the mix, for use with binoviewers the light path inside the binoviewer is quite long and the amount of back focus available outside the telescope is limited so a low profile, low height focuser with a good racked length is often needed.

The starting point is to sketch out a scale drawing of your telescope, you already know the telescope mirror or objective size and focal length plus the maximum back focus available from the point it leaves the telescope tube so with these dimensions inked in you can add the ray path to the drawing and then directly scale off the drawing for the width of the focuser tube at the furthest inbound point that the focuser can travel.

The focal point of the binoviewer objective is added to the diagram and the distance between the outer edge of the telescope tube and the mounting flange of the binoviewer gives you the profile height of the focuser plus half its travel distance, you then choose the bore size that does not obstruct the light cone fully wound in with the profile size that is equal to or less than this measurement.

It is not very often that you will see binoviewers on a Newtonian because the back focus distance is normally just not sufficient without moving the primary up the tube and fitting a larger secondary, refractors seem more accommodating.

If you start with the dimensional diagram you can work out if the back focal length will make binoviewing possible with the 250 PX and then determine which bore size will not introduce vignetting or cut-off into the light cone and which profile height focuser will allow your chosen eyepieces to reach focus.

William.

[cyborg421]

I should have mentioned that it is for a 100/800 refractor that I am buying not my Dob.

What will I see differently with the different focussers?

My eyepieces at the moment range from a 6mm Delos to a 31MM T5.

I just need to know if for that scope there is an advantage of one size focuser over the other.

Simon

[ian_bird]

William

Stonking answer mate! I learned a lot from that. Thank you for your reply - that's what I call sharing knowledge!

Much appreciated.

Ian