Extension for straight through viewing,,,

[mark81]

Hi All,

If i wanted to use my ST80 for a bit of straight through viewing - is it simply a matter of attaching an extension tube?

If so, would this do the trick?

https://www.firstlightoptics.com/adapters/astro-essentials-80mm-125-focus-extension-tube.html

Many thanks

Mark

[Ruud]

An extension tube will give you an upside down view (rotated  over 180°). If the tube is too long you can no longer focus on infinity. The one you link to would most likely work just fine.

In combination with a diagonal, an extension tube can be used for close range focusing. I use one to turn my scopes into long distance microscopes.

There are 45° prism erecting prisms, which bring you halfway to straight through. The image quality suffers somewhat from them, which you may not notice at low magnifications.

Straight through porro and roof prism assemblies are used in binoculars. I don't know if these are also made for telescopes. Again, the light path through them might get too long to reach focus at infinity, but if you focuser has plenty inward travel you are probably OK. If not, use a Barlow instead. A Barlow moves the focal plane of the telescope out.

The tube you link to looks a lot like a Barlow with the lens removed. If you have a longish Barlow you can experiment a bit, with and without the lens.

 

[mark81]

@Ruud Thanks for the info...

[Louis D]

Since most 1.25" mirror diagonals have an optical path length between 75 and 95 millimeters, that 80mm extension tube should provide just about the correct distance for focus.