Is it that simple?

[Kevdan]

Hi all,

Just a hypothetical question, Is the primary mirror in my skywatcher (150 x 750) the same as the primary mirror in a 150 x 1400 ?

If so is it just a case of extending the tube length to get a longer focal length and then re-collimating. And if so what is the maximum focal length you can go to without distorting the image. Only reason i'm asking is cause I see quite a few lesser quality longer scopes on ebay and other sites so was wondering wether the tubes can be cut and shut.

Probably a daft question but if you dont ask you'll never know.

Kev.

[Cornelius Varley]

The 150/1400 has a spherical mirror with a focal length of 1400mm. Within the focuser is a lens called a Jones/Bird lens which corrects the lens for spherical aberation and allows the telescope to use a shorter tube. The 150/750 has a parabolic mirror and is better optically than the cheaper to make spherical mirror.

Peter

[ronin]

Is the primary mirror in my skywatcher (150 x 750) the same as the primary mirror in a 150 x 1400 ?

No. The focal length of yours is 750 mm and that is where the image is formed. On a 150x1400 the focal plane is at twice the distance ie 1400 mm and that is where the image is formed for that scope.

The profile cut in to the glass will be different. Yours will have approximately twice the depth of glass removed compared to the 1400 at the centre.

Just because they are they same diameter means nothing.