If the tube is 400 mm long, then what you have is a scope which should
have an additional lens at the back of the focuser which acts like a
Barlow lens. The idea is to provide 1000 mm of focal length in a
shorter and more manageable tube. The basic focal length of the mirror
is probably indeed 400 mm (though this is a bit short -- 500 mm would
be more likely), but the lens is intended to increase the effective
If this lens is missing, however, you could find that the telescope
focus point is different from what is allowed for, with the result
that you can't focus it at all. Alternatively, if it is present, it's
possible that someone has removed it in the past and replaced it
backwards, which would affect its optical performance.
So, check without an eyepiece to see if there is a lens somewhere in
the focusing barrel. If there is, does it look as if it has been
removed at some point? If so, I can't give any practical advice as
I've never tried dismantling one of these scopes, so it's up to you to
do something about it.
If there is no lens there, suspect that it has been removed and lost,
in which case try experimenting with holding the eyepiece farther away
from the barrel to see if you get a focused image. If this is the
case, all you can do is to try to buy a suitable extension tube to
allow you to get a focused image, assuming you can't send the scope
back and complain that it doesn't work.
Collimation is probably not the answer. A mis-collimated scope usually
focuses, but just doesn't give a very satisfactory image.
Please report back!
On 4 Apr, 14:15, robgtr <robin.broadb...@virgin.net> wrote: