In article <JeVte.4$Yb7.47@xxxxxxxxxxxx>, mchale@xxxxxxxxxxxxxx says...
> Olganitsch <hintauo@xxxxxxxxx> wrote:
> > Best way to approach this is to not pay too much - try one out, that is.
> > Optically they can be superb to good, but Ive never seen or heard of
> > a bad one.
> > Mechanically I have never seen or heard of one with image shift ....
> > irrespective
> > of what those experts who never owned one say from their arm chairs and
> > toilet seats...
> > They are NOT in the league of scts. These are f/15 systems. As such they
> > are not bright. When collimated correctly, and yes you can collimate them,
> > they provide superb planetary and lunar detail but they would not be
> > particularly good for stellar work, even doubles work because they simply
> > are not bright. They do resolve fine doubles with losts of contrast, but you
> > pay in brightness.
> Umm, F/ratio has no impact on the brightness of an image when an instrument
> is used visually. It does have an impact on exposure times when used
> photographically but that is because the image scale is larger. In general
> brightness of an image is determined by magnification. While the minimum
> Magnification of the scope is not going to be as low as an f/6-f/10 system;
> it is hardly terribly high (figure about 10X/inch of aperature). In any case
> magnification won't reduce the brightness of stars, only non-stellar objects.
Note that the word "brightness" can be ambiguous. It can mean total
brightness, or surface brightness. While the total brightness is
unaffected by the magnification, the surface brightness will be invesely
proportional to the square of the magnification, in one particular
> In fact for a long time achromatic refractors (where f/15 use to be the rule)
> were favored instruments for stellar work, particularly for splitting doubles
> where magnification was important.
> Therefore, if the 7" Mak is dim don't blame it on the focal ratio. Something
> else is causing it.