f ratio?

[rusirius]

When you look through a scope with a central obstruction you don't see a black hole in the middle of the image, but its effects are spread out across the entire field. This dims the image slightly and can cause a lost of contrast, a lot of planetary detail is low constrast so the smaller refractor can give more constrasty and pleasing planetary vews then a larger Newt/ Cat but it cannot resolve more detail.

For instance, if you look at Saturn though a small apo you will often see the different coloured banding on the planet look more impressive and vivid than in a larger Newt/ CAT, this is the low constrast detail. If you then try looking at Cassini (and Enkle if you have a large scope)division then you will see that the larger scopes are more suited to resolving these smaller, high constrast details.

Using my 8in LX90 at f10 and 2000mm focal length I cannot make out any colours viusally on Saturn, although they can become apparent when using the LPI. The Cassisni division is tricky but can be just made out on a good night with a 12 or 9mm eyepiece.

My wife has a Skywatcher Evostar 120 Achromat at f8 and 1000mm focal length. Having compared the two, Saturn seems sharper and crisper with the Skywatcher than with the LX90 and I could clearly make out the Cassisi division with a 5mm eyepiece. There seems to be some colour visible but I'm not sure whether this is down to chromatic aberration though?

When I tried to view Mars dueing opposition in December 2007 all I could see was a pinkish red blob, while others were takling about the details the could observe. We did't have the Skywatcher then to compare but I concluded then that the LX90 was a poor planetary scope.

The LX90 is better than the Skywatcher when it comes to DSOs and has an overall brigher sharper image which is presumably down to the greater aperature and hance resolution.

It will be interesting to see how Jupiter compares.....

[Ajohn]

That more or less goes along with my experiences rusirius - that's why I can't understand why some one hasn't bought the 120 I have for sale.

On the comment about no hole in the image in a scope with a central obstruction - Had a surprise when I tried an extremely low magnification on a small mac cas. Using an 80mms eyepiece I could see a black hole in the middle of the image. It was a large black hole too.

John