Celestron Astromaster 130

[voitty]

Hi Guys, I am just new to all this stuff so I have a few questions for you. My tele is an F5 which I asume means focal length, can someone explain what this means and the difference between different all the F ratings. I am also not overly happy with the celestron plossl eyepieces quality wise considering this is I guess the most important part on the tele to give me a clear sharp view (on a perfect clear night sky) so how can I work out what will be a good size eyepiece and what is a good qualty one that will give me good focus on say jupiter as I think anything else will be a bit of much for the 130. The moon looks awsome through the 10mm & 20 mm and I can get a pretty good view of Jupiter with 4 moons but not much detail, will a filter help or is a better eyepiece the way to go Thanks alot Andrew.

[GeoffW1]

Hi,

Yes, it does have to do with focal length. F5 is the focal ratio, or F-number, of the scope. This is the focal length divided by the aperture. So if you have a 130mm scope, then your focal length would be 650mm.

It is handy to know your focal length, because the magnification of your image is found by dividing the focal length of the scope by that of the eyepiece. So if you used a 18mm EP you would get a magnification of X36.

Note that there will be a theoretical limit to how much magnification your scope can handle, and still produce a viewable image. This is roughly 2 times the scope aperture in mm, so for the above scope that is about X260, implying use of a 2.5mm EP ! For various practical reasons (such as atmospheric disturbances and optical aberrations) it is very difficult to view higher magnifications, and you will mostly find you are limited to about X200. In fact, most enjoyable viewing comes with even lesser magnifications. So for Jupiter, say about a 4mm eyepiece at most.

I have found Plossl lenses to be best in 25mm and above, and I quickly discarded the shorter focal length Plossls when starting out. There is an enormous choice in better quality shorter EPs, and the newer ones like Baader Hyperions have a wider field of view which is easier to get your eye onto. Good EPs appear often for sale here on IIS Trade.

I feel myself that almost any scope can benefit quite a bit from a good EP, and you can keep the EPs when you succumb to aperture fever and move to a larger scope :lol:

As with camera lenses, the focal ratio expresses the "speed" of the scope, and F5 is a fastish scope. In fact, it is uncommon to find scopes much faster than F4 (although the 8.4 metre LSST is F1.2 !)

Like anything else, fast scopes have their pros and cons. A fast scope has a shorter focal length and is therefore more compact and more portable. However these scopes are more prone to optical aberrations which degrade the image, especially if it is of an object like a planet.

Most Dob scopes are about F5 though because this gives the shortest lightest tube which can still be made to work well enough in that optical design. Lots of good quality refractors on the other hand, are about F10 to F12. That gives an image which is not as bright, but better quality.

I am not an astrophotographer, so I will leave the consideration of that and the F ratio of a scope to those more informed :thumbsup:

Cheers