Please explain

[stopusinmyid (Tammy)]

Hi Peoples,

: I have a little 5" scope and I am just wondering if a 10" scope doubles the size of what I am looking at or is it the EP's that do all the work.

Also, I was having a look at Jupiter this morning and I seemed to have trouble focusing it properly using the Barlow 2x or the 6mm Planetary EP. Even when we have a look at Saturn we seem to have the same problem. We get nice clear views with the 10mm and am just wondering what the problem may be . It doesn't help everytime we try to focus the whole lot shakes.

[Davros (Lauren)]

A 6mm eyepiece may be pushing your scope too far, beyond its capabilities. A 10 inch scope does not double the size of your objects but it does take in a lot more light making them easier to see. Eyepieces magnify your objects and better quality eyepieces will make things a lot clearer. Sounds like you may need a better scope mount as well. If you've got the bug a dobinson mounted Newtonian may be a good idea in 8 to 10 inches. With my 10 inch dob i can resolve the bands of Jupiter and all of its moons.

[michaellxv (Michael)]

A 10" scope will gather 4x as much light as your 5" which will make your image brighter, it will also get you more resolution in what you see.

Magnification is determined by the focal length (FL) of you scope divided by the FL of the eye piece that you use.

Try to use too much magnification for your scope and the conditions and you will get a very poor view.

[mithrandir (Andrew)]

The rule of thumb for the maximum usable magnification M is 2x the aperture in millimetres. So a 4" = 100mm gives 200 times.

Now the magnification of a telescope and eyepiece combination is given by M=F/f where F is the focal length of the telescope and f the focal length of the eyepiece.

Turning this around, the minimum f is given by F/M which in your case is F/200.

You didn't say what the focal length of the telescope is, but assuming ideal conditions and given that the 10mm works F can not be more than 2000mm, and that the 6mm does not work means it is at least 1200mm.

So for example an 8" SCT - aperture 203mm and focal length 2032mm - supports about 2x203=406 times magnification which you get with a 2032/406=5mm eyepiece.

You can only use these maximum magnifications if the seeing is good.

Andrew

[esky (Craig)]

In addition to the great advise previously, it could be that your scope is hard to focus because it hasn't been collumated properly. This is more noticeable when you use shorter focal length eyepieces. An easy to understand analogy of this would be making a small, already grainy picture larger. It will be even more obvious that it is grainy when it has been magnified.

Collumation isn't really tricky once you know what you are doing. Plenty of good tips around this forum. It should be done often for best results, some would say every time you use your scope.


*just noticed you have an astromaster 130EQ in your signature block. That has a focal length of 650mm. So, your 6mm eyepiece on its own gives you a magnification of 108.333X. The barlow doubles that. Now because your aperture is 150mm, you should be able to push your scope to 300X using the rule Andrew talked about (but remember the perfect conditions caveat. Light pollution, the moon, turbulence all affect the seeing). Given that I have confirmed it is a Newtonian Reflector you have, definitely have a look at the collumation and compare the view you have to what it looks like around the new moon.