Ok, Noob Question

[Kain]

Doesn anyone know in MM that focal length of my Dob lol.. (How embrassing)

I know it's F5, so what would that be? I mesured the tube and thats 1220mm long :S

Kain

[GazOC]

Is it 10"?

If so, 10" is about 250mm (or 254mm)

So 250mm x f5 = 1200mm

[Kain]

So to find a power of an EP, it's 1200 / 20mm for example?

Kain

[GordonCopestake]

yup a 20mm eyepiece is x60

[Kain]

Ahh, i'm slowy getting he hang of these numbers

Kain

[GordonCopestake]

It'll give you a 3.3mm exit pupil which is quite large. An "optimal" eyepiece would be a 12mm as that gives you a 2mm exit pupil which i find most comfortable. It also gives you a useful magnification of x100. To reach your theoretical maximum magnification (on a good night) you would need a 3mm eyepiece or a 6mm 2x barlowed

[Kain]

Thanks for that Gordon

Kain

[Kaptain Klevtsov]

Just to chip in to clarify the numbers a little as I have the T shirt as it were.

The f5 that you see bandied about is actually f/5 as in focal length (f) divided by 5. So a 100mm f/5 'scope has a focal length of 500mm (aperture (100mm) X f number (5) =focal length).

10" converted to mm is 250 for your 'scope, f/5, therefore 5 times 250mm = 1250mm.

Magnification is focal length of the 'scope divided by the focal length of the eyepiece so you need to know the focal length for all sorts of stuff. Image scale for imaging goes mainly on focal length, thats why camera lenses are described by their focal length first. The f/ number tells you how "efficient" the system is at collecting the light.

For a couple of examples:-

My OMC140 has a focal length of 2000mm therefore a f/ number of 14 and a bit. Thats poor for imaging as the amount of light gathered is spread very thinly and the exposure time needed is pretty big.

The 300mm Skelescope is f/5.3 therefore it has a focal length of 300 X 5.3 = 1590mm so the exposure time required is much less because of the different f/ number, but the image scale is not dramatically different.

What this means is that I can choose either 'scope and get roughly similar image scale, but with the Skelescope I need much shorter exposures.

I'm not doing this very well, but some of it might be usefull.

Captain Chaos

[Astroman]

Excellent decription, CC.

[FLO]

Excellent decription, CC.

Absolutely 8)

Bookmarked it for future use