same aperture diferent focal length

[Jazzmatazz]

Hi guys,

I was looking for a OTA and found within budget both this options:

Orion Telescope N 203/1000 OTA Orion Telescope N 203/790 Astrograph OTA

Can you explain me in terms of observation, the diferences between them? the 790mm is more expensive, i thought it would be otherwise...

thanks!

[Peter Drew]

The 203-1000 telescope is a standard general purpose unit intended mainly for visual observation.

The 203-790 telescope is a specialist design most suited to astrophotography.

Quality short focus telescopes are often more expensive. 

[Jazzmatazz]

but whats the diference between the two, looking through the same Eyepiece??

[Astro Imp]

The longer focal length will give greater magnification but less field of view than the shorter focal length assuming the same eyepiece in each.

BTW magnification=focal length of scope/focal length of eyepiece.

[John]

The 203-790 could well have a larger diameter secondary mirror which would reduce contrast a little when viewing the moon and planets. The F/3.9 focal ratio of the 203-790 will mean that the collimation "sweet spot" is smaller, coma will be more apparent and eyepieces will need to be even better corrected.

[jefrs]

It has not been said but an f-stop number (of any lens system) is focal.length / iris.diameter by definition i.e. 790/203 = f/3.89... (f/3.9)

By rearranging the eqn focal.length / f-stop = iris.diameter

The f-stop number is of course a measure of total light capability (not corrected for lens transmission which is the "T-stop")

and the iris diameter will be the mirror/lens diameter, almost but not quite same thing as aperture.

A low f-number is "fast" and better suited for photos (which can be enlarged)