confused about focal lenghth

[crbazzer]

i'm new to all this so please be patient, let me try and explain my confusion. i have a skywatcher 130P scope, it states its focal lenghth is 650mm,the actual lenghth of the tube is about 600mm long.my confusion is:- my dad has a seben star 114, its a simple reflector like mine but smaller, its tube lenghth is only 400mm long but it states its focal lenghth is 1000mm. is the focal lenghth determined by the manufacturer in some way or can you actually measure the distance of tube etc to find out what it actually is yourself. not sure how they come up with the sizes,i know its the distance the light is reflected to the focussing point but that doesn't add up if you measure it. any help would be great,

thanks.

[jgs001]

The focal length is determine by the objective of the system... in the case of the reflector the primary mirror. As you've noted your tube is about the same length (allow for the reflection out to the eyepiece and you should find it's spot on). The Seben on the other hand, will have the equivalent of some form of barlow in the focuser to increase the effective focal length.

[assasincz]

I may be wrong but I believe that some manufacturers actually mount an extra barlow optical element in the focuser tube to make the focal length of those short-tube reflectors longer.

Perhaps somebody more qualified will clarify

[John]

I may be wrong but I believe that some manufacturers actually mount an extra barlow optical element in the focuser tube to make the focal length of those short-tube reflectors longer.

Perhaps somebody more qualified will clarify

Thats exactly how they do it. It's not a great optical design in my opinion - it makes collimation very difficult for one thing.

The Skywatcher 130P will be much better optically than the Seben - but don't tell your Dad !

[Cornelius Varley]

The Seben telescope is design known as a Bird-Jones. In the focuser is a corrector lens that compresses the focal length of the mirror (1000mm) into a shorter tube. As stated earlier they difficult to collimate and optically are not as good as a Newtonian.

Peter

[ollypenrice]

The replies above are correct but remember that the tube on a Newt only repesents part of the focal length. If you took out the secondary the converging beam (light cone) coming from the mirror woud focus (come to a point) at whatever is the distance of the mirror's focal length. That would usually be just outside the end of the tube.

However, the converging beam is reflected out at 90 degrees through the focuser before it comes to a focus in the direction it was going.

Olly

[Damo636]

I used to have a Celestron 127 with the barlow in the focuser to a achieve the 1000mm fl. It was a nightmare to collimate.

[crbazzer]

Thanks guys for all your quick replies, my confusion is completely clear now. I can now get on with some more important stargazing, allthough i will be back with more questions shortly no doubt.

cheers chris