draw tube lense

[lancerde]

Hi folks

why does my telescope have a lense at the bottom of the draw tube.

Thanks JC:icon_scratch:

[LeeB]

some scopes reduce there tube length by having a secondary lens, your scope would not focus withouit it.

[Tribal-Wolf]

Some also have an image erecting lens fitted in so you can use the scope for terrestrial use, Celestron Astromaster and Powerseekers I think have them

[E621Keith]

The purpose of the lens depends on your scope

In a budget reflector the lens in the focuser is usually a barlow to increase the scope focal length and to hide poor optics.

In top end scope, they are usually field flatteners for correcting field curvature, e.g. Petzval refractors, Modified Dall Kirkham, EdgeHD.

[Cornelius Varley]

What telescope is it ?

Peter

[lancerde]

Its a TS 150mm 1400 reflector. Not very expensive. Thank you for all the replies.

JC

[Tribal-Wolf]

It is a barlow to increase the focal length of the scope

This is a similar scope from same company

[lancerde]

Thanks. Yes different colour but exactly the same scope.

[Cornelius Varley]

This type of telescope is known as a Bird-Jones. The mirror is spherical with a focal length of 1400mm. The lens is the focuser is there to correct spherical abberations and to compress the light path down to a shorter length.

Peter

[lancerde]

Thanks Peter

Is it possible to set up the secondary with the lens removed from the focuser? It is easier to see this way, or must I leave it in to achieve a correct collimation?

JC

[Cornelius Varley]

Thanks Peter

Is it possible to set up the secondary with the lens removed from the focuser? It is easier to see this way, or must I leave it in to achieve a correct collimation?

JC

The telescope won't won't without the corrector lens.

Peter

[lancerde]

Thanks Peter