What is an extension tube used for

[Neilsermk1]

I purchased a second hand Skywatcher 150 Newtonian and it came with a 37.5 mm. tube with 2 grub screws in it.. As a complete newbie I don’t know what this is for. Any help would be greatly appreciated. Best regards

Neil

[Zeta Reticulan]

If your scope has a Crayford focuser it is probably to help certain focal length eyepieces to attain focus. It's not uncommon to not have enough back focus with Crayfords, so the extension provides that.

[Neilsermk1]

Would this also be used with a DSLR to attain focus

[Star Struck]

I cannot get any eyepieces to focus on my SkyWatcher 200 Newtonian without it.

 

This YouTube video will help answer the question about getting a DSLR to focus with your scope.

 

 

[Peter Drew]

Hello Neil, welcome to this forum.

Astrophotography is such a popular aspect of amateur astronomy these days that manufacturers are increasingly configuring telescopes so that a camera will come to a focus. The camera focuses closer to the telescope which means that eyepieces focus further out, the extension tube makes up the difference.

 

[Neilsermk1]

This is starting to make sense now, thanks for all your help. Having now also watched the video on the link I can see how it is used . Thanks again to all who responded 

[Louis D]

17 hours ago, Peter Drew said:

Hello Neil, welcome to this forum.

Astrophotography is such a popular aspect of amateur astronomy these days that manufacturers are increasingly configuring telescopes so that a camera will come to a focus. The camera focuses closer to the telescope which means that eyepieces focus further out, the extension tube makes up the difference.

 

Does this mean they are using oversized secondaries to fully illuminate the eyepiece's field of view given the high profile of the focuser?

[Cornelius Varley]

2 minutes ago, Louis D said:

Does this mean they are using oversized secondaries to fully illuminate the eyepiece's field of view given the high profile of the focuser?

The PDS versions do have a larger secondary.