What does this mean please - requires additional 20mm focus travel

[Angelika]

I may purchase a multiple filter wheel and the description says it requires additional 20mm focus travel. Please forgive my ignorance, does this mean I will also need an adapter of some kind?

Thank you for your help

[Merlin66]

It just means that you need to allow 20mm for the width of the filter wheel between the camera and the focuser.

You may already have this within the current focusing range....

[Angelika]

I will only be using the filter wheel with eyepieces, will that make a difference??

[yeti monster]

Yup! The thickness of the filter wheel will move your EP out by that amount (20mm), so you'll need to be able to rack the focuser tube in the extra 20mm to regain focus after fitting.

[Angelika]

The thickness of the filter wheel will move your EP out by that amount (20mm), so you'll need to be able to rack the focuser tube in the extra 20mm to regain focus after fitting.

How do I do that ?

[Merlin66]

When you normally use your eyepiece you use the focusing knobs to get it into good focus; if you change your eyepiece you normally need to refocus using the same knobs....

With the filter wheel in position the thickness of the body is "pushing" the eyepiece further out, so to compensate you have to re-focus and move the focuser 20mm towards the tube. If you normally use any spacers/ extension pieces/ diagonals etc these should be removed before adding the filter wheel.

(If there's insufficient travel available in the focuser - unfortunately you have a problem? )

HTH

[Angelika]

This is making sense, thank you... so I won't know if I have insufficient travel in the focuser until I have the filter wheel in place... now I need to find someone with the same scope and a filter wheel...

[John]

If you bring the scope to focus (on an astro object) without the filter wheel, then note / measure the remaining amount of inward focuser travel, you will know whether you have the necessary 20mm available.

[jimmyjamjoejoe]

Just stick your usual eyepieces in the focuser and bring them into focus, then remove the eyepiece and measure the focuser tube sticking out. If it's sticking out further than 20mm, then you should be fine.

[ollypenrice]

Beware, though. Filterwheels are designed primarily for use with cameras. You will need to screw an eyepiece holder onto one side of the wheel and a nosepiece on the other. Both of these are likely to have a small shoulder or flange which will add a couple of mm to the 20mm I suspect.

Olly