Anyone know where I can get a 2" 25mm extension tube?

[great_bear]

The new mirror in my scope seems to have a slightly different focal length than the previous one.

This means I need to get a 25mm extension tube for the 2" focusser. 35mm seems common, as does 50mm - but I need a 25mm one.

Anyone know where I can get one?

[Spile]

Would there not be enough tolerance in the 35mm to allow you to achieve focus?

[AstroKeith]

There is a minimum length to extension tubes as it has to be able to accommodate the whole of the 2" nose of the eyepiece. 35mm is probably the shortest you'll find.

[great_bear]

2 hours ago, AstroKeith said:

There is a minimum length to extension tubes as it has to be able to accommodate the whole of the 2" nose of the eyepiece. 35mm is probably the shortest you'll find.

D’Oh! Of course! That makes sense now - thank-you. 

[great_bear]

3 hours ago, Spile said:

Would there not be enough tolerance in the 35mm to allow you to achieve focus?

Unfortunately not - it’s that pesky non-parfocal 17.3mm Delos and the extra infocus that it requires!

In light of the comment from @AstroKeith I guess I’ll just move the primary forward instead; it’s easy enough to do. 

[Mandy D]

53 minutes ago, great_bear said:

Unfortunately not - it’s that pesky non-parfocal 17.3mm Delos and the extra infocus that it requires!

In light of the comment from @AstroKeith I guess I’ll just move the primary forward instead; it’s easy enough to do. 

It might not be possible to get an extension tube, but you could get a collar machined in aluminium to effect a 25 mm spacing. It might not be the perfect solution, but I have done this in the past. You just need to take care to ensure the eyepice is properly secured in the focuser.

[AstroKeith]

I believe Televue do a special "in-focus adapter" that brings the 'pesky' Delos par focal.

An alternative I did for my reticule eyepiece that had the same problem is get an old 1 1/4 to 2" adapter and saw off the top flange so it would go further in the focuser. I used a grub screw to hold the eyepiece in the adapter.

[great_bear]

2 hours ago, AstroKeith said:

I believe Televue do a special "in-focus adapter" that brings the 'pesky' Delos par focal.

The downside is it’s pricey, and that the design necessitates an Allen key rather than a thumbscrew to secure the eyepiece (great suggestion nonetheless!):

 

(probably worth noting that it’s only parfocal when used with a paracorr, but is nonetheless still useful for reaching focus with eyepieces like the 17.3mm and 14mm Delos which some scopes can struggle with)

Edited November 5, 2022 by great_bear

[great_bear]

Mirror now moved - hoping this doesn’t cause a problem reaching focus in the opposite direction 🙂

Looks like it’ll be a couple of nights before it’s clear enough outside for me to try…

Edited November 5, 2022 by great_bear