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Prime focus - why the need for an extension tube?

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Old Friday 11th January 2019, 22:46   #1
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Join Date: Jul 2008
Location: Gloucestershire UK
Posts: 118
Prime focus - why the need for an extension tube?

Hello, I am just setting up to try prime focus digiscoping (Acuter 20-60x80 scope, Canon eos760D). I intend to use a 2x (possibly 3x) Barlow. Whilst looking at the equipment used for some of the prime focused, digiscoped pics shown in various user galleries here I see that most have included an extension tube (often 2"). I am assuming that this is placed between the Barlow and the camera. Can anyone clue me up on the following:-
A)What is the purpose of the tube?
B)How do I know which length of tube I require?
C)How are tubes - particularly the end fittings - specified? Do most tubes come with a T thread at each end to screw into the Barlow and T ring on the camera?
D)How do I check out that the tube is working optimally to do what it should do?
Any other advice and suggestions relating to prime focus digiscoping with a Barlow and a DSLR would be most gratefully received, thank you in advance, Pete
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