Cutting optical tube [ celestron 60AZ ]

[jonzo]

Hello, I Have cut down my telescopes tube from 700mm focal length to about 350mm focal length. Now when I try to use it, It wont focus. Is the primary objective lens too small now at 60mm? I have tried using my DSLR at prime focus and still no luck...

Help

Jonny.

[Dave Lloyd]

Err

[Tinker1947]

Hmmm, some photo's might help as what you have typed make me think your be needing a replacement tube.....???

[happy-kat]

Have you tried your highest magnification barlow with your camera or eye piece to see if that reaches focus?

[tekkydave]

Hello, I Have cut down my telescopes tube from 700mm focal length to about 350mm focal length. Now when I try to use it, It wont focus. Is the primary objective lens too small now at 60mm? I have tried using my DSLR at prime focus and still no luck...

Help

Jonny.

Why?

[John]

Cutting your tube length by that amount means that the focal point of the objective is nowhere near where the eyepiece / camera will be which is why you can't bring the scope to focus. Cutting the tube does not change the focal length of the objective lens. People do cut their scope tubes down a little to allow devices like bino viewers to come to focus but the shortening is just a few cm not anywhere near 50% of the tube length.

[Cornelius Varley]

Your telescope will still focus at the same point, this is now about 250mm beyond the the telescope. Unfortunately. because you have cut the telescope tube down to 350mm the telescope is now too short to reach the correct point of focus. The function of the tube is to hold the objective lens, which has a fixed focal length and the eyepiece at the correct distance between them to achieve focus. Cutting the telescope tube down doesn't affect the focal length of the telescope.

[jonzo]

@TINKER it's a shorter version of the celestron 60az from 700mm to 350mm

@happykat i'm going to try that

@tekkydave I wanted to try and make it a small refractor for 'wider FOV' AP

@john ah I see, What would you suggest me doing with the scope? I still have the extra 450mm tube laying here

[jonzo]

@Cornelius Varley oh ok so its useless now

[Cornelius Varley]

@Cornelius Varley oh ok so its useless now

Probably. You could try replacing the old tube with a new aluminium tube of the correct length or using a sleeve (match the ID of the sleeve to the OD of the existing) to hold the two cut pieces together.

[John]

I think Peter has sumarised your options.

[Cornelius Varley]

There was one other option. File under B 1 N

[John]

There is a whole 60mm 700mm focal length refractor tube on here for sale for just £18.00:

http://www.astroboot.co.uk/AstroBoot/telescope-and-astronomy-stuff.html

[Tinker1947]

Hmmm, some photo's might help as what you have typed make me think your be needing a replacement tube.....???

When i typed this i figured it was trashed.......

[Moonshane]

if your focuser drawtube is quite long, you could cut off a section maybe an inch or two wide and use this as a sleeve (you may need to also cut it lengthways to allow it to slip into the tube). You could then drill both sides of the joint and fixed with rivets / short self tappers.

[michaelmorris]

If you try and patch the two halves together you're highly unlikely to end up with correctly collimated (aligned) optics.  If replacing the tube, you'll have to check the alignment is spot on.

[Mr Spock]

This is one thing you can't fix with duct tape

[Dave Lloyd]

Any chance of a d.i.y solution where you could make an adjustable holder for the objective?

[Mukv]

which end did you cut off?

[barkis]

Jonzo, with all due respect, I think you have been a little naive.

Why on earth did you do such a thing, and why did you not seek advice before embarking on such 

a devastating action. The focal length of any telescope cannot be changed by altering the tube length alone.

Ron.

[alexjj67]

which end did you cut off?

Damn you beat me to it 

[SkySurfer]

Get another lens for the piece you have cut off, and make a pair of bins.... :)

SS

[LeeB]

could always put a hinge on it...