Shortening my Telescope tube

[Legin (Nigel)]

Hello,

This is more of a question for ideas at this point. I have an Orion 12XTG scope and I considering shortening the tube by 5 cm.

It seems pretty straight forward to do be it with a bit of risk. If anyone has any suggestions, ideas advice or otherwise please let me know.

I am a bit worried about messing my scope up. But I also figure it is easy enough.

I want to perform some astrophotography on my telescope but the focus will not go in far enough. I only really want to photograph the moon and the planets.

Aside from a bit of light loss and extending the focus. What else would shortening the tube do to the scope.

Is it literally a case of just measuring the correct distance from the back of the tube then cutting the tube. I assume you use a hacksaw or something similar if you don't have an angle grinder.

The whole idea seems very easy though the cutting bit will be a bit of a pain.

I just thought to ask first before I take a rather drastic step.

[malclocke (Malc)]

Have you tried with a barlow lens / powermate?

My 10" GSO dob doesn't reach focus with a DSLR, but it does with a barlow. And I'd say you will almost certainly be wanting to use some kind of focal extender for planetary so it might be worth trying that before you start on the surgery.

The other thing that is usually recommended is trying to extend your collimation bolts as far as they will go to bring the mirror further up the tube, and also consider a low profile focuser. It all depends how far out you are from reaching focus though.

[Legin (Nigel)]

Did some testing today and managed to remove a short little ring like piece from my CCD adapter. It is like a 3 mm extension tube of some sort I took it out and now I have about 5mm of play on scope when it is in focus.

Joy....

Not everything will focus. My DSLR still will not. But alas my scope was designed for visual work so I am kind of asking for something it wasn't designed to do.

Alas a nice Barlow is next on my list and maybe a low profile focuser down the track.



Now alas if I could just get a clear night. I got the CCD camera three days ago and cloudy, cloudy, cloudy. Managed to get some sunspots yesterday though the focus wasn't perfect it was almost right.

Oh and that tip about the collimation screws helped a bit I guess all together I managed to pull another 5-10mm of focus which really helps.

[rogerco (Roger)]

You are not specific about your cameras. The DSLR will have problems because of the large distance between the rim and the sensor. Small planetary CCD cameras generally have the sensor much closer to the mount rim and they can focus. The DSLR is not the best for planetary work anyway except in video mode.

If the mirror cell will fit further up the tube I would not be cutting the tube I would just drill a few more mounting holes 50mm up the tube. That way you can restore it if you want to sell it later.

Remember by moving the primary further up the tube the secondary may be too small and you will experience some clipping. If you only have a 1:1 focusor then going for a low profile 10:1 one might be a double improvement.

[Legin (Nigel)]

My second attempt at Jupiter.

I will be getting a Barlow this week so yippee. I will probably get a 2.5 x then down the track a 4 or 5 x for those times of excellent seeing. Plus the avi approach will help in this regard.

The scope itself is great for visual astronomy. Be it in need of near constant collimation. As I move it in and outside it get a bit roughed up.

As it turns out I can now focus my CCD camera being a DMK41 it has a much short focal point than my DSLR. I tried an Orion Solar System Colour imager and while it was fine the DMK 41 seems much better and has far more manual control.

The CCD camera seems better suited for astrophotography than my DSLR so I will call this one a win

[linkpng (Alfred)]

As others have said, think twice (3x) before cutting bits off your tube. Please consider all other options first, like fitting a lower focuser, using a slight Barlow such as 1.3x. The same Barlow lens has different effects depending on where it is placed. Have you looked around for a very low helical focuser? Or can you mount part of the focuser inside the tube? There is always about 15 mm free space inside the tube that would not cause vignetting.

If you really need to cut metal, in principle another option is moving the focuser and spider down and to the opposite side of the tube. All the old holes can then be covered nicely with eight screws and one plate (even thin black plastic will do). The scope can then be fully restored at any time to its original configuration for visual use. It will be very easy to swap between visual and CCD use.

Keep in mind that bringing the (heavy) primary up will also drastically alter the balance, so that the alt bearings have to be moved. Moving the lighter focuser, spider and eyepiece assembly will have less effect. Note that once you cut off the 50 mm it will be difficult to put it back on nicely for when you like to go back to visual use or sell the scope later. Cutting that large diameter of thin metal smoothly without bending the tube is also not a trivial task.

[skfboiler]

I found this out on another forum. You can screw off the top part of the focuser. The remaining part is threaded. Then you can screw on the t-ring and camera directly to the focuser now. They say this should provide adequate focus for DSLR cameras. I tried it with my focuser and sure enough, I can screw my camera onto the focuser with the T-ring. I have not had a chance to attempt pictures yet.