Thanks for the tip Gary.... I've already bought a 35mm 2" extension tube - so whichever way I reduce the distance, it will be by 35mm - will be plenty, with some to spare....
Hi Lee...wrap a length of wide paper ribbon around the circumfirence of the tube so it overlaps...then tape it to the tube..makes a nice straight edge to guide you!
Cheers!
Wouldn't it be easier to move the spider and focuser down a bit? Four new (little) holes for the spider and one new (big) hole for the focuser. If you put the focuser on the opposite side to its present position you could have a reasonable interchangeable setup--maybe have 2 focusers and just move the spider up or down.
Geoff
Nope,
A couple of hundred years of Amateur Telescope making has told the astronomical community it is infinitely easier to shorten the tube of a newtonian telescope in this situation than to move the focuser and the spider.
To be perfectly honest, as Gary Beal indicated, it should take less than 30 minutes with an angle grinder to cut the tube to the required length and drill new holes to fasten the primary mirror cell. More import than the time issue, however, his scope will look like it has been done semi professionally rather than look like something a German sniper has used for target practice, with unused holes in it everywhere.
CS-John B.
PS: Lee, when you cut the tube, take your time and make sure you don't "burn the steel". You could also use a Dremel to cut the tube. I cut 1.2mm stainless steel with my dremel. It takes a while but does a neat job.
Just a thought....what about making another tube complete with spider, secondary and focuser then just swap the primary over...I know it's a lot more work..but you'll retain the original tube..
I think the extension tube in the original tube soounds much easier.... For the work involved in making a tube, focuser etc, I'd just buy a new scope....
Well, I have done it..... 35mm of metal sliced from the bottom of the Newt.... made the most of a visit from my father, and we attacked it together.
Now I need some clear skies to test - I have nothing terrestrial at infinity focus here....
Now - where's my cheshire! I think I may need it.....
Don't look at the pic if you are squeamish (about your beloved scope!)....
That would be a brave estimate!
No - find the focus point of the Newt like this. Rack the focuser all the way in - I removed the 1.25" adaptor as I intend to use a 2" T adaptor on the camera.
Point the scope at the moon - take a piece of white card and hold at the focuser opening, move back and forth (along a tape measure held against the focuser) and you will see when a focused image is produced - this is where you want the CCD in the camera. In mine it was 36mm from the end of the focuser.
Since the CCD sits about 42mm back in the camera body (measured with carefully lowered toothpick to the side of the CCD), and the T-adaptor takes up 13mm or so, I worked out I needed *at least* 20mm or so extra in-focus.
Because a 35mm extension piece is readily available, I decided on taking 35mm off the tube, this way the camera should easily obtain prime focus, and the 35mm extension tube should allow all my EP's to work exactly as before.
You have a nice big moon tonight - should be easy to work out where the focal plane is relative to the end of the focuser.
I'm going to test the camera on the Newt tonight with the full moon, hope to post a report in this thread before too long.......
First light for the new-length Newtonian.....
Rudimentary testing only tonight - reached focus no problem visually with the 35mm extension piece in focuser, and DSLR (D70) focused without.
Image of the moon - not great, but focused - taken at the end of my driveway about 2hr after moonrise in a stiff breeze - scope now seriously needs rebalancing of course, so I had to hold the tube still while the shutter opened. Also forgot to take the thing off ISO 1600 from the other night
At least it focuses!
Will recollimate soon, then hopefully will punch out some DSO's....
Mike - I'm in the process of putting together an article about the adventure.
Hope it's all going well with the new setup! I just went out and purchased a 400D. I plan to use this with my TAL 200K and also 8" Dob (both scopes interchanged on EQ6). Cutting the tube sounds quite drastic as I would have expected a low profile focuser to do the trick
Lee, I see you at least took the precaution of getting someone else to do the cutting after removing the safety guard How nicely finished does the edge look after cutting with the grinder?
A guy I know put a larger diameter disc on the same size grinder you were using - the extra tip speed and forces generated caused the thing to fly apart - he's now minus one eye.
It's done!
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Initial test successful!
How nicely finished does the edge look after 
