First DIY 5" refractor

[Cam (Cam)]

I purchased mounted 127mm 700mm glass from Surplus shed. Had a mate who had a tube from an old reflector that will fit if I machine the threads off the cell. Im starting off with a focuser I got for free that came off a 60mm Meade refractor to test the glass. If its ok I will spring for a moonlite or sim. What Im worried about is cutting the tube length. Where do I have the focuser tube when measuring? in? out? half way? Do I allow for the diagonal? if so how much? Hope someone can take the time to advise. Thanks Cam

[dannat (Daniel)]

If you're going to be using a diagonal you definitely need to add its length to the train, is it a 2" unit (they're longer than 1.25")
if your not mounting a camera you can prob rack focuser half way with diagonal in place to measure the length.
Also check the lens itself to dbl check it's actually 700mm f/l

[Cam (Cam)]

Thanks for your reply. I would like the option of being ale to use a camera. Its a 1.25" at the moment. If I decide to upgrade to a moonlite focuser I guess it will be 2" although my eyepiece collection at present only consists of 1.25"

[JC (Julian)]

Originally Posted by Cam

I purchased mounted 127mm 700mm glass from Surplus shed. Had a mate who had a tube from an old reflector that will fit if I machine the threads off the cell. Im starting off with a focuser I got for free that came off a 60mm Meade refractor to test the glass. If its ok I will spring for a moonlite or sim. What Im worried about is cutting the tube length. Where do I have the focuser tube when measuring? in? out? half way? Do I allow for the diagonal? if so how much? Hope someone can take the time to advise. Thanks Cam

Hi Cam;
You might have to recut the tube when fitting a 2 inch focuser & diagonal, you can measure the focuser length and a 2 inch diagonal will add another 80 to 100mm that will need removing;

"Last column is light path length added by diagonal in millimeter.

added = 222 - raw + offset

Code:
raw(mm) Diagonal-name size(inch) offset added(mm)
222.0 no-diagonal 2.00 0.0 0.0
114.5 WO 2.00 0.0 107.5
112.0 OPT 2.00 0.0 110.0
115.5 TAK 2.00 0.0 106.5
118.0 AP-MaxBright 2.00 0.0 104.0
109.0 Baader Mirror 2.00 0.0 113.0
156.0 Baader-T2*1 1.25 8.5 74.5
147.5 Baader-Amici*1 1.25 8.5 83.0
109.0 TV-Everbrite*2 1.25 8.5 121.5"

http://www.cloudynights.com/ubbthrea...2/Main/3313600

Cheers; Jules

[Cam (Cam)]

Thanks for that. Im still not exactly sure where to have the focuser when measuring if I want to use it for visual as well as photography. How do I check the glass is actually the stated 700mm fl?

[ZeroID (Brent)]

Take it out in the sun (infinite distance light source sort of) and burn a few ants off. Use a tape measure or mount it on a rod. You are only trying to ensure it is 'about' 700mm, not measure it accurately. The focusser should take care of the rest. You can always fake the scope up on a piece of wood before hand as well to geta better idea of how it will assemble.
I always make the tube\OTA too long. Easier to cut a bit off than glue a bit on. The other method is to make a cell to hold the lens that fits(slides) inside the tube and adjust to suit then fix.

I have a 2" focusser and a suitable tube for a 100mm ..... but no suitable lens .
One day it will come together ....