Shortening tube on refractor

[AstroNebulee]

Hi

I would like to purchase a skywatcher st80 but a thought came to me. 

I have an old 80mm F10 meade refractor scope as pictured. I was wonder could I just shorten the tube to make a 400m refractor similar to the st80. 

I can take the lens mounting off the scope and shorten towards to focus tube end and reinstall the lens mounting back on. What I'd like to know is the possible and feasible to do and where do I take the measurements to and from to make it into a faster refractor. Do I measure from the lens cell down or from the ficus tube up. Its probably simple yo someone just liked to know I get it right. The tube is metal. 

I have attached some images to see. If any more information needed just ask. 

Or am I just being a total idiot in thinking this. 

Lee 

Edited February 9, 2023 by AstroNebulee

[Stu]

4 minutes ago, AstroNebulee said:

Hi

I would like to purchase a skywatcher st80 but a thought came to me. 

I have an old 80mm F10 meade refractor scope as pictured. I was wonder could I just shorten the tube to make a 400m refractor similar to the st80. 

I can take the lens mounting off the scope and shorten towards to focus tube end and reinstall the lens mounting back on. What I'd like to know is the possible and feasible to do and where do I take the measurements to and from to make it into a faster refractor. Do I measure from the lens cell down or from the ficus tube up. Its probably simple yo someone just liked to know I get it right. The tube is metal. 

I have attached some images to see. If any more information needed just ask. 

Lee 

It doesn’t work like that unfortunately. The optics remain the same focal length, so all you will end up with is the same scope that probably needs an extension to reach focus! Don’t do it! Get an ST80  

[CraigT82]

If you chopped the tube down the lens is still an F/10 and the eyepieces will still come to focus 800mm beyond the lens. You can’t change the focal length… you’d need to regrind the lenses! 

[AstroNebulee]

Just now, Stu said:

It doesn’t work like that unfortunately. The optics remain the same focal length, so all you will end up with is the same scope that probably needs an extension to reach focus! Don’t do it! Get an ST80  

Thank you Stu for confirming this. My brain sure does work in a duff way to most normal people 🤣

Lee 

[AstroNebulee]

1 minute ago, CraigT82 said:

If you chopped the tube down the lens is still an F/10 and the eyepieces will still come to focus 800mm beyond the lens. You can’t change the focal length… you’d need to regrind the lenses! 

Thank you Craig 👍

[Stu]

Just now, AstroNebulee said:

Thank you Stu for confirming this. My brain sure does work in a duff way to most normal people 🤣

Lee 

Don’t worry Lee, you aren’t the first to suggest this and sure as heck won’t be the last 🤣

[jmrp]

Lee - 

it can be done - if you are intent on cutting the F10 tube you can get 
an F5 telescope using the following information and a commercially 
available 0.5x focal reducer on the focuser tube end:

D80F10 to D80F5    this is just one example - cut 100mm off tube    
if this does not work for you - you can use a 100mm extension tube
at the focuser (no focal reducer) to restore original focal length                                                                                                                                                                                                                                                                                                                                    

Focal Length Objective             800mm    
Focal Length Reducer(FR)         200mm    
FR dist behind objective lens   600mm    
Back Focal Length                    100mm
Effective Focal Length              400mm
Total Length                             700mm

Bob

                    

[Peter Drew]

I'm not disagreeing but you could probably get a used 80mm f5 telescope for around the price of a focal reducer.  I wouldn't be too optimistic about the field characteristics of a telescope at the severe reduction of a 0.5x FR.    🙂

[AstroNebulee]

Thank you guys. I have ordered a st80 from FLO so no cutting the Meade tube for me and now I'll be owning a much raved about st80. 

Lee

[Louis D]

3 hours ago, Peter Drew said:

I'm not disagreeing but you could probably get a used 80mm f5 telescope for around the price of a focal reducer.  I wouldn't be too optimistic about the field characteristics of a telescope at the severe reduction of a 0.5x FR.    🙂

How hard would it be to find a matching doublet and the correct distance to place it in the tube to make a poor man's Petzval or Aplanat design?

[Peter Drew]

To give good results?. quite hard I expect.     🙂