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Fixing/modding cheap scope help needed

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#1 Mrcloc

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Posted 09 July 2020 - 05:46 AM

The first telescope I bought wasn't a known brand or anything. Optics were always ok, but there were obvious cost-cutting construction bits. I upgraded, but I want to use the old scope as a fun tabletop dobsonian type that the children can also use. It also might not be bad for wider views. I dusted the box off the other day and got to work aligning the optics (that's a big mission, but I'll get it right). When I was tinkering, I remembered that I had tried this before, but couldn't think why I didn't finish. I then found the bit that made me stop - a barlow fitted into the focuser. That's why it's a 1000mm Newtonian only about 500mm long. I took out the barlow and tried to get focus without it. The memories of doing the exact same thing all came back. Oh, and I do want to mention that I would prefer a 500mm scope.

 

Right, so as it turns out, the focuser needs to be further in than it can be to get focus. That has two problems - obviously it can't focus is the first, but the other is the obstruction of the focuser tube in the OTA itself. I would like to know if it's possible to correct this, and what the easiest (or most practical) way is. If I move the secondary closer to the primary, I also have to move the focuser, but I'm also going to lose aperture (which I suppose isn't the worst thing ever). If I move the primary (cutting the tube is probably the easiest way here), then I guess I only lose a little aperture. But what if I construct a focuser which can bring the eyepiece closer to the secondary?

 

If there are any other ideas, I'd love to hear them. Maybe I should try turning the barlow around - what effect would that have? I probably couldn't get focus then either. I'll give that a try sometime.



#2 doublevodka

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Posted 09 July 2020 - 06:13 AM

Sounds like a Bird-Jones design - http://astrowiki.jmh...Jones_Telescope

 

Unfortunately I don't think there's any way to get rid of the barlow / corrector as it's designed to work that way. Well unless you replace the primary completely, but for that amount of fuss and cost is it really worth it?

 

This video may help with collimation though - youtube.com/watch?v=5yLJh31bWNQ



#3 Mrcloc

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Posted 09 July 2020 - 06:15 AM

I think it's just an achromatic doublet... But then I'll have to keep it in.



#4 Mrcloc

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Posted 09 July 2020 - 06:17 AM

Sounds like a Bird-Jones design - http://astrowiki.jmh...Jones_Telescope

 

Unfortunately I don't think there's any way to get rid of the barlow / corrector as it's designed to work that way. Well unless you replace the primary completely, but for that amount of fuss and cost is it really worth it?

 

This video may help with collimation though - youtube.com/watch?v=5yLJh31bWNQ

Thanks. Yes, I'll keep it in. I was looking at some lens simulations, and it turns out it is just a corrector. That's fine - I'll keep it in. Then the only challenge is collimating. I got rid of my laser collimator when I sold my 8" dobsonian.



#5 cookjaiii

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Posted 09 July 2020 - 04:29 PM

https://www.cloudyni...erseeker-127eq/

 

Here's an extensive thread on modifying a Bird-Jones telescope.  Most of it may be beyond your interest, but surely you will learn a lot.

 

Good luck.




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