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Refractor won't focus - do I need an extension tube

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#1 Bigal1817

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Posted 14 July 2020 - 02:55 PM

My grab and go 4" won't focus with a 2" 30mm eyepiece, do I need an extension tube?  I tried using the 35mm extension tube that came with my 8" dob but I suspect it's not long enough.  My refractor is a 600mm fl and 102mm aperture. 



#2 Xeroid

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Posted 14 July 2020 - 03:03 PM

Was that "no focus" event  WITH or WITHOUT a diagonal ?


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#3 junomike

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Posted 14 July 2020 - 03:05 PM

Try without the Diagonal to determine if it's in-focus you require..


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#4 Bigal1817

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Posted 14 July 2020 - 03:05 PM

It was without the diagonal.  The diagonal only fits 1.25" ep so I removed it to try and use with the 2" ep



#5 Dave McCrary

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Posted 14 July 2020 - 03:10 PM

You will need a 2 inch diagonal then.


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#6 Bigal1817

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Posted 14 July 2020 - 03:18 PM

Thank you.  I am curious, why do I need a 2" diagonal?  



#7 JohnBear

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Posted 14 July 2020 - 03:39 PM

 

You will need a 2 inch diagonal then

OR an extension tube equivalent to the extra light path thru the diagonal.

 

Hope that helps.


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#8 Couder

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Posted 14 July 2020 - 04:03 PM

Before you buy anything, just try looking at something (maybe the moon?) and pull the eyepiece out, keeping it as close to lined up as you can. If you can get to focus, you need an extension. If you can't, then you need in-focus.


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#9 Bigal1817

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Posted 14 July 2020 - 04:07 PM

That's very helpful, I will try that.  Thank you!



#10 JohnBear

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Posted 14 July 2020 - 04:08 PM

Yo don't have to wait for nightfall either.  Just a focus on anything a couple miles away.


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#11 JamesMStephens

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Posted 14 July 2020 - 04:12 PM

Thank you.  I am curious, why do I need a 2" diagonal?  

The diagonal is the same thing as an extension tube, but with a 90-degree bend!


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#12 SonnyE

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Posted 14 July 2020 - 06:31 PM

Thank you.  I am curious, why do I need a 2" diagonal?  

Is that a trick question? flowerred.gif

 

Well, because nobody makes a 2"  down-funnel to fit a 2" eyepiece into a 1 1/4" diagonal.

But everybody supplies a spool to fit a 1 1/4" into a 2" hole.

 

When I bought my diagonal, I didn't even consider a 1 1/4 " Diagonal. I went right to the highest transmittance section.

And I got a Williams Optics. Because I knew I might one day want to use it with a camera.

Incidentally, all my eyepieces are 1.25".

 

I used Couder's suggestion many years ago to determine extension length. But I was using a camera.

It actually works very well.


Edited by SonnyE, 14 July 2020 - 06:32 PM.

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#13 sanbai

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Posted 14 July 2020 - 10:56 PM

Thank you. I am curious, why do I need a 2" diagonal?

Because using a refractor without a diagonal causes discomfort, to put it mild.

If your eyepiece is 2", then you need a diagonal that accepts 2" eyepieces. For the others, use an adapter 2"->1.25", which may come with the diagonal.

I remember getting last year my first (used) 2" eyepieces and not having yet the diagonal... What a wait! I still tried, but it was a pain. Luckily I didn't wait for long, an excellent one popped in the classifieds.

Edited by sanbai, 14 July 2020 - 11:00 PM.


#14 Phil Sherman

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Posted 15 July 2020 - 10:56 AM

Eyepieces usually consist of an eyepiece assembly and a screw on tube that goes into the focuser. If you have some additional 2" eyepieces, you can remove a couple of these tubes from other ones and use them as extension tubes to figure out how much extension you'll need.



#15 SteveG

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Posted 15 July 2020 - 06:59 PM

Most modern refractors with 2” focusers don’t have enough outward focus travel to use without a diagonal (or extension tube). As others have noted, get yourself a 2” diagonal, which typically comes with a 2” to 1.25” adapter.


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