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what is the extension tube for on a diagonal for

beginner equipment Meade SCT
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#1 SDAngler

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Posted 23 January 2021 - 06:17 PM

Just ordered a 2" diagonal for my 10" Meade LX200 Classic SCT.  The diagonal will screw directly on the back of my OTA, but there is also an extension tube that comes with the diagonal that adds about an inch spacing between the tube and the diagonal.  Is there any benefit from using the extension tube other than maybe not hitting my head on the back of the telescope?  Is there an optical benefit?  Should I use the extension tube?   What is the purpose of this extension tube?  I've attached a photo of what I am describing.  Any advice would be much appreciated.

 

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#2 sevenofnine

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Posted 23 January 2021 - 06:42 PM

Extension tubes are usually used to reach focus with some eyepieces or barlow combinations. 


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#3 SeattleScott

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Posted 23 January 2021 - 07:36 PM

I wouldn’t use it unless you can’t reach focus without it for some reason.

Scott
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#4 SDAngler

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Posted 23 January 2021 - 08:38 PM

Sorry....I don't understand the term "reach focus".  Does that mean the focus knob could be blocked and I won't be able to reach it, or does it mean I can't reach, as in achieve, focus (get it into focus) unless I use this extension tube when using a barlow or certain eyepieces. Perhaps due some reason for the need for additional space in between the diagonal/EP and the back of the telescope to properly focus an object.  I'm a newbie and don't understand the terms yet...



#5 PNW

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Posted 24 January 2021 - 12:55 AM

The term reaching focus refers to achieving focus. Sometimes the focuser will reach it's limits before you achieve focus. That 1" extension tube will allow you to compensate if needed. You may need extra space between the SC Adapter and the eyepiece.  Normally you won't need it. Your SCT has a lot of turns before it limits out. The only way to tell is to try it out.


Edited by PNW, 24 January 2021 - 01:06 AM.

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#6 ravenhawk82

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Posted 24 January 2021 - 04:31 PM

Consider that the telescope comes in to focus at a set distance behind the back of the tube. I don't know what yours is specifically, but what matters is that you can't really change this distance. The purpose of a focuser is to increase or decrease the length of the tube to allow different eyepieces and cameras to reach this focus point.

If you want to look through it visually, you'll probably be using a diagonal to hold your eyepiece. The diagonal adds a certain amount of length that the light needs to travel through to reach your eyepiece. In this case, you *probably* won't need the extension tube. However, lets say you want to mount a camera. Now that you're not using the diagonal the tube length is effectively shorter and you  can't rack the focuser back enough for your image to be in focus. This is where the extension tube comes in. Now you can increase the tube length so that your camera is sharp.

I used the camera vs eyepiece comparison because this is the situation in which I most often find myself needing to add or remove an extension tube. It all depends greatly on what specific eyepieces, cameras, correctors, and flatteners you're using though. If your focus goes all the way back and it's not in focus yet, add the extension. Conversely if the opposite happens and you can't push the focus in enough, remove the tube.

Hope this helps!


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