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AT72EDii, GSO Plossl 32mm, won't focus (refractor newb)

refractor beginner observing eyepieces
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#1 wookieking

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Posted 12 January 2018 - 09:24 PM

Have a brand new at72edii from our fine friends at astronomics. Picked up a Agena Plossl 32.

Put eyepiece in scope, and the only way to focus stars is to fully extend the focuser and pull the eyepiece out a smidge.

My refractor experience is decades behind me, and this probably will make a "top 10 newb questions" list, but what is going on?

Do I need a diagonal for length? Am I horrendously mismatching eyeiece to tube?

Thee t-ring and adapter used for my Nikon and AVX newt have the same focus problems in the opposite direction.

But that setup works with the newt.

Please help a refracto-newb on what should be first light.

#2 gene 4181

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Posted 12 January 2018 - 09:33 PM

  Diagonal or extension tube



#3 petert913

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Posted 12 January 2018 - 09:37 PM

Yep,  you need to insert a diagonal or use an extension tube.  That will fix you up.



#4 jay.i

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Posted 12 January 2018 - 09:38 PM

You will need a diagonal, or an extension tube as gene said. I have the AT72EDII as well and it will not come to focus for any of my eyepieces without one. I didn't get binoviewers to come to focus either, actually. But yes, you'll need to eat up some of the light path with a diagonal. A prism diagonal has the shortest path (I think), a mirror diagonal will have a longer path. I use a large 2" Astro-Physics mirror diagonal and have no problems bringing any eyepiece to focus.



#5 wookieking

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Posted 12 January 2018 - 09:40 PM

Thanks to both. I thought that might be the case. The over correction by the Nikon/newt setup was throwing me.

But foregoing the direct view and spending more time with the Nikon/backyard combination has yielded the the first pin pricks of light!

Back into the cold and thank you again.

#6 jallbery

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Posted 13 January 2018 - 08:49 AM

Telescopes create an image at the focal plane.  When you use an eyepiece, the focal plane needs to align with the eyepiece's field stop location, which is in the barrel of the eyepiece.   When you attach a camera, the focal plane image needs to be inside the camera at the location of the sensor.

 

So if we eliminate a diagonal, when using an eyepiece, the focal plane needs to be INSIDE the focus tube.   And when using a camera, it needs to be OUTSIDE the focus tube.

 

Now, most visual users of a refractor will use a diagonal, so it isn't unreasonable for a refractor design to assume the use of a diagonal.   If the user really wants to go straight through, he/she can add an extension tube.  Why not just make the draw tube longer?  That can lead to vignetting, particularly with faster refractors.

 

Newts are typically even more challenged for focuser travel: you don't want the focus tube sticking back into the light path, and the higher profile you make your focuser (to allow for more travel), the bigger secondary required.   So Newts are often designed primarily for either visual or imaging.   A visual newt may lack sufficient travel to get the focal plane outside the focus tube.   An imaging newt may require an extension tube to be used visually, particularly with specific eyepieces.

 

If you have a Newt that was designed for visual use and want to use it with a camera, a common solution is to move the primary mirror forward a distance sufficient to allow the focuser to position the focal plane at the location for the camera.  A lower profile focuser might also due the trick. If you only need a few millimeters, it may even be possible to use the collimation adjustments to move the entire mirror forward enough to allow for the required focus point.




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