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Difficulty in Focusing

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#1 malaylahiri


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Posted 23 October 2021 - 01:27 AM



   I am Malay from New Delhi. India, a newbie here.  I have recently purchased a 10 inch GSO DOB.  However, ever since its assembly, I find the focusing extremely difficult.  I have also collimated it using Astromania next generation laser collimator and the scope is perfectly aligned with the finderscope.  Still no luck.  I have tried the focusing with 30 mm GSO lens. What I am doing wrong?  I hope there is no problem with the focusser itself. Please advise.  



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Posted 23 October 2021 - 01:31 AM

Malay, perhaps you need to use an extender, or perhaps you are using one that needs not to be used. Your scope probably includes at least one extender. Try different combinations that will change the length of the optical path. Good luck.
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#3 gitane71


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Posted 23 October 2021 - 06:10 AM

faster focal ratio scopes have a small range in which they focus.  It might help to use a Barlow, getting rough focus, and then moving  the eyepiece up and down in the barlow (so you wouldn't be using an ? f/4.7 but more like an f/9.4).  Also, has your air been stable?  turbulent air might make things more difficult.  If you are observing shortly after sunset and there is a lot of concrete or asphalt that needs to cool off, you might try later at night when the scope and landscape have cooled off more.  

#4 maroubra_boy


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Posted 23 October 2021 - 06:49 AM

Hello Malay,


Welcome to CN! :)


I am very familiar with your 10" scope as I had one, and I am also very familiar with your 30mm eyepiece as I still have one.


When you try to focus, are you finding that the focuser draw tube is not coming out enough to reach focus?  This is a common thing to happen, not so much because of a problem with the telescope or eyepiece, but the telescope itself is the same one made by GSO for photo so it is built with photo in mind.  The situation for us using eyepieces is the draw tube needs to come out more so eyepieces will focus.


DHEB made the right suggestion in getting an extension tube.  However, there is a very quick and easy way to verify that the draw tube needing to come out more is the solution!  With the 30mm eyepiece in place & the draw tube out all the way, loosen the eyepiece locking set screw & slowly pull out the eyepiece and see if this bring the object into focus.  If it does then a short extension tube is the easiest solution.


That it is a fast focal ratio scope is irrelevant & a Barlow is not a help.  Actually understanding the quirks of photo vs visual in the fabrication of mass production scopes helps solve a simple situation.


Malay, you will often also find big differences in the focusing position for different eyepieces.  Some will require more out-travel of the draw tube other eyepieces more in-travel.  More out-travel is easy to solve if like you the draw tube doesn't come out enough.  More in-travel is more difficult to resolve.  But let's do this simple trick first of slowly & carefully pulling out the eyepiece.


Oh, and if the eyepiece does need to come out a little, you can still use the scope this way.  Simply tighten the eyepiece locking  set screw on the pulled out eyepiece.  The scope will still work fine.



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#5 Bjorg63



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Posted 23 October 2021 - 07:28 AM

I have a similar 10" Orion. A couple of my eyepieces need a little more out travel in the focuser in order to reach focus. I loosen the set screw in the focuser, pull the eyepiece out as needed to reach focus and then a little more out, push focuser in a little and then tighten the set screw. This allows me to fine tune the focusing. No way I will achieve best focus otherwise.

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#6 sevenofnine


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Posted 23 October 2021 - 11:33 AM

Welcome to C/N! welcome.gif You have very good answers to the problem so far. I will just add try an easy target like the Moon first. After that you can just relax and enjoy. Best of luck to you! ubetcha.gif

#7 Paul Sweeney

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Posted 23 October 2021 - 12:41 PM

If you do need more out focus and don't have the extension, you can try tightening the primary mirror screws. You need to compress the springs all the way, then recollimate. Moving the mirror down the tube will move the focus point closer to the tube. On my GSO 12" dob, I had to move the mirror up the tube to reach focus.

#8 SteveG


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Posted 23 October 2021 - 01:43 PM

As noted, you don't need to buy anything. Just lift the eyepiece out of the focuser by about 9-10 mm. Tighten it there and you should be able to focus it.



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#9 malaylahiri


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Posted 25 October 2021 - 02:55 AM

Thank you all.  

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