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New Focuser made New Problem

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#1 Rgross2112

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Posted 16 September 2020 - 09:13 PM

I recently replaced a screw focuser with a Orion 13039 2-Inch Crayford-Style Telescope Focuser on my 10” Dobsonian.  Found that the 2” 40mm lens worked great but my 1.25” 12mm would not come into focus.   The Crayford does have a higher profile and when I took the Crayford focuser down to its lowest setting it still needed more to become focused.  Will a focal reducer work in this case for my 1.25” lens or am I looking at a new build here?  Any help would be appreciated

 



#2 213Cobra

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Posted 16 September 2020 - 09:23 PM

I recently replaced a screw focuser with a Orion 13039 2-Inch Crayford-Style Telescope Focuser on my 10” Dobsonian.  Found that the 2” 40mm lens worked great but my 1.25” 12mm would not come into focus.   The Crayford does have a higher profile and when I took the Crayford focuser down to its lowest setting it still needed more to become focused.  Will a focal reducer work in this case for my 1.25” lens or am I looking at a new build here?  Any help would be appreciated

You might try the Tele-Vue In-Travel 2">1.25" adapter. It might give you just enough added infocus. -Phil


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#3 wrnchhead

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Posted 16 September 2020 - 09:27 PM

Welcome to Cloudy nights!

 

If the focuser is too tall, it's just incompatible. How close to focus are you? One possible work around is bring the primary as far forward as you can by loosening the collimation bolts (assuming strong enough springs to hold well in that position) 

 

New build..is this one you built or a commercial one?



#4 Rgross2112

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Posted 16 September 2020 - 09:50 PM

I hand built it about 30 years ago when I lived in Rapid City SD. Joined a local astronomy club and one member offered to teach me how to hand grind the mirror. It took a year to complete doing it one day a week for an hour or two each time. The tube is a painted concrete forming tube that I hand painted constellations on. There is some sentimental value to it. I’m afraid though the space available at the primary mirror will not be enough.
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#5 wrnchhead

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Posted 16 September 2020 - 10:04 PM

That's cool! We would all love to see pictures! So with sonotube (I have never built one) could you just shorten the tube at the primary end and reinstall the cell? 



#6 Bill Weir

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Posted 16 September 2020 - 10:38 PM

You can either get another new focuser or just move the whole cell forward in the tube. With the eyepiece adaptor removed using the 12mm hold it in the opening and lower it in wile looking until you reach focus. Do this with the scope pointed towards Polaris so the star will stay in place. Then note how far inside the draw tube it is. This is approximately how much you need to move the cell forward. Bring it forward even a bit more because the 12mm might not be the future eyepiece needing more out focus. I’m sure you know how to move the cell forward seeing as you installed it in the first place. In the end it may mean needing a larger secondary but one thing at a time.

 

Bill


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#7 Starman1

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Posted 17 September 2020 - 12:07 AM

If it's a small diameter 1.25" eyepiece, Astrosystems has a negative height adapter that allows the eyepiece to seat as much as 1/2" below the top of the focuser.

If that's enough, it's a cheap solution.

Otherwise, re-drilling the tube and raising the cell is the solution.  You'll have to add a counterweight to the outside of the bottom of te tube to compensate for the raising of the center of gravity that results.

 

I find this ironic since 99% of the time when focusers are changed, the new focusers are lower and the people don't have enough OUT travel to get to focus.


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#8 wrnchhead

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Posted 17 September 2020 - 12:13 AM

I find this ironic since 99% of the time when focusers are changed, the new focusers are lower and the people don't have enough OUT travel to get to focus.


Which is ideal, cause an inexpensive spacer is the solution. (Plus gets your draw tube out of the OTA)
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#9 Rgross2112

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Posted 17 September 2020 - 07:54 AM

Thank you guys, you all have given me several options to consider.  It hit me last night when I read the suggestion about moving the primary forward I could cut a portion of the tube assembly and reattach the primary.  I just replaced this last month when I discovered my anchors were cracked.  I went with a thicker piece of wood.  Looks like I’m going to detach it and cut the tube after some close focal measurements.  Will see if a new secondary mirror will be needed.  I really appreciate all of your input and suggestions on this.   I will try to get some photos posted here, right now I’m getting a warning that my file is too large.  Rush




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