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8" Dob Focuser Question

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#1 Matty S

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Posted 21 November 2019 - 05:56 PM

Hi. My 8" f6 Celestron Starhopper Dob's focuser has gummed up to the point of being unusable. So rather than do a tear-down, clean and re-lube of the original 1.25" r&p focuser I was wondering if I swap it out for a nicer 2" Meade r&p focuser that I got for another project which has since gone awry.

There is quite a bit more back focus on the Meade unit and it stands taller off the tube, don't know if this would cause an issue? Just thought I'd ask before cutting into the Sonotube... thanks!

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#2 Vic Menard

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Posted 21 November 2019 - 06:37 PM

Looks to me like the Meade is about an inch taller fully racked in. If your eyepieces all focus more than an inch above the 1.25-inch focuser--you're good to go. If not, perhaps you can move your primary mirror forward an inch or so. Of course, if you only have 1.25-inch eyepieces, you'll also need a (low profile) 2- to 1.25-inch adapter. You should also determine how much the focuser drawtube protrudes below the focuser when the drawtube is almost fully racked in--and whether or not it will protrude too far into the tube.

 

Finally, you'll need to figure out if your secondary mirror will tolerate a taller focuser...


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#3 Matty S

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Posted 21 November 2019 - 06:55 PM

I doubt enough of my ep's focus that high to make it worthwhile. Thanks for your input. Sounds like alot of tinkering to me, especially since it works fine with the original focuser. I have other newts with 2" focusers. Looks like I'll be rebuilding/lubing that old focuser, no worries there.

Didn't hurt to ask. Thanks again!



#4 Asbytec

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Posted 21 November 2019 - 08:42 PM

Looks to me like the Meade is about an inch taller fully racked in. If your eyepieces all focus more than an inch above the 1.25-inch focuser--you're good to go. If not, perhaps you can move your primary mirror forward an inch or so. Of course, if you only have 1.25-inch eyepieces, you'll also need a (low profile) 2- to 1.25-inch adapter. You should also determine how much the focuser drawtube protrudes below the focuser when the drawtube is almost fully racked in--and whether or not it will protrude too far into the tube.

 

Finally, you'll need to figure out if your secondary mirror will tolerate a taller focuser...

All good points. Use a diagonal calculator (link below) to see if the secondary is sufficient size and plug in the numbers. Do some tinkering by attaching it to see. Then try some eyepieces focused at infinity. 

 

https://www.bbastrod...om/diagonal.htm



#5 Jon Isaacs

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Posted 22 November 2019 - 06:05 PM

Hi:

 

This is the difficulty I see:

 

I have had three of the 6-8 inch Sonotube starhoppers.

 

The secondary mount is a single vane that uses one screw from the focuser mount. You would have to figure out how to remount the secondary.

 

It is does have three screws for adjusting the tilt but some arrangement to center the secondary in the 2 inch focuser would be required.

 

I always figured I just use a 4 vane spider but that's additional hassle and expense.

 

Jon




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