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Help needed with secondary mirror

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11 replies to this topic

#1 Itz marcus

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Posted 22 September 2019 - 09:58 AM

Hi,

Just got an Antares 2.6 inch secondary. I glued it to the stalk with silicone spaced with matchsticks. The problem is that it is thicker than the previous mirror and I have tightened the center screw all the way and it is still slightly too low. What can be done to raise it? Is there a shorter stalk that is made? I have a 12 inch Orion intelliscope truss scope. 

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Itz



#2 Allan Wade

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Posted 22 September 2019 - 03:33 PM

I did exactly the same with my old Orion XT12g. Before gluing the new secondary to the stalk I cut back the stalk from the  end the bolt threads in to. That gave me enough travel to get the secondary under the focuser. I don’t remember how much I cut off from the stalk, it was a long time ago.

 

It’s going to be a PITA but you are going to have to remove the mirror from the stalk and cut it back and then re-attach it again. I think that’s the safest option to guard against damaging your new secondary.


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#3 Itz marcus

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Posted 22 September 2019 - 05:43 PM

Hi,

How did you cut it?

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Itz



#4 Itz marcus

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Posted 22 September 2019 - 05:44 PM

Hi,

Or would it be possible to buy a lower profile stalk (do they make one?) The one I have is 1 inch on the shorter side.

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Itz



#5 Asbytec

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Posted 22 September 2019 - 07:37 PM

I am not sure the usual suspects, Astrosystems and Protostar, make secondary holders to fit commercial Dobs using the existing holes. 

 

Options seem to be cut the existing holder to size (is it hollow?), fabricate one using 2" PVC tube cut to fit, or try to find a replacement. Maybe something like this (truthfully, though, it's almost the same as 2" PVC):

https://www.apm-tele...y-mirrors2.html

 

Here's an idea how to use PVC, though you probably only need the holder and not the rest of this design.

http://conradhoffman.com/secondary.htm

 

Browsing the web you run across these...

https://www.sidereal...-mirror-holder/



#6 Starman1

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Posted 22 September 2019 - 11:53 PM

Hi,

Just got an Antares 2.6 inch secondary. I glued it to the stalk with silicone spaced with matchsticks. The problem is that it is thicker than the previous mirror and I have tightened the center screw all the way and it is still slightly too low. What can be done to raise it? Is there a shorter stalk that is made? I have a 12 inch Orion intelliscope truss scope. 

Clear Skies

Itz

Actually, you only need a drill and a few minutes.

Remove the spider and secondary.

Drill new holes in the tube about 1/2" above the current holes and mount the spider in the new holes.

If you want to, though it's not necessary, cover the old holes with small Avery dots (they now come in vinyl as well as paper).


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#7 Allan Wade

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Posted 23 September 2019 - 07:10 AM

Hi,

How did you cut it?

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Itz

I just trimmed it back with an angle grinder cutting wheel.

 

I’m not aware of anyone making the same type of stalk in a smaller length.

 

Don’s idea of moving the whole assembly up the tube is a good one as well. If you are ever considering installing a filter slide, then rotate the spider around 45 degrees as well while you are moving it. That way the space under the focuser is clear to fit the filter slide without a spider vein in the way.



#8 Pinbout

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Posted 23 September 2019 - 08:00 AM

I prefer getting a round maple dowel from HD and cutting a new stalk.

 

I hate how the stalks are threaded on and prefer a thru bolt that's a little more beefier. 



#9 Itz marcus

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Posted 23 September 2019 - 12:48 PM

I prefer getting a round maple dowel from HD and cutting a new stalk.

 

I hate how the stalks are threaded on and prefer a thru bolt that's a little more beefier. 

Thanx was thinking about doing that.



#10 Itz marcus

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Posted 23 September 2019 - 12:51 PM

Hi,

Ok right now I got tyhe scope collimated but try as I might I can't get the primary centered in the secondary. It is well collimated and the star test is definitely better than b4. Does it make a difference if the primary is not centered on the secondary or as long as the primary is fully visible in the sight tube (catseye) that would be fine?

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Itz



#11 Starman1

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Posted 23 September 2019 - 03:48 PM

As long as the entire primary is visible in the secondary, you're fine.

The purpose for centering it is to provide maximum illumination dead center and equal illumination on every side.

If a tiny difference in illumination is present from one edge to the other, would you, could you, see it?  Maybe not.

But, you need to be sure ALL the primary is visible at the focal plane, plus a bit more, so you are not stopping the scope down.



#12 Asbytec

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Posted 23 September 2019 - 07:08 PM

The reflection of the primary does not have to be centered in the secondary, but it should be centered under the focuser with good axial alignment. 


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