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Still wrestling with new secondary

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#1 wrvond

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Posted 17 July 2017 - 12:41 PM

I mounted the original stalk (an aluminum cylinder with elliptical slice at one end and threaded hole for center screw at the other end) in the center of the new mirror. I was pretty sure that stalk was mounted in the center of the old mirror.

However, when I installed the assembly onto the spider, I couldn't get the mirror completely centered under the focuser. The stalk is slam up against the spider with only just enough room to collimate the mirror, and the center of the mirror is maybe 3 mm below the center of the focuser.

Tried the scope out Saturday night. The seeing was poor with too much sky glow. I only got a glimpse of Jupiter before it vanished behind the house.  Saturn was a white blob with wings, though at one point I was able to resolve it into a sphere with one big ring around it. Still bright white.

The stars were tack sharp, especially in the 24mm Pan. The sky to the ENE was velvety black scattered with diamonds. It was quite beautiful to look at, but there were no objects in that area above the horizon to try to view.

 

So my question(s) -

Regardless of the change in major or minor axis size, the center of the mirror is the center. If the stalk was mounted on the center of the old mirror, the new mirror should line up under the focuser the same. The only thing I can conclude is that the stalk should not be attached to the center of the mirror. The original mirror had an offset of 3.048mm. Does this mean the center of the stalk should be attached 3mm below the center of the mirror?

 

I used aquarium sealant to attach the mirror to the stalk in an effort to avoid off gassing issues. Is there an adhesive that is preferred over aquarium sealant?

 

The stalk is pretty small, and I have a hard time getting three distinct dabs of sealant on there. The original mirror was attached to the stalk using double sided foam tape. It held just fine for several years. Should I consider going back to foam tape?

 

Thanks.



#2 Matthew Paul

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Posted 17 July 2017 - 01:53 PM

Are there any markings on the old mirror from the double sided tape? Is it offset? If the original mirror was offset, I would offset the new one as well. Perhaps it would be easier to calculate the distance that the mounting plate needs to be offset from the edge and mark it to be reinstalled, rather than trying to find the center.

 



#3 Nils Olof Carlin

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Posted 18 July 2017 - 03:38 AM

You write: "Regardless of the change in major or minor axis size, the center of the mirror is the center." Things are not so simple - a secondary mirror has a geometric center but also an optical center, offset from it.
You give sparse details, but I believe you put in a new secondary (of different size? Is it thicker than the original?). To achieve a centered fully illuminated field of view, you should use the optical, not the geometric, center. This is automatically done when centering with a sight tube. If so, the secondary should be mounted offset away from the focuser in order to make the optical center (and thus the optical axis) centered in the tube - not really critical enough to mess with, but if you mount a new one, you might as well offset properly.
You write: "the center of the mirror is maybe 3 mm below the center of the focuser." How do you measure this? With 3 mm offset, the geometric center should be 3 mm down the tube from the optical axis and the optical center.


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#4 wrvond

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Posted 18 July 2017 - 09:07 AM

It does look pretty sparse, doesn't it?

The OEM elliptical  (63mm minor axis) is being replaced by an Antares elliptical (66mm minor axis) retaining the stock spider and stalk.

 

spider And mirror
 
The original mirror was fastened to the stalk with double sided foam tape. I am attaching the new mirror to the stalk with aquarium sealant using toothpicks as spacers.
 

IMG 1725

 

I mounted the stalk to the geometric center of the mirror back. I'm reasonably sure the stalk was attached to the geometric center of the original mirror as well.

When I installed the new mirror and spider assembly in the tube, I couldn't get the geometric center of the elliptical directly under the geometric center of the focuser. With the stalk adjusted all the way up against the spider's plate, the geometric center of the elliptical was approximately 3mm closer to the primary (calibrated eye measurement).

Specs for the XT10g call for 3.048mm offset. When I adjusted the spider vanes, I set them to the same length (spider plate in the geometric center of the tube), so no offset there. I didn't introduce any offset at the vanes because the mounting holes in the tube are all equidistant from each other. Offset of the vanes would result in two vanes being angled, which I think is probably undesirable.

Performance would seem to indicate that everything is working well, and that I am intercepting the entire cone of light. There is no fall off or vignetting that I am able to detect. However, I am bothered by the stalk being so close to the spider plate.

Things I didn't take into account:

I don't know if the new mirror is thicker than the old. I'll have to get them both out where I can measure them.

The difference in thickness between foam tape and silicone. I'm going to assume the foam tape was thinner than the gap introduced by the toothpicks.

 

I'm thinking that if I move the stalk lower on the mirror back, that will give me the gap I'm wanting between the stalk and the spider plate, but it will also move the center of the mirror closer to the focuser, which probably isn't good since the manufacturer specs indicate the mirror should be offset away from the focuser 3mm in the first place.

I was looking at the spider vanes and noticed they are not all attached to the plate in the same way. At least one was mounted facing the opposite direction of the others. It occurs to me that the 3mm offset may be introduced in this way. I'll have to take another look at that. I may have the spider incorrectly oriented, and may need to rotate it in the tube. That's as good a place to start as any.

 

I have learned that aquarium sealant is an appropriate adhesive for this application, and foam tape has been known to fail when exposed to heat.

 

 



#5 sixela

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Posted 18 July 2017 - 11:55 AM

Your problem is probably that the Antares is a lot thicker than the stock secondary, which means the centre of _the face_ is a lot closer to the primary.

 

Is the focuser collimatable? Because you could point it slightly towards the primary without too many drawbacks. If it's a Moonlite, you can certainly let Muhammad move to the mountain rather than the mountain to Muhammad.

 

But I can't help but feel that you have actually not offset the secondary, from the photo you posted.


Edited by sixela, 18 July 2017 - 12:00 PM.


#6 wrvond

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Posted 18 July 2017 - 12:05 PM

Your problem is probably that the Antares is a lot thicker than the stock secondary, which means the centre of _the face_ is a lot closer to the primary.

 

Is the focuser collimatable? Because you could point it slightly towards the primary without too many drawbacks. If it's a Moonlite, you can certainly let Muhammad move to the mountain rather than the mountain to Muhammad.

 

But I can't help but feel that you have actually not offset the secondary, from the photo you posted.

It's a MoonLite, but I'm not aware of anything that makes it collimatable.

I've been reading that most builders don't believe offset is necessary in anything this small. Maybe I'll pull it and just start over. Determine where the center of the focuser and the center of the primary intersect and adjust to put the center of the elliptical there.



#7 Nils Olof Carlin

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Posted 18 July 2017 - 12:10 PM

I have a spider + diagonal setup looking like yours (but not in use!). Placing it hub down on a flat table, the vanes are very different heights, the difference some 5 mm. The vanes are fixed to the hub by 2 screws each - likely yours are, too, but the pic is too dark to tell. If so, you could perhaps shim at the screws to tilt the vanes a bit and get the hub out a few mm. Else find 2 that seem to tilt the wrong way, and exchange them (they will be rotated by 180 deg - PS I just tried with mine and it does the trick!).

My secondary is clearly mounted (with double sided tape) with offset some 3 or 4 mm in the right direction. If you do not (and I see no reason to remove and mount it again!), the only consequence is that the optical axis (collimation done) is offset at the level of the secondary, and the optical aim tilted by a fraction of a degree from the tube axis. No real problem, even with digital setting circles). 



#8 wrvond

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Posted 18 July 2017 - 08:04 PM

Thanks for looking into that. They are all held in by two screws each. I'll take a close look at that as soon as I can.



#9 sixela

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Posted 19 July 2017 - 03:38 AM

moonlite_focuser_newtonian_triknob.JPG

 

The little hex screws next to the bolts that pull the focuser onto its base (and the scope) are little "push" collimation screws. With these you can point the focuser to the current location of the secondary if you run out of possibilities to move it under the current focuser axis.


Edited by sixela, 19 July 2017 - 03:40 AM.

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#10 wrvond

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Posted 19 July 2017 - 08:41 AM

moonlite_focuser_newtonian_triknob.JPG

 

The little hex screws next to the bolts that pull the focuser onto its base (and the scope) are little "push" collimation screws. With these you can point the focuser to the current location of the secondary if you run out of possibilities to move it under the current focuser axis.

I bet I read that when I installed it and had completely forgotten about it. Thanks!  waytogo.gif



#11 wrvond

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Posted 24 July 2017 - 08:07 AM

Problem solved. 

It occurred to me that the vanes were not all facing the same direction at the attachment point on the center plate. Originally I had thought offset was introduced by lengthening and shortening the vanes. Turns out the offset is set by the way the vanes are mounted to the central hub. All I had to do was rotate the semblance 45 degrees and suddenly everything fell into place.

IMG 0072
IMG 0075

 


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#12 Starman1

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Posted 24 July 2017 - 02:09 PM

Problem solved. 

It occurred to me that the vanes were not all facing the same direction at the attachment point on the center plate. Originally I had thought offset was introduced by lengthening and shortening the vanes. Turns out the offset is set by the way the vanes are mounted to the central hub. All I had to do was rotate the semblance 45 degrees and suddenly everything fell into place.

Huh?  The offset of a secondary mirror has nothing to do with the vanes unless they are of unequal length and not attached at 90° from each other around the periphery of the tube.

The center of the center bolt in the secondary holder should be dead center in the tube.

Offset is accomplished one of two ways:

1) Offsetting the glued-on secondary at the secondary "post"

2) Centering the secondary completely between the tube walls and letting the "New Model" of collimation (Uni-directional offset) tip the optical axis toward the focuser to create the necessary offset.

It is not accomplished by adjusting the vanes.




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