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how to set sling length on dob ?

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#1 jonstarrysky

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Posted 12 October 2013 - 07:07 PM

The picture shows the back of my mirror box, with a cable sling supporting a 35mm thick ZOC 18" mirror.
So by eye I'd say the mirror needs to move up a little to be centered in the mirror cell, by adjusting the sling turnbuckle. How is this adjustment best measured to get just-so??

I was thinking of marking the mirror center on its rear side with a black dot (the mirror is soon to be taken out for other work). Then some how working out where the center of the 18 mirror supports lie, using the center bolts of the three floating metal bars.....

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#2 Mirzam

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Posted 12 October 2013 - 07:18 PM

If you can collimate the telescope it's close enough.

JimC

#3 derangedhermit

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Posted 12 October 2013 - 10:16 PM

Of course you want the outer ring of support points to be equidistant to the edge of the mirror. If you don't think eyeballing it gets you close enough, just measure from each of the two closest points of each of the 6 triangles. You shouldn't have to adjust the sling length often.

I assume something like 1/16" is good enough. You are right in trying to only center the mirror on the cell, not anything else.

#4 jonstarrysky

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Posted 13 October 2013 - 10:51 AM

Surely the mirror needs to be accurately centered in the 18pt cell as far as possible, especially for a thin mirror. PLOP assumes the mirror is centered. The idea isn't to equally distribute mass, but rather to position gravity related deformation in such a way as to reduce error in the mirror's figure away from being a perabola. The two things being subtly different, i.e. deformation that just causes a change in the effective focal length without destroying the mirror's figure isn't an issue. So with the mirror off center, deformation will inevitably adversely affect the mirror's figure. Just adding some annotations to the picture, it looks like my scope was set up slightly off center....

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#5 JimMo

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Posted 13 October 2013 - 12:31 PM

My sling looks exactly like yours, is it a Webster cell? I took an old wood yard stick and cut it exactly the size of the inside of my mirror box. I marked a line at the exact center of the ruler and every once in a while I use it to eyeball if the mirror has moved or not. In the six years I've had my dob I've adjusted the turnbuckle a couple of times. Eric Webster told me to get it set and then duct tape the turnbuckle to keep it from moving but I never did. Besides, it only has to be close to get good collimation.

#6 derangedhermit

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Posted 13 October 2013 - 02:39 PM

Surely the mirror needs to be accurately centered in the 18pt cell as far as possible[...] Just adding some annotations to the picture, it looks like my scope was set up slightly off center....


You just found one good way to more accurately center the mirror on the cell. Take a full-frame photo from several feet away, and use an image processing program to draw circles. Zoom in and count pixels to measure distances. This is similar to a technique someone here came up with for accurately placing a center spot.

#7 jonstarrysky

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Posted 13 October 2013 - 02:58 PM

Jim, yep its a Webster cell, so your reference point is the geometric center of the mirror box. I'll probably move mine up to where I think the mirror is centered with respect to the flotation cell triangles. The limitation is actually the top "keeper", perhaps this can be removed to file down the flat side. Or to replace with a thinner sheath of material placed around the keeper bolt.....

#8 JimMo

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Posted 13 October 2013 - 04:49 PM

Jim, yep its a Webster cell, so your reference point is the geometric center of the mirror box. I'll probably move mine up to where I think the mirror is centered with respect to the flotation cell triangles. The limitation is actually the top "keeper", perhaps this can be removed to file down the flat side. Or to replace with a thinner sheath of material placed around the keeper bolt.....


I had the same problem, I just flattened one side of the dowel on a belt sander. My cell was the first one before they reintroduced the D14. Except for the truss hardware, altitude bearings, and Webster's beautiful finger joinery my scope is a Webster clone that Eric was so kind to help me with. He even called to ask about my dimensions and torque equations when the D14 was being redesigned.






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