Focusing a Button-Hook mounted Feed Horn

[esteveW]

I'm installing a 10' mesh BUD which has the Chaparral Corotor feed horn on a Button-Hook mount. Since this button-hook tube comes out of the center of the dish. I'm unsure of there the feed horn should be pointed.

When I took the dish apart, I marked all the components so I could reinstall at the original focal length (and the adjust from there). However, I did notice that the Corotor/feedhorn was somewhat off center and appear to be pointing some distance off the dish center. Perhaps 6-10" off center. It wouldn't take much to bend the mount tube and I suspect how the feed horn is aimed. The focal distance doesn't appear to have been changed since origanal instalation a the old site.

If I must aim it at a point off center, how do I ensure that the Feed Throat is perpendicular to the dish surface. One of the methods I have seen used a rod, attached to the Scalor, deep in the feed horn. Since my feed horn is a Corotor w/ a Polarity Probe in the middle or on the Servo shaft. I would be afraid of bending something inside there.

An recomentation would be appreciated.

Steve

 

[Anole]

Our man Stogie5150 has been working on fine-tuning his BUDs recently.
He has two threads going about a 12' Paraclipse and an 8½' Birdview.
From chatting with him, it sounded like he was getting superb reception on both.
Today, he's on a trip to help another member pick up a dish, but when he returns, I expect some more posts detailing his latest tuning tricks.
(you might want to stockpile: a laser pointer, a tape measure, and a 2x4 long enough to span your dish)

In the mean time, perhaps other folks here have some clever tricks of their own.
Or, you could look on the Geo Orbit site; it's got a wealth of BUD-related info.

 

[BelowZero]

esteveW,

If you haven't all ready, check out the FTA Manuals section on this forum, there is a copy of a Chaparral Corotor II feedhorn instruction manual there.
Take a look at steps 7 and 8.

BelowZero

 

[14karat]

Being a Corotor, the feedhorn should be removable from the scalar ring.
If you can find an appropriate size pipe to slide through the scalar so you can view down to the center of the dish, that will get you real close.
I used the string method across the face of the dish (N-S/E-W) to form a crosshair at the center, then slid a tight-fitting pvc tube through the scalar and used it to scope the center of the dish.
Once I had it that close, I had to use a laser to make it better... and I know I'm still a bit off...

 

[esteveW]

"There in lies the Problem"!. The center of the dish is occupied by the Button-Hook mount tube. Should the dish center still be considered the "Main Axis of the Parabola. (physically it is, but what effect does the tube actually have any effect or should I just ignore this conflict and still use it as my focal point?)

As I think about this, maybe I should first make sure the feed horn/scaler are centered on the focal point (by reshaping the button hook offset, then follow 14karat's pvc pipe procedure.)

Thanks for all the replies. everyone keeps the thought process going.

Steve

 

[Greg Mueller]

This might give you an idea.

Align-O-Tron

I used to manufacture these gizmos as a side business, till the Chinese wouldn't sell me lasers anymore. The problems are similar with telescopes, movie projectors and satellite dishes. Collimation

I am thinking about scrounging up enough stuff to make one for the Chaparral

 

[Hermitman]

The buttonhook should be adjusted so the feedhorn is dead center and pointing directly at the center of the dish. Adjust your feedhorn's f/d ratio in the scalar ring then adjust the focal distance by moving the buttonhook in or out. That will give you a good starting point for fine tuning the dish. The buttonhook attachment fitting at the center of the dish has no impact on the signal since the lnbs/feedhorn is blocking the signal from the satellite. The satellite signal reaching the feedhorn is coming from the curved part of the dish. Good luck.

 

[zaxxon]

Hermitman is correct, dead center, that is how mine is set up and it gives a perfect picture.

 

[14karat]

"There in lies the Problem"!. The center of the dish is occupied by the Button-Hook mount tube.

My bad - wasn't even thinking about the buttonhook mount being the center of the dish... I can see that being a dillema since there's really nothing to 'aim' at (how do you tell if the buttonhook tube is perfectly straight?)

I know it's not an answer, but I'd have to convert it to a 3 or 4 post support...

 

[sergei]

If you check with Skyvision they may still carry the laser centering tool, I got mind from them a couple years ago. Works really good at centering the feedhorn, but its not cheap.