Can you guys ID this Dish ?

[a33]

A friend of mine has an article on measuring the accuracy of a dish
Dish accuracy checking with lasers ...de N6CA

Interesting test!
It is not quite clear to me in what dimension he measured the 'focus travel', though.

Was that perpendicular to the reflected beam, at/near the focal point?
Or was that in the focal length-dimension (along the line Vertex to Focus), as might be an interpretation of his first drawing?

In the latter case a small abberation near the dish center would indeed easily cause a big 'focus travel' outcome, I presume.
But I'm no expert on this, so it is just something I'm wondering.

Nevertheless, great testing and experimenting!

Greetz,
A33

 

[Inclined Orbit]

Interesting test!
It is not quite clear to me in what dimension he measured the 'focus travel', though.

Was that perpendicular to the reflected beam, at/near the focal point?
Or was that in the focal length-dimension (along the line Vertex to Focus), as might be an interpretation of his first drawing?

In the latter case a small abberation near the dish center would indeed easily cause a big 'focus travel' outcome, I presume.
But I'm no expert on this, so it is just something I'm wondering.

Nevertheless, great testing and experimenting!

Greetz,
A33

In the link the dish under test is a prime focus and the laser would be mounted exactly 90degrees to the sliding thing that runs across the face of the dish. With reflective tape or the recommended plastic ruler, the laser will converge in roughly the same spot as you move it across the face of the prime focus dish. Chip used a clear plastic ruler sticking straight out of the dish so he could see the focal point in at least 2 dimensions. The link was meant to measure the surface accuracy of a dish and you would tell that by how much the focal point moved around as you slid the laser across the dish.

My version of the setup would use the same laser and sliding gizmo on an offset dish and you would find the focal point by varying the angle of the laser pointing into the dish. Instead of the 90degrees in the link I would start with a common angle for an LNB or feed arm like maybe 22 degrees and with the laser pointing at an angle into the center of the dish find that focal point in space where you would expect the feed to have been originally. Then move the laser across the face of the dish and see where the focal point goes. If it moves away from the first focal point change the laser angle until you find one that keeps the laser at a single constant focal point as you move the laser across the dish. That will be your actual focal point to build an arm or support for your feed.

Calculations are a good start to find the focal point but you will never really know until you get a feed mounted and play with the focus. This laser exercise will prove out where the focal point is and take some mystery out of it, especially if your not comfortable with the math.

 

[a33]

Calculations are a good start to find the focal point but you will never really know until you get a feed mounted and play with the focus. This laser exercise will prove out where the focal point is and take some mystery out of it, especially if your not comfortable with the math.

Of course, 'the proof of the pudding is in the eating', as they say!
It is always good, to get affirmation of an assumption or of a calculation, by some sort of visual or measurable result.

I must confess, I don't possess a laser.
But I can put dots on a dish surface, and I have a measuring tape and caliper with vernier. And some computers with parabola-equations in them.

(Edit: I've only seen one dish, where the calculations gave some unexpected outcomes. Alas, the dish owner has not analyzed further why the unexpected outcomes occurred.
The calculations were right, but something on the measuments was not OK? Or something must not have been parabolic in shape? I cannot tell...)

Greetz,
A33