Cassegrain Vs Gregorian

[koansrc]

From what I'm reading the difference between the two is the direction of the 2nd reflector ( concave V convex ). I'm also reading that for offset dishes the curve is inward facing and therefore Gregorian. For prime focus dishes the curve is outward (Cassegrain). Can anyone discuss this ? and perhaps (if it's accurate) explain why offset dishes need an inward facing curve and primes an outward ?


:-snowman :-gnome :-sman2 :-pachrist :-carrot :-sball

 

[Robbo]

From what I'm reading the difference between the two is the direction of the 2nd reflector ( concave V convex ). I'm also reading that for offset dishes the curve is inward facing and therefore Gregorian. For prime focus dishes the curve is outward (Cassegrain). Can anyone discuss this ? and perhaps (if it's accurate) explain why offset dishes need an inward facing curve and primes an outward ?


:-snowman :-gnome :-sman2 :-pachrist :-carrot :-sball

I think a little more reading up is required.I'm going to.
The reason being, as I understood it, whether the dish is offset or prime, is completely independent of whether it is cassegrain or gregorian.

So, what I'm saying is, you can have cassegrain or gregorian for both offset and prime focus types. I may be wrong.

Robbo

 

[Robbo]

This document explains a lot:-


_http://www.w1ghz.org/antbook/conf/Multiple_reflector_antennas.pdf

and confirms what I said in my previous post.


Robbo

 

[CROSSBONES]

I have never seen a prime focus Gregorian, always off set:-csmile

 

[koansrc]

I have never seen a prime focus Gregorian, always off set:-csmile

Me neither. All photos of dual reflector prime focus dishes are Cassegrains. That's why I'm curious about the difference between offsets and gregorians.

I'm wondering whether the 2nd reflector on a prime focus dish (cas or greg) wiould actually increase reception ( by focusing stray rays that never make it to the feed ) or reduce reception due to the shadow it will obviously cast.

The very informative link/ .PDF Robbo71 posted and the excel file I think attempt to answer this question. But it's a bit over my head and needs some more understanding.

It seems that for my 3.0m prime I would need a subreflector with min diameter 44cm, an f/d 0,33 and 16,8cm focal.


:-snowman :-sman2 :-sugar :-Cbigsmil :-pachrist :-carrot :-fcm

 

[Robbo]

The very informative link/ .PDF Robbo71 posted and the excel file I think attempt to answer this question. But it's a bit over my head and needs some more understanding.

I agree, rather complex, eapecially when you are not quite sober.:-HoHo

I just looked at the pictures
but from those you can clearly see the difference between Gregorian and cassegrain, and shows (theoretically at least) that both are possible in either offset or prime focus configurations.


44cm does sound large though. But the little arcon sweety dishes are cassegrain prime focus, with the sub-reflector diameter being approx 1/5 of the dish diameter, and they supposedly perform well for their size.

 

[koansrc]

I agree, rather complex, eapecially when you are not quite sober.:-HoHo

Ha! Merry Christmas!

the little arcon sweety dishes are cassegrain prime focus

I would love to try one of those minidiscs as a 2nd reflector on my 3.0 m CM. I wonder if anyone has actually had a go.

 

[patmos]

I understand this will be beneficial to large dishes (10m upwards). As percentage of subreflector to main will be less. on a 15 m dish the sub appears very small for a cassegrain. The advantage is beam reshaping that's why there is more gain with these types of antennas.

 

[johnsattuk]

I have never seen a prime focus Gregorian, always off set:-csmile

4.9 mtr and 305 mtr :-hny

 

[Robbo]

Ha! Merry Christmas!




I would love to try one of those minidiscs as a 2nd reflector on my 3.0 m CM. I wonder if anyone has actually had a go.

I don't think it would work, as the subreflector needs to be elliptical rather than parabolic. With a parabolic subreflector I reckon that the rays would miss the sweety's cassegrain subreflector.

 

[koansrc]

the subreflector needs to be elliptical rather than parabolic

call me sober, but I don't understand the terms elliptical and parabolic. What do you mean ?

 

[koansrc]

4.9 mtr and 305 mtr :-hny

from the photos it's not possible to distinguish if they are cassegrain or gregorian

 

[johnsattuk]

from the photos it's not possible to distinguish if they are cassegrain or gregorian

Do you mean that 'you' are unable to determine

They are both gregorian as a bit of research shows. The 305 mtr being the very famous 'Aricebo' gregorian

 

[koansrc]

Do you mean that 'you' are unable to determine

They are both gregorian as a bit of research shows. The 305 mtr being the very famous 'Aricebo' gregorian

Thanks for the clariffication. I don't think it is only "me". From the photos alone I'm pretty sure noone can distinguish. As you say, a bit of research is needed. :-thumbup

 

[Robbo]

call me sober, but I don't understand the terms elliptical and parabolic. What do you mean ?

They are different geometric shapes, with a different equation and properties.

_http://www.analyzemath.com/parabola/Equation.html

Note, I am not a mathematician, so I will leave you to play with the tutorials given in the link.

Robbo

 

[koansrc]

I don't think it would work, as the subreflector needs to be elliptical rather than parabolic. With a parabolic subreflector I reckon that the rays would miss the sweety's cassegrain subreflector.

Ok so a parabolic dish is round and an offset dish is elliptical. Now since the 3m CM is round I think the 2nd reflector would also have to be round.

Checked my smaller offset gregorian dish and sure enough the 2nd reflector is elliptical as is the main dish. It makes sense.

I'm starting to think this might be worth trying ... Anyone got a minidisc for sale ?

:-snowman :-thumbup :-sugar :-pachrist :-carrot

 

[Robbo]

Ok so a parabolic dish is round and an offset dish is elliptical. Now since the 3m CM is round I think the 2nd reflector would also have to be round.

Checked my smaller offset gregorian dish and sure enough the 2nd reflector is elliptical as is the main dish. It makes sense.

I'm starting to think this might be worth trying ... Anyone got a minidisc for sale ?

:-snowman :-thumbup :-sugar :-pachrist :-carrot

Yes, prime focuses are round offsets are not, they are oval, but to the satellite ( the dishes that are higher than they are wide) look like a circle to the satellite. But both offsets and prime focus are parabolic. The offset is a small section of a much larger prime focus dish.

Why the sky mini-dishes are such a stupid shape, does anyone know?

I was referring to the shape of the cross-section of the sub-reflector. I.e the subreflector has to be correct shape so that when placed past the focal point, that it would reflect all the rays back to the lnb. A parabolic shape I don't think would do it, as this works for rays that are coming in along its axis, whereas with the gregorian, they are coming out of a point ( the dish focal point).

 

[PaulR]

Why the skyminishes are such a stupid shape, does anyone know?

As you probably already know, the bigger a dish is, the smaller the acceptance for the satellite becomes - which is why it's such a swine to get big dishes adjusted accurately.

The satellites in the Clarke belt are generally 3 degrees apart so, providing the dishes acceptance angle is less than this, when aligned properly it will reject the adjacent satellite signals.

When you get to dishes the size of the minidish if they were made as a "straight" 3cm dish it wouldn't reject the neighbouring signals so the minidish has the extended "ears" to provide the adjacent satellite rejection.

 

[Robbo]

Oh, yes I see, thanks Paul, so its just to give it a narrower beamwidth in the azimuth direction, whilst still keeping the dish relatively small overall.

 

[johnsattuk]

Checked my smaller offset gregorian dish and sure enough the 2nd reflector is elliptical as is the main dish. It makes sense.

The second reflector is elliptical, but ratio of major to minor axis is less than the large dish. This is because it is at an angle to the main dish so that the beam can be directed downwards to the LNB, which is positioned to be out of the way. It is usually approx half the dish offset.

If the subreflector was at the same offset angle as the dish, (as is the case with a Prime dish) it would be round, but the LNB would then shadow some of the dish, thus negating the advantage of the offset.