Disaster!!

[Vipersan]

Hi friends ..
The dish has now been up for several months since it was blown down ..

This morning I re-strung the dish in situe ..and discovered it to be not quite as perfect as it was when down on the floor and then first re-mounted ..
In other words ..it appears to have moved /settled ..
The discrepencies are minor ..but since I am always after maximum signal I propose to try and fix this in situe ..

Here's what I am considering ..
I'm attaching a diagram showing how the dish is mounted to the polar mount back ring at six points ..
Currently the L plates are hard up against the mount ring and bolted up ..
If I can ascertain which direction the dish is actually out ..and which petals do not perfectly conform ..

Maybe it would be possible to slacken off ..one L mount plate at a time ( the relevant ones )..insert a thin plastic or metal packing shim ..and re-tighten the U bolt ..
Anyone think this is a bad idea ..
Nothing done so far ...at this point no more than an idea..

rgds
VS

 

[Vipersan]

Hi friends ..
The sun is finally shining ..so took the opportunity to begin the Dish Eye Project ..

Took me a couple of hours to remove the dish centre plate ...a new plate will be made which carries the camera better..
But for now ..a quick lash up ...
Better cabling required and a removeable sun cover for days like today ..to prevent ccd burn ..

I chose to locate it where I did because this area of the dish is in shadow from the lnb ..so isn't going to affect the gain ..

...also considering trying the dish in cassagrain mode at a later date ..assuming I can find or make a suitable sub
reflector ...but for now its the camera eye ..
I figure ..useful for
1 dish security..
2 LNB postioning and alignment
3 Obstacle locator ..trees etc

The camera is a 2nd hand self leveling camera ..but appears to require some adjustment as it tilts a little ..

Anyhow ..
Here are some quick caps taked from my home security system ..to which the camera has been patched in ..

rgds
VS

...more mad than scientist I think..

 

[Vipersan]

..Hi friends ..
Camera housing is starting to take shape ...

All plastic construction..

Consisting of a plastic bung from a poster tube..
A short section of black plastic drainpipe..
A screw top plastic storage jar ..
The plate screws on to the storage jar thread to provide an anchor to the dish ...for the whole assembly ..and is made from the base of a CD cake container..
The space between the storage jar and drainpipe is filled with window frame sealant ...

Still need to cut the eye hole in the red lid for the camera ..and pack the camera inside ...then bolt to the dish ..and finally seal the base plate white plastic bung ..

but will leave that job for tomorrow..
Pleased with it so far tho'

rgds
VS

 

[Vipersan]

Well ..
The camera is now more or less centrally mounted ...
Some minor adjustment needed ....but even with the positioning errors ...its plain to me that the lnb mount isn't quite at the focus.

So looks like the Dish eye is already proving its worth...

Working on the assumption that the stringing/crosswires ...currently forming a triangle ..would theoretically form the triangle roughly at the centre and in line with the focus ..
The lnb is North and west of the true line..
and part of my sky at 40West is obscured by tree..

comments welcome..

rgds
VS

 

[Lazarus]

No comments, as this is all "input" for me - but plenty of praise and congrats not only for the entertainment but primarily for your success.

 

[Vipersan]

Cheers Gordon ..
I guess what I was after ..was just confirmation that my thinking re ..the cross wires and lnb misalignment ..were correct ...

But certainly its gonna be a useful tool..
rgds
VS

 

[Captain Jack]

Your ingenuity knows no bounds VS. Amazing thread!

 

[Vipersan]

...Ok friends ..
A bit more off the wall thinking..
..and I've no doubt this has been discussed many times before ..

Since there is a lnbf sized hole at the middle of my PF dish .....I can't help being curious about the possibility of trying the old girl in Cassegrain or gregorian mode ..
So here goes ..
Presumably I'd need a small parabolic sub reflector ..which when flipped would be cas or greg ..both variations capable of refocusing the signal to the hole at the centre of the Alcoa..
Since the alcoa is a parabola ...nothing special about the shape of the sub reflector ...other than it too would be a parabola ..
...I guess my next question is ..would the sub reflector be approximately the same diameter as a scalar plate from a corotor...?

I assume the lnbf could be fixed at the centre of the alcoa hole ...
The sub reflector suspended between the legs of the tripod ..the moved in and out ..to achieve focus ...(refocus)
Is that too simple ?
Im assuming there would be an optimum point at which the sub reflector would illuminate the alcoa totally but that would not neccessarilly be at a point which would refocus at the same distance back to the centre hole of the alcoa..

..Or have I got this totally wrong ..and the sub reflector is not a parabola at all ...and is shaped otherwise ..in order to not only illuminate a specific dish ...but also to achieve focus at the middle of the dish ..where in my case the hole is ..

This is just curiosity on my part ..but would appreciate input from any of you with knowledge of these things.
rgds
VS

 

[Vipersan]

....Ok been doing a bit of digging ..and it would appear that the basic sub reflector shape is not a parabola as I first thought ..
..but a hyperbola ..
So not an 'off the shelf ' shape ..and less easy to find and adapt something to experiment with..

doh!

still ..it was just an idea..
rgds
VS

 

[PaulR]

IIRC correctly the Cassegrain uses a hyperbolic second reflector and the Gregorian a parabolic.

Wouldn't the reflector shadow the main dish and reduce signal though? Or are you planning on using a reflector that smaller than the LNB? You'd have to put the sub reflector closer to the dish than the LNB position simply so that the sub reflector can do some focusing of its own.

 

[Vipersan]

Hi Paul ..
The plan ..in theory ..would be to use a similar sized sub reflector....to the currently used scalar plate ..

The extra efficiency of a re-focussed beam ..is _supposed_ to overcompensate for the shadowing effect of the sub or indeed the scalar plate..

At least ..thats what I read ..
rgds
VS

 

[PaulR]

The extra efficiency of a re-focussed beam ..is _supposed_ to overcompensate for the shadowing effect of the sub or indeed the scalar plate.

My thoughts are that there a few differing reasons for using a sub-reflector.

Firstly, you can have a longer focal length folded up in the length of a normal dish. This gives a higher gain (but reduced side lobe rejection of course).

Then, there's the reason of keeping the LNB close to ground so that you can cool it with liquid nitrogen. Astronomers want the lowest noise figure for their sky searching.

The Arecibo bowl gives another reason. Obviously the bowl can't move but the sub-reflector can to give a small amount of directionability (is there such a word?).

Often there will times when two, or even three, of the reasons will be invoked. There are probably other reasons that I haven't thought of as this is just stuff off the top of my head and I might be wrong on a few things.

 

[Vipersan]

Thanks for your thoughts Paul ..
Certainly its a complex subect ..
Just as an aside ...I've been concentrating on getting the lnb dead centre as the setup currently stands ...
During the process I have been trying to work out how much out of focus my 6 petal dish currently is ..
Looking for deaf petals etc ..

To this end I cut a triangular sheet of polystyrene roughly the shape of 1 petal ..
Monitored the signal on Petagon channel ..approx 6.4 db currently ..
I then went oround the dish covering each petal in turn ..and recording the drop in signal ..

the drop varied between petal coverings ...but never more than 0.3db on any of the six petals covered ..
I'm a little confused ??
Does this mean the dish is fairly accurately focused ...?
Or does it mean the dish is evenly deaf ?

I thought I'd get bigger drops ..
perhaps 0.5 > 1db for each petal ..

but it looks like its behaving like an optical lense ..
IE ..for a dirt spot on a camera lense ...the overall brightness diminishes ...but not by much ..and the image effectively doesn't see the dirt spot ..and appears to create an illusion of being able to see through the dirt spot as though its not there..
Any thoughts ..
anyone ?
rgds
VS

 

[PaulR]

One thought I have is that as it's C band then accuracy just isn't as important as Ka band. Maybe it IS focused enough.

 

[Huevos]

Firstly, you can have a longer focal length folded up in the length of a normal dish. This gives a higher gain (but reduced side lobe rejection of course).

Yes... but if you are just putting the subreflector at the focal point you haven't changed the focal length at all. To change the focal length (to longer) you would need a shallower parabola. With a proper Cassagrain design there is no convergence between the primary and secondary reflector, nor at the secondary reflector. The first point of focus happens at the LNB (or eyepiece in the case of a telescope). This is in contrast to a Gregorian design where there is a focal point between the primary and secondary reflectors, and a second focal point at the LNB (or eyepiece in the case of a telescope).

_http://en.wikipedia.org/wiki/Cassegrain_reflector
_http://en.wikipedia.org/wiki/Gregorian_telescope

 

[Vipersan]

Yes... but if you are just putting the subreflector at the focal point you haven't changed the focal length at all. To change the focal length (to longer) you would need a shallower parabola. With a proper Cassagrain design there is no convergence between the primary and secondary reflector, nor at the secondary reflector. The first point of focus happens at the LNB (or eyepiece in the case of a telescope). This is in contrast to a Gregorian design where there is a focal point between the primary and secondary reflectors, and a second focal point at the LNB (or eyepiece in the case of a telescope).

_http://en.wikipedia.org/wiki/Cassegrain_reflector
_http://en.wikipedia.org/wiki/Gregorian_telescope

...So the idea that you could place a sub reflector closer to the dish ...ie further in than the focal point ..effectively collecting a wider beam then focusing that to a sharper point even closer to the dish centre isn't practical ...?

rgds
VS

 

[PaulR]

Yes... but if you are just putting the subreflector at the focal point you haven't changed the focal length at all.

Well I didn't say anything about the position of the sub-reflector but it would have to be before the original focal point otherwise it wouldn't be able to do any focusing. Hence what I said about having a longer focal length within the profile of an "ordinary" dish. I left unsaid he need for a shallower dish, unfortunately.

The first point of focus happens at the LNB (or eyepiece in the case of a telescope). This is in contrast to a Gregorian design where there is a focal point between the primary and secondary reflectors, and a second focal point at the LNB (or eyepiece in the case of a telescope).

Now that's interesting, I had overlooked that point. Ta!