Vanadium 50 said:
I would like to see the Crab. This seems impossible.
It does seem impossible, but it can be possible. You will need to balance your resource investment in several dimensions if you are to stand a chance. My recommendations are for a minimum cost, minimum time entry into Crab pulsar RA.
Vanadium 50 said:
I don't buy that. Or any single parameter number. In principle, I can trade dish size for exposure time.
I have already pushed that trade in my estimate of how to cross the impossible to possible boundary. I have put together a list of waypoints that can get you that 30 Hz accumulated result. You can reduce the dish, but it will make the signal processing significantly longer and more difficult. It is easiest to start with a dish that is low-cost and possible, something that an amateur could make, that would not be redundant tomorrow.
Dish area is important because that is really the lowest cost investment that will get you ahead in s/n ratio. Why a 15 metre dish? Because anything beyond that is too difficult to construct. 15 m can be built a little sloppy and still work. 15 m can be aimed without too much trouble. Building a smaller dish will take a similar amount of management and number of components, but the square law says, go on adding another 1 metre annulus, until it is too floppy, or will be damaged by strong winds, even with the lower windage of a chicken wire mesh surface. More on a dish at the end.
You will want to avoid the expense of developing and running cryogenic receivers at the focus, so will benefit from a stack of Peltier effect coolers for chilling the LNA front-end, through to the first down-converting mixer and IF line driver. Peltier is now the lowest cost cooling for the maximum advantage.
The advantage of an SDR is that it provides a channel that is highly flexible and dynamic in frequency. That never happens with RA, where you are restricted, by interference, to operate in quiet bands specifically reserved, allocated to RA. You should use the full bandwidth available in the band, to gather the maximum energy. You do not need a DC to daylight SDR to do RA, it is too hot and too noisy.
By using crossed dipoles at the focus, you get more energy from the same dish aperture. H & V polarisation requires a two channel (synchronous) receiver, with a single 1st LO, which can then also produce LH & RH signals. That cannot be done with an off-the-shelf SDR, which will generate too much heat for the cooling system at the focus.
OK, so pulsars produce more energy down in the 608 to 614 MHz RA band than at 1420 MHz, but you must aim for a 1420 MHz dish initially, because you can't go backwards to build another dish later for 1420 MHz. What you start to invest in, will be a long-term constraint on your RA.
How to build a low-cost and safe 15 m dish?
I would build an offset fed dish, that was hinged on an edge, close to the ground on a wide circular turntable. It would approximate a triangle, with three circular corners and three straight edges, for which the magic number is; 10=3
2+1. In polar coordinates, the aperture would be;
Radius = 7.5 + Sin(3*theta) * 7.5 / 10.
That beam pattern is indistinguishable from a circular aperture.
It could be built safely, flat on a concrete slab, as an open octet truss, NOT as a heavy hub on a post with petals. All measurements would be vertical, from the flat concrete slab into the structure, using a stepladder where needed, with the critical key points painted onto the slab first.
The focus would descend to about 1 m above the ground when the dish was declined below the horizon, so you could work on the supports and receivers safely, without a cherry picker.
It would be excellent for tracking slow and steady targets creeping along near the horizon, but not good for targets that pass directly overhead, which would require high speed motors on the turntable.
Such a dish also has amateur radio applications.
