# Presentation: 703420

## Parabolic Cylindrical Reflectors

**Parabolic Cylindrical Reflectors**

**John Bunton**
**Fermilab, 10-11 July 2008**

## History

**Cylindrical Reflector      (64m dish = 3,200 m2)**
- 1958 - 178MHz,  Radio Star Interfer. 10,000 m2,
- 1967 ~400MHz
**Northern Cross 31,000 m2**
**Ooty****	****	****   16,000 m2**
**Molonglo ****	****   40,000 m2**
- 1980 843MHz MOST 19,000 m2
**In particular Molonglo and Parkes both built in **
- same decade (61 and 67),
- same country and
- cost about the same
- BUT Molonglo had more than 10 times the area
**Collecting area for cylinders is cheap**

## Molonglo Breakdown

** ****Fixed NS transit cylinder**
** ****22,000 m2**
** ****Steerable EW cylinder**
-  **18,000 m2**
** ****Line feeds – 3.2km**
- EW arm fixed pointing – at transit
- NS Mechanical phasing elements, delay switching, multiple beams
- large amount of electronics
**cf Parkes almost none**
** ****Parkes design frequency 3GHz – Molonglo 1.4GHz**
- Parkes was ten times the cost per square metre

## Historical Cost Comparison

## So What Went Wrong?

**Initially Molonglo very successful**
- At one time had detected greatest number of Pulsars
**But up grades need lots of electronics**
- Only one major one 403MHz to 843MHz
- 352 LNA
- 88 phased combiners and down converters
- Dual 44 input Butler Matices
- Etc ...
- Cooled receivers not possible (100K, now 60K)
**Parkes has had a new receiver each year (only one needed)**
- Very lowest Tsys possible at all times (20K at 1.4GHz)
- Bandwidth increase with time (1GHz in some bands)
- Molonglo still 4MHz
- Other frequency ranges developed 0.15 to 22 GHz
- Dual Polarisation
- Multibeaming (has now detected half known pulsars)

## The Future

**Parkes has reached the limit in**
- Tsys  (limited by factors other than LNA noise)
- Bandwidth  (becoming limited by feeds not electronics)
- No beams   (limited by size of focus cabin)
**Where to now**
- For mainstream astronomers – SKA
**SKA needs to be all things to all astronomers**
- Must have full sky coverage – steerable reflectors
- Want 0.3 to 22 GHz or more
- This is too much for one antenna (low, mid and high components)
- Wide field of view – reference design is FPA on dish
- Wide bandwidth ~50% of observing frequency
**Consequently large number of receivers**
- 5000 to Millions
**Will have same problems as Molonglo**

## Some Engineering

**Number of beams Nb needed for a given field of view (filled aperture antennas) **
- Nb = number of antennas x number of beams per antenna
**Independent of technology**
**Everyone wants large Aeff and FoV... SO**
**Everyone will have a large number of beams to process**
**And cylinders are not disadvantage because they need a large number of receiver.  EVERYONE does.**

## Why Not Cylinders for the SKA

**Cheapest collecting area**
**Same electronics problem as the rest**
**BUT**
**Not sold well (my fault)**
**Astronomers not comfortable with “outmoded” technology**
**Perceived poor polarisation response**
**Non circular beam that rotates on sky**
**1 and 2 now over come by fresh approach from this community**
**4 Jeff cut the Gordian knot by going to a transit instrument**
**3 Still needs work to prove – maybe transit cancellation**

## FoV – Effective  Area Trade-off

**Substitute equation for number of beams Nb into equation for survey speed gives**
- Nb = number of antennas x number of beams per antenna
**N****b****.BW is proportional to electronics cost**
**Thus survey speed ******** Effective area x cost of electronics**
**This suggests that we may want to reduce the number of beams per antenna and increase number of antennas**
- RF beamforming for galaxy detection

## Concept Cost (estimates) (BW = f /4)

**Aperture array from SKADs  FoV 240 sq deg**
**Dish 192 element FPA – FoV 30 sq deg @1.4GHz (f****-2****)**
**12m Steerable cylinder **
-  full electronic beamforming,  Fov 120 sq deg @1.4GHz (f-1)
- RF beamforming (0.3m), FoV 20 sq degree @ 1.4 GHz (f-2)

## Thank you

**ICT Centre**
- John Bunton
- Senior Principal Research Scientist
- Project Leader – ASKAP Digital Systems
**Phone:** +61 2 9372 4420
**Email:** john.bunton@csiro.au
**Web:** www.csiro.au/org/ICT.html

**Contact Us****Phone:** 1300 363 400 **or** +61 3 9545 2176**Email:** Enquiries@csiro.au  **Web:** www.csiro.au