Transcript Presentazione di PowerPoint

E. Verroi and G. Naletto
To a brief summary
The main problem is the instability of count rates in the 4 channels
Bad possibility of pointing the star ( since we are “blind” )
 Low focusing control
 Great noise contribution for long periods variable objects
Why ?
First hypothesis...
Shape of the
beam section on
the first lens of
the optical train
for a well
centred source
First hypotesis...
In this case we would see
differences beetween the 4
channels BUT stability in the
global countrate (4 ch sum)
THERE IS ANOTHER
PROBLEM
...Another instrument
A time tagging photon counting array returns data like this:
Only two stars have been analyzed (gzccnc216-pg0911)
 We don’t know the sky conditions for these 2 acquisitions
 Neither their positions in the sky
Integration of the signal over 20
minutes of acquisition
 Gz-kind star and Pg-kind star 
Pg are about 4 times brighter
than gz but their noise
(thermal + sky ) is about half.
Data analysis
Data analysis
Gaussian fit FWHM
Pg: x 3.6”--- y 3.9”
Gz: x 2,4” --- y 2,5”
Look out
!! This is not the seeing !!
During the whole acquisition (20 min for each star) an optical
system with the aperture of AQuEYE (3 arcsec entrance
diameter) would acquire only:
53% for gz
33% for pg
In the worst case an 8” pinhole is necessary to collect about 90% of
the photons during the acquisition!
Fraction of collected
photons as function of the
pinhole diameter for
Pg0911
This is the seeing:
Binning the data we can simulate the seeing
pg0911 with integration
time = 0.5 s
Pg 3s T-bin
Pg 1s T-bin
Pg 0.25s T-bin
Gz 8s T-bin
Pg 0.5 s T-bin
Gz 1s T-bin
Only a part of the problem is caused by auto guidance system.
Centroid position of the star for 8s time bin for pg0911
The auto guide jumps are of the order of 0.5” but also with a
“perfect” guide we don’t collect enough photons:
Fraction of photons collected as a function of the entrance aperture
angular diameter for several measurements of seeing for a perfect
guiding and well pointed instrument
For this reason we choose to increase the pinhole dimension for
IQuEYE as we will see later
Simulation of the signal for the four
SPADs as AQuEYE would see it...
original
simulated
IQUEYE
The Italian Quantum Eye for NTT/TNG
Assumptions:
Telescope: focal length = 38.5 m; focal plane scale factor = 187 µm/arcsec;
f/10.85
Assumed FoV to be collected: 5 arcsec = 0.935 mm on telescope focus
Needed total magnification (100 µm diameter SPAD): smaller than 1/10
IQUEYE Optical Concept
Focal reducer
Aqueye like single arm
It has been considered a focal reducer with two couple of lenses, with a
magnification of 1/3.25, and reducing a 5” telescope image (935 µm) to 290 µm
(FWZH).
Then, a pyramid splits the beam in four arms, and the following lenses further
magnify the spot by 1/3.6, bringing the 5 arcsec spot size to 80 µm diameter
(geometric magnification).
IQUEYE Optical Performance
The end
The end
Summary of preceding episodes...
The aim of this activity is to realize a “prototype” of the instrument QuantEYE,
proposed for application to the focal plane of OWL .
QuantEYE is a very fast photometer, dedicated to the observation of celestial
targets in the photon counting regime at 1 GHz max count rate
Our resources: 182 cm Ekar telescope in Asiago mounting AFOSC (Asiago
Faint Object Spectrograph and Camera )
AFOSC is a focal plane instrument applied to the telescope focus
with which the beam is collimated, filtered/dispersed (grism)
and refocused (with a 0.58 magnification).
Summary of preceding episodes...
Owing to the limitations we have on available resources, we decided to subdivide
the telescope pupil in only four parts ( instead of the 100 previewed for QuantEYE )
focusing each of them on a dedicated 50 m SPAD detector.
Summary of preceding episodes...
AFOSC focus
Specular Pyramid:
Splits the beam
in four parts
towards each
SPAD
Couples of Doublets:
Focus the beam on the
SPAD’s detector
Optional
Filters
S.P.A.D.
Single photon
avalanche diode
Summary of preceding episodes...
Our optics send a 180 m spot covering the
whole pin-hole in 50 m in the image plane.
50 m is the spad’s detectors diameter
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