AQuEYE analisys of problems: guiding and seeing 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
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
...Anotherinstrument 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 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%forgz 33%forpg
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 0.5 s T-bin Pg 0.25s T-bin Gz 8s 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
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).
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 mm spot covering the whole pin-hole in 50 mm in the image plane. 50 mm is the spad’s detectors diameter To the presentation