Pointing performances - 9th ASTRI Collaboration Meeting

9° ASTRI general meeting
Universidade de São Paulo
Instituto de Astronomia, Geofisica
e Ciencias Atmosferica
Towards the upgrade of the ASTRI
structure following the consolidation
of the requirements
Enrico Giro – INAF OAPD
for the ASTRI Collaboration & the CTA Consortium
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
1
Outline
•
•
•
•
•
A bit of history
Requirements
Pointing concepts for Cherenkov telescopes
Next steps
Redesign and pointing performances
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
2
A bit of History
• ASTRI has been funded since 2011 for three years. All
contracts had to be signed before the end of 2014.
• Contracts for design phase started in 2011, document
delivery was in early 2013.
• Building phase started at the end 2013.
• The telescope of Serra La Nave has been built on
requirements defined by CTA in 2011-2012 (when
defined).
• For this reason changes in requirements later than 2012
were not taken into account on prototype.
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
3
Pointing performances
Pointing performances can be obtained online or post
calibration. Astronomical telescopes typically need an
online calibration because of the small Field of View
(FoV) of their focal instruments and use of slits for
spectroscopy.
In Cherenkov data acquisition the pointing
performances are typically post calibration.
Post calibration needs to track the real position of the
telescope during event triggering.
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
4
Requirements
Error budget on pointing evolved since 2011. But just now we are still
“consolidating” requirements. This was a main issue in the design of ASTRI
prototype. It is very difficult to deal with requirements and if evolving and
if overestimated.
At the present these are the requirements.
Working conditions
Ambient temp. range
Gradient
Max wind
El. range
-15deg +25deg
7.5 deg/h
36 km/h
25-89.2 deg
Favorable conditions
Are these ones compatible?
Gradient
Max wind
El. range
3 deg/h
20 km/h
35-85 deg
Pointing CTA
Pointing SSTs
Pointing 1 SST
< 5 arcsec per axis (systematic)
< 7 arcsec (rms space angle)
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
5
Requirements (II)
Just a comparison:
An optical telescope “old
fashion” as the ESO 3.6 m has
a pointing error of 5 arcsec
(online calibration).
The telescope is INSIDE a
dome. Wind limit OUTSIDE is
72 km/h.
Estimation on the secondary
could be about 25% of outside
wind (18 km/h) and 7% on
primary cell (5 km/h) .
The comparison is not fully
unfair because ASTRI is a 2
mirror telescope.
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
6
Requirements (III)
(http://www.mpi-hd.mpg.de/hd2012/pages/presentations/Hofmann.pdf)
Is it this the origin of pointing requirement?
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
7
MAGIC calibration system
A CCD mounted on the center
of the dish permits to image
the camera focal plane, its
LEDs, the sky and except during
observations, the target in the
center of the focal plane.
Differential distances between the target on the focal plane,
the target in the sky and LEDs permits to evaluate the real
pointing of the telescope. Data acquisition of the CCD
camera must sampled with the same frequency we want to
know the pointing performances (i.e. at least at the main
oscillation frequency).
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
8
SST 2M telescope and pointing
The new configuration of ASTRI SST-2M implies to reconsider the
strategy to calibrate the pointing of the telescope. A direct imaging of
camera and sky is difficult and moreover it cannot track for tilts of
secondary mirror impacting on pointing performances.
Three options are possible:
• Using the Cherenkov camera to have a snapshot of the stars in the
field of view during data acquisition (variance method)
• Using a devoted CCD camera in the central PDM of the focal plane as
“guiding” camera for pointing purposes
• An online calibration in the same way used for astronomical
telescopes.
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
9
Direct imaging with Cherenkov Camera
Pros
• No additional hardware to be installed
• The full focal plane is used for scientific acquisition
• High sensitivity in detector response
Cons
• Low focal plane sampling with respect to pointing
requirement
• Need of accurate metrology for the camera focal plane
• High noise due to unresolved background stars
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
10
Direct imaging with Cherenkov Camera (II)
MAGNITUDE
ZETA TAURI 3.01
114 TAU
4.88
121 TAU
5.37
121 Tauri
Zeta Tauri
VARIANCE (ADC2)
CRAB
114 Tauri
sampling = 2048
Rate= 80 Hz
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
Credits: O. Catalano
11
Direct imaging with Cherenkov Camera (III)
DSS field of the CRAB convolved for ASTRI PSF
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
12
CCD camera in the central PDM
Pros
• Devoted hardware for pointing
• High sensitivity in detector response
• High focal plane sampling with
respect to pointing requirement
Cons
• High Noise due to unresolved
background stars
• Limited field of view for sky
coverage
• Central FoV subtracted to scientific
acquisition
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
13
Online Calibration
In this case the same strategy of the optical telescopes is
adopted. Calibration of flexures (using for example
Tpoint) on a stiffer telescope permits to know in real
time where the telescope is pointing without imaging
the FoV.
Pros
• The telescope position is know in real time
• Reliable system
• In the case of SST-2M a safer way to achieve
requirements
Cons
• Weight of the telescope
• Structure design pointing oriented and not PSF
oriented
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
14
Next steps
In case of negligible noise, expected centroid performances are of the
order of 6 arcsec. But this is not a fully realistic case. So it is will be
necessary:
• To use synthetic images originated from GSC2 to have a realistic
simulation of the ASTRI FOV with at least a limit magnitude of 14th.
• For the case of variance method the real geometry of the Cherenkov
camera must be know.
• The real shape of the PSF must be know (from prototype
characterization) also in windy conditions.
• To simulate small shifts (~ 6 arcsec) in synthetic images to test the
sensitivity of the astrometric solution
• To estimate sky coverage for typical targets necessary to reach
pointing requirements.
• Check estimations directly on the prototype.
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
15
Redesign and pointing performances
ASTRI structure (GEC) has been designed for online calibration. In fact at design phase
we had not:
• estimation of sensitivity for pointing calibration with the variance method
• Mechanical space for a CCD camera in the center of the FoV
For this reason masses of prototype are on the side of the astronomical telescopes
Miniarray redesign have to deal with new requirements (Empty column)
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
16
Redesign and pointing performances (II)
On September 2014 a talk of Punch was on “CTA Questions arising
on the Wind Requirements impact on Observing time”.
Project Office is thinking to change requirements? Is it really necessary?
Is it correct relating the observing time for favorable condition?
Could be a better approach to calculate how many sources need for
favorable conditions?
PLEASE
In few months we are going to redesign the telescopes for the miniarray, so
we NEED at redesign kick-off:
• To be sure pointing requirements are consolidated
• To be sure that requirements are not overestimated
In a different situation we have only one chance:
We have to select requirements we consider correct
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
17
Summary
• Pointing performances are a critical issue for mass
dimensioning of the SST-2M.
• Different pointing strategies could influence
dimensioning of the main structure of the
telescope
• The present design of the prototype has been
driven by old requirements defined well before
the building phase.
• An accurate and balanced definition of
requirements on pointing is an essential aspect of
the redesign phase aiming to decrease the mass
of the SST-2M.
• In few months the redesign phase will start and at
that point the final requirements must be settled.
Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015
18
THANKS!
Enrico
N.
Surname
Giro – INAF
Affiliation
– OAPD
– Conference
- ASTRI general
– Place,
meeting
Date #9– Bologna, 24 Feb.2015
19