Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 11

Towards the upgrade of the ASTRI structure following the consolidation of the requirementsEnrico Giro – INAF OAPDfor the ASTRI Collaboration & the CTA Consortium

Universidade de São PauloInstituto de Astronomia, Geofisicae Ciencias Atmosferica

9° ASTRI general meeting

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 2

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 3

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 4

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.

Pointing performances

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 5

RequirementsError 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 -15deg +25deg Gradient 7.5 deg/hMax wind 36 km/hEl. range 25-89.2 deg   Favorable conditions     Gradient 3 deg/hMax wind 20 km/hEl. range 35-85 deg   Pointing CTA < 5 arcsec per axis (systematic)Pointing SSTs Pointing 1 SST < 7 arcsec (rms space angle)

Are these ones compatible?

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 6

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 2mirror telescope.

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 7

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 8

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 9

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 10

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 11

Direct imaging with Cherenkov Camera (II)

Zeta Tauri

121 Tauri

114 Tauri

CRAB

MAGNITUDE ZETA TAURI 3.01114 TAU 4.88121 TAU 5.37

sampling = 2048Rate= 80 Hz

VARIANCE (ADC2)

Credits: O. Catalano

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 12

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 13

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 14

Online CalibrationIn 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 15

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 16

Redesign and pointing performancesASTRI structure (GEC) has been designed for online calibration. In fact at design phasewe 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 telescopesMiniarray redesign have to deal with new requirements (Empty column)

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 17

On September 2014 a talk of Punch was on “CTA Questions arisingon 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

Redesign and pointing performances (II)

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb.2015 18

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 19N. Surname – Affiliation – Conference – Place, Date 19

THANKS!