A wide field spectrograph/camera

transcript

Roberto Ragazzoni, Carmelo Arcidiacono, Jacopo Farinato, Marco Dima, Emiliano DIolaiti, Giorgia Gentile, Demetrio Magrin Valentina Viotto, Matteo

Quasimodo, Fernando Pedichini, Emanuele GiallongoINAF – Padova, Bologna and Roma Observatories

October 22nd, 2008Sede di Monte Mario

Background…• If you think to (seeing limited) wide field

you immediately think to a Prime Focus…

Background…• If you think to (seeing limited) wide field

you immediately think to a Prime Focus…

…and in fact we built two of them… whose optics works from U to H for half degree Field of View with 0.2arcsec optical quality

Scaling factors…• We are in the transition to ELT era…• …so we should be used to break the factor 2

syndrome…• We want a FoV 2..3deg in diameter rather than a

0.5deg one…• …that is up to 36 times more FoV then LBC…• …or it is 9x4=36 times again more photons than

a 1deg on a 4m telescope!• We want it with Multi-Object, Multi-Slits or Multi-

IFU spectroscopic capabilities…

So, what exactly we want…?• We want an LSST with, in addition, 3rd

dimension (spectroscopy) too.• We want it at a price and a timescale of an

instrument rather than of a telescope• We want it feasible with today national

technological capabilities• We want it with more industrial

involvement to gain more momentum on raising funds

Is this possible…?

Scaling factors…

• In 1984 Richardson et al. wrote a paper on the scaling subject…

• Size of the first lens scales linearly with the FoV…

• …leading to a 4.9m lens for a 3degs Prime Focus…

• weighting about 100tons!

Surprised????

And still the refractive part ofthe camera remains evenmore difficult and challengingas what we call a “conventional” Prime Focus…!!!

Size compared with a normal (???) man…

Would you really allow this person to do a WFoV camera for you???

…new challenge requires new solution…

What is difficult in a W-FoV imager?

• The fast focal ratio (smaller than the Cassegrain stations) Prime Focus or Focal reducer

• Wide Field of View (a tautology??) use big optics

• Correction of off-axis aberration Several optical elements to control aberrations (with aspheric too)

• A large array or mosaic Big buttable CCDs

What is difficult in a W-FoV imager?

• The fast focal ratio (smaller than the Cassegrain stations) Prime Focus or Focal reducer

• Wide Field of View (a tautology??) use big optics

• Correction of off-axis aberration Several optical elements to control aberrations (with aspheric too)

• A large array or mosaic Big buttable CCDs

VLT or LBT….Secondary mirror modifiedto achieve an F/4 in the usual position…

A trapped F/4 unit…

Just an explanatory picture

Long F/ focal plane

Short F/ focal plane

Extensive segmentation

• Each unit has the same optical elements as any other else

• Correcting devices are mounted in a different fashion

• Achievable FoV of a single element crucial for defining the instrument

• Designs lies in the range 2..3arcmin in size

4.9 sq. degs 3.14 sq. degs 4.0 sq. degs

1000 to 2000 cameras

1 arcsec diameter

Multi Object Spectroscopy too!!• The physical size where fibers are to be located

is much smaller than the mechanical size where the positioner is to be placed

• Ideal condition for mechanical solutions (almost doable with off-the-shelf components)

• Ratio of size is ratio between long and short focal ratios (2 to 5, in our designs)

• Array of small FOSC spectroscopic cameras with a number of moveable multi-slits

F/4F/3 F/2

Options for spectroscopy• Multi-fiber or multi-IFU

– Simpler mechanically and optically– Requires a dedicated spectrograph(s)– Blind pointing & fiber losses– Spectral resolution and band flexible

• Multi-slit– Requires double relay & more mechanics– Pointing is safe and straightforward– Spectral resolution and band limited & fixed

• In both cases multiplexity is in the few to 10 thousands range

It is movie time…

What a detailed Phase A study should contain…?

• Science cases… here we are!!!• Pipeline, archiving and data reduction plan• Cost vs. implementation plan (ramping up of the

covered FoV?)• Secondary mirror feasibility, mounting design

and balance requirements on the telescope• Precursor experiment? (an off-axis camera)Specific for VLT• How to preserve the VLTI optionSpecific for LBT• Location (fourth bent Nasmith)• Justification & plan for double foci

References• SPIE proc.6272, 25 (2006)• SPIE proc.6269, 187 (2006)• SPIE proc.5492, 121 (2004)• A napkin on a “restaurant” in Copenhagen (2002)

How to go ahead…?• Setting up a consortia or running alone?• VLT, LBT or still keeping the two options open?• Science is probably straightforward but, why not to

“count us”…??? A WorkShop soon on the subject?• A real, INAF endorsed, preliminary design phase, similar

to the one for LBC.

Considerations• This is undoubtely a HUGE projects• Requires a detailed true design to go ahead• Requires to setup a real conosrtium both inside

and outside Italy• Requires connection with data management• One shot is about 1000 x 2kx2k pixels• Requires sort of automatic operations• One spectroscopic shot requires placing a few

thousands slits. It must be automatic to a large extent!

List of issues (preliminary)• VLT vs. LBT issue:• LBT can be trapped or normal Cassegrain. (no way… trapped is by far the

best but just we have to show up this)• In normal Cassegrain there is need of an additional foci position and of a

larger M2 and M3. • Point out masses, sizes involved and feasibility. • In case of VLT arguments about where is the overall cog, and effects on the

interferometry. • About LBT discuss how the two channels can be diversified. • Wavelenght, modalities, number of slits, etc…• Comparison with LSST (2.6deg is the threshold for having the same A

Omega). • How much can be the optical filling factor (dead strips between cameras. • Also recall that due to Cassegrain blurring there is an overlap with

vignetting)• There is a sort of analysis carried out by Giorgia but it is still floating!!!

List of issues (II)

• Optical Design:• Number of lenses. • Goal quality expected to be 80% in

0.3arcsec?? • Mass production of the lenses. • Is there any possibility of inside the FoV

correction to make the single FoV much larger??

• We actually lack a REAL optical design!

List of Issues (III)

• Spectroscopy:• Concept. Review multi fibers just to point out

that is a problem to find out the objects. • Mechanisms, positioning devices. • Preparation to observations.• Where and how to place slits in automatic

manner.• In this moment there are conecpt and fake

movies (even not finished!)

List of issues (IV)• Mechanical design:• Overall design. • Modularity• Accessibility. • Filters exchange• Maintenance• Piping• Overall derotation• Secondary mirror• Can we proceed without a baseline optical design?

Issues (V)• Detectors:• Point out that the area or pixel number is

invariant. Mass production. Quality tes. Certification. Testing? Thinned back illuminated or not? Can me move out from conventional CCD technology?

• Controllers:• Engineering. Remotely programmable.

Adressable. WiFi?? Certification, testing, wquality again.

Issues (VI)• Active Optics:• Just a few modules or additional little cameras for

making ActOpt at a few edges. To be checked/studied• Background:• Just to recall that there could be a background issue.• Data Management:• How much data get out? Picture of the bathroom with a

lot of data everytime you flush it!!!! Yellow claim one do not confgrid all the single images unless requested. Maybe this can be done anyway in a pipeline delayed?

Issues (VII)• International Issues:• VLT and LBT are so different. For LBT Germans claimed

interest and Americans denied it. Competition with LSST. Peculiarity of spectroscopic option.

• Management:• Definition of WPS• Criogenic devices:• Thermal budget. Pletier cooled?• Money:• How much it can really cost? Where can we rob the

money?

Furthermore…

• Science!• We all agree, BUT there is not a well

defined structure• Which must be OUR role?• There will be a meeting somewhere,

hopefully the May 13th, 2008• Will be presented at Ringberg• Will be a poster (sigh…) at Marseille…

First steps

• Define nominal figures• Figure out an optical design• Define a mechanical framework• Setup a scientific environment• Aim to set up the cost!• Still the project need firm and outstanding

documents/papers• Who does what???

Open to free discussion

• What do you think?• Which part do you like?• Which is your opinion into the VLT vs. LBT

issue?• Who we wish to involve into this?• Need to setup a minimum goal with

deadline!!!

A wide field spectrograph/camera

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