Pg 1 2012 SALT Meeting: SALT 2GI SALT Second Generation Instruments (2GI): The Half-Offner Spectrograph As a Spectroscopic Building Block Darragh O’Donoghue.

pg 1

2012 SALT Meeting: SALT 2GI

SALT Second Generation Instruments (2GI):

The Half-Offner Spectrograph

As a Spectroscopic Building Block

Darragh O’DonoghueSouthern African Large Telescope

in collaboration with

Chris ClemensUniv. of North Carolina

pg 2


SALT 2GIs

Can be one or more items from:

• Additional spectroscopic capability (wider λ, IFU(s) …)

• Adaptive optics and instruments to match

• Telescope wider field and instruments to match

• Specialised instruments (coronography, Fourier Transform

Spectrometers ...)

This is by no means an exhaustive list.

Today I am going to talk about a new kind of optical spectrograph which can be used in a wide variety of ways, and present one specific example of how it might be used on SALT.

However, the talk is NOT a specific instrument proposal. It is rather to show a new kind of instrument which may be of interest to those with specific science cases which could be met by this instrument along with an appropriate front end. I will certainly be paying close attention to the radio projects to be talked about for the rest of the day.

pg 3


The Horror Of The Modern Optical / NIR Spectrograph

On today’s 10-m class spectrographs, you will find instruments which have / are:

• Physically large: typically ~2 metre in linear dimension so themechanics are difficult (need to keep optics in place to 25 microns)

• Optics are large and heavy: 250 - 300 mm in diameter

• Numerous elements: 12 – 20 lens elements is commonplace. Many …

• … lenses are made of CaF2, the crack cocaine of the optical designer: Expensive Fragile Vulnerable to thermal shock or thermal gradient shock

• The most desperate, UV light-craving addicts will use BaF2 oreven NaCl as lens substrate material.

• Antireflection coatings cost an arm and a leg

I have a right to criticise: my design of SALTICAM includes many of the above (but has not had optical problems – my explanation for this is “SALTICAM did not push the limits of the technology and I was lucky”).

pg 4


Gemini Multi-Object Spectrograph (GMOS)

GMOS on Gemini is an example:

pg 5


Optical train of Gemini Multi-Object Spectrograph

GEMINI Multi-Object Spectrographs (GMOS)

pg 6


GEMINI Multi-Object Spectrographs (GMOS)

Optical diagram of Gemini Multi-Object Spectrograph

1 metre

pg 7


IMACS on Magellan

600 mm

pg 8


Physically large with lots of large and delicate glass

~ half the lenses are either CaF2 or NaCl

pg 9


… Worse Horror Is To Come …MOBIE: Low/med resolution

spectrograph for the 30-Metre Telescope (TMT).

pg 10


I have been thinking for a number of years:

“Can it be simpler?”

Offner imaging spectrometer designs seemed like a promising avenue to explore for a solution.

pg 11


1973 Patent for Abe Offner’s 1-to-1 Relay:

pg 12



Concave spherical primary mirror

Identical concavetertiary mirror

Convex secondary mirror

It has EXCELLENTimage quality as it Is corrected for all5 primary optical

aberrations

Folding flatmirror

pg 13



… is imaged on to a silicon wafer

Application:lithographic

production of printed circuits:

Light passing through a scanning mask …

pg 14


Chris Clemens is one of only 3 commercial suppliers of astronomical VPH gratings in the world, having spent 10

years building up a fabrication facility and learning how to make them.

Chris and I discussed Offner-type designs for some years. My ongoing question to Chris was:

“Can you make a convex VPH grating?”

In 2012 February, his answer was “Yes”.

But what are VPH gratings and why are they relevant to Offner spectrometers?

pg 15


Transmission gratings using DCG (dichromated gelatin) processed after exposure to a laser hologram fringes.

This imprints a modulation in the refractive index, as planes of constant n.

Volume Phase Holographic gratings:astronomical dispersers of choice

pg 16


Tuning VPH gratings for efficiency: the ‘super blaze’

Traditionalsurface relief

Super blaze curve

VPH at specific incident angle

Rotate the grating and the peak efficiency “travels along” the superblaze

curve

pg 17


The first curved VPH grating design

pg 18


The first “Half Offner”-type spectrograph design

pg 19


The first “Half Offner”-type spectrograph design

pg 20


Layout of the mini prototype

Proof Of Concept Prototype: 2012 May-JuneDesign, procurement, commission and try-out in 6 wks

Resulting spectrum

• 1 x off-the-shelf spherical mirror, 50 mm in diameter, from Optosigma• 2 x off-the-shelf meniscus lenses from JML Optical …• … turned into a curved VPH grating in Chris’ lab• Some optical bench fixturing• Detector: white paper (max sensitivity with the lab window blinds down)• Later we got hold of a decent CCD camera but we only got eyeball estimates

of performance (due to time limits)

20mm

pg 21


Developing The Half Offner Into A Real Instrument

• Design a full-sized, useful, astronomical instrument of interest for SALT.

For the real SALT, the constraints on better performance include:

SALT has moderately poor seeing: 1 arcsec rare; 1.4-2.0” typical. Median seeing was believed to be ~1 arcsec. Image slicing would help enormously.

SALT is slow to set up, and has limited track time so shutter-open time is squeezed. Reducing spectroscopic setup time will help substantially.

Multi-object spectroscopy on SALT is difficult and exacerbates the previous point.

Many SALT proposals are for spectroscopy of single objects.

Many SALT proposals would like to do ultraquick “point and shoot” type spectroscopy (ToOs, transient follow-up or short exposure-many object programs) but are frustrated by poor shutter-open efficiency due to long setup time.

pg 22


• In response to the above, we (Chris Clemens, Gerald Cecil and I) have come

up with the following concept for SOAR and SALT for transient follow-up: IFU-fed for image slicing to beat seeing and minimize setup time. Single

IFU to begin with for single object programs, expansion to many for multi-object spectroscopy. Multiple IFUs to be deployable.

Resolution 3000 covering the wavelength range 380-900 nm Individual spectra will span 140 - 200 nm. 3 channels covers it all.

This is NOT a SALT 2nd Generation instrument proposal per se. It is a rather a prototyping and characterization design. It may prove to be a competitive SALT 2GI. Or another variation of the spectrometer as the basic unit, in a design aiming at other science goals. More prototyping work is needed first.

• So … the immediate aim is to build a full-scale prototype of the basic spectrometer for laboratory characterization: • To measure image quality and resolution• To measure efficiency• Assess overall instrument performance (ghosting, stray light etc.)

Developing The Half Offner Into A Real Instrument

pg 23


IFU-fed R=3000 Spectrograph

• This is an R = 3000 spectrograph covering λλ 380-900 nm

• Detector is 25 mm in the spectral direction and 10 mm field in the spatial• Single spectrum covers ~170 nm. A 3-channel system does it all in 1 shot

pg 24


R=3000 Spectrograph

“Spot diagrams” show instrument has excellent image quality

-5 mm

0 mm

5 mm

Wavelength (nm): 520 580 630 690

Spatialfield

30 microns =2 pixels ontypical CCD

pg 25


R=3000 Spectrograph

But you promised us a 2 element spectrograph …

Two field lenses were added by Offner to his relay in a later patent to widen the field of view. Ours arise for the same reason and yield an even more compact instrument. They are small and made of any glass which transmits the light (BK7 or, for UV, fused silica – the two most common glasses. No CaF2, NaCl or anything fragile and expensive).

One version of this design uses 2 off-the-shelf field lenses costing $300.

Field lenses

pg 26


R=3000 Spectrograph

But … what about the focal reducer?

This compresses the scale and speeds up the beam by a factor of 2 to reduce the size of the detector (and the optics). It uses 8 lenses, the largest being 12 mm in size and all the rest under 7 mm. BUT … half the lenses are CaF2.

Another focal reducer I have designed is much better: it is half as long and uses 1 glass type (e.g. BK7 or silica). No CaF2. It may be worth patenting

so I don’t want to show it ye.

pg 27


The IFU

Gerald Cecil (UNC) has been developing, in collaboration with Josh Bland-Hawthorn (U of Sydney), IFUs positioned by efficient robotic positioners: “Heavily fused” and “Lightly fused” Hexabundles comprising 61 x 100

micron fibers in a hexagonal packing arrangement. They currently prefer the lightly fused devices for astronomy: they

report 84 per cent fill-factor if etching the ends of the fibers is done. One can purchase a device now for $12 500 for a device now

This technology may need to become more mature.

We are currently thinking of, as an alternative (or preferably), a scheme similar to the GMOS IFU by Allington-Smith et al. (2002) which uses lenslets to feed fibers. These will have 100 per cent fill factor.

pg 28


Heavily-fused Hexabundles: FRD problems

pg 29


Lightly-fused Hexabundles: 84 % fill factor

pg 30


Rough concept for lenslet array: 100 % fill factor

100 mu = 0.45” on sky

4” on sky

.

.

.

.

.

81 x 75 micron fibers

lenslets

fib

ers

9 x 9 lenslet + fiber array

pg 31


• Optics: 140 mm spherical mirror $ 3000 50 mm curved VPH grating $ 7000 2 x 70 mm field lenses $ 5000 Focal reducer $ 5000 Optics coatings $ 5000 Integral field unit $ 10000

• Mechanics: $ 3000

• Electronics & software (grating changer): $ 5000

• Detector & software: 1 x 25 mm Andor EMCCD: $ 37000

$ 80000

Cost Of The R=3000 Spectrograph Hardware

pg 32


Conclusion

The Half Offner is a compelling new kind of spectrograph which is: Very simple Very compact Very inexpensive

and might turn into the spectrograph design of choice in the future if its initial promise pans out.

One way or another, a full-sized lab prototype of the basic spectrometer (no IFU and no focal reducer) will be built here at SAAO and more quantitative information about its performance should be in hand by the second half of next year. Specific instrument concept proposals should be available by then as well.

pg 33


pg 34


Pg 1 2012 SALT Meeting: SALT 2GI SALT Second Generation Instruments (2GI): The Half-Offner Spectrograph As a Spectroscopic Building Block Darragh O’Donoghue.

Documents

gi salt

salt meeting

metre slide

gi imacs

nacl slide

gi worse horror

north carolina slide

optical designer