Download The Dragonfly Telephoto Array

The Dragonfly Telephoto Array
Pieter van Dokkum
Bob Abraham
Allison Merritt
Jielai Zhang
Imaging the night sky
• Over past ~40 years factor of ~100 improvement for point
sources:
• Imaging mag 25 —> 30
• Spectroscopy mag 21 —> 26
• However, low surface brightness sky remains relatively
unexplored: limits have not changed much in past 30+ years!
Tal et al 2009 - low surface brightness survey of nearby ellipticals
28-29 mag/arcsec2
David Malin - early 1980s (?)
Low surface brightness science
• Faint galaxies: dwarfs have very low surface brightness
Koposov et al 2015 - satellites of Magellanic Clouds
Low surface brightness science
• Stellar halos and substructure / tidal debris around galaxies
Bullock & Johnston
Low surface brightness science
• Lot of other things!
Comets
Light echos
Intra-cluster light
Galactic
cirrus
Why so little progress?
• Building bigger telescopes / more sensitive instruments does
not help
• Limitations have to do with design choices that make
large, fast telescopes possible
VISTA telescope
What is needed?
• Signal from structures that are much bigger than the seeing
scale: high sensitivity requires small focal ratio
• Night sky, and many objects in it, are >1000x brighter than
the hoped-for regime: need high dynamic range and
therefore well-behaved PSF
Small focal ratio
• Fast telescopes exist: modern reflectors have f=1-1.5, to
provide a wide enough field for their large aperture
LSST
optical
design
Sloan
telescope
COST: large secondary mirror
Small
focal ratio
—> large obstruction in light path
• Moves energy to the wings
of the PSF
• Support structure causes
diffraction
Mirrors are bad, too
• Mirrors themselves also cause scattering, because of microroughness and dust
cterization of scattering in an optical resonator
o
0.71
o
0
o
-0.64
o
-0.85
o
0
o
1.00
Figure 2.2: Figure consisting out of 25 CCD images of the scatter from a single mirror.
n the center an obscuration blocks the direct beam. The speckles are formed by scatter
due to surface roughness of the mirror.
speckle pattern is not caused or influenced by edge-diffraction of the mirror as the
r of the spot (Airy-disk) on the mirror (at L1 = 36 cm away from the pinhole) is small
L1 /D ≈ 2 mm) as compared to the size of the mirror. Furthermore, the spot on the
Klassen 2006
c .
igh-latitude galactic dust, which reflects
ic disk (e.g., Sandage 1976; Witt et al.
that any of the green-colored structure
ster light. Faint ICL structures can indeed
om M87, as was previously observed in
e
To emphasize the impact of the scattered star light, we performed one reduction of the data without any reflection removal
or star subtraction. This is shown on the left in Figure 8. The
difference between the properly star-subtracted image and
the reduction without star subtraction is shown in Figure 9. The
most significant contribution to the excess light comes from
eighth-magnitude stars to the west of M87. In total, the
Net effect: reflecting three
telescopes
produce
complex
PSFs,
which
flux from foreground stars in the field is equivalent to a single
2
star of magnitude
M V ¼ 3:7.
discussed
in § 3, since at least
limit low surface brightness
studies
toAs~29
mag/arcsec
1.2% of the star light is scattered to radii beyond 2.4′, there is
SLATER, HARDING, & MIHOS
PSF of conventional telescope
•
1270
1272 SLATER, HARDING, & MIHOS
FIG. 4.—Two 450 s exposures of Arcturus, with the star positioned in opposite corners of the field of view. The optical center is toward the top right of the image on the
left, and toward the bottom left on the right image. The images saturate to black at μV ¼ 21:2.
tion indicates that the window is 0.6 cm away from the CCD as
axis from bright stars. The ghost is the result of light from a star
expected. The reflection off the top of the dewar window was
reflecting off the CCD, traveling back up the telescope, reflectfaint enough to be far below the noise level on realistic foreing off the corrector, and ultimately returning to the CCD. This
ground stars near the science fields, and so we do not attempt
ghost was masked out of all calibration and science images. The
to model it. Similarly, multiple bounces between the reflecting
most distinct reflections beyond 2′ are caused by the dewar winsurfaces and the CCD will contribute a small amount of light at
dow and the filter, and appear as bumps in the PSF at roughly
large radii, but using the measured reflectivities, we calculate
2.5′ and 10′ from the star. These are the reflections that will be
that all of the secondary reflections will have surface brightmodeled and removed.
The sources of the reflections can be confirmed by using the
nesses fainter than μV ¼ 30 mag arcsec"2 in our Arcturus
size of the reflections to determine the extra path length traveled
images, well below our detection limit. A summary of the reby the reflected light. The size of the two largest reflections corflecting surfaces and the brightness of their reflections is prerespond to a reflecting surface 3.6 cm away from the CCD,
sented in Table 1.
which matches the position of the filter. The bottom surface
These reflections have been modeled in Zemax, an optical
of the filter (the side closest to the CCD) causes the brightest
ray-tracing program, to confirm their source and the causes
FIG. 6.—Same
Fig.
4 after
the reflections
PSF. subThe images
at μVbetween
¼F
22:1,
half
brightness
of the images
inmodel
Fig. 4. of
difference
between
the
reduction
with
no and
star
IGroughly
. stars
10.—Cumulative
histogram
reflections,
andimages
can as
beineasily
seenremoval
in allofthe
example
images.
of saturate
the offset
andthetheir
reflections.
The
The top surface of the filter produces a much fainter reflection,
the telescope included all optical surfaces in the light path, in-
Slater,
Harding,
of the scattered light
observed
from stars&inMihos 2009
Ideal telescope
• Ideal telescope has no mirrors and an unobstructed light
path
refractor!
Refractors
• Except for solar telescopes, refractors have been dead for
astronomy for a century
• But they are alive and well in the real world!
Refractors
• Except for solar telescopes, refractors have been dead for
astronomy for a century
• But they are alive and well in the real world!
Refractors
• Except for solar telescopes, refractors have been dead for
astronomy for a century
• But they are alive and well in the real world!
Superbly coated, perfectly baffled,
optically-fast (f/2.8) refractor
Latest generation of telephoto lenses
• Many optical elements (bad) …
microcrystalline alumina film cancels reflections arising from
the the
microcrystalline alumina film. The idea is that the microcry
nd the
refractive index transition from 1.0 to 1.4, and the
r antiintermediate layer, functioning as a single-layer antim the
reflective coating, reduces reflections arising from the
1.84
nefit of
refractive index transition from 1.4 to 1.84. The benefit of
1.56
Microcrystalline this approach is that the intermediate layer’s refractive index
e index
Alumina
1.4
nt lens
and thickness can be adjusted to match different lens
Many optical elements (bad) refractive
…
indexes.
of the
Figure 8 shows the refractive index profile of the
1.0
special220nm
coatingsanti-reflective
(good) coating formed on the first lens element. In
ent. In … BUT have
68nm
x, the
order to match the glass’s 1.84 refractive index, the
Thickness (nm)
ickness
microcrystalline alumina film was designed with a thickness
Intermediate
layer
Lens
•
•
Refractive Index
Latest generation of telephoto lenses
Fig.8 Refractive index profile of SWC
2.0
Standard multi-coating
Angle of
incidence
1.4
1.2
1.0
0°
15°
30°
45°
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
Wavelength (nm)
(b)
nce of (a) SWC and (b) Multi–coating
650
700
Reflectance (%)
0°
15°
30°
45°
Reflectance (%)
le of
ence
1.8
1.6
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
“Sub-wavelength” coating
Angle of
incidence
0°
15°
30°
45°
400
450
500
550
600
Wavelength (nm)
650
700
(a)
Fig.9 Measured reflectance of (a)
The Dragonfly Telephoto Array
• First test: in basement
paperclip
toy flashlight
M51, with single telephoto lens
(Megantic dark sky preserve)
NGC7626 group
400mm f/2.8 II
(20 minutes)
NGC7626 group
400mm f/2.8 II
(20 minutes)
standard lens
Canon f400 f/2.8 II lenses have ~10x less scatter
than the (superb) Burrell Schmidt
Surface Brightness (mag/arcsec2 )
12
Log10 (Normalized Flux)
1
0.01
10-4
Burrell Schmidt
Dragonfly Array
14
16
18
20
22
Burrell Schmidt
24
Dragonfly Array
26
28
10-6
0
10
20
30
Radius (arcmin)
40
50
0.1
0.5 1.0
5.0 10.0
Log10 Radius (arcmin)
Abraham & van Dokkum 2014 [Dragonfly overview paper]
Also (independent confirmation): Sandin et al 2014
50.0
• Dragonfly is currently a
0.46m, f/0.89 refractor
• 2x3 degree field of view
• 2.9’’ pixels
• Fully robotic - operates
every clear night
• Dragonfly is currently a
0.46m, f/0.89 refractor
• 2x3 degree field of view
• 2.9’’ pixels
• Fully robotic - operates
every clear night
Dragonfly can* reach ~32 mag/arcsec
2
No stellar halo around M101
a
13
b
van Dokkum et al 2014
M101
M31
* After subtracting stars and significant binning. May not apply to all areas. Observe responsibly: results obtained in the past offer no guarantee for the future.
Early science results (not for this meeting..)
Seven dwarfs in the
M101 field
(Merritt et al 2014)
No stellar halo
around M101
Ultra-diffuse galaxies
in the Coma cluster
Plans
• Completed survey of ~10 nearby luminous spiral galaxies results very soon (Merritt et al, Zhang et al)
• Upgrade! Will be going from 10 to 50 lenses
• Upgraded array will be equivalent to refractor
with aperture of 1 m and focal ratio of 0.4
Summary
• Telephoto lenses offer way to (finally!) image the sky at
surface brightness levels >29 mag/arcsec2
• Dragonfly Telephoto Array has been operating for ~1 year;
already several surprising results. Upgrade under way!