WHAM

WHAM-SS DR1 Public Release


The Wisconsin H-Alpha Mapper (WHAM) is an astronomical observatory tailored to study wide-spread, diffuse optical emission from the Milky Way. Here at our site you can find an overview of the project, its specialized Fabry-Perot spectrometer, and more technical detail about the installation. Then, explore the science we’ve been doing, or download our publicly released Hα survey. Finally, delve into WHAM-related publications that discuss our discoveries and results in more detail.

WHAM was built and operates due to support primarily from the National Science Foundation.

The grand machinery of galaxies is set in motion by some of the most basic forces in the universe. The gravitational energy between atoms and molecules in interstellar gas is continually converted into light, heat, and high-speed particles in the cores of stars. The by-products of this ongoing process become the essential ingredients of planets and life itself.

The formation of stars and the release of matter and energy during their lifetimes regulates the inevitable crush of gravity, setting up a complex feedback loop that allows galaxies to continue this cycle for billions of years.

The Wisconsin H-Alpha Mapper (WHAM) group is studying one important component of the interstellar medium (ISM) in our own Milky Way to help answer important questions about how galaxies work.

  • Where does the energy produced in the star-forming regions of our Galaxy go?
  • How does that energy propagate away from these birth sites?
  • How does this energy change as it travels, and how is it deposited back into the Galaxy?

The disk of the Milky Way contains a thick (many thousands of light-years) layer of ionized gas, dubbed the Warm Ionized Medium (WIM). This layer appears to be powered by ongoing, active star formation.

Ultraviolet and X-ray light leaking from the dense star-forming regions in the disk appears to be the primary source of energy. But some evidence suggests that a portion of the power may come from converting energy released in supernovae, which occur on average about once a century in the Milky Way. Due to the WIM’s diffuse nature – only about 100,000 atoms per cubic meter – it is difficult to detect and characterize with traditional astronomical instruments.

WHAM is a custom-built observatory designed for studying the WIM in detail. It has produced the first map that traces not only the distribution but also the motion of the gas. To achieve this goal, WHAM obtains spectra instead of images of very faint Balmer-alpha (Hα) emission from ionized hydrogen. Its primary mission is to produce the first spectral, all-sky survey of this emission from the Milky Way.

While on Kitt Peak in Arizona, WHAM obtained the data for the northern portion of this survey and followed up with a variety of other projects to explore the properties of the WIM and the origin of the energy needed to sustain it.

In 2009, we moved WHAM to Cerro Tololo in Chile so that it can observe from the southern hemisphere and complete the all-sky survey. We will then return to exploring the detailed physics of the WIM as well as gas associated with two of the Milky Way’s satellite galaxies, the Large and Small Magellanic Clouds. Most of these ongoing projects combine new WHAM emission-line observations of elements other than hydrogen (primarily sulfur, nitrogen, oxygen, and helium) to measure the physical conditions of the gas and explore the processes involved in powering the WIM.

A spectrometer is an essential tool that astronomers use to examine the intensity of light as a function of wavelength (color). The resulting curves can reveal a tremendous amount of information about the physical properties and motions of the object emitting the light as well as any material through which the light shines. Like the quality of digital imaging, the ability of a spectrometer to record finer pieces of wavelength space is known as its spectral resolution. Advances in physics and optics over the last century have made many types of spectrometers available for use in astronomy.

One of the most common in use today is the diffraction grating spectrometer. These devices achieve their best performance and highest resolution when fed light from tiny angular sources containing as much light as possible. Thus, telescopes with better angular resolution and larger collecting area are capable of supporting higher quality output from a grating spectrometer. But this performance scaling of a grating spectrometer is not as ideal for studying the spectrum of astronomical objects that are not point-like. In particular, for a source like the Warm Ionized Medium (WIM) that emits optical radiation which covers the sky, they are extremely inefficient. Fortunately we have other tools at our disposal!

A Fabry-Perot “ring-image” spectrum of a Th-Ar calibration lamp taken with WHAM.

One alternative instrument, the Fabry-Perot spectrometer, is particularly useful for studying sources that extend over large portions of the sky. Unlike the grating spectrometer, its resolution is not limited by the size of the light cone entering the instrument. Instead, it can accept a large cone of light and still achieve excellent spectral resolution.

As a result, it is a great instrument for studying features that are distributed over the sky, including the earth’s atmosphere, dust in the solar system, and the interstellar medium of our Galaxy.

At the left is a (false-color) image of a thorium-argon laboratory calibration lamp that was taken with WHAM’s Fabry-Perot spectrometer. The yellow rings are two atomic emission lines from the characteristic spectrum of a heated thorium gas. In these spectral images, larger rings have shorter wavelengths (i.e., are bluer).

A plot of the intensity versus wavelength of the two Th emission lines recorded in the image above.

We convert the ring image to a more traditional two-dimensional spectrum by averaging image pixels in an annulus at a particular radius (or wavelength) to create one data point in the figure to the right. Fabry-Perot spectrometers map equal spectral intervals to equal area intervals, so that each of these spectral data points is calculated from equal area annuli—bins with the same number of image pixels. These spectra form the primary data set for all of our WHAM science.

Fabry-Perot spectrometers can be used in several ways when coupled to telescopes. WHAM is configured so that all the light from the sky within a “beam” (or cone) having an angular diameter of one degree (about double the width of a full moon) is analyzed by the spectrometer. In this mode, we loose any information about the spatial distribution of the light on the sky within that beam, but we gain a tremendous amount of sensitivity to very faint emission. By stepping the beam across the sky, we can still construct a map of the WIM where each beam is like a pixel in the resulting image. Since the WIM covers the whole sky from our vantage point inside the Galaxy, structures tend to have large angular extents and WHAM’s 1°-resolution is adequate for tracing structures throughout the ionized medium. The tradeoff in spatial resolution nets us huge increases in sensitivity over traditional imaging projects as well as the ability to collect spectra, which are essential to uncover the dynamics of the gas in the Milky Way as well as some insight into its three-dimentional structure.

The Wisconsin H-Alpha Mapper is a specialized instrument to study the distribution and kinematics of diffuse, ionized gas in the Milky Way. WHAM consists of a custom built 0.6-meter telescope and a dual-etalon 15-cm Fabry-Perot spectrometer housed in an environmentally protective trailer. In its primary spectral mode, WHAM records a spectrum spanning 200 km/s (about 4 Å near Hα) with 8-12 km/s velocity resolution from a one-degree, spatially integrated beam of the sky. With its large-aperture design and modern CCD technology, WHAM can detect emission as faint as 0.05 Rayleighs in a 30-sec exposure. For gas at 10,000 K, this observed intensity corresponds to an emission measure of about 0.1 cm-6 pc, more than 10 million times fainter than the Orion Nebula.

WHAM’s first major product was the Northern Sky Survey. By fully mapping the Hα emission within about ±100 km/s of the Local Standard of Rest (LSR) over the portion of the Galaxy visible from Kitt Peak, we can now explore the spatial and kinematic structure of the warm, ionized component of the ISM in our Galaxy. This new survey of ionized gas is analogous to previous surveys of neutral hydrogen made through the 21-cm radio line. Using both surveys, we can now also explore the relationship between the diffuse ionized and neutral gas. An example of what we hope to learn from such methods can be found in Reynolds, Tufte, Kung, McCullough, & Heiles, 1995, ApJ, 448, 715.

WHAM is a completely remote and robotic observing facility. We have taken special care to allow all aspects of operation to be controlled from any location. Our remote capabilities are based on telescope control software written by Jeff Percival that is also used by the WIYN telescope. Although the instrument is located in Arizona, nearly all WHAM observations have been operated remotely from Wisconsin. For a typical night, an observer plans and schedules targets before the evening begins then configures the instrument and initiates the observations. WHAM collects the data and closes up for the night autonomously.

WHAM spent a year in Wisconsin (November 1995 – November 1996) at Pine Bluff Observatory for testing and software development. It then moved to Kitt Peak, Arizona on November 19, 1996 and began the Hα survey, which took about two years. With the map of Galactic Hα emission under its belt, WHAM gathered data from Kitt Peak to explore the detailed physics of the ionized ISM. Optical emission lines from He, S+, N+, O, and O++ are routinely observed by WHAM.

In April 2008, WHAM was removed from Kitt Peak. The instrument spent six months in Madison refurbishment and upgrades. It was installed at the Cerro Tololo Interamerican Observatory in Chile in March of 2009. From Chile, the WHAM group is completing the Hα survey as well as continuing to explore other optical emission lines and the extended gaseus structures associated with the Magellanic System.

The WHAM project is funded primarily through grants from the National Science Foundation with additional support provided by the University of Wisconsin Graduate School, the UW Department of Physics, and the UW Department of Astronomy. Much of the hardware was built and assembled by the University of Wisconsin Space Astronomy Laboratory and the Physical Sciences Laboratory.

Research Topics

Milky Way

Emission Line Maps of the Milky Way

On August 8, 1996, WHAM obtained its first test map from the Galaxy during final testing at Pine Bluff Observatory in Wisconsin. Shortly after, it was moved to Kitt Peak and began it’s primary task: mapping the entire sky in Hα. Most of the data for the northern sky was obtained during 1997 and 1998 while the southern sky was observed in 2009 and 2010. After Hα was well in hand, we began mapping select portions of the sky in other emission lines, including [S II] 6717Å, [N II] 6583Å, and Hβ (see Emission Line Studies). Some multi-wavelength maps of interesting regions of the sky have been published (see our publications list for several selections). The Hα survey is available in the Survey section of this site.

WHAM’s velocity-resolved maps nicely complement the narrow-band filter imaging projects such as the Virginia Tech Spectral Line Sky Survey and the Southern Hα Sky Survey Atlas (SHASSA).

Intermediate- and High-velocity Clouds

Emission-line studies of HVCs provide many new clues about the nature of these elusive objects. Until recently, HVCs and IVCs were studied almost exclusively through maps of the 21 cm line of neutral hydrogen and a handful of absorption line studies toward more distant objects. We can tune WHAM’s velocity window to observe gas outside the ±100 km/s window about the local standard of rest covered by the sky survey. We have made many observations toward known neutral and highly-ionized HVCs, finding ionized gas in nearly all those observed to date. Hα and [S II] 6716Å from several of these regions have been published (see Papers for details). Hα emission from intermediate-velocity gas often appears in the primary survey, which is also highlighted in a few of our publications.

H II Regions

Aside from being interesting studies in their own right, H II regions can be used as probes for the ionizing radiation of their parent star(s). Since interstellar hydrogen is particularly efficient at attenuating radiation shortward of 912Å, direct observations of the far-ultraviolet radiation from hot stars is rare. WHAM fills an interesting niche here by detecting faint H II regions around isolated main sequence and evolved O and B stars (e.g., Reynolds et al. 2005). Since we can also map these regions in other emission lines, these new finds may be good constraints for those trying to model the spectrum of hot stars.

Magellanic System

LMC & SMC

We will be mapping emission in and around these nearby neighbors to see how far their ionized gas extends. The kinematics revealed by these spectral observations may uncover some surprises and should allow us to study connections between the neutral gas in and around the galaxies and any extended ionized structures. WHAM’s one-degree beam is too large to provide a great amount of insight on the internal structure of the galaxies themselves.

Bridge

21 cm observations have shown that a tenuous neutral structure connects the galaxies. Some ionized material and even early stars suggestive of recent star formation has been detected in the portion of the Bridge closest to the SMC. With WHAM, we will be able to detect and map any ionized gas associated with this feature to very faint emission measures.

Stream

An impressive, ~100°-long tail of gas trails the clouds’ orbit through the Milky Way’s halo. Prior studies have shown there is both ionized gas and Hα emission at certain locations along the Stream, but the full structure has only been surveyed in H I to date. One of our major priorities from Chile is to provide the first survey of the Stream in Hα and to carry out several comprehensive multiline studies along its length. There are many outstanding questions about the origin of the Stream and its evolution, dominant gas phases, ionization structure, and interaction with the halo of the Milky Way.

Leading Arm

Out ahead of the galaxies’ orbit, a less organized collection of neutral gas proceed their passage through the halo. No ionized emission has been detected from this structure yet, but WHAM should provide the most sensitive search to date.

Optical Emission Lines

Since most of the Hα emission we detect arises from hydrogen recombination, atomic physics dictates a set ratio of Hα to Hβ emission from ionized interstellar gases. Although it is a slight function of temperature, near 10,000 K, the ratio is about 3:1 in favor of Hα. However, interstellar dust absorbs more blue light than red so that ratios greater than this are typical in observations. Observed ratios of Hα/Hβ provide an interesting probe of dust in front of and within the ionized gas. Madsen & Reynolds 2005 present our first application of this technique toward a hole in the local dust toward the northern inner Galaxy, which generally has a substantial amount of obscuration.

[S II] 6717Å & [N II] 6583Å

These lines are nearly as bright as Hα in the WIM and are good tracers to discriminate between the diffuse background emission and H II regions. Combined with Hα, they begin to trace the physics of this ionized phase in addition to its distribution. In both the Milky Way and other spiral galaxies these lines tend to increase in intensity relative to Hα as the Hα emission decreases. In several of our papers, we propose that these rises are due to increasing temperatures. Both the collisional excitation of theses forbidden lines and the recombination that produces Hα are functions of the gas density squared (the emission measure). However near 10,000 K, the emissivity of the forbidden lines increases much more rapidly with temperature than that of Hα decreases. Thus, the smooth increase of [N II]/Hα and [S II]/Hα ratios with decreasing Hα intensity seems to indicate a gradual rise in the gas temperature. This argument can be taken one step further since, in many cases, the decrease in Hα intensity is due to a decrease in electron density. For example, as we look toward regions above the Galactic plane, the Hα intensity is decreases smoothly with distance from the plane due to the exponential scale height of the ionized layer. In this particular case, we then infer that the temperature of the WIM rises into the halo of our Galaxy.

[O I] 6300Å

Due to similar first ionization potentials, the fraction of neutral and singly-ionized oxygen and hydrogen are locked together from charge-exchange reactions in many astrophysical plasmas. WHAM detected this line from the WIM for the first time near the Galactic plane. Measurements of this line relative to Hα provide a good estimate of the average fraction of neutral oxygen (and thus hydrogen) along the line of sight. See Reynolds et al. 1998 and Hausen et al. 2001 in our publications list for details.

[N II] 5755Å

Like the 4363Å “auroral” line of [O III], this upper level transition from the isoelectrically similar [N II] spectrum provides a direct measurement of the temperature of an ionized region. Since [O III] is quite faint in the WIM, the 5755Å line is more likely to be detected from the diffuse background. Although the emission is still very faint, we have several detections that confirm that the WIM has high temperatures for photoionized gas, particularly when compared to diffuse H II regions ionized by single stars (see Reynolds et al. 2001 and Madsen et al. 2006 in our list of papers).

He I 5876Å

Using WHAM, we have detected this line for the first time from the WIM (see Steve Tufte’s PhD thesis and Madsen et al. 2006). This recombination line probes the degree of helium ionization in the WIM. Comparing the helium ionization fraction to the hydrogen ionization fraction yields valuable information on the spectrum of the WIM’s unknown source of ionization.

[O III] 5007Å

Emission from the WIM of this classic H II region line had only been detected in the Galactic plane (b = 0) prior to WHAM. Observations of this gas at even higher latitudes provides upper limit measurements of the contribution of 5007Å emission from hot, Galactic coronal gas. Madsen et al. 2006 provides a recent summary of some detections and upper limits of [O III] from the WIM.

Earth & Solar System

Geocoronal Studies

Unfortunately for Galactic observers, the earth provides it’s own Hα and Hβ emission line which varies with time and location on the sky. However, this emission is precisely the interest of a group of Wisconsin and Embry-Riddle aeronomers. In collaboration with Susan Nossal, Ed Mierkiewicz, and Fred Roesler, nearly every photon collected for the WHAM all-sky survey is being used for scientific research. WHAM observations of the geocoronal line are helping to shape models of the earth’s exosphere, the very outer reaches of our atmosphere.

Lunar Sodium

Smith and collaborators (Smith et al. 2001, 1999) discovered an extended tail of lunar sodium atoms over 400,000 km long with an all-sky imaging device. Several processes may be responsible for this atmosphere including thermal desorption, photo-desorption, ion sputtering and meteoric impact ablation. However, the relative importance of these processes remains uncertain, both with regard to spatial and temporal trends. Collaborators Ed Mierkiewicz and Michael Line used WHAM to map morphology and velocity distribution of this extended lunar sodium exosphere (Line et al. 2012). When compared to models, these spectral observations can be used to infer the initial velocity distribution of the sodium atoms escaping from the moon.

Comets

Hyakutake: In collaboration with Frank Scherb and Fred Roesler the WHAM group collected [O I] 6300Å, Hα, Hβ, and NH2 data in March and April of 1996 during the close passage of the comet. The [O I] data provided a sensitive measurement of the water production rate in the comet. The Hβ data revealed the first detection of this line from a comet and, combined with the Hα data, provide interesting information on the solar Lyβ emission line and how it affects the comet.

Hale-Bopp: This spectacular comet was also observed by WHAM in February – April of 1997. The [O I] distribution around the comet was mapped out to explore water production rates. The water ion, H2O+, was also observed this time, and provides a sensitive tracer of the comet’s ion tail. Using WHAM’s extremely narrow-band imaging mode (~12 km/s passband), we obtained a data cube of velocity slices, which may provide detailed information about the motion of ions down the comet’s tail.

The WHAM Sky Survey (WHAM-SS) is available after two decades of the dedicated work of many and funding provided by the National Science Foundation. Use of the data is open to all researchers; however, it is important to properly attribute all use of these data.

The all-sky survey paper is still being prepared for submission (Haffner, L. M., Reynolds, R. J., Madsen, G. J., et al. 2020, in preparation). Until this work is accepted,  a concise reference for the new southern data is:

Haffner, L. M., Reynolds, R. J., Madsen, G. J., et al. 2010, in ASP Conf. Ser., 438, ed. R. Kothes, T. L. Landecker, & A. G. Willis, 388.

Consult the Northern Sky Survey (NSS) release publication for full details of the facility, survey strategy, and reduction methods:

Haffner, L. M., Reynolds, R. J., Tufte, S. L., Madsen, G. J., Jaehnig, K. P., & Percival, J. W. 2003, ApJS, 149, 405.

When using the survey data, please add an acknowledgments to the effect of:

The Wisconsin  Mapper and its Hα Sky Survey have been funded primarily by the National Science Foundation. The facility was designed and built with the help of the University of Wisconsin Graduate School, Physical Sciences Lab, and Space Astronomy Lab. NOAO staff at Kitt Peak and Cerro Tololo provided on-site support for its remote operation.

More technical background on the WHAM instrument and facility can be found in these references:

Tufte, 1998. PhD Thesis, University of Wisconsin
Haffner, 1999. PhD Thesis, University of Wisconsin

The WHAM Sky Survey is available as two datasets. The first is a kinematic release containing more than 49,000 spectra covering the whole sky revealing Galactic Hα emission between roughly -100 to +100 km/s (LSR) with 12 km/s velocity resolution. The second dataset contains only the velocity-integrated (-80 to +80 km/s LSR) intensities of these pointings. As a convenience, the kinematic release contains the integrated dataset in separate fields.

You can browse a sample of the WHAM-SS through a set of pre-generated figures. These are suitable for use in presentations or posters with proper attribution.

Release Versions

Public releases are tagged with a major label (DR#), a survey version (YYMMDD), and a revision date (YYMMDD). The initial release is labeled DR1. Using these designations, the base name for public survey files follows: wham-ss-LABEL-vVERSION-RELEASE. For example, wham-ss-DR1-v161116-170111 is a file generated on 01/11/2017 from the DR1 survey base version 161116. See the documentation below for more details.

Documentation 

This document describes the WHAM-SS and the format of the data. Each data file also includes a README in the header or in an IDL variable. Please read them carefully before contacting us with questions.

Kinematic Survey (DR1-v161116-170912)

  • IDL SAVE [89 MB]
  • FITS Binary Table [87 M]
  • FITS Cube [gzip, 359 M] (Interpolated, Regular Grid)
  • *** NOTE: The FITS cube (only, not the IDL or FITS binary table) from the 170317 release was missing a final scaling factor (from instrument units to Rayleighs). Intensities derived from that cube need to be divided by 22.8 for the units to be in Rayleighs. Or, rerun analysis using the newer releases above.***

Integrated Survey (DR1-v161116-170912)

Release History

DR1-v161116-170111 : Initial public release.

DR1-v161116-170222 : Add integrated survey release in FITS image format. Tweak data file header documentation.

DR1-v161116-170317 : Add kinematic survey release in FITS cube format. No changes to other formats.

DR1-v161116-170912 : Fix scaling in FITS cube format. No changes to other formats.

The following grid of images contains a variety of velocity-integrated Hammer-Aitoff representations of the WHAM-SS. Each contains a slightly different logarithmic scaling (as noted in the colorbar for each image) to maximize the structure displayed for the velocity slice. Each slice is presented with six different center longitudes (0° through 300°) and in several color tables.
ColorBrewer Orange-Red (CB-OrRd) 
Velocity Range Map Center [°]
[LSR km/s] 0 60 120 180 240 300
-80 to +80
(“Total”)
PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-90 to -70 PNG PNG PNG PNG PNG PNG
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-70 to -50 PNG PNG PNG PNG PNG PNG
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-50 to -30 PNG PNG PNG PNG PNG PNG
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-30 to -10 PNG PNG PNG PNG PNG PNG
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-10 to +10 PNG PNG PNG PNG PNG PNG
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+10 to +30 PNG PNG PNG PNG PNG PNG
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+30 to +50 PNG PNG PNG PNG PNG PNG
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+50 to +70 PNG PNG PNG PNG PNG PNG
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+70 to +90 PNG PNG PNG PNG PNG PNG
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ColorBrewer Reverse BW Ramp (CB-Greys) 
Velocity Range Map Center [°]
[LSR km/s] 0 60 120 180 240 300
-80 to +80
(“Total”)
PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-90 to -70 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-70 to -50 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-50 to -30 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-30 to -10 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-10 to +10 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+10 to +30 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+30 to +50 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+50 to +70 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+70 to +90 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
Red Temperature with black background 
Velocity Range Map Center [°]
[LSR km/s] 0 60 120 180 240 300
-80 to +80
(“Total”)
PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-90 to -70 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-70 to -50 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-50 to -30 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-30 to -10 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
-10 to +10 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+10 to +30 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+30 to +50 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+50 to +70 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF
+70 to +90 PNG PNG PNG PNG PNG PNG
PDF PDF PDF PDF PDF PDF