Northern Sky Narrowband Survey: DR0.2 (Draft)

Data release DR0.2 contains the first complete image set with the emission line [OIII] at 500.7 nm and the [SII] doublet at 671.7 nm and 673.0 nm from instruments 6a and 6b, as well as updated results for Hα (instrument 3). A list of changes compared to DR0.1 con be found in section Changes.

The pre-processed data published here was calculated on 47 overlapping tiles with a size of 31° × 31°. Compressed 8-bit color images can be accessed in sections Overview images and Tile images. Full dynamic range images with linear intensity can be retrieved from an HiPS (Hierarchical Progressive Survey) repository. In order to assess the reliability of the data, sections Data acquisition and processing and Artifacts and limitations provide a brief description of the image processing and a detailed description of all known artifacts.

Overview images

The overview images shown below are 212° stereographic projections of the entire surveyed region. The projection type was used because it preserves shapes. Maximum image resolution in the JavaScript viewer (click on the images) is 30″ at the center (δ=90°) and about 11″ at boundary (δ=-16°). In the navigation mode of the Javascript viewer (press the 'N' key or click the 'N' button) it is also possible to browse to other views on this site (including the tile images).

Click on the images to load a high-resolution (1.3 GP) versions using a JavaScript viewer.

Overview image of DR0.2 of Northern Sky Narrowband Survey in [OIII], H-alpha, and [SII]
False-color composite where [OIII] is mapped to red, Hα is mapped to green, and [SII] is mapped to blue. The brightest stars are added to visualize the regions contaminated by strong continuum light. (Aside from that, the image does not contain continuum light.)
Overview image of DR0.2 of Northern Sky Narrowband Survey in H-alpha, blue continuum, and red continuum
False-color composite where Hα (without continuum) is mapped to red, blue continuum (including emissions of OIII and HII) is mapped to green, and red continuum (without Hα but with some SII emissions) is mapped to blue. Emission nebulae appear predominantly reddish, while reflection nebulae range from green to blue. Stars are partially subtracted to make the faint reflection nebulae visible.
Overview image of DR0.2 of Northern Sky Narrowband Survey in H-alpha
Legend for Overview image of DR0.2 of Northern Sky Narrowband Survey in H-alpha

Pseudo-color composite, i.e. image is colored according to Hα intensity (without continuum) as shown in the legend. Units in the legend are Rayleighs. The brightest stars are added in white to visualize the regions where Hα intensity is uncertain due to contamination by starlight.

Overview image of DR0.2 of Northern Sky Narrowband Survey in RGB without H-alpha.
(Almost) true-color image without Hα. Stars are partially subtracted to make the faint nebulae visible. Most objects in the image are reflection nebulae. Their color is relative to the average star color, which was used for white balance.

Hα was subtracted from the red channel using a factor that was determined empirically to prevent underflow. Because Hα and SII emissions strongly correlate, this also eliminates some — but no all — SII light. Furthermore, the green and blue channels contain [OIII] and Hβ emissions. This explains the colors of the bright emission nebulae visible in the image.

Note that the ecliptic plane appears as a double band due to the high-pass filtering effect of the background estimation.

Tile images

Links to the 47 tile images with a resolution of 10″ can be found in the following table. The size of each tile is 31° × 31°, with an overlap of at least 5.3°. Columns 2 and 3 of the table contain the center positions of the tiles in ICRS and J2000 coordinates, respectively. (The difference can be neglected in this context.) For each tile, three color mappings are available (links in columns 3 to 5):
OHS
False-color image showing emission lines [OIII] (mapped to rad), Hα (mapped to green), and [SII] (mapped to blue)
HBR
False-color image showing Hα (mapped to red), blue continuum (mapped to green), and red continuum (mapped to blue)
Hα
Pseudo-color image of Hα. The links [Hα Legend] refers to a legend that shows how the image was colored according to the Hα intensity in Rayleighs.
Preview images of the tiles can be loaded by clicking on the header of columns 3 to 5.

ID DEC in ° RA in h:m OHS HBR Hα
000_006 76.1222:17 [OHS] [HBR] [Hα]   [Hα legend]
000_005 76.1218:51 [OHS] [HBR] [Hα]   [Hα legend]
000_004 76.1215:26 [OHS] [HBR] [Hα]   [Hα legend]
000_003 76.1212:00 [OHS] [HBR] [Hα]   [Hα legend]
000_002 76.128:34 [OHS] [HBR] [Hα]   [Hα legend]
000_001 76.125:09 [OHS] [HBR] [Hα]   [Hα legend]
000_000 76.121:43 [OHS] [HBR] [Hα]   [Hα legend]
001_011 50.3823:00 [OHS] [HBR] [Hα]   [Hα legend]
001_010 50.3821:00 [OHS] [HBR] [Hα]   [Hα legend]
001_009 50.3819:00 [OHS] [HBR] [Hα]   [Hα legend]
001_008 50.3817:00 [OHS] [HBR] [Hα]   [Hα legend]
001_007 50.3815:00 [OHS] [HBR] [Hα]   [Hα legend]
001_006 50.3813:00 [OHS] [HBR] [Hα]   [Hα legend]
001_005 50.3811:00 [OHS] [HBR] [Hα]   [Hα legend]
001_004 50.389:00 [OHS] [HBR] [Hα]   [Hα legend]
001_003 50.387:00 [OHS] [HBR] [Hα]   [Hα legend]
001_002 50.385:00 [OHS] [HBR] [Hα]   [Hα legend]
001_001 50.383:00 [OHS] [HBR] [Hα]   [Hα legend]
001_000 50.381:00 [OHS] [HBR] [Hα]   [Hα legend]
002_013 24.6223:09 [OHS] [HBR] [Hα]   [Hα legend]
002_012 24.6221:26 [OHS] [HBR] [Hα]   [Hα legend]
002_011 24.6219:43 [OHS] [HBR] [Hα]   [Hα legend]
002_010 24.6218:00 [OHS] [HBR] [Hα]   [Hα legend]
002_009 24.6216:17 [OHS] [HBR] [Hα]   [Hα legend]
002_008 24.6214:34 [OHS] [HBR] [Hα]   [Hα legend]
002_007 24.6212:51 [OHS] [HBR] [Hα]   [Hα legend]
002_006 24.6211:09 [OHS] [HBR] [Hα]   [Hα legend]
002_005 24.629:26 [OHS] [HBR] [Hα]   [Hα legend]
002_004 24.627:43 [OHS] [HBR] [Hα]   [Hα legend]
002_003 24.626:00 [OHS] [HBR] [Hα]   [Hα legend]
002_002 24.624:17 [OHS] [HBR] [Hα]   [Hα legend]
002_001 24.622:34 [OHS] [HBR] [Hα]   [Hα legend]
002_000 24.620:51 [OHS] [HBR] [Hα]   [Hα legend]
003_013 -1.1223:09 [OHS] [HBR] [Hα]   [Hα legend]
003_012 -1.1221:26 [OHS] [HBR] [Hα]   [Hα legend]
003_011 -1.1219:43 [OHS] [HBR] [Hα]   [Hα legend]
003_010 -1.1218:00 [OHS] [HBR] [Hα]   [Hα legend]
003_009 -1.1216:17 [OHS] [HBR] [Hα]   [Hα legend]
003_008 -1.1214:34 [OHS] [HBR] [Hα]   [Hα legend]
003_007 -1.1212:51 [OHS] [HBR] [Hα]   [Hα legend]
003_006 -1.1211:09 [OHS] [HBR] [Hα]   [Hα legend]
003_005 -1.129:26 [OHS] [HBR] [Hα]   [Hα legend]
003_004 -1.127:43 [OHS] [HBR] [Hα]   [Hα legend]
003_003 -1.126:00 [OHS] [HBR] [Hα]   [Hα legend]
003_002 -1.124:17 [OHS] [HBR] [Hα]   [Hα legend]
003_001 -1.122:34 [OHS] [HBR] [Hα]   [Hα legend]
003_000 -1.120:51 [OHS] [HBR] [Hα]   [Hα legend]

HiPS (Aladin, Stellarium, ...)

Selected data is made available as HiPS (Hierarchical Progressive Survey) and can be viewed with tools like Stellarium, Aladin, or Aladin lite. The following table contains links to the HiPS datasets, which can be imported into the viewers or clicked to load them in Aladin Lite. The link names are the last part of the HiPS/IVOA ID, which start with simg.de/P/NSNS/DR0_2/; for example, full ID of the OHS composite is simg.de/P/NSNS/DR0_2/ohs8. Descriptions of the rows and columns can be found below the table.

Channel(s) Linear intensity
FITS 		Variance Compressed intensity
PNG 		Color
PNG
Hα halpha sn-halpha halpha8
[OIII] oiii sn-oiii oiii8
[SII] sii sn-sii sii8
Continuum vc sn-vc rgb8
OHS ohs8
HBR hbr8

Linear intensity FITS
FITS HiPS with linear intensity and full (uncompressed) dynamic range. Units of emission line channels are Rayleighs (R); unit of the continuum HiPS is Rayleighs per nm (R nm-1).
Compressed intensity PNG
Monochrome PNG HiPS with a non-linear tonal curve and compressed dynamic range for easy visualization
Variance
Estimated square noise (variance) normalized to 30″ square pixels. This is approximately the smallest structure size that can be safely resolved (distinguished from stars) in Hα. (The variance depends on the size of the area over which a signal is integrated. Therefore, such noise information is only usable if that size is specified.) The maximum resolution is 12.9″.

The data is derived from the total sample variance during stacking. Units are the square of the intensity units, i.e. R2 for emission line images and R2 nm-2 for continuum. Channel mixing, such as from continuum and Hα subtraction, is correctly accounted for. However, it should be noted that noise is not the same as error; i.e., artifacts are not taken into account. Nevertheless, the accuracy of the processed images is primarily determined by photon noise, whether from contamination or from the signal of interest. This noise is correctly reflected in this dataset.

Note that the HiPS may contain very large values (instead of infinity) at undefined pixels, usually caused by sensor saturation near bright stars, which may complicate visualization. On the other hand, variance allows for scaling by averaging (unlike reciprocal (square) noise, which would correctly handle undefined pixels).

Color PNG
Color composites as HiPS with non-linear tonal curve and compressed dynamic range
Hα, [OIII], [SII]
Emission lines Hα (656.3 nm), [OIII] (500.7 nm), and [SII] (671.7 nm and 673.0 nm), respectively. The maximum resolution of intensity HiPSs is 6.4″; usable resolution is about 10″.
Continuum
Visible continuum without Hα, but with other emission lines. The color HiPS is an (almost) true color visualization with partial star subtraction. All other HiPSs are combinations of all three color channels (red, green, and blue) with full star reduction. Maximum resolutions are 12.9″; usable resolution is about 20″.
OHS
False-color composite of the emission lines [OIII] (500.7 nm, mapped to red), Hα (656.3 nm, mapped to green), and [SII] (671.7 nm and 673.0 nm, mapped to blue). The brightest stars are added to visualize the regions contaminated by strong continuum light. The maximum resolution is 6.4″; usable resolution is about 10″.
HBR
False-color composite showing Hα (mapped to red), blue continuum (mapped to green), and red continuum (mapped to blue) Emission nebulae appear predominantly reddish, while reflection nebulae range from green to blue. Stars are partially subtracted to make the faint nebulae visible. The maximum resolution is 6.4″; usable resolution is about 10″.

Data description

This section provides additional details on the datasets used in the results presented above.

Hα

Hα data was captured with instruments 1 and 3. The usable resolution is about 10″

Hα was background-corrected and intensity-calibrated to Rayleighs using WHAM data; see Haffner et al., 2018 and HWAM-SS DR1. For continuum subtraction, the lower-resolution red and green channels from this data release have been used. To minimize artifacts around stars, continuum subtraction was applied after subtraction of (the same) stars, see the image processing section for details and the artifacts section consequences.

[OIII] and [SII]

[OIII] and [SII] data were captured with instruments 6a and 6b, respectively. The usable resolution is about 10″. Continuum was subtracted from the raw data in the same manner as described above. Intensity is approximately calibrated to Rayleighs using the method described in Intensity calibration of [OIII], [SII], and continuum. For SII, this is the sum of the intensities of the doublet at 671.7 nm and 673.0 nm.

Continuum

Continuum data was captured with instrument 2. The usable resolution is about 20″. Intensity is approximately calibrated to Rayleighs per nm (R nm-1) using the method described in Intensity calibration of [OIII], [SII], and continuum.

Unless otherwise stated, Hα was subtracted from the red channel with a factor that was determined empirically so that no underflow occur. Because Hα and SII emission strongly correlate, this also eliminates some — but no all — SII light. No attempt was made to subtract emission lines (like [OIII] and Hβ) from the green and blue channels. Thus, none of the continuum channels is totally free of emission lines.

Intensity calibration of [OIII], [SII], and continuum

During stacking, images are calibrated to reference stars. With the given intensity calibration for Hα and assuming that the average star color is white, this allows an approximate intensity calibration for the other channels. This simple method suffers from the following errors:
  1. The filters have not yet been measured. Ratios of continuum vs. emission line photon flux are estimated from datasheets.
  2. The average star color (within the spectral range of interest) is not always white (e.g. reddening near the galactic plane).
  3. Inherited errors from the reference channel Hα
Nevertheless, under the given conditions, this method should be more accurate than intensity calculations that involve transparency estimation (atmosphere, optics, filters) and/or quantum efficiency estimations (the sensor manufacturer Sony does not publish this information). It is planned to improve the current method using direct intensity information of the calibration stars -- if such data can be found.

Data acquisition and processing

Each point was recorded by dozens short-time (up to 60s) exposures using multiple instruments. The camera pointing coordinates lie on a grid whose size is slightly smaller than 50% of the field height and 33% of the field width. This technique allows for detecting and rejecting outliers (e.g. satellite tracks and filter reflections) and minimizes the impact of varying field angles (e.g. filter transmission variations and optical aberrations). These issues are discussed in detail in the section Artifacts and limitations.

For each tile, a reference source list with 32,000 stars from the PPMXL catalog (Roeser et al., 2010) is calculated using stereographic projection. The single exposures are aligned (with nonlinear distortion terms) to typically more than 1,000 stars from this reference list, which is also used for intensity calibration so that the average star color in the results is white. The stacking process is iterative, with results from previous iterations used to sort out outliers and help estimate the background. In order to maximize the signal-to-noise ratio, each single exposure is weighted according to the estimated noise. (This noise estimation is also responsible for underweighting hotpixels and badpixels. Photon noise of objects can be calculated from the results of the previous stacking iteration.)

The final stacking result of Hα has the highest resolution and is therefore used to extract point sources. Based on this source list (and assuming these point sources are stars), stars are subtracted from all stacking results. After that, continuum subtraction (from Hα) and Hα subtraction (from red continuum) are applied. (Performing continuum subtraction before star subtraction would cause too many artifacts around stars.)

The star-, continuum- and Hα-subtracted images are background-corrected, either using reference data (WHAM data for Hα) or by suppressing frequency components that can't be detected by the cameras (all other channels; see section Suppression of large structures due to background estimation).

FITS HiPSs are calculated directly from these results. All 8-bit color images (Overview images, Tile images and 8 bit HiPSs) are dynamic range compressed (non-linearly high-pass filtered) and tonal curve corrected. Furthermore, the brightest stars are re-added in order to visualize the regions that suffer from contamination by star light.

Artifacts and limitations

Suppression of large structures due to background estimation

The background for all channels except Hα had to be estimated from the darkest regions within the field of view because no absolute reference data are available (for Hα, WHAM data can be used). This method of background estimation also suppresses large homogeneous structures. Thus, the results can be considered as bandpass filtered, where the lower resolution limit depends on the field of view (and the number of background approximation terms), and the upper limit depends on the optical resolution. This effect is responsible for the ecliptic plane appearing as a double band in the continuum images rather than as a single broad band.

The upper detection limit for structures is about 3° in all directions. Objects that are smaller in at least one direction, like filaments, can be safely detected.

Reflections between filter and sensor

Reflections between interference filters, which are mounted in front of a lens, and imaging sensors cause artifacts like the ones depicted below.
Artifacts caused by reflections between imaging sensor and interference filter mounted in front of the lens.
Single exposure showing a double reflection (onion pattern) of a bright star on an imaging sensor and an interference filter. The artifact is point-symmetrical around the normal of the filter, approximately at the center of the image.

These reflections occur at wavelengths where the interference filter is partially transmissive. Fully blocked wavelengths do not reach the sensor, and fully transmitted wavelengths are not reflected by the filter. The intensity of the artifacts, therefore, depends on the ratio of partially transmitted wavelengths to fully transmitted wavelengths. This is why such artifacts are only significant with narrowband filters.

Because each point is observed at different field angles (see section Data acquisition and processing), these artifacts occur at different positions and can be sorted out by the stacking software with a certain probability. This probability is proportional to the error (intensity of the artifact) relative to the noise floor. As a result, residuals of the reflections appear as spots in the emission line channel around bright stars, as can be seen in this example.

Satellite trails and other temporary effects

Similar to the filter reflections described in the previous section, such temporary effects can be at least reduced by the outlier filter of the stacking software. How well this works depends on how many images the disturbances appear in:

PSF variations

The star subtraction software has a limited ability to handle variable PSFs (point spread functions). PSF variance across the image field is controlled by a camera pointing grid, which is more than 3×2 times smaller then the field of view. There are two effects which cannot be fully compensated for and thus cause artifacts: Both kinds of artifacts should be reduced in the future as the fraction of images captured with the improved filters increases over time.

Starlight contamination and limits of the star subtraction

Star subtraction cannot perform miracles. Even if the PSF were known exactly, the signal-to-noise ratio near bright stars becomes very low due to photon noise caused by the starlight. In practice, in addition to these errors, inaccuracies in the PSF (see previous section) are also amplified at bright stars. Regions where the subtracted result becomes too uncertain are therefore interpolated. In general, artifacts around bright stars must be expected.

Two strategies are used to handle this situation:

  1. Re-adding brightest stars: In some data sets (e.g. all color images), the brightest stars are re-added with reduced intensity. This provides a natural visualization of the regions contaminated by starlight.
  2. Providing variance data: Variance data is made available in the form of FITS HiPSs. This data accounts for photon noise and is therefore a reliable measure to assess contamination by starlight.

Elimination of small Hα sources

As described in image processing section, the Hα data are used for star extraction because this channel has the highest resolution. The drawback of this choice is that tiny Hα sources that appear as points are misinterpreted as stars and are eliminated too. In practice, this limits the spatial detection limit to about 30″.

Continuum and Hα subtraction in saturated regions (near M42)

Saturation occurs in the brightest parts of the Orion Nebula (M42). Continuum subtraction would cause artifacts. Visually, this is fixed by interpolation (but it does not make the results more valid).

Variable stars

Because images are captured over a long period and input images are weighted based on (photon) noise, variable stars appear distorted in the stacking results. This distortions lead to tiny disc-shaped residuals after star subtraction, as shown in this example.

Remarks and discoveries

The Northern Sky Narrowband Survey reveals numerous objects that cannot be found in standard catalogs or in the SIMBAD database. (The JavaScript viewer supports plotting of catalog objects and SIMBAD queries.) These objects are collected in a list of uncataloged objects. A few spectacular discoveries are:

The ecliptic plane appears as a wide double band in the continuum images (due to the high-pass filtering effect of the background estimation). Furthermore, the geosynchronous orbit is visible as a thin trail at a current epoch declination of δ=-7:25° (observed from a geographic latitude of 51:11°3; ICRS system is tilted about 0.15° relative to current epoch equatorial system).

Statistics

More information about the camera array used for the data acquisition can be found on the instruments page.

Here is some additional statistical information:

Hα [OIII] [SII] Continuum
Acquisition period 2018/11/10 to 2025/03/27 2022/12/13 to 2025/04/04 2022/12/13 to 2025/04/04 2018/11/10 to 2022/09/04
Total exposure time 6027 h 1664 h 1816 h 2084 h
Number of single exposures 3.63 × 105 1.00 × 105 1.09 × 105 1.68 × 105

Changes

Below is a list of changes compared to DR0.1:

Outlook

Unless unexpected problems arise, results utilizing high-resolution continuum data (from instruments 5a-5c) should be published in second half of 2026 or first half of 2027. This will be data release DR1. The use of the high-resolution continuum data should significantly improve the quality, as it allows for a more accurate continuum subtraction and star detection. (Currently, Hα is used for the latter purpose because it provides the highest resolution. As a result, small emission sources may be misidentified as stars.)

If there are delays, a DR0.3 may be released in the meantime.

Acknowledgements

I acknowledge the use of WHAM data for background correction and intensity calibration of the Hα data. The Wisconsin H-alpha Mapper (WHAM) and its H-alpha 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.

This work made use of PPMXL data for alignment and calibration.

References

 
L. M. Haffner, R. J. Reynolds, S. L. Tufte, G. J. Madsen, K. P. Jaehnig, and J. W. Percival. The wisconsin hα mapper northern sky survey. The Astrophysical Journal Supplement Series, 149(2):405, dec 2003. [ DOI | http ]
 
L. M. Haffner. WHAM SS Data Release, 2017. [ http ]
 
S. Roeser, M. Demleitner, and E. Schilbach. The PPMXL Catalog of Positions and Proper Motions on the ICRS. Combining USNO-B1.0 and the Two Micron All Sky Survey (2MASS). The Astronomical Journal, 139(6):2440–2447, June 2010. [ DOI | arXiv ]

Monitor calibration

For optimal display, the monitor should be calibrated so that as many shades of the grayscale shown below are distinguishable. At least the 4% steps should be separated by every monitor. The 2% steps are visible on better monitors. To distinguish the 1% steps, a HDR monitor is required.

gray scale

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