INGS is a compendium of 143 stellar-type spectra formed from spectra of stars of similar type from 3 sources:

Published References making use of these spectra

Click to expand plots
Click here for list, click on item to see spectrum \\ Click here for list, click on item to see fluxes     Click here for the gzipped tar file of fluxes

Only HD25975 appears in published NGSL and SpeX data, and overlays very well. [Additional unpublished SpeX overlap with NGSL is here].
Otherwise there is NO overlap of specific stars within these sources, but there is considerable overlap of stellar type, luminosity and metallicity.
Temperature and luminosity class coverage is good, with NGSL having better coverage of earlier types, and SpeX having better coverage of later types.
Metallicity coverage is good for solar, metal-weak and very metal-weak (broadly defined as [Fe/H]~0.0 for solar, w~-0.6 for 'weak' and ww<-1.2 for very-weak)
but there are fewer available spectra of metal-rich stars, labeled 'r' corresponding here to 0.2<[Fe/H]<0.4.

The INGS library is intended to be useful for comparing various observational system bandpasses, throughputs and exposure times, and potentially for population synthesis.

Stellar spectra from the available NGSL and SpeX libraries were grouped by temperature, gravity and metallicity, similar to the groupings in Pickles 1998.
but adjusted as appropriate for the spectral types in NGSL and SpeX, and extended with additional metal-weak groupings.
Types from SIMBAD were also checked, and used to select appropriate IUE spectra for the INGS spectral-type groups.

Temperature, gravity and metallicity measurements of the NGSL library include the measurements from Heap & Lindler (2010) and Koleva and Vazdekis (2012), which generally correlate well;
both are listed in
NGSLList together with the adopted stellar group assignments for the INGS spectral-type in the last column.
The SpeX type-calibrations were used to help assign those spectra to appropriate spectral-type groups, and listed under SpexList.

The INGS library is well defined on the GK dwarf and giant sequences, but has relatively sparse input type-coverage for early types.
The adopted IUE, NGSL and SpeX spectral-type groupings are listed in this spreadsheet, and the INGS types are illustrated graphically as Mbol vs. LogTe and Mv vs. V-I.
The spectral type basic data, colors in UBVRI and ugriz are listed for Sloan Primed and Sloan Survey (Unprimed)
where Mbol and Msun have been assigned to each type, LogTe and [Fe/H] follow from the spectral selection tables, and BCV and hence MV=Mbol-BCV are measured from the spectra.

The INGS library does NOT show individual stars on a common uv--ir grid, but it does show representative spectral types on such a common grid.
Comparison with CALSPEC spectra for various types of stars is generally good, as illustrated for Vega, for the G0V star P041C, for the M7V star VB8,
and for the O5pv flux standard BD+75 325 (which is possibly variable, but see Landolt & Uomoto 2007).

Heap and Lindler described how they addressed the problem of flux loss of NGSL spectra through the STIS narrow slit with a series of "bowing" corrections in NGSL_talk.pptx
Residual effects may pertain for two direct NGSL comparisons with the STIS New Spectrophotometric Standards, for the metal-weak FV BD+29 2091 and for metal-weak G8III HD009051.
The new STIS Calspec spectra (taken through a wide slit) sometimes exhibit more flux in the red than do the NGSL spectra, taken through a narrow slit but corrected for slit losses.

The direct comparison for the SDSS standard subdwarf BD+17 4708 exhibits flux differences throughout the spectrum, but primarily exhibits NGSL flux loss in the red.
The magenta curve, 1 from 1150A to 5375A, rising to 1.1 at 10150A then FLAT thereafter, shows the empirical red correction which has been applied to some INGS library spectra, to "twist" up the I-band fluxes of some NGSL combinations, so that when combined with SpeX spectra, they properly match Tycho/2MASS BTVTJHKs fluxes, as illustrated by blue and red flux error bars in the spectral plots.

This empirical NGSL correction has been applied to 24 (or 17% of 143) combined spectra, as listed in ingsList. The majority of NGSL spectra appear to be well calibrated, and combine well with SpeX calibrated spectra for stars of similar type. The correction curve has also been used to redden the I-band flux of the latest NGSL M star spectra so they combine with later M SpeX spectra to combine to give the correct fluxes, as measured by 2MASS Flux overlays.

Spectral Combination Procedure

All the input spectra are well flux and wavelength calibrated, and the SpeX spectra have been corrected for atmospheric absorption.
For each spectral-type, relevant NGSL and SpeX spectra were normalized in the wavelength range 8500A to 9500A, splined onto the common output wavelength grid, and overlaid.
A combined spectrum was formed with the SpeX values tapering in over the range 8200A to 8260A, and NGSL values tapering out over the range 10090A to 10150A.
The uv flux of this combined NGSL/SpeX spectrum was measured in the wavelength range 2900 to 3100A, and selected IUE spectra normalized to that value over the same range.
A further combined spectrum was formed with NGSL values tapering in over the wavelength range 1690A to 1750A, and IUE values tapering out over the range 3010A to 3070A.

Despite the fact that spectra from different stars are combined, the groupings are designed to, and generally do exhibit small dispersion between component spectra,
in both spectral shape and features. In particular the overlay between NGSL and SpeX is generally very good, including for the ir-Ca II triplet.
As mentioned above, some NGSL spectra had their red-fluxes increased prior to combination with SpeX, to force agreement with Tycho/2Mass fluxes of a component star.

Synthetic magnitudes were calculated on several photometric systems (Pickles & Depagne, 2010) and the final spectral fluxes (and errors) scaled for all spectral-types to Vmag=0.
The combined individual spectrum of HD25975 has a measured synphot synthetic Vmag of 6.09, an rmag of 5.83, and 2MASS JHKs mags of 4.59, 4.16 and 4.09 respectively.

The list of spectral-types, colors and basic data are given in ingsColors.sp or ingsColors2.sup - Individual spectra can be viewed by clicking on the links in these files,
where the type-spectrum (black), error (cyan) and input-dispersion (red) are plotted from 1150 to 7200A, and from 6800 to 25000A.
All the text files of INGS spectral data can be downloaded as a gzipped tar file where each spectral-type file (like ings_009_a0v.dat) consists of 4 columns:

  1. Wavelength in A, in steps of 1.5A from 1150 to 3100A, then in steps of 2.5A to 5500A, in steps of 3A to 10150A and in steps of 5A to 25000A
  2. F(λ) in erg/cm2/s/A normalized to Vmag=0 (except for HD25975) for the relevant type spectrum - plotted in black in the spectral plots
  3. Flux error in F(λ), derived as averages of the input catalog flux errors - plotted in cyan
  4. Sigma of the input spectra, ie. the dispersion of the input spectra included in the combination - plotted in red

This INGS library compilation is significantly better than a previous HILIB library (Pickles 1998) in providing a finer, adaptive wavelength grid,
good flux-calibration and space-based uv-optical spectra unaffected by atmospheric extinction. The coverage in the infrared from SpeX spectra also provides
good atmospheric feature removal and flux calibration, with superior type-coverage, particularly for later types.
INGS spectral types earlier than F, with no SpeX coverage, use HILIB fluxes beyond 1.1um. Similarly the uv coverage for late-types may use data from HILIB.

Known Problems

The normalization of the uv-optical component (at V) of late M types is difficult because of the relatively sparse NGSL coverage for these types, but the I--K regions are well defined.
Some atmospheric contamination in the INGS library spectra can be present around 9500A because of averaging the NGSL and SpeX libraries over that region.
The combined spectra are interpolated across the H-K 1.8 to 1.9um atmospheric region, and sometimes across the J-H 1.36 to 1.4um atmospheric region.
Some late-M spectral-types or early supergiants have poor coverage in NGSL and/or SpeX catalogs, but have been emulated by including spectral types that bracket the desired spectrum,
leading to a representative spectrum with larger than desirable input dispersion (last column, red in the plots). The value of including some of these types is debatable, but they are retained for now.

Please refer to this website if you use these data; comments to apickles at lco.global are welcome,
updated 2017-12-07