The Interactive 2MASS Image Service provides fast interactive viewing and retrieval of a single 2MASS Image Set. The service allows both Atlas and Quicklook image sets to be retrieved, where each "Quicklook" image corresponds to a 2MASS Atlas Image that has been lossy-compressed with hcompress, using scale = 4.0 and quant = 1.0 parameters. In addition to returning restored lossy-compressed images in FITS format, the Interactive 2MASS Image Service creates a JPEG of the desired 2MASS Image, overlayed with filter glint/persistence artifact locations and a coordinate grid.

This help page provides a guide for adjusting the mapping between the J, H, or K_s magnitudes of FITS Image pixels and the RGB intensities of displayed pixels.

Color Mapping Image Parameters

pixel min

This is the minimum FITS pixel value that was considered when mapping FITS pixels to RGB intensities for visualization. All FITS pixels with values below this minimum were considered to be equal to the minimum for the purpose of color mapping.

pixel max

This is the maximum FITS pixel value that was considered when mapping FITS pixels to RGB intensities for visualization. All FITS pixels with values above this maximum were considered to be equal to the maximum for the purpose of color mapping.

Taken together, the pixel min and pixel max parameters give the range of FITS pixel values that can be differentiated between in the RGB color space of the 2MASS JPEG.

The generated JPEG is a greyscale visualization of a FITS Image, so in this context color mapping is the process of converting 16bit integer or 32bit floating point FITS pixel values to one of 256 discrete shades of grey. This is an inherently lossy operation; however, allowing the color mapping function, pixel min, and pixel max to be adjusted gives you some control over where you wish to spend the limited visualization "precision" available.

Color Mapping Inputs

By changing the values of the Color Mapping Inputs and clicking on the Submit button, you can regenerate the 2MASS JPEG according to a color mapping of your choice. The various inputs are described below :

mapping

This input allows one of four different color mapping functions f(P_fits ) -> P_jpeg to be specified:

lin

f(P_fits ) is linear in P_fits

log

f(P_fits ) is logarithmic in P_fits

loglog

f(P_fits ) ~ log(log(P_fits ))

equ

Indicates that histogram equalization is to be performed on the 2MASS FITS Image. If this mapping function is chosen, the range min and range max inputs will be ignored.

invert

If the invert option is un-checked, f(P_fits ) is constructed such that f(pixel min) = black and f(pixel max) = white.

If the invert option is checked, the mapping is inverted: f(P_fits ) is constructed such that f(pixel min) = white and f(pixel max) = black.

range min/max

The range min/max inputs allow specification of pixel min/max in one of several ways :

• The input value is interpreted as an actual FITS pixel value unless it ends with '%': in this case it is interpreted as a percentile on the histogram of FITS pixel values.
• If the input value is one of the literal keywords min or max, then the pixel min/max is set to the actual minimum/maximum FITS pixel value in the 2MASS Image.
• It is also possible to use a simple arithmetic expression for each value: arg [OP arg], where arg can be either min, max, or a value optionally followed by a '%'. OP must be either + or -. See the examples below.

If the color mapping function has been set to equ (histogram equalization), the range min/max inputs are ignored.

Examples:

• range min : 2%
  range max : 98%
  

  Sets pixel min such that 2% of all FITS pixel values are below it. Sets pixel max such that 98% of all FITS pixel values are below it.

• range min : min
  range max : max
  

  Sets pixel min to the smallest FITS pixel value. Sets pixel max to the largest FITS pixel value.

• range min : min
  range max : 1e5
  

  Sets pixel min to the smallest FITS pixel value. Sets pixel max to 100,000.

• range min : min+100
  range max : 99%-50
  

  Sets pixel min to the smallest FITS pixel value, plus 100. Finds a value V such that 99% of all FITS pixel values are below it, and sets pixel max to V minus 50.

Other Inputs

show artifacts

This input allows the filter glint and persistence artifact location markers to be removed from the generated JPEG. When checked, artifacts are overlayed on the JPEG. Otherwise, they are omitted from display. This option is not available if no artifacts were found to cover the 2MASS Image retrieved.