Inspecting pixels

Four things in atlas report numbers rather than pictures: the hover readout, the statistics panel, the FITS header panel, and the histogram window. All of them read the raw array, not the rendered image, so none of them move when you change the display scale.

Hover readout

Resting the cursor on a pixel puts its index and its count on the right of the status bar:

(341, 169)  27,922

The indices are 0-based, x across and y down, so the pair reads directly as data[y, x] in whatever you are inspecting the frame with. This is numpy’s convention rather than DS9’s 1-based one, and y counts down because atlas draws the first row of the array at the top of the tile.

The count comes from the raw array, so it is a detector count and does not move when the display scale changes. Blank pixels (NaN or inf) read as a dash rather than as a number, and colour frames report one sample per channel.

A frame is usually shown smaller than it is, in which case several data pixels share one screen pixel and the readout names one of them. It always names the pixel whose count it shows.

The readout is always available: it needs no configuration, adds no panel, and does no work at all until the cursor is over a frame. It sits beside the status messages rather than replacing them, so neither overwrites the other.

Tip

While a live stream is running, the readout re-reads the hovered pixel as each frame arrives, so resting the cursor on one pixel shows its counts changing. When tiling, it reports whichever tile the cursor is over, prefixed with that frame’s name, which need not be the current frame.

Statistics

The statistics tool adds a dock panel summarising the current frame’s pixel values: mean, median, standard deviation, min and max, plus the pixel count.

atlas image.fits --enable statistics

tools:
  statistics: true

The figures come from the raw array, not the rendered image, so they describe detector counts and do not move when the display scale changes. Blank pixels (NaN or inf) are excluded and reported separately, since a single NaN would otherwise make every statistic NaN.

Why the panel has an update rate

Recomputing is not free. The median alone costs about ten times the other statistics put together, roughly 30 ms on a 2048x2048 frame. So while a live stream is running the panel refreshes at update_hz (2 Hz by default) rather than on every displayed frame:

tools:
  statistics:
    enabled: true
    update_hz: 2.0      # between 0.1 and 60

Switching frames by hand still recomputes immediately, and a hidden panel does no work at all, so closing the panel is a real way to give the display back its time budget.

Header panel

The header tool is the one tool that is on by default. It docks on the right and shows the FITS header of whichever frame is current, following your selection as you move between frames.

Hide and show it from the View menu, or switch it off entirely:

atlas image.fits --disable header

Histograms

The histogram tool adds Tools → Histogram…, which opens a window plotting the pixel-value distribution of every frame currently on screen. In tile mode that means all the tiles, one curve each, which is the quickest way to see that two frames really do share a bias level.

atlas image.fits --enable histogram

The histogram window has its own Log count axis checkbox, independent of the frame’s scale. A pixel histogram is usually dominated by a single sky or bias peak, and a log count axis is what makes the faint tail visible. Changing it affects only the plot, never the image.