Bin vs. Block vs. Zoom ![[New]](../imgs/new.gif)
DS9 has a few options to reduce or increase the size of the data being displayed.
Bin is only available when displaying a FITS table, i.e., an event file. By default, DS9 will bin the center 1024x1024 pixels at a resolution of 1 pixel per bin. For example, for a Chandra ACIS event file, bin=1 will mean each pixel is 0.492". Users can increase the bin size (e.g., bin=2), which will decrease the resolution. So, continuing with the same example, bin=2 means each pixel is 0.984". Users can also choose to change the image size from 1024x1024 and recenter the image to a different location. Under the Binning Parameters options, users can also choose to filter on additional columns in the event table, e.g., Energy or Time. When displaying images, the Bin menu will be unavailable and grayed out.
Zoom is the most common option to (de-)magnify the image. The way DS9 works is that for each pixel in the display buffer, it displays the, singular, closest pixel in the original data. So, when zoom=1, there will be a 1-to-1 mapping between displayed pixels and pixels in the original data. When zoom=2, there will be a 2x2 block of pixels in the display that will all map to 1 pixel in the original data; the image will be magnified. When zoom=0.5, a single pixel in the display will map to a 2x2 block of pixels in the original data. However, DS9 will only display the value from 1 (the closest) pixel. This results in a subsampling/down-sampling of the original data. Total flux is not preserved. This over-sampling/down-sampling is very fast.
Block, located under the Analysis menu, is the third option to (de-)magnify an image. With block=1, each display pixel is the sum (or average) of 1 original pixel. With block=2, each display pixel is the sum (or average) of 2x2=4 original pixels. This changes the spatial resolution of the data while preserving the total flux (with the 'sum' option). Setting block=0.5 also works: each display pixel maps to one-quarter of the original pixel. Pixels are not treated as integrated quantities, so all the flux is assigned to the pixel "closest" to the center. This will introduce rows and columns of 0's in the displayed image. Blocking is slower than zooming.
All three operations can be used on the same data (assuming an event table). The order is Bin, Block, and then Zoom. Generally speaking, it is best practice to zoom in when you want to magnify an image and to block out when you want to shrink an image.