Making HDR images of deep space objects in IRIS


Imaging some DS objects like M42 - the Orion Nebula overexposing some areas is nearly inevitable. In M42 bright trapezium region is easy to overexpose when catching the much fainter nebulosity around it. In post processing its however possible to correct this problem by using not overexposed trapezium region from an image made on much shorter exposure. Such images are quite often called HDR image - high dynamic range.

HDR images are made out of 2 or more images of the same area - each image on different exposure time to catch either faint or very bright areas. Combining them in to one HDR image can be done in multiple apps, like Photoshop but those are not free. IRIS however is free and has a ready to use feature that will merge all images into a HDR image.

Overexposed core of M42
HDR image with fixed core

IRIS is a quite powerful but not so "easy" to use application for image processing. Download the ZIP file and extract the application. When you run it you should get something like this:

IRIS main window
IRIS main window
Start with app settings. In File choose Settings. For the Working path select folder in which you have the images you want to work with.
IRIS settings
IRIS settings
Next start the IRIS terminal using the button from the top navigation bar (the one marked yellow on the first screenshot). Most of IRIS features are controllable through that terminal. You enter a command and it will be executed in the "working path". In our case it will be HDR making.
IRIS terminal

Making a HDR image in IRIS

At start we need few images shot at different exposures. Those images must have the same dimensions and must be aligned vs each other (Nebulosity can do that). When you have such files (like FITS files) you may start making a plain text config file - DESC.LST. Put the filename and relative exposure time in every line, like so:

align_z300 1.00000
align_z60 0.20000
align_z30 0.10000
align_z5 0.01666
In my case "align_z300" is "align_z300.fit" and it used 300 sec exposure. "align_z60" was at 60 sec, which is 0,2 compared to the 300 sec exposure. You don't specify file extensions in this file. When the file is ready you may run MERGE_HDR in the IRIS terminal (enter the command and press enter):
This should make an HDR image ready to be save and processed as you like. If anything is not correct (image dimensions, errors in config) the command will fail. The last command "value" - 1000 is an intensity threshold. For 16-bit DS images it should be quite low (even lower than 1000) to prevent faint signal from being lost. For day light images it's usually bigger - check this tutorial.
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