What image software is needed to process the ASI STUDIO TIF files.

The ASI Studio image output. Provides a .Jpg image file, a .fit image, and a .tif file.
What image processing app is needed to use the .tif file?

The image processing apps I have do not stretch the image enough to display anything but the brightest stars and very little nebulosity in the image.

Does it primarily only work with Photoshop or Pix insight?

aruckle

aruckle
Hi, aruckle!
Our software ASIVideoStack can open and process .tif file.

There are a number of programs that can process .TIF files.

Photoshop and I believe Lightroom (both expensive).
PixInsight (also expensive)
Affinity Photo (very Photoshop like and only costs $70 and does not require a subscription). It can process just about any format including .FITS files.
SIRIL (free and can process .TIF plus .FITS images). It does a great job of stacking comets (can stack stars and comet separately and use other software to recombine them into one image)
GIMP (free but the interface takes a little getting use to)
RawTherapee (also free)
There may be others