Astrosharp Astronomical Sharpening Tool

This is one of over 100 articles in the author’s Astronomy Digest.

Pixinsight can use a plug-in called ‘Blur Exterminator’ which employs an AI based deconvolution tool to sharpen images.  It is excellent, but there is a cost of 230 euros for Pixinsite  and $99 for Blur Exterminator to enable it to be used.  A free software program has recently become available which also uses AI devolution to sharpen astronomical images.  Only available for Windows, it is very simple in use and has just been made able (May 2023) to accept RGB Tiff files.

The ‘Lazy Geek’ has compared its results with Blur Exterminator and finds the sharpening a bit aggressive. It is easy to ‘tone down’ the result by blending it with original image as described below pertaining to the over sharpened image of M13. Stars against a bright background can sometimes show a darkened ring around them. As I describe below, there is a far better way of using Astrosharp than applying it to a complete image which eliminates this problem.

The program is large and takes some time to download with detailed instructions given at the end of the article, but let’s first describe its use.

Using Astrosharp

As described below, the unzipped files are placed in the ‘C’ drive in a folder called’Astrosharp Main’

Right Clicking on the execute file in the program folder and then on ‘Open’ opens up the following image.  The first time it is used, Windows will not, at first, allow you to open the program.

There is only one slider that can be adjusted but, in general, the author advises leaving it at the default ‘325’ position.

One simply then browses to select the image to be sharpened – a 16  bit monochrome or RGB Tiff. (It currently says that only monochrome Tiffs are acceptable.  This is no longer true.)   It advises one to select unsharpened, calibrated, files.

The program then works away with a gradually filling task bar.  When complete it shows thumbnails of both the unsharpened and sharpened images.   The ‘Download Sharpened File’ box at the base of the program window is simply clicked upon to save the sharpened image in the desired folder.  One must select the folder in which it is to be saved (it is not necessarily that from where the image has been selected) and append ‘.tiff’ to the filename before saving.

That is all there is to it! 

The program reduces the star sizes and ‘cleans them up as seen in the two crops of the M13 image below and, I think, applies some local contrast enhancement to the image as seen in the sharpened image of the Andromeda Galaxy to which it gave a subtle, but worthwhile, improvement.

Crop from image of M13
After Astrosharp has been applied

The best way to use Astrosharp

It may be that the best way to use Astrosharp is to separate out the Nebulae and stars using Starnett++ (now accesible from Siril -see my M31 processing article as to how to set this up). Then apply Astrosharp to the ‘Nebula’ and (possibly) ‘Stars’ images separately.

Astrosharp applied to the M31 Nebula image

Original M31 image
This image has an empty alt attribute; its file name is A50-1024x696.jpg
Th nebula image derived using Starnet++ and having applied Astrosharp

I find that as well as reducing the star sizes, Astrosharp can increase their brightness. I like the stars to become less obvious in an image that is focussing on a nebula such as M31. This is not a problem. The enhanced Stars image is copied and pasted over the Nebula image and the two combined by flattening them using the ‘Screen’ blending mode. The stars brightness is simply controlled by adjusting the ‘Opacity’ slider (0% = no stars, 100% = full brightness.

[An alternative to using Astrosharp to reduce the star sizes is to use the free program Images Plus which, in its special functions, has a star size reduction tool which does not brighten the stars.]

The ‘Stars’ have been reduced in size using Images Plus as an alternative to Astrosharp and added to the nebula image using the screen blending mode, adjusting the opacity to suit.

Astrosharp’s author advises that one should only apply the program to images that have not had any sharpening previously applied.  I did actually try giving additional sharpening  to an already sharpened image – as seen in the images of M13 below – and the result was, as suspected, somewhat over the top.  However, one can adjust the amount of sharpening applied quite easily.  Using any of the image processing programs such as Adobe Photoshop (very expensive now), Affinity Photo (excellent and not too expensive) or Glimpse (free), one simply copies and pastes the sharpened image over the less or unsharpened image and, in the ‘Normal’ blending mode, adjusts the opacity to suit.  (0%, unsharpened, to 100% fully sharpened.)

Presharpened M13 image

Over sharpened image having applied Astrosharp
Sharpening reduced using layers and opacity in Normal blending mode

There is a very useful YouTube video about it.

Downloading and setting up the program

One opens this website:

and clicks on the green Code box.

Then Click on ‘Download ZIP’.

This is a very big file of 460 Mbytes!

When scanned, click on ‘Open’ and the zip file will have been saved into the ‘Downloads’ folder.

Click on ‘Extract All’  and select the ‘C’ drive – the extracted files are 947 Mbytes in size and will go into a folder called ‘Astrosharp Main’.  A total of ~32,000 items!

Open the ‘C’ drive and select ‘Astrosharp Main’.  Right Click on the Astrosharp Ikon and click on ‘Open.   The program window will appear.