Without the proper Aquatint screen known as the stochastic screen I had to use Photoshop. My images for the show had some rich blacks and I did not want to take any risk of loosing them due to open bite. Open bite occurs when the positive covers the film completely from UV light when exposing the image, leaving the film soft and causing the wash out during developing so that the plate is no longer capable of holding ink. 
For some of the images the Diffusion Dither 300 worked well for others not so much.

For the image below it worked well. First of all the image is quite big, 70 x 53 cm, when unfolded.

Day in, day out. Photopolymer, 2016,  Photo Helen Tago

It took a lot of trial and error but the final file I sent to the printing centre looked like this:

Original file, 70 x 53 cm

Magnified to a 100% it looked like this:

I applied the Diffusion Dither after calibrating the file to the desired size. If you apply it before, the raster will be resized too.

The reason that the original file and the printed etching look so different comes from the fact that each step in the process makes the image more contrasty.

My process:

1. I started with resizing my image to the final format

2. To make it easier to imagine the final outcome: Layer--New Adjustment Layer---Hue/Saturation. 
In the menu I then dragged the saturation to  -100.

3. To get the desired darks and highlights in nuance I then chose: 
Layer--New Adjustment Layer---Curves. In the second dropdown menu I selected Red, Green and Blue one by one. Playing with the curve of each color until satisfied. Keep in mind that the image needs to be less contrasting then you want the final outcome to be. Experimenting and testing is always  needed.

4. The next step: Image--Adjustments---Curves I reduced the value of black down to 80-75%,  the light tones drop out also, so the diagonal is also moved to 15% to leave some dots in the whitest whites.  

5. Then I changed the Mode: Image---Mode--Bitmap. I changed the Output resolution to 300 pixels/inch and used Diffusion Dither. (This is not the only way to get a printable image, but it was the case for my specific work I describe here) 

Ideally you would need to print on an Inkjet printer onto a transparency. I have achieved relatively nice results with my regular laser printer but the quality in the commercial printshop tends to be much better.