Automatically Fill Circular Mask to Canvas And Automatically Centre A Mask - PSE 2020

Thank you very much Greg. I tried this and it works fine. It will enable me to create a selection the exact size of the canvas. The other good point is that if I move the selection with my mouse near the centre, it will automatically snap to the exact centre without any difficulty (bravo Adobe!). If it is a smaller circular selection than the size of the canvas, then I will have to use the method in Jeff's post to centre it.   

I need to understand how to do this to achieve a fisheye lens effect with photoshop elements. First I make the image and canvas an exact square, then apply the spherize filter. Then I need to use a circular mask without any feathering and copy the centre of the spherized image to a new layer. The finishing touch is to darken the outer circumference of the sphere with a blurred black inner stroke with transparency set to taste.

Thank you Jeff and Greg for all your help. 

Kind regards

Chris

Thank you very much Jeff. This work-around works fine. Evidently, from what I can see, it's not possible to directly centre a mask on the canvas. Using this method, the mask has to be made into an object on a separate layer first, then it can be centered on the canvas using the move tool and Align Centre/Middle. Afterwards, the object can be converted back into a selection using control/left click on the layer icon. Then the object can be deleted if its not needed leaving just the circular selection.

Next I need to find out how to make a circular mask to precisely fill the canvas. Eg if my canvas is 4000 x 4000 pixels, I want the circular mask to be 4000 x 4000 pixels. 

Thanks again for all your help.

Kind regards

Chris