This makes for easier image retrieval when visually comparing with new images of the candidate galaxy. Since the DSS images are not taken with HOU source telescopes, the plate scale, image field size, and orientation will not be the same. When two or more images of the same candidate galaxy have been obtained from the same telescope, further image processing can proceed. In particular the tutorial on Finding Asteroids provides clear, simple steps for subtraction. Subtract the sky value from both images.
Subtractions come out the best if they are done in the above order.
Have your reference and newer images available for downloading and the location coordinates of the SN ready.