Tweening my solar images to produce GIF animation - a forgotten process :-(

I do solar imaging in H-alpha. sometime these are just still images and other times we take a series of images and do an animation of a sunspot or prominence.

The animation proces is simple. Take a bunch of images,import them into a stack, align the images, make frames from layers and tween between the frames to produce the GIF animation. This is simple but can take hours to get everything just right.

Here is the problem. I remember being able to tween between frames all at once. That is, select say 40 frames, ask it to tween say 4 intermediate frames and it is done, all at once. That doesn't work any more. Now I have to do this one frame at a time. This is time consumming and prone to mistakes.

Either I forgot how to do this, all at once tweening, or somehow, it can't be done in PS any more. I hope it's the "forgot."

If anyone knows how to do the all at once tween, please let me know. It would be greatly appreciated.

Thank you in advance for your help.

Eric