Help with Exposure Times

[Keith1]

Hello,

I am relativley new to imaging and are having very limited success. Could someone confirm that my understanding below is correct please.

I managed to get a image of the Ring Nebulae, more by luck than judgement. I have an ETX125 telescope and Canon 450D ccamera. I took 75 5 second pictures and stacked them in DSS. This was the largest time I could expose for without stars trailing.

However, when I tried M27 the other week, I can not see this on the camera at all. I have scoured the net and found that although M27 is the same magnitude as M57, the fact that it is larger, means that it will actually be seen as far duller from a visual perspective.

My theory goes that if the object is double the size, then it will appear a 4 times as dim.

Therefore, if I captured M57 with ISO 1600 and 5s images, then would it make sense then that to image an object double the size (hence 4x as dim), then I would need to expose for 20s at 1600 ISO.

Appologies for the long message, Any advice regarding this is appreciated.