I currently own a Contax 139 which, unlike many other models, doesn't feature a switch you can lift the mirror with. I'm planning to do some night stars bw photography with very long exposures (i.e. about 4- hours).
Could the mirror movement itself blur the picture?
If so, does the blurring effect anyhow worsen or decrease if the exposure is very long? Does adding a weight to my tripod help mitigate the effect?