Dielectric coatings are extremely durable and shouldn’t ever really degrade. Enhanced aluminum or silver coating will fail, typically losing significant reflectivity after 10-20 years. For a 30 year old diagonal it probably isn’t dielectric, and could probably use a recoat.
Some will talk about SCT coatings lasting a long time, especially in dry environments. Yes a 30 year old SCT can still give good views. But typically when people recoat their ~40 year old mirrors does make a detectable difference in brightness. Or when they compare them side by side with a newer model. Generally it isn’t a night and day difference, but every 10% helps. Also, SCT is basically a sealed tube. It would be hard for dust to get up in there even when changing eyepieces, unless you are observing the ground. A diagonal is exposed and dust/pollen/dirt can settle in there every time you swap eyepieces. Not a ton of exposure but it adds up, and the diagonal will typically need cleaning long before a SCT primary mirror.
Consider that the enhanced aluminum coating was probably 95-96%. Dielectric are 99%. So if the coating even degraded a mere 5-6% reflectivity over 30 years, that would be 10% difference compared to a new dielectric.
Consider how many times you have cleaned the diagonal. I hear a good rule of thumb for reflector primary mirror is recoating the mirror after every three cleanings. Maybe this is excessive, especially if you are very careful how you clean the mirror. But if you have cleaned your mirror once every three years, that would be ten cleanings.
Once I had my nearly new VX10 with 97% coating out next to an old Coulter. My scope gave much brighter views. Coatings do matter. Granted the Coulter probably was around 88% originally, and a primary mirror has much more exposure than a diagonal mirror. I expect the difference would be more subtle in your case. But I expect there would be a detectable improvement in transmission.
Edited by SeattleScott, 27 October 2021 - 04:27 PM.