I started to have problems with my PST about 18 months ago . The main problem was not being able to pick out any detail on the surface no matter where I turned the etalon ring. Thinking it was the Rust problem starting I put it away and forgot about it. Having got the scope back out today, I had a look and the objective lens seems clear blue and transparent with no sign of rust. My question is. Is the rust only visible on the objective lens or can it elsewhere in the optic train.
As far as I am aware the PST ‘rust’ problem only occurs on the main objective lens and not elsewhere. Since you mention that your objective is ‘blue’ then this would indicate you do not have the rust problem. Even with low activity in hydrogen alpha, such as at the moment, you will always be able to see the chromospheric network that occurs all over the disk – see for example the second image (right hand side) on page http://www.petermeadows.com/astroimaging/ (this Ha image was taken with a PST).
You should slowly adjust the etalon ring and always ensure you have good focus until you see the network. You may need to move the etalon back and forth and few times. If the solar disk is uniform without any texture than you are either off frequency or there is a problem with the etalon.
With your PST did you every see any prominences/filaments etc? Is it still under warranty?
So it used to work and now it doesn’t, you are saying? There is a clear difference between the images it gave more than 18 months ago and now, even allowing for the decline in sunspots?
All solar equipment does degrade with time, and always needs servicing eventually. If there’s no detail, then one of the filters inside is not performing correctly, and likely needs replacing, which can only be done by Coronado.