Jason,I am unfamiliar with the IDAS LPS filters for camera lenses for astro work,What does it do ?? I am interested in using my 400 L prime F 5.6 for more astro work (one year-when the clouds clear
).
I think I saw Doug selling his 300 F4,I will have to get in touch with him to get some pointers.
The 300 L prime,is a very good lens,It was a hard choice for me,when I had to choose between it and the 400.
To the OP,just stick with canon,do not buy those non camera brand lenses,in the end they cost more than a L,the amount of frustration I have had with them,cost me a lot of money,wasted time,and missed out on opportunities I can never get back.
A canon non IS F4 70-200 are very reasonably priced at present,for what you want it does sound like a 300 L would be more suitable.
C