As I discuss in this answer, each of the four Vedas consists of four portions: Samhitas, the core part of the Vedas which consist of hymns heard from the gods; Brahmanas, commentaries on the Samhitas which provide instructions on the proper conducting of important rituals; Aranyakas, which provide instructions for rituals meant for forest-dwellers and hermits; and Upanishads, which consist of conversations between teachers and students which clarify the philosophical message of the Vedas. But when someone says, e.g. "I read the Rig Veda" without qualification, they usually mean the Samhita of the Rig Veda, because the Samhitas are the core part of the Veda which are of divine origin; the verses of the Samhitas come from sages known as Dhrishtas (literally "seers") who heard them directly from the gods, and then these verses were compiled by a sage named Krishna Dwaipayana Veda Vyasa (or Vyasa for short). The Brahmanas, Aranyakas, and Upanishads are works that were mostly later added on as supplements and commentaries for Vyasa's Samhitas.
But there is one Upanishad that has a very different origin - the Isha Upanishad, AKA the Ishavasya Upanishad. It isn't some supplement or commentary, it's actually part of one of the Samhitas themselves; it's the 40th Adhyaya of the Vajasaneyi Samhita, which is the Samhita of the Shukla (white) version of the Yajur Veda. (Here is another translation of the Upanishad.) This means that the Isha Upanishad isn't just someone's philosophical teachings like the other Upanishads, rather it's a hymn heard directly from the gods.
So my question is, who is the seer of the Isha Upanishad, i.e. the sage who heard it from the gods? If I had to guess, I would guess that it's the sage Yajnavkya, who's associated with the Vajasaneyi Samhita in general, but I'm not sure. Also, since the verses of the Vedas are usually addressed to some god or the other, who is the Isha Upanishad addressed to? The last few verses are addressed to gods like Pushana and Agni, but who are the earlier verses addressed to?
This is the sort of information that can be found in the Anukramanis, a set of systematic indices for the Vedas which, among other things, documents the sage who heard each verse from the gods, and the deity that each verse is addressed to. In this answer, I compiled the sage and deity information for most of the hymns from the Anukramani of the Rig Veda Samhita, but the Yajur Veda Anukramanis don't seem to be so easily accessible. Apparently there is one Anukramani for the Vajasaneyi Samhita, as described in this book:
The third Anukramani, that of the Madhyandina-Shakha of the Vajasaneyaka, is ascribed to Katyayana, who is mentioned also as the author of an Anuvakanukramani. It gives the names of the poets, the deities, and the meters, for all the verses of the Samhita, including the Khila (Adhyaya 26-35), and the Sukriya portions (Adhyaya 36-40).
That seems like exactly the work I want, because it has information for Adhyaya 40, which is the Isha Upanishad. So does anyone know whether Katyayana's Anukramani of the Madhyandina Shakha is available online, or if it's even been published in book form, whether in Sanskrit or in English?
EDIT: I posted a seperate question to try to find the Anukramani information for the Vajasaneyi Samhita.