Which index did you search?
Further info: Philemon Blethin MA, is listed on the Clergy of the Church of England Database (1619 – 1642). Admission as vicar to Dorstone, Hereford is 12/10/1621. He married Susana Godwin on 21 April 1610, she being the daughter of Dr Godwin, possibly Franciscus Godwyne (1562-1633). Francis Godwin was later the Bishop of Hereford and he had associations with Matherne, Monmouthshire. Francis apparently wrote the first story about space travel in English literature: The man in the Moone: or A Discourse of a Voyage Thither by Domingo Gonsales, the Speedy Messenger, which was published posthumously in 1638 (source: Encyclopaedia Britannica). He was the great uncle of Jonathan Swift (Wikipedia).
Children of Philemon and Susana are:
1. Susana b. 21 July 1613; bpt. 26 July 1613
2. Ann b. 1614
3. Godwin b. 3 January 1615; bpt. 7 January 1615
4. Theodore b. 14 March 1616; bpt. 23 March 1616
5. Francis b. 22 February 1620?; bpt 3 March 1620
All Monmouthshire. Please treat these dates with a little caution as the entries do not appear to be in sequence with at least one date heading missing.
(source: Ancestry)
Further, there are papers listed on the National Archives index as being held at Lambeth Palace dated 16 October 1655. Title: Dorstone Vicarage Hereford. Description: James Whiting. The death of Philemon Blethin. Thomas Dainty, stationer, of St Michael Quern, London.
Dainty was also a bookseller – maybe a business connection with Theodore the bookbinder?
Of course, none of the above may relate to your telescope maker, but at first sight it does look rather promising and, without any other candidates, is worthy of further research.