First Telescope

I made my first telescope from cheap optics almost 15 years ago when I was in 10th grade. The basis information to use ophthalmic optics for this purpose came from a Russian popular science book “Ghar main Antariksh” or “Space in Home” by Florenti Rabiza.

For objective, I purchased a +1.5 diopter¹ (650 mm FL) 65 mm ophthalmic glass lens from an optician “Baliwalla and Homi” in Mumbai. I however regretted later for buying +1.5D since “focus to diameter ratio” was low and it introduced considerable amount of spherical aberration in the telescope. For a common telescope, it should be more than 15. F ratio² is mostly referred as f/10 or f/15 which is basically an indication of diameter or aperture of the objective compared to its focal length.

¹Power of a lens (Diopter or D) = 1/focal length (in meter) = 100/focal length (in centimeter) = 1000/focal length (in millimeter)
²F ratio = Focal length/Diameter = 650 mm/65 mm = 10

If you want to make your own telescope then I would advice buying +1 D, 50 mm lens. This will have a F ratio of 20 and will have less spherical aberration.

I did not make eyepieces for this telescope, instead I purchased them from Tejraj & Company for 150 Rs. at that time. A 12 mm and 25 mm. I also purchased a 3X variable barlow at same price.

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