DIY 6-Element 33mm Eyepiece

[Rocket_the_Raccoon]

Béla Szentmártoni (1931-88) was a Hungarian amateur astronomer and telescope maker. He founded the Albireo Amateur Astronomy Club in 1971 and have made one hundred mirrors ranging from 4" to 10". On the August 1967 issue of Sky and Telescope (http://alpha.dfmk.hu/~albireo/szm/skylap.gif), he published his making of a 6" rich-field Newtonian with an eyepiece design of his own, giving a TFOV of 1.5 degrees. The original Szentmártoni eyepiece formula consisted of 3 doublets, all with a focal length of 65mm. The spacing of the field group elements is 2mm while the spacing between the field group and the eye lens is 4mm. The modified version above retains the basic orientation of the 3 doublets and spacing ratio of 1:2. However, the focal lengths and actual spacings are different (as shown in the diagram). 
( Further Info : https://hu.wikipedia.org/wiki/Szentmártoni_Béla , http://alpha.dfmk.hu/~albireo/szm/atm.htm )

Using the 3-lens formula from http://www.astronomyboy.com/eyepieces/ep_calc.shtml, the EFL of this modified version is 33mm. The TFOV is calculated by (field stop / objective focal length) x 57.3, which is then used to obtained the AFOV based on the EFL, AFOV = TFOV x (objective focal length / EFL of eyepiece). Finally, eye relief is calculated based on AFOV and the 7mm human pupil size using the formula from http://www.mira.org/ascc/pages/lectures/fabform.htm.

For a 1.25" 33mm eyepiece, an AFOV of 43 degree is not considered wide in today's standard. However, one have to note that the AFOV of any eyepiece is limited by the size of its field stop and many 1.25"  eyepieces in the 30~32mm range have smaller AFOVs than specified. For example, Baader Classic Plossl 32mm has an AFOV of ~47 degrees (https://stargazerslounge.com/topic/179005-baader-classic-32mm-plossl-meets-vixen-30mm-npl-plossl/) and the Skywatcher Super Plossl 32mm has an AFOV of 43 degree (http://astro-okulare.de/English/skysp32e.htm).

As for image quality, this 33mm Modified Szentmártoni is not just sharp but also sharp to the edge without distortion on my f/5.7 refractor. I believe it will pretty much replace my veteran Omcon 25mm Plossl (Celestron silver-top equivalent. fully coated only) as a great finder eyepiece and an indispensable accessory to enjoy rich star fields.

Ingredients :
- two 25mm doublets with focal lengths of 100mm scavenged from a pair of 10x25mm compact binoculars
- one 21mm doublet with focal length of 80mm scavenged from a pair of 8x21mm compact binoculars
- plastic objective retention ring from the 10x25mm binoculars as the spacer of the two 25mm doublets (field group)
- plastic part from the eyepiece of an old 10x50mm binoculars as the eyepiece top that holds the 21mm doublet (eye lens)
- 1" copper pipe coupler as eyepiece outer barrel
- metal ring from the StarPointer of a Celestron Astromaster 70AZ (intended to be the field stop, not used)
- rubber eyecup from another eyepiece (not used)
- rubber eyecup from an old pair of 10x50mm binoculars
- chalkboard stickers to make the inner barrel of the eyepiece and for fitting elements with different diameters
- E6000 crafting glue 
- black marker pen for darkening the edges of the doublets

Lessons Learned :
- Though the total time needed to build the eyepiece is less than an hour, I spilt it up to several days in order to let the E6000 glue fully dried.
- When glueing a lens to a plastic part, the glue should be put on the edge of the lens, not the plastic, to avoid excessive glue spreading onto the lens surface.
- The use of an internal eyepiece barrel enables more precise placement of the field group and avoids the mess of glueing the field group to the copper barrel.
- The chalkboard sticker, which was rolled up to make the internal barrel, took out the need of painting the inside of the eyepiece with flat-black paint.

   My DIY Eyepiece Set : 33mm, 18mm, and 10mm