First, let’s get one item out of the way: if what you want is the final product (ie: the telescope) and you are not overly interested in the building process, do yourself a favor and buy a commercial product from an established manufacturer: you will save time & money and avoid bad surprises. If you wish to buy, the premium brands are well known: Takahashi, TEC, AP, Televue, just to name a few. Nowadays, the mass market manufacturers also make excellent optics at bargain price: Skywatcher, Explore Scientific….
If the journey is the destination and you want to build a refractor for the pleasure of building, here is the path I followed. It was a fun journey.
Most objectives I could find online for ATM (Amateur Telescope Building) are doublets. For some reason triplets seem very hard to come by. One of the most reputable source for objective is Istar. The Istar objective come with a magnesium alloy collimation cell, which is very valuable, as a good collimation cells is difficult and expensive to build. Their H-Alpha line in particular, for solar observation, is very interesting…
However, for this project I went cheap (at least cheap for the aperture) with a 127mm Surplus Shed doublet, as the goal was to experiment with the build and I was not overly concerned with the end result (I have a 10 inch Newtonian of equivalent focal). There is a bit of a back story to the Surplus Shed lens: when the US camera manufacturer Wollensak (very popular in the 1950s and 60s) went bankrupt, a small shop, Surplus Shed bought their designs and trade name. Today Surplus Shed have some of the Wollensak objectives manufactured in India by batches, and sell them as “Wollensak brand, Quality optics since 1899”. Every now and then, a batch is made, imported and sold at a very sweet price on their website in the US. I took advantage of one of those batches and bought a 127mm, focal 1,200mm, f/9.4 doublet.
I went for the f/9.4 version (rather than the f/5 version also proposed by Surplus Shed) as the slower doublet has better expected correction. The exact design of the objective is not documented on the Surplus Shed website (they sell, however a CD with a catalog of all the Wollensak designs), but I suppose it is a classical crown/flint, and one can get a fair idea of the performance by looking at the achromatic chapter of any ATM book (for example Smith/Ceragioli/Berry, chapter 5).
There are various discussions on Cloudy Night regarding the cell being mounted backward, or sometimes having the lens upside down. Mine was backward.
I simply ordered an extruded aluminum tube from Onlinemetals: 6″ Outside Diameter x 0.125″ walls x 5.75″ Inside Diameter cut to 48″ length, which I spray painted black inside and outside.
I cut two plywood rings to sandwich the cell barrel, with just enough play to be able to collimate with 6 push screws (3 pairs of 2), at 2, 6 and 10 o’clock, taped in the aluminum tube. Those 6 screws center and tilt the aluminum lens barrel within its cell. This is very simple, easy to make, and it works well. I am not saying it is the design of the year, though. I cut the wood rings with a rooter, using the Berry method as describe in his book “The dobsonian telescope”. Very simple and, as Berry says, very satisfying to cut perfect circles of wood with ease.
I used a Moonlight focuser, mounted on a plywood ring, also cut with the rooter.
The mounting rings are some stock 6″ rings from Explore Scientific.
I calculated the baffles using a python script made for the occasion.
The python scripts calculates all the baffles for you and also provide an image with the design. The baffle calculation is “maximum baffle”, that is the baffles prevent seeing any tube wall from the focuser. There is another method to calculate baffles, called “minimum baffle”, where one is allowed to see some some tube wall from the focuser, as long as the part of the tube seen is in the shadow of a baffle.
Then I designed the baffles in Openscad and 3D-printed them in TPU. The design is opensource, here. This is easy, fast, the baffles are extremely accurate, and being made of a slightly flexible material (think tire rubber) they fit in the tube even if the tube is not perfect. This saves a ton of headache! I cannot insist enough: if you make your baffles of wood, or aluminum, or rigid plastic, either you’ll never get them down the tube, or they’ll have to be loose and won’t fit properly. TPU being elastic but relatively rigid, you can design a tight fit, yet still be able to deform them to go down the tube easily.
Oh! and baffle make a huge difference. Aim outside and look through the tube without, then with baffles. Seeing is believing.
The dust caps can also be printed of TPU.
First I remove the objective and collimate the focuser, using my Telecat (or any sight tube). The goal is to make the sight tube stop just a bit bigger than the aperture stop, from the observer view point. Another way to collimate the focuser is to insert a collimation laser in it and put a sheet of paper with a 6″ circle coincidental with the 6″ aperture. When the laser spots hits the center of the circle, the focuser is collimated.
Focusing the objective cell is a bit more tricky. It can be done with an holographic attachment on the laser, or more simply by use of the penlight method (which I found in Berry’s older book, chapter 8.10 “Build your own telescope”). Shine a pen light in the focuser and look through said focuser. You see 3 reflections, corresponding to the 3 surfaces of the 2 lens. Collimate until the reflections collapse in one.
End up with a star test collimation.
I did a few optical tests putting the lens on a tripod at the lake.
The optical prescription being a doublet, chromatique aberration is expected: a lens will not focus different colors in exactly the same place because the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths). In practice, for doublets, we expect a magenta halo around bright objects. This effect is certainly present in the picture below:
The images also showed some vignetting on a full frame sensors, coming, I believe from the 2 inch focuser and the 2 inch to e-mount adapter. After all 24×36 has a diagonal of 43.27 mm so a long 50mm diameter drawtube, further obstructed by the 2″ adapter is expected to vignette.
The images were reasonably sharp. Now this homemade lens has 2 elements in 1 group. I tested it side by side, on the same camera, to my reference, the Sony 70-200mm F/2.8 GM OSS II, which has 17 elements in 14 groups and is an exceptionally sharp lens. Well, the Sony is certainly sharper, but also 10 times the price.
So I will conclude saying the doublet is performing within expectations.