Whitepeak Observatory, Tacoma, WA

Building a 120mm f/5 Widefield Achromatic Refractor (RFT)

In the winter of 2007 I obtained a 120mm f/5 lens taken from a WWII vintage pair of US Navy "Big eye" 20x120 giant binoculars. (example of the binocular model this lens came out of-US NAVY image) I bought the lens for 50$ and decided to make a RFT (Rich Field Telescope) out of it. This would be my first telescope building project.

I chose 6" galvanized steel stovepipe for the main tube and for the dewshield a 12" piece of 7" phenolic tube from "Public Missiles" a model rocket supplier.(thanks again to the ATMer who provided me with the tip leading to this source.) For the reducing rings, since i don't have access to a large lathe, i had to make these out of wood, which in the end worked just fine, allowing for a nice snug fit for all the components. I bought a used 2" short draw focuser, the tube rings and the dovetail. The finder was a remnant off the 80mmf11 package which originally came with what is now the guidescope on my observatory scope. Here's a pic of what i started with, after rough cutting the reducing and trim rings and before painting:

We have a big (scary!) abrasive wheel cutoff thingamajig out in the barn so i used that with a jig to help cut the steel tubing nice & square:

Here's a couple of pix of the painting process. All "rattlecan", primed and finish coats. (Krylon, which i like because it dries fast enough that gunk floating around doesn't have as much time to mess things up.) The finish coat was Krylon Appliance epoxy. I prepared the galvanized tube by sanding with 220 grit and then wiping down right before painting with regular household white vinegar, then primed and finish coats were applied. This must have been the right approach as the paint ended up adhering very well. The "stovepipe" i used was the old-fashioned kind which has a seam which "unhooks" and comes apart. This was actually nice as it allows opening up the tube which makes painting the inside with Krylon Ultraflat really easy. This tube is also the right size where a coffee can lid fits perfectly as a lens cover (these lids were also used to make nice thin baffles).




Baffle

The phenolic tube i used for the dewshield had one negative aspect: the spiral winding left crevices up & down the tube. This wasn't a problem for the inside (which is flocked) but for the outside these groves required a lot of time spent filling and sanding and filling and sanding--ack!--to get a smooth finish for painting. I used all stainless fasteners.

Anyway, here's the completed scope:

and another view:


Brief first light:

Set up the scope and my 20x80 binoculars and compared views. The FOV using a 30mm 2inch 70 degree AFOV eyepiece was a full 3.5 degrees true. I found the light gathering of the scope to be noticeably superior to the binoculars-- as it should be. So even though the coatings on this old lens are inferior to a more modern lens, they still do better than the binos, which is what i was hoping. (I calculate the binos should give about 110mm telescopic aperture equivalent and since the scope's working aperture is about 117mm, these results seem consistent with what i'd expect.)

There is more CA apparent from this lens than seen in my 6" f/8, but this i expected. The quality in a quick star test seems good seen at 32x/inch using a 4mm abbe, uniform inside & outside with crisp symmetrical outer rings; however there is a *touch* of astigmatism present. I took a lot of care to measure and align everything when i assembled the scope--which paid off as it came out dot-on collimation-wise. Overall, i think this lens has promise for the rich field use i wanted it for in the first place.

If you have any questions/comments re; a similiar project you are working on or planning, feel free to write; i'm happy to share any knowledge garnered from this project. mclark@cityastronomy.com


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