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Antares 2"-1.25" Twist-lock Adapter

Posted by Joe Metzger   04/28/2008 12:00AM

Antares 2"-1.25" Twist-lock Adapter
I admit it. I’m a gadget guy. I’m always on the lookout for accessories that I think will enhance my observing experience, so I obtained one from Agena AstroProducts for evaluation. The idea of a twist-lock adapter seemed like a good one, and I was anxious to see how it would work.


A few days later, the adapter arrived in the mail. My initial impression was that it was well made and nicely finished. I immediately noticed a couple of things I wasn’t expecting. Firstly, the barrel was smooth – no safety undercut. Secondly, the adapter’s compression ring was plastic rather than brass. After inspecting the twist mechanism design, I discovered that a flat compression ring really wouldn’t work properly, and that plastic was probably a good cost-effective material for the ring.

Some other features of the adapter are a black barrel interior, and a knurled twist ring. If the twist ring is removed completely, the threads are T-ring compatible, which is nice. The adapter is also threaded for 2” filters. With the exception of the compression ring, the adapter is of all-metal construction, and the twist lock operates very smoothly.


To test the holding power of the adapter, I inserted a 13mm Baader Hyperion eyepiece, which weighs about 14 oz., tightened the twist-ring, and then held them upside down and gave a little shake. I was surprised when the eyepiece slid partially out of the adapter. I tried it several times, with the same result – the eyepiece came out of the adapter about 1/8”, and then met enough resistance to stop it from dropping out completely. What was happening was that the safety groove on the eyepiece did not line up exactly with the compression ring when the eyepiece was fully inserted into the adapter. When the compression ring was tightened, it was riding on the shoulder of the groove, which didn’t provide enough surface for the ring to grip properly. When I shook them, the ring slipped off the shoulder and into the safety undercut, where latent tension compressed it just enough to prevent the eyepiece from coming all the way out of the adapter.

Obviously, the slippage was a problem. I then tried lining up the compression ring with the safety groove before tightening, which, although it meant the eyepiece was not fully inserted into the adapter, I figured would at least allow the compression ring to tighten fully and solve the slippage problem. To my further surprise, I was unable to tighten the compression ring enough to grip the eyepiece. When fully tightened, the compression ring was just slightly too big.


I decided to try cutting a little off one end of the compression ring, hoping it would then compress more, and grip the eyepiece. I snipped about 1/16” off (see photo), filed it smooth, and tried again, this time with success! The compression ring was now able to get a solid grip on the eyepiece barrel in the safety undercut. Unfortunately, this meant that the body of the eyepiece was sitting about 1/8” above the surface of the adapter (see photo). Esthetically not very pleasing, but at least it worked.


Since there seems to be no standard for where on a barrel a safety undercut should be placed, or how wide or deep it should be, I can’t say whether the adapter will have the same problem with other eyepieces. Other than what I have described, I think the adapter is a nice accessory if you are using several smooth-barrel 1.25” eyepieces during an observing session, and you want a quick and easy way to insert and remove them from a 2” focuser or diagonal. However, I wouldn’t recommend the adapter for use with heavy eyepieces, binoviewers, or cameras with safety-undercut barrels unless you take the time to shorten the compression ring, and always make sure the compression ring is fully seated in the safety undercut.

UPDATE (5/1/2008): I have been told that Antares has modified the design of the adapter and now all new adapters are shipping with shorter compression rings. - J.D.