Wednesday, December 23, 2020

hacked the t-ring

I hacked the t-ring for the Canon EOS camera. 

I had always noticed some play in the t-ring but didn't think much about it until recently. The recalled the issue during the Jupiter-Saturn conjunction event I participated in even though the slop was not a factor for the planetary live views.

Where did I buy this generic ring? Couldn't remember. Clearly sometime shortly after coming by the Canon digital single-lens reflex in 2012.

Back in June this year when I was imaging in Vulpecula for double stars, I thought, "This is bad." The slight rotational play in the t-ring and the camera body meant the camera angle could change. That must not be allowed to happen if attempting accurate position angle measurement with the camera on the telescope. (And if it does happen, one must then calibrate each photo!)

Today I did some quick research into the matter. A Google search on the phrase "play in t-ring" yielded a few interesting hits.

The first was for the Baader Planetarium zero-tolerance protective Canon DSLR T-Ring available listed on the Ontario Telescope website. I had a look at the product page and pictures. It sounded like it was made to exacting tolerances to reduce both rotational and octagonal shifts. 

The octagonal issue is important to imagers after perfectly flat fields. I experienced this issue with my image of Albireo, the double star, in late 2012 which Dietmar analysed to in fact show just how much the camera was not square.

I followed another link from the search engine results page, this time landing in a Cloudy Nights discussion. Back in April, in a thread titled Canon EOS T-Rings that don't wiggle, people were talking about solutions. Different t-rings were discussed with remarks on fitment.

Perhaps I need to get a high-quality t-ring. The Baader unit is not inexpensive. Would it fit the 40D?

All this got me thinking... 

Grabbed the 40D body and t-ring. There didn't seem to be discernible tilt (I still wonder if the flat field issue with beta Cygni was due to sag). However, the rotational play I eyeballed at about one degree. Could I get rid of this?

Suddenly, I thought, "What about a gasket?!"

I tripped to the kitchen in search of some elastic bands. When unwrapping various grocery products, I keep the rubber bands, tossing them to a shelf by the sink. Happily, I found a pile. I retrieved them all and returned to the workshop. I was very pleased to have some fine, small bands that looked rather promising. I slipped a thin blue band into the track of the t-ring and remounted the assembly to the camera body. 

t-ring with rubber gasket

Holy Universe! It worked! Wow. The t-ring was now very snuggly held in place. There was no play due to the compressive band. The rotational resistance was now so great it was in fact easy to unthread the nose piece from the ring.

I think this will work very well. Another hack completed, solving a nagging problem with imaging. Better, more consistent results are in store for double star projects.

This simple fix has eliminated the rotational problem. Perhaps it will help with octagonal alignment but I'm less concerned with that.

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