With everything starting to line up with visual observations, it’s time to start focusing on what I really want to do and that is astrophotography (AP).  First the equipment needed in addition to the telescope, a camera of course.  I’ll be using my Canon T3i t start with.  The telescope has an inherent property of elongating the stars around the edges of the sensor.  To compensate for that, I’ll be adding to the optical tube assembly (OTA) a flatter-reducer designed for it.  It’s an Orion ED80 .85x reducer-flattener.  It changes the focal length of the OTA from 600mm to 510mm.  This also changes the speed of the OTA from f/7.5 to f/6.3  This give me a wider field of view as well as lets in more light along with flattening the entire image so stars appear nice and round across the entire sensor.

This brings up the need for back focus.  All DSLR cameras are built to have 55mm of back focus.  That is the distance from the sensor to the flange (lens mount point).  Initially there was some confusion as to where I needed to measure from so, I went to my discussion group and posted the following question. 

The Canon T3i has 44mm of distance from the sensor to the mount point on the body, according to some research on the web. The T-Ring specifically for the Canon has 11mm, as measured by my ruler, once it is completely threaded to the reducer corrector. Do I need any of the multitude of extension tubes that I bought to achieve the needed back focus of 55mm as 44mm plus 11mm = 55mm? My whole confusion at this point, do I measure from the sensor or do I measure from the mount point on the body of the camera?

I got the following response

There is a reason that many reducers/flatteners/coma correctors specify 55mm as their design back focus. It is because 55mm is the depth of *ALL* DSLRs when they have their correct T-ring installed. The whole point of designing the devices with a 55mm back focus is so that you don’t need any other adapters. The measurement is from the mounting flange to the sensor.

The T-mount standard was developed by Tamron (hence the “T”), a maker of SLR lenses. They wanted to make their lenses interchangeable among different brands of cameras, so that a photographer who used both Canon and Nikon cameras, for instance, didn’t need to buy two different lenses of each focal length. To do that, they needed adapters for each camera mount type. They also needed the optical spacing to be the same for all cameras, even though each manufacturer had their own body depth, so that the lenses would focus properly. The solution was to have the mount adapter compensate for the body depth. The result is that all standard T-rings make their camera 55mm deep.

I followed up with one more question.

Thank you for the info Kathy, that will get logged into my journal. One more question if you don’t mind. What devices or when would there be a need for extension tubes?

And got this response

There are focal reducers / flatteners that require 85mm or 105mm of back focus. To use one of these with a DSLR, you would need 30mm or 50mm of extension tubes, respectively.

Or, with your 55mm reducer, you might some day decide to use it with a dedicated astro camera that uses 12.5mm of back focus. So then, you would need 55 – 12.5 = 42.5mm of extension tubes.

I replaced my DSLR with a filter wheel / CCD combo. My coma corrector required 57mm of back focus: 2mm for the necessary thread adapter and the standard 55mm for a DSLR. To make up for the missing camera body depth with the new equipment, I had to use 19mm of extensions.

These came from Kathy, one of the co-administrators and quite the accomplished astrophotographer.  I can only hope to take images that might compare to hers someday.