A Smaller Upper Cage

I discovered while designing my 13 inch f/3.0 telescope that the upper cage's inner diameter can be made smaller than thought. As I projected light from the primary, I noticed that it cleared the inner diameter of the upper cage by a good margin.

Many of us fit the upper cage into the mirror box, so a smaller upper cage means a smaller mirror box, which results in a smaller rocker. This can save 10 - 15% of overall weight. On a 100 pound telescope, that's 10 to 15 pounds less telescope to lug about..

The cost for the smaller upper cage is a 2% loss of light at the edge of the low power field of view. That corresponds to 1/50 magnitude light loss.

My primary is 13.2 inches, so I made my upper cage 13.5 inches inner diameter, a good inch smaller than I would have otherwise. It works great!

Consider an order of magnitude greater light loss, say 20%. A 20% light loss may seen dramatic, but the eye's semi-logarithmic response can hardly tell the difference. To put it in context, it's like lowering the magnitude limit of your telescope from (let's say) 16.5 to 16.2 magnitude. And this light loss is at the extreme edge of the field of view at lowest power, which means that the sky background is as bright as the night sky is to the unaided-eye. Experienced observers know that high magnifications are required to push a telescope to its limits. The field of view at high power is much tighter, with none or almost none light loss. The result is that the 20% light loss is insignificant.

The following two images demonstrate the vignetting from the diagonal and tighter upper end. You can see that the diagonal’s vignetting is far greater than then smaller upper cage’s vignetting.
This image shows that the entire primary mirror fits within the diagonal.

This next image is from the extreme edge of the field of view given a 100 degree eyepiece at 7mm exit pupil. You can see the diagonal’s edge to the top in the image is vignetting part of the mirror’s edge. This amount of light loss is represented by the extreme edges of the illumination profile graph. If you look closely at the bottom of the primary mirror, you’ll see a very thin strip of very dark brown – that’s the vignetting caused by the tight upper end. The area of this strip is insignificant compared to the primary’s edge clipped by the diagonal.

Enlarged schematic showing areas of vignetting:

So save some weight, save your back and make your upper cages smaller!

Mel Bartels, Oct 2010