Telescope Reviews: Which C5 Extension Needed?

Revisiting this subject. last we spoke I said, I tried this but took mine off the C5. When looking back over my data that led to my decision to take it off, I found an error. So I retested this arrangement this weekend.

What I was concerned about is, am I able to see the entire field of view in every eyepiece that I want to use? There may be some slight darkening at the edges to to lack of full illimination, but where I come from, that's not a deal breaker.

Here's what I did.

Picked 4 eyepieces a 7.5, 12.5, 18 and 24mm. Set up a meter stick about 125 feet away. Using the eyepieces in a TV85 refractor of known focal length, recorded the extent of visible field (on the meter stic, measured in mm) for each eyepiece. OK so far easy enough, but why do this. Two reasons, you'll see.

Next I brought out my C5 with the standard 1.25" diagonal. ran all 4 eyepieces through the C5/1.25"diag and recorded the extent of fov again.

I then compared the reading for each eyepiece in the refractor to the reading in the C5. Got near exactly the same ratio of drop in fov for all 4 eyepieces. Good, consistent data for 4 eyepieces. So what that shows so far is only one thing changed, the POWER. And because power changed, all 4 eyepieces have exactly the same ratio reduction in the fov. Variances here were on the order of 0% to 1%.

Step 3 - I changed out the diagonal on the C5. I put on my 1.5" extension and my Astrotech 2" SCT diagonal. Ran all 4 eyepieces thru this set up and again, with less than 1.5% variance, I got all 4 eyepieces to drop by near exactly the same ratio of decreased fov. Good, consistency again. This proves once again, the only thing that changed is power.

So what did I find:

Assuming that my TV85 refractor is exactly F=600mm, then:

my C5 with a standard visual back and a 1.25" TV dielectic diagonal operates at a focal length of F=1367mm.

my C5 with a 1.5" long extension and a 2" Astrotech dielectric diagonal operates at a focal length of F=1540mm.

Next I pulled out two wide field 2" eyepieces. Having proved the F=1540 of this arrangement, these I compared by calculating the expected fov, based on measured field stop diameter, a valid and fairly accurate method. I came within 1% of expected fov when using a 27mm Panoptic and about 1.5%-2% for a 40mm TMB Paragon. The actual Tfov for each of these eyepieces worked backwards gave an effective Afov of 64 for the Pan and 66 for the TMB. BTW, the fov in the TMB was just wider than the meter stick, so I had to estimate it at 1100mm, perhaps accounting for a portion of the very slightly larger error.

This means there was no fov cut by vignette in the large 2" eyepieces. I didn't really check for light fall off, only for Tfov. So, while I might expect some light fall-off due to less than full illumination, there appears to be NO complete cutoff of fov.

I'm going to try this out for a few months. F is longer, but that's OK, I have lots of decent/very good eyepieces to get the powers I want. And I'll get longer eye relief from some of my longer eyepieces. The benefits are, I also have four 2" eyepieces that I can now use, AND this diagonal and twisted screw locked arrangement will be far more stable to hold my denkmeier binoviewer. Other than the longer than specified F, I don't see any disadvantages, and even that is pretty minor since this is not really my wide-field scope.

edz

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