Does it make sense to use a 2” diagonal on a Celestron C5? The consensus seems to be no.

Why not? Well, the rear axial port on a C5 is approximately 25mm in diameter, far smaller than the opening in a 2” diagonal. This, in turn, would suggest that eyepieces with 40mm+ field stops (say a Nagler 31mm Type 5 or Explore Scientific 30mm 82 degree) would be quite limited - an argument that makes a fair amount of sense to me.

But from time to time I do use a 2” diagonal and an ES 30mm 82 on my C5. Whenever I do this, the field seems far wider than it should be - if the axial port were actually limiting the field of view.

So today I decided to *attempt* to measure what was going on.

I set up my late-model C5 and a TV 85 approximately 60 feet from a wall where I’d taped two yardsticks end to end. I wanted to measure the visible field of view using various eyepieces, and then compare the ratio of the measured fields with the ratio of the corresponding theoretical fields of view.

Here’s my logic, such as it is . The shortest FL eyepiece I’d use was a TV 11mm DeLite, whose field stop of 11.7mm wouldn’t be constrained by the C5’s axial port. I’d set the measured field of view for the 11mm DeLite as my baseline, and then see if the measured FOV for wider field eyepieces (with field stops in excess of the 25mm axial port) scaled up as fast as they theoretically should. If not, my assumption would be that the axial port did indeed limit things.

On to the measuring!

First up was the C5 with a 32mm Meade plossl and an 11mm Tele Vue DeLite, in a 1 1/4” diagonal. I calculated the theoretical field of view at 1.24 degrees for the 32mm Meade, and .54 degrees for the 11mm DeLite. This gave a theoretical ratio of 2.30 (Meade FOV as a multiple of the DeLite).

I then used the yardstick to measure the actual FOV through the eyepieces, and got 14.15” for the 32mm and 6.3” for the 11mm. This resulted in an actual ratio of 2.26, which seemed close enough to the 2.30 to indicate there was no limitation imposed by the axial port (see table below). An expected result.

Next I tried my TV 85 with an ES 30mm 82 (43mm field stop) and the TV 11mm DeLite. In theory, the clear aperture of the TV 85 shouldn’t limit the FOV observed with the ES 30mm. Using the same approach as above, I came up with a theoretical FOV ratio of 3.67 (ES as a multiple of the DeLite), and a measured ratio of 3.56. Again, close enough given my less-than-exact measurements to suggest that actual equaled theoretical, and to seemingly confirm that this measurement methodology worked OK.

Then it was time to try the 2” TV diagonal on the C5. As a first step, I tried to estimate the focal length of the C5 with the 2” diagonal. I measured the FOV through the 32mm Meade Plossl at 12.75”, which was roughly 89% of the FOV I’d measured using the same eyepiece with a 1 1/4” diagonal.

Based on this, I estimated that the C5 with 2” diagonal was operating at about a 1400mm focal length, instead of its standard 1250mm. I used this FL to calculate the theoretical FOV on the C5 using the various eyepieces.

Finally, I went ahead and measured the field of view on my yardstick while using a 30mm ES 82, 20mm ES 100, and 11mm TV DeLite. As you can see in the table below, in all cases the ratio of the measured FOV very closely tracked the ratio of the theoretical FOV - which was not exactly what I’d expected.

I didn’t try to measure how bright the image was at the edge of the field vs. the center. There was probably some light fall off, but it wasn’t dramatic and it wasn’t hard to read the numbers at the edge of the field.

I don’t doubt that there are shortcomings with this simple approach. In any event, I’d love to have the optical experts out here on CN either a) point out the mistakes I’ve made while trying to measure this, or b) explain the following: why doesn’t the small axial port on a C5 act as more of a limiter on the field of view?

Thanks!

Paul