TFOV

[bingevader]

I use an approximate TFOV value for some of my EPs for star-hopping.

Without getting too nerdy and seeing as for my usage it's an approximation anyhow, only as a star-hopping guide, is there much difference between the two calculations for TFOV?

TFoV = EP AFoV / magnification of EP

TFoV = (field stop dia. / scope focal length) X 57.3

Most of the on-line calculators seem to use the first equation.

Neither would be as accurate as working out the TFOV using the drift method, but then I'm not needing to be that accurate.

I guess I've answered my own question really!

Are there other applications that require that level of accuracy?

Cheers

[Stargazer_00]

None that im aware of. I use the first one for eyepieces and the second one to work out what the maximum potential TFOV a scope can offer.

[Stu]

I just compared a number of the Televue eyepieces using both formulae and for most the calculations were very similar. However, for the 31mm Nagler and 41mm Panoptic the results were different by approx 0.3 degrees based on a 480mm focal length refractor, not insignificant. I assume this is because the field stop is limited by the physical size of the barrel. The 24mm Panoptic is also different by 0.18 degree for similar reasons in that it is getting to the limit of the 1.25" barrel.

Stu

[Scooot]

With my 17.3 the first calculation comes out at 1.038, the second at 1.012. Not many manufacturers publish the field stop info so that's probably why the first is used the most.

[Stargazer_00]

In this circumstance I would trust the more complicated equation. The simple fact is the stated AFOV onn an eyepiece isnt always the exact AFOV (well maybe it is in Televues). The field stop does not lie, you can measure it with a ruler. It's much more difficult to measure AFOV accurately by eye. you have to rely on the drift method and then you have all sorts of variables. Ideally something right on the eclipctic to mitigate most of them.

[michael.h.f.wilkinson]

Most calculators use the first equation because the magnification and AFOV of the Ep are generally specified on the box, but the field stop not always. The second equation is the better, as the apparent field of view quoted is essentially the radius of the circle you see through the EP. This does not take distortion (pincushion in all wide angle I have seen) into account and overestimates the actual section of the sky visible through the scope. The field stop diameter tells you how much of the image plane of the scope is actually seen. As most telescopes have hardly any distortion (compared to the EP).