Message posted by lone wolf on June 15, 2004 at 18:45:06 PST:
Looking this over, I forgot to mention a two things, namely field of view and hyperfocal point. The field of view (FOV)is basically the angle required from the point of the observer to see the distance object. I find FOV to be more meaningful than magnification, a term typically used in binoculars. To figure out what FOV is required, simply compute the angle based on a topo map. Use Nellis as an example. Since the USAF acquired the land where the observers used to park, there are basically two locations for viewing, namely from the front at Las Vegas Blvd, and from the back at Hollywood Blvd. The front viewing spot is more or less here: The Hollywood observation point is at N36.21817 W115.02509, shown here: Televue has the eyepiece equations here: The hyperfocal distance is where to shoot to get maximum depth of field. [We are back to photography here, not spotting scope use.] For Tikaboo, everything is pretty far away and virtually at infinity as far as the optics are concerned. However, for a small refractor (gay 600mm) and a 4x barlow, if you focus on an object merely 3 miles away, the depth of field will extend to infinity. For Tikaboo shots, I focus on the last set of hills that you see in the typical shots. I believe these are about 10 miles away, so if those hills are sharp, everything behind them will be sharp. You could try to focus on buildings on the base, but due to the distance, the image is not sharp and thus it is hard to tell when the focus is optimal. It is easier to focus on something 10 miles away, i.e. the hills. You can always check this focus by looking at the base and seeing if you can alter the focus to make it look sharper.
A long plane on the runway (such as a B1B), would required a FOV of 3.4 degrees, or a 600mm lens. For photography, this would be prime focus (i.e no additional magnification) for most refractor telescopes. For a small fighter plane at the same distance, even a 1200mm won't fill the image. The FOV should be about 0.6 degrees. Note that with a photograph, you can generally pluck out the tail number, so you don't need to fill the image plane, but if you want to record tail numbers with your eye, it is best to get close to the target FOV.
Planes near the DOE ramp need a FOV of about 1 degree. Fighters on their ramp need a FOV of about 0.5 degree.
You need to select the pop-up. The FOV depends on the style of eyepiece. [You don't have to buy Televue eyepieces, since the chart is pretty generic, i.e. just look at a Plossl to get an estimate FOV.] If you add a barlow, the FOV is reduced by the gain of the barlow, i.e. 2x barlow has half the FOV. For Groom viewing from Tikaboo, I'd say keep the FOV in the 0.8 to 1 degree range since the viewing quality won't warrrant a smaller FOV.
In Reply to: Re: Telephotography posted by Jason on June 14, 2004 at 18:48:14 PST:
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