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Old Thursday 14th December 2017, 20:18   #26
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The Swaro extender was introduced chiefly to offer a higher magnification option with the BTX. It gives the BTX 55x with the 65 and 85 mm objectives and 60x with the 95 mm.

The extender also works beautifully with the ATX, turning a 95 into a 50-120x zoom scope.

A big binocular would have its advantages, but good ones are very expensive (even compared to the BTX) and much larger and heavier.

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Old Saturday 16th December 2017, 04:00   #27
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Originally Posted by jring View Post
Hi Alan,

nice to hear we agree that long focal length EPs tend to have lower apparent field of view due to the limitation of the true field by the field stop diameter, as I wrote before.

It just seems we have a different definition of soda straw view - I have only heard this as a moniker for very low afov views like in a 40mm ploessl limited by the 1.25" barrel or other simple EP designs.

Regarding your bins - if the majority have 65 deg afov, you seem to like wide field bins - like me.
Unfortunately in my modest collection only the great Nikon E2 8x30 with 8.8 deg tfov, the also quite nice KOMZ 6x24 with 11.4 deg tfov and an old and not very good JB22 (Itabashi Kogaku Kikai Seisakujo Inc.,Tokyo) 7x50 with 10 deg tfov make the criterion...


I agree that all things being equal, and at a given magnification wider apparent field is better. However, I think more in terms of actual field of view, both for astronomical and terrestrial use. For instance, consider looking at the Double Cluster in Perseus. In a large telescope, say a 28" reflector, even a low power eyepiece with very wide apparent field is not going to frame the object as pleasingly as the same eyepiece in a smaller instrument (assume similar F ratio). The minimum magnification of the large instrument will be quite high giving a very small actual field.

I once experienced an extreme case of this. We were looking at M13, the globular cluster in Hercules using the Mount Wilson 60" telescope. I thought we were pointed to the wrong object, because all I saw was an extended open cluster. Then I looked through the "finder scope," an 8 inch Schmidt Cassegrain, and sure enough there was the familiar object. It was just that with the extreme magnification of the 60" we had resolved the core of M13 and couldn't see its shape/boundaries.

My soda straw analogy was again largely referring to actual field of view. For instance, if you are looking through a telescope and have a 1 degree actual field of view, there are something like 20,000 unique places to point the telescope in a hemisphere. Thus finding something in the sky and pointing the telescope at it is like searching for something "through a soda straw." That is why we tend to use finders or low power eyepieces on smaller telescopes to get onto the object we want to view.

One last example. My 7x50 binocular have just over 50 degree apparent field of view, and my 15x45 binoculars have about 67 degree apparent field of view, yet it is considerably easier to find things in the 7x50 than in the 15x45, because the 7x50 has a 7.5 degree actual field of view and the 15x45 has a 4.5 degree field of view. The 7x50 actually sees 2.8=(7.5/4.5)^2 as much sky as the 14x45.

Again, I agree with you that wider apparent field is pleasing, and also different observers have different preferences for what sort of magnification and eyepiece designs they prefer.

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Old Saturday 16th December 2017, 16:27   #28
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It may be that the Seeing or steadiness of the air in the San Francisco Bay area is never good. In which case a good telescope won't really help.
However, sometimes it might be good.

A colleague has just bought a Skywatcher Evostar 120mm ED Pro refractor (1160 Sherwoods for instance) and Sky Tee altazimuth mount (~400).
This seems good value for an Apo or semi Apo 900mm focal length f/7.5.
The specs say it will take up to 360x magnification, which I see no reason to doubt.
I await his reports.
Some say the dovetail bar is too short etc. and I think one may need counterweights as the mount will support up to two 15Kg telescopes.

I would choose this over a Swarovski 95 or Kowa 88mm.

With regard to large binoculars or a binocular adapter the problem is that visitors don't get the IPD or the dioptre correction right and I think they are much better off with a single eyed view.

It would not surprise me if the view at night over the Bay at say 10 p.m. would be good.
There is no reason not to view then as there must be lot to see in a brightly lit city.

Last edited by Binastro : Saturday 16th December 2017 at 16:30.
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Old Sunday 17th December 2017, 06:22   #29
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We are likely to visit the Bay Area in the next few weeks. I will probably bring my small spotting scope along as I hope to visit Point Reyes in addition to meeting my newest grandchild in Berkeley. Perhaps I will have a chance for an East Bay view of San Francisco while I am there. If so, I will report back.
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