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Old Thursday 14th December 2017, 19:18   #26
kabsetz
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Joachim,

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.

Kimmo
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Old Saturday 16th December 2017, 03:00   #27
ailevin
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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...

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Joachim,

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.

Alan
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Old Saturday 16th December 2017, 15:27   #28
Binastro
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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 15:30.
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Old Sunday 17th December 2017, 05:22   #29
ailevin
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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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Old Tuesday 13th February 2018, 15:11   #30
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My T.V. repair man and antenna expert told me that Malta can be seen from Sicily, and I note also vice versa.
The distance is about 60 miles.
Probably 20 or 30 years ago for him. He used to repair Alfa Romeo cars.
He cannot sleep without the T.V. on all night. For me I need silence.
I found this out as we both watch Inspector Montalbano and the name Camilleri was mentioned in Sicily, and I know this surname is used also in Malta. It may be they understand each other's language?

A newspaper report from someone in Malta in 1922 with a 2.5 inch refractor say trees and buildings were seen and a church and steeple.

It seems that it is more common to see the lights of Sicily at night from Malta, a photo shows very many over a wide expanse, than seeing Sicily in the daytime when conditions are usually worse.

So clearly in the right conditions ten miles is a doddle.
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Old Wednesday 14th February 2018, 09:18   #31
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Originally Posted by Binastro View Post
My T.V. repair man and antenna expert told me that Malta can be seen from Sicily, and I note also vice versa.
The distance is about 60 miles.

A newspaper report from someone in Malta in 1922 with a 2.5 inch refractor say trees and buildings were seen and a church and steeple.

It seems that it is more common to see the lights of Sicily at night from Malta, a photo shows very many over a wide expanse, than seeing Sicily in the daytime when conditions are usually worse.

So clearly in the right conditions ten miles is a doddle.
Hi David
According to internet sources at sea level the distance to the horizon is about 3 miles, on top of a hill 100ft high its about 12 miles. But we can see the coast of Ireland from The Oa on Islay at sea level and that is about 25 miles. Still, I would have thought 60 miles would need viewing from a fair altitude.

Lee
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Old Wednesday 14th February 2018, 14:51   #32
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Hi Lee,
Strictly speaking at sea level the horizon distance is zero.
A person with eyes 5ft 6 inches above sea level the horizon is 3 miles.

At 60 miles the height needed to see the sea the other end is about 2,200 feet assuming no mirage effects.
However, a person at 550 ft height above sea level would see a building say at 550 ft the other end.

Sicily and Malta I think are quite hilly.

I had two astronomers from Malta turn up about 1980.
The had made a fine telescope from a 6 inch aperture objective from a periscope from a scrapped British submarine in Malta. I think there was a lot of this stuff lying around in the 1950s and 1960s.
They paid all of 5, possibly a Ross objective?

Unfortunately it broke and they wanted a replacement for near a fiver. I had nothing to offer them.
6 inch aperture good long focus objectives were expensive in 1980.
I gave them some quite good Plossls, so they wouldn't go away empty handed and maybe some other goodies.
I found about 30 of these Plossls, which cost a total of flumpence. I have no idea what they were for.
A local machinist made 30 brass mounts for them and they were good eyepieces, fairly short focus.

There are quite a few good astronomers on Malta.

The photo shown of Sicily from Malta at night clearely shows lights probably up to 70 miles distance along a very wide vista.
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Old Wednesday 14th February 2018, 15:12   #33
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It seems that the highest point on Malta is only 550ft above sea level.

However, in southern Sicily there are several mountains around 3,000ft or a bit more.
It may be that even the southern Sicily coast has cliffs. If so this may be where the wide vista of lights are situated.
Sicily has quite a lot of high mountains.

P.S.
Another source list cliffs in Malta at 830ft above sea level.

I often viewed Southend on sea or maybe Westcliff from the north Kent coast with my 25-40x55 Broadhurst Clarkson draw tube scope. 22 miles.
My viewpoint was about 35ft above near sea level. However, the tide sometimes went out about 500 yards.

Last edited by Binastro : Wednesday 14th February 2018 at 19:21.
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Old Friday 16th February 2018, 17:20   #34
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Oupus, I was out yesterday (16 Feb) standing about 300' W. of the Atlantic shoreline overlooking a bay between this barrier island and the mainland - about 1/2mi to the West. You would have thought I was in the Australian desert for the rippling in the air. You may better luck at altitude... but I would borrow a pair of binocs or a scope, like Ailevin said, before making the investment.

Jim
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Old Friday 16th February 2018, 21:40   #35
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Hi Jim.
What was the approximate temperature of the water, the land and the air?
Was there any wind?
Was the pressure changing rapidly or constant?
What time of day also?

Local conditions vary enormously.

As an astronomer, I look at the weather. If conditions are poor I don't even bother to get a scope out, even if clear.
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Old Tuesday 27th February 2018, 22:03   #36
aeajr
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Originally Posted by opus914 View Post
I live in the east bay area of San Francisco and have a view of the city across the bay, a distance of about 10 miles (as the crow flies, as measured by Google maps). Not sure whether to go with a spotting scope or binoculars. Any recommendations? Thanks!
Opus,

I am curious if you ever got your scope and what you purchased.

What level of detail were you trying to see. As this is a bird forum, were you trying to identify birds at 10 miles? Where you successful?
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