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Old 10-12-2010, 10:40 PM   #3076
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Good gravy, that is a lot to remember. If it is OK with you I would like to start a file so I can go back and read without having to try to find your tips each time. We really do appreciate you doing this for us buddy.

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Old 10-12-2010, 10:42 PM   #3077
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That's all good information to know. Thank you!

I hope this isn't a stupid question, but does the focal ratio of 4.5 in a telescope mean the same thing as a camera set at f4.5?

How do companies get away with advertising telescopes with such high magnification? I'm glad to know that those numbers don't mean anything because it sure would be easy to fall for their tricks otherwise.
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Old 10-12-2010, 10:46 PM   #3078
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Good gravy, that is a lot to remember. If it is OK with you I would like to start a file so I can go back and read without having to try to find your tips each time. We really do appreciate you doing this for us buddy.
Jim, if you should end up getting yourself a copy of Terence Dickinson's book "Nightwatch", all this and a hundred times more will be in there. I can't recommend it highly enough. It's sort of the "bible" of amateur astronomy.

One of its' best features - of which there are many - is that it has sky maps with the locations of dozens of the most interesting objects, as well as all the constellations. No matter where you may be on earth, all the info is perfectly valid, although most is directed at what is called the mid northern sky. That's you and me.
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Old 10-12-2010, 11:04 PM   #3079
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That's all good information to know. Thank you!

I hope this isn't a stupid question, but does the focal ratio of 4.5 in a telescope mean the same thing as a camera set at f4.5?

How do companies get away with advertising telescopes with such high magnification? I'm glad to know that those numbers don't mean anything because it sure would be easy to fall for their tricks otherwise.
Hmmm...that's a good question. The ratio on your camera refers to the f-stop, which I do not believe has anything to do with the focal ratio at all. What that has to do with is the amount of light you want to let into the camera. So on a very bright day you let in less light, both by the f-stop and length of exposure - say 1/1000 of a second for example, and f/10...and for a very dull day, you might have the f-stop at 2.8 and the exposure at 1/100 second.

So focal ratios in a telescope refers to a physical length.

As for the advertising of such ludicrous powers, they are actually telling the truth. They might give you a 4mm eyepiece and a 3x Barlow lens with a skinny little scope of perhaps 600mm focal length. This actually yields a power of 450x.

The only problem is that all you could possibly hope to see through something like that would be fuzz.

Here's a general rule of thumb for how much power any telescope can handle. Fifty times the aperture in inches (let's say 4" for example) so 50 x 4 = 200. In order for a 4" scope to be able to handle a magnification of 200x would require exquisite optics, the best eyepiece and perfect "seeing". In the real world you can cut that fifty down to twenty five times.

Theoretically, my 10" scope could swallow 500 power. Not a chance. No matter how clean the optics, or how good the eyepiece - and I have some of the very best in my arsenal - I would consider myself lucky to get 250x clearly. You will almost invariably find that 100x or lower will give you the best views.

There are targets you will want higher power on - looking at the cloud belts on Jupiter for example - and many others. But as the power goes up, the brightness of the object goes down.

Last edited by cocobolo; 10-12-2010 at 11:07 PM. Reason: spelling
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Old 10-12-2010, 11:51 PM   #3080
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You were talking about gravity the other day, I got a question, is the bigger the planet or star the greater the gravitational pull on an object?

Lord willing I will get that book.
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Old 10-13-2010, 12:07 AM   #3081
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You were talking about gravity the other day, I got a question, is the bigger the planet or star the greater the gravitational pull on an object?

Lord willing I will get that book.
As a general rule...yes.

Take our moon for example. It is much smaller than earth, and has only about 1/6 the gravity that earth does.

Jupiter on the other hand, is far bigger, and it's gravitational pull stretches out for literally millions of miles.
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Old 10-13-2010, 12:35 AM   #3082
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You were talking about gravity the other day, I got a question, is the bigger the planet or star the greater the gravitational pull on an object?

Lord willing I will get that book.
Here's the book your looking for,
http://www.amazon.com/NightWatch-Pra...6948087&sr=8-1
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Old 10-13-2010, 01:45 AM   #3083
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Thanks DD, I appreciate the link, I bookmarked it. I read the first page and the first sentence answered a question I just asked Coco. That is a must have book. Thanks again buddy.

By the way DD, I like your new avatar
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Old 10-13-2010, 08:11 AM   #3084
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Gravitational pull is a function of the mass of the objects and the distance between them, if I recall.
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Old 10-13-2010, 11:46 AM   #3085
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Gravitational pull is a function of the mass of the objects and the distance between them, if I recall.
This is going to take some thinking for me, I am a little thick. I do appreciate it Jl.

I got a question I have been curious about for years. I know basically how a maginet works to draw metal to it and I know gravity draws objects. How does gravity work? I remember something back in science class but not much. Can anyone explain how gravity can have a pull on an object that isn't metal?

Be easy on me now, I am not the sharpest tool in the shed.
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Old 10-13-2010, 08:31 PM   #3086
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Originally Posted by jiju1943 View Post
This is going to take some thinking for me, I am a little thick. I do appreciate it Jl.

I got a question I have been curious about for years. I know basically how a maginet works to draw metal to it and I know gravity draws objects. How does gravity work? I remember something back in science class but not much. Can anyone explain how gravity can have a pull on an object that isn't metal?

Be easy on me now, I am not the sharpest tool in the shed.
Jim, your question raised my curiosity level considerably. I understand the effect that gravity can have between one object and another, assuming that one of the objects has a substantially measurable gravitational pull.

To that end I just looked on several physics sites to see if I could find some sort of standard answer. I could not. Lots of discussion, up to and including black holes, which get so dense I simply do not understand it. Even light cannot escape from a black hole.

From our perspective, let's just be thankful that Jupiter is out there keeping the space around the Earth clear of marauding rocks and so forth...at least for the most part.

Do you remember the fairly recent comet which broke up into many pieces from the gravitational pull of Jupiter and crashed into it's surface? It was one of the Shumaker-Levy comets.
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Old 10-13-2010, 08:56 PM   #3087
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Jim, your question raised my curiosity level considerably. I understand the effect that gravity can have between one object and another, assuming that one of the objects has a substantially measurable gravitational pull.

To that end I just looked on several physics sites to see if I could find some sort of standard answer. I could not. Lots of discussion, up to and including black holes, which get so dense I simply do not understand it. Even light cannot escape from a black hole.

From our perspective, let's just be thankful that Jupiter is out there keeping the space around the Earth clear of marauding rocks and so forth...at least for the most part.

Do you remember the fairly recent comet which broke up into many pieces from the gravitational pull of Jupiter and crashed into it's surface? It was one of the Shumaker-Levy comets.
Man, this just gets better and better, I had no idea that Jupiter did that but I can sure see how it does. I am not sure I remember the comet crashing into Jupiter, was anyone able to see that happen? That had to be really exciting, I know it would have been for me.
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Old 10-13-2010, 10:22 PM   #3088
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Man, this just gets better and better, I had no idea that Jupiter did that but I can sure see how it does. I am not sure I remember the comet crashing into Jupiter, was anyone able to see that happen? That had to be really exciting, I know it would have been for me.
We knew that this comet was going to hit Jupiter, and all sorts of telescopes were trained on the planet in the hope of seeing something.

Comets are nothing more than a collection of ice and mud basically, and many of them are pretty small. Comet Hartley 2 if I recall is thought to be only about 1 kilometer across.

I do have the magazines here with the impact story well written up...let me see what I can find and I'll get you more details. As the comet approached Jupiter, the extreme pull that Jupiter exerted on the comet literally pulled it to pieces.
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Old 10-14-2010, 12:01 AM   #3089
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Coco's astronomy tip of the day (No. 9)

Eyepieces...the real meat and potatoes of how you see what you see.

Any telescope is worthless without an eyepiece. And luckily, there are literally thousands of different eyepieces out there for every possible astronomical need. But not to worry, you only need a few.

Yes, I'm sorry, no single eyepiece will fill the bill for everything. But a small collection of low cost, high quality eyepieces will get you off on the right foot.

The two basic sizes of eyepieces are 1 1/4" - which is by far the most common - and 2", which is designed (for the most part) to fit larger telescopes. Yesterday, we touched briefly on eyepieces. So, let's delve a little deeper now into these cunning little creations.

Eyepieces vary in cost from cheap ($10 or so) to very expensive ($700+) and this would be for a single eyepiece! But don't worry if you are not an Arabian Sheik with a country full of oil wells, some of the best eyepieces are truly reasonably priced.

Any given eyepiece will provide you with what is called "the apparent field of view". AFOV for short. Typical fields are 50 to 70. At 70 AFOV you are entering what is known as wide field territory. Plus, as the AFOV gets wider, the price usually climbs...rapidly! What all this means is that as the AFOV gets wider, you can see more of the sky at the same power.

Here is a small collection of eyepieces.
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Old 10-14-2010, 12:08 AM   #3090
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The eyepieces above range from 6.4mm to an astonishing 72mm. The small one fits a standard 1 1/4" barrel (in the focuser) and the big one (72mm) fits a 2" barrel.

I stumbled across the big e/p on the web one day and thought it must be an error, as the biggest size I had seen mentioned before was 50mm. But after an email to the seller, I was assured this was quite correct.

This is a very low power e/p and gives terrific views of larger objects...like the Orion nebula for example.
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