Pls help me to ans some qes regarding scopes

[Jingguo]

Hi Guys, Jingguo here. Thanks 4 helping me with those ans d previous time. I came across this problem recently while reading something on scope. 1st) How does focal ratio affect us when we view the scope? 2nd) what does it means by "rich view" telescope?? 3rd) What does it mean by maximun theoretical field of view?? n how does d angle affect our visual?? 4) How does d eyepiece affect the telescope's true field of view? 5) How do the magnification of the scope be caculated?? n how it affect our visual?? 7) How do we test the optical quality of the scope?? what does it mean by inside and outside-of-focus star test? thanks alot

[nF]

For question 1) Refer to http://www.singastro.org/viewtopic.php?p=1080

For question 2) A "rich view or rich field" telescope is a low power telescope with immense fields of view that shows good contrast between the stars and the background sky, stars show up as brilliant sharp points of light. It offers some of the advantages of binoculars where large areas of the sky may be scan through them.

Will try to answer the others questions later in the afternoon, need to catch some winks.

[ChaosKnight]

Suggested answers:

(1) Focal ratio is the ratio of focal length to aperture diameter. It helps when it comes to photography. A low focal ratio (eg. f/4) scope will require shorter exposure time for a picture than a higher (eg f/9.8) one. Usually a low f-ratio is an indication the scope is meant to be a rich-view scope, because the magnification is lower than one of similar aperture but longer focal length.


(2) Rich-view usually means the telescope will give low magnification, but nice, bright images when it comes to extended objects.

(3) i don't know.

(4) Each eyepiece has an apparant field of view that is fixed. The actual field of view when using the eyepiece on the telescope is the apparant field of view divided by the magnification.

(5) Magnification is the focal length of the telescope divided by the focal length of the eyepiece. Higher magnification only affects extended objects. The object will appear bigger but dimmer, because the light is spread out over a larger area. Stars still look the same because they are not considered extended objects.

(7) Put your telescope lens/mirror in a glass of sugar solution. If you see rainbow colors coming through then the lens/mirror is damn good. Nah.....just kidding. Don't do this.

(8) I used this last time to check for proper alignment of my Newtonian mirrors. A defocused image of a star should be a perfect circle with the shadow of the secondary mirror exactly in the center. I think the inside/outside focus star test can be used to test if the cell holding your mirror/lens is so tight it distorts your optics. Inside of focus, the image is an ellipse. Outside of focus, the image is an ellipse with major axis perpendicular to the first instance. Not too sure on this though.

[ChaosKnight]

Is there something wrong? Howcome all my number 8 (eight) appear as face-with-sunglass?

[Airconvent]

Is there something wrong? Howcome all my number 8 (eight) appear as face-with-sunglass?

because the system recognises an "8" followed by a ")" and a man with sunglasses!

[weixing]

Hi,

1st) How does focal ratio affect us when we view the scope?

Basically, the focal length is use to determine the magnification of your scope. A short focal length scope can generally reach low magnification more "easily" than a longer focal length scope and a long focal length scope will reach high magnification more "easily". Below are some example:
Scope A: 500mm focal length - eyepiece: 40mm=12.5x, 20mm=25x, 10mm=50x and 5mm=100x
Scope B: 1500mm focal length - eyepiece: 40mm=37.5x, 20mm=75x, 10mm=150x and 5mm=300x.
So if you are mainly observe planets, a long focal length scope is more suitable for you since you will normally use high magnification. Also a short focal length scope is generally more expensive than a long focal length scope of the same aperture, because it is more difficult to manufacture a short focal length scope.

2nd) what does it means by "rich view" telescope??

Normally is call as Rich-Field (wide Field-of-View) scope. It is generally a scope with short focal length, so it normally is use in low magnification.. therefore have wider Field-of-View. Quality of scope is not in the requirement, so the quality will not be good if you get a cheap one.

3rd) What does it mean by maximun theoretical field of view?? n how does d angle affect our visual??

Not quite sure what you mean... but I think you mean the maximum Field-of-View that you can get from a scope. The maximum Field-of-View is generally depend on how low your magnification can reach and the apparent Field-of-View of that eyepiece. A wider Field-of-View can let you see more area in the sky without lowering your magnification.

4) How does d eyepiece affect the telescope's true field of view?

See formula: True Field-of-View = Apparent Field-of-View / Magnification

5) How do the magnification of the scope be caculated?? n how it affect our visual??

See formula: Magnification = Focal length of Telescope / Focal length of Eyepiece.
The higher magnification you use, the object image is bigger, dimmer and True Field-of-View is smaller than at lower magnification.

7) How do we test the optical quality of the scope?? what does it mean by inside and outside-of-focus star test?

A star test can be use for checking telescope optics, but requires careful examination to reach a correct interpretation. I think the best way to test is to compare it with a good scope of the same aperture, focal length and using the same eyepiece. The inside focus mean very slightly inside the focus point and outside focus is mean very slightly outside the focal point...ur...understanding?? If not, don't know how to explain... May be someone will give a more clear explanation.

Hope I answer your questions. Have a nice day.

[sojourn88]

you may be interested in a little EXCEL file I wrote.. I have uploaded it into one of the yahoo groups.
http://groups.yahoo.com/group/Stellarvu ... epiece.xls