how much more can you see with a 90mm over a 60 mm refractor telescope?

Answer 1

My guess would be 50% more

Answer 2

In terms of stellar magnitudes you will gain about 1 magnitude. You will get some extra help in seeing dim deep sky objects, too.

Equally important, however, is resolving power: the ability to see two close objects as distinctly double rather than as a single object. The ability to "split" a double star is a case in point. This resolving power is called the Dawes Limit, named after an amateur of the 19th century. In seconds of arc it is 116/objective in mm. So the DL for your 60mm is 1.93 arc seconds. Objects closer together than this will be seen as a single object. Your 90mm has a Dawes Limit of 1.29 arc seconds.

To see what this might mean, let's do a back of the envelop calculation.

The diameter of the moon = 3476 Km
Apparent size of the moon = 1800 arc seconds (30 arc minutes)
So, 1 Km = 1.93 arc seconds.

You 60 mm objective will not resolve mountains or craters which are closer together than 1 Km, but your 90 mm will. In truth, modern quality optics can usually exceed the Dawes Limit (it is experimentally determined, after all). So, you will get, very roughly, 50% more detail with the 90mm than with the 60mm (1.93/1.29).

Will you be impressed by this. I would be careful about it. 90mm is really better than 60mm, but impressively better? Probably not. If you are, however, a double star watcher, or a variable star watch, then you will appreciate the 90 more than the 60.

As someone else pointed out, you will be even more gratified if you jump to a 150mm or 200mm reflector.

HTH

Charles

Answer 3

You get 2.25 times the light gathering and 2.25 times the resolution.

That said, you can see slightly dimmer stars, but the real action would be planetary and nebula observation - 2.25 times the detail. Think of it like this - would you rather have a 15 inch monitor at 800x600 or would you rather have it at 1024/768? The image in your eye is like the size of the monitor (15 inches) the resolution is like the pixil count - that's a function of the area of the mirror or lens of a scope.

With stars, you can resolve much anyway - they all look like dots.

On the other hand, with nebula, the more magnification you have, the dimmer the object will look - you are taking the same amount of light from the nebula and spreading it out over a larger area on your retina. It will still be brighter in the larger scope, but the dimming effect of the magnification will wash that out.

Planets however don't really dim much - there is alot of light to work with.

Answer 4

Understand that magnification and view area are different.
The diameter of the lens will dictate what you see.

Area of circle 1 = π r²
Area of circle 1 = π (60 x 10^-3)²
Area of circle 1 = 0∙011 309 733 ... m²

Area of circle 2 = π r²
Area of circle 2 = π (90 x 10^-3)²
Area of circle 2 = π (90)²
Area of circle 2 = 0∙025 4469 ... m²

Now get the ratio of the circles.
Circle 1/ Circle 2 = 0∙011 309 733 ... m²/0∙025 4469 ... m²
Circle 1/ Circle 2 = 2∙25

The ratio is; 1 : 2∙25 that is, you will see 125% more with the bigger lens then the smaller lens.

Answer 5

Fat Albert, don't guess.

It is the area of the object lens that is the factor not the linear dimension. people are so one-dimensional. Light enters the lens over an area.

So, the 90mm to 60mm, is like 90x90 to 60x60

or simply 9x9 to 6*6

i.e. ratio of 81 to 36

i.e. over twice the resolving power.

It means you could see stars about 1 magnitude dimmer with the 90mm. That could treble the number of stars you could see.

Answer 6

The light gathering power is the ratio of the apertures so the ratios of apertures of the 90 to the 60 is about 2.25. Theoretically you should be able to see things that are one stellar magnitude lower. In realty, they're both pretty small so the difference won't be dramatic.

Answer 7

Hi. The resolution will be better in the larger objective, all else being equal, so slightly finer lunar detail and planetary detail. A jump up to a 6" or 8" reflector would be more dramatic.

Answer 8

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Answer 9

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