Why higher magnification for planets?

[gary19]

OK, I admit it, I just don't understand why you'd use something like a 5mm eyepiece to give you higher magnification for planets but use a 25mm for lower magnifications for DSO's.

To my reckoning the DSO's would need higher magnification because they are so much further away.

What am I missing here?

There I've done it and published my ignorance.

[toml42]

Despite being further away, DSOs tend to be much, MUCH bigger, so they subtend a greater angular size, the andromeda galaxy, for example, takes up an angular size equivalent to 6 full moons!

[Earl]

The higher the magnification the smaller the area you are looking at.

Planets are small so the higher magnification makes them fill more of the eyepiece.

As toml42 says DSO objects even though much fainter are larger in the sky and the smaller visable area will stop you from seeing the whole thing.

[GazOC]

Also, every time you double the mag you make the objects 4 times dimmer because planets are generally brighter than DSOs they can take more magnification before they become too dim.

[Digz]

The easiest thing for me is to remember our planets are in our Solar system within the Milky Way (our galaxy) if your looking at the Andromeda Galaxy (clue is in the name) then there will be similar solar systems with similar planets contained within. Hopefully this will begin to show the relative size differences and hence magnification requirements.

Of course everything said above is spot on and you must remember that I an new to this as well

[brantuk]

The moon is several hundred miles diameter and only 1/4 of a million miles away. Whereas Messiers can be thousands of light years high and wide and millions of light years away. A 5mm trained on M42 would just show a blur of light - trained on the moon however you'd see some nice fluffy craters

[just looking]

Thanks Gary for starting this thread

Fasinating read

Thanks for this guys,

Learning things on here all the time

so Pleased I joined but yet so much info to take in.

chez

[sardaukarr]

handn't thought about it that way i always thought you use less mag for more light

[great_bear]

I always thought you use less mag for more light

That's a consideration too, yes.

[LukeSkywatcher]

Reading this thread i cant help but think of this:

YouTube - Father Ted - On Holiday

[gary19]

That Father Ted made me laugh! Thanks. Small vs further away.

[Digz]

I cant get onto youtube at work, but 'Im guessing its the conversation about the cows?

'These cows are small whereas those cows are faaar away!'

[great_bear]

That's the one... (good guess!)

[Digz]

I love that scene, so funny. Im gonna have to take a look when I get home

[themos]

Typical planet is arc-seconds wide, typical DSO is arc-minutes wide (apart from "planetary nebulae", I guess). One arc-minute is 60 arc-seconds.

[acey]

Everyone agrees about the value of high magnification on planets; with DSOs it's more controversial.

As has been said, many galaxies are of large apparent size, and magnifying a galaxy makes it dimmer. But magnification also makes the background sky look darker, so that the contrast of the object against the sky remains the same (and is generally pretty low).

The eye detects low-contrast objects best when they are of a certain size (roughly half a degree, or the size of the full Moon seen with the naked eye). Too small or too large, and the object may not be seen at all. If you want to see an entire (relatively nearby) galaxy as a bright blob then low power is best, as this will deliver the brightest image. But if you want to see fine details in galaxies, or galaxies that are very far away (and hence of very small apparent size) then higher power should be used in order to make things large enough to discern at low contrast.

A sensible procedure is to acquire the object at low power, then work up to the highest useable power. I mostly look at small, distant galaxies, and mostly use the same high magnification I would use on planets. At low power these galaxies often are simply not seen - they're too small. On nearby galaxies I use high power in order to see details.

Stephan's Quintet, for example, is a group of five galaxies squeezed into about five Jupiter-widths. The inner spiral arm of M51 is one or two Jupiter-widths from the galaxy's centre. These call for high power.

On the other hand, if you're looking for the large, low-contrast galaxy M33 in a light-polluted sky, then you may want to use the lowest possible power, because if the object is too large when seen through the scope then the eye will be unable to detect it at all. Similarly, in a dark sky, large low-contrast objects such as Barnard's Galaxy may only be visible at low power.

[gary19]

Acey, that I can understand. If you want to see the detail in (relatively) small galaxies you will want high power. Larger galaxies will require lower power to obtain a wide field of view and get the contrast.

[abumuhannadh]

Reading this thread i cant help but think of this:

YouTube - Father Ted - On Holiday

i got this trying to view that

"This video contains content from MyVideoRights, who has blocked it in your country on copyright grounds."

damn.

[acey]

Acey, that I can understand. If you want to see the detail in (relatively) small galaxies you will want high power. Larger galaxies will require lower power to obtain a wide field of view and get the contrast.

You got it.

Putting it a bit more technically, when you look at an extended object with a telescope at its lowest useable power, the surface brightness as seen through the scope is (theoretically) the same as the surface brightness as seen with the naked eye. Magnifying further will reduce the surface brightness further (by the square of the magnification increase).

So the brightest view is the one you get with the lowest power, and in real life it's not equal to the naked-eye surface brightness but only about 70% or less (because of light loss in the telescope).

But magnification lets us see things more clearly, so there's a trade-off between brightness and contrast sensitivity. One rule of thumb (suggested by Nils Olof Carlin) is "raise magnification until the object is about half a degree, or until the background sky is so dark you can't see the field stop - whichever comes first".

Increasing aperture gives you the ability to take the same light and magnify it over a larger area. So a larger aperture telescope shows more detail, and fainter galaxies, than a small aperture one. What counts as medium or high power for a small scope (with consequent loss of brightness) may be the lowest useable power for the larger scope, which therefore gives a much brighter view.

You can find your scope's Brightness Factor (and limiting magnitude, among other things) by plugging in the numbers here:

N.A.A. Telescope Math Calculator

I haven't studied it too closely but I think they assume 70% as the transmission factor, so if you put in your aperture and the focal lengths of scope and lowest power eyepiece, you should get a figure of 0.7 or less. Key in your higher power eyepieces and the number will go way down. With my 12" scope I do most galaxy viewing using a 6mm TMB Planetary (the same eyepiece I use for planetary viewing). This gives images with a surface brightness only 4% as much as the naked-eye view. But there's no way my naked eye can see something as small as Stephan's Quintet or the spiral arm of M51. The 6mm eyepiece makes these things big enough for me to see clearly, so it's a fair trade-off. If I were to use the same eyepiece in a 6" f8 scope I'd get a Brightness Factor of only 1.5% and the views would probably be too dim to be useful.

The relationship between magnification, contrast and DSO visibility was studied very extensively by Roger Clark in his classic book Visual Astronomy Of The Deep Sky. His work is very mathematical but the upshot is that he found there is an "optimum magnification" for DSOs. In practice you just work up the powers until you hit it. Clark's website has more details:

Visual Astronomy of the Night Sky

[sardaukarr]

hehe lets have a screaming contest

[ringz]

Arghhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

[tarqs101]