Viewing Jupiter.

[steven_w82]

Hi all!

Was reading up about what I can expect to see though a beginner scope, and got quite excited when I read the following quote 

http://www.gizmag.com/top-five-astronomical-objects-new-telescope/25600/

'Even though Jupiter is currently about 630 million km (390 million miles) away from Earth, 40X magnification will make it appear the size of the Moon in the night sky'

My scope is the Celestron travel scope 70mm refractor.  It comes with 3 eyepieces, a 20mm, 10mm, and 4mm.  The focal length of the tube is 400mm, so dividing 400 by 10 yields 40x magnification.  It was nowhere near the size of the moon for certain, although I could make out the disk of the planet and it's moons!  Did I get my simple maths wrong in some way, or is it just a really dodgy article?

[Qualia]

Warning: Math is not my strong point, so what I write may be completely wrong, but however tentative, let's give this a go

I imagine what the writer of the article has done is to say to himself: on the one hand, the full Moon subtends a half-degree of sky (1800 arcseconds). However, Jupiter only subtends about 45 arcseconds of sky. To go from 45 arcseconds to 1800 arcseconds, all I have to do is divide the latter by the former and I find that I only need to magnify Jupiter by 40x to make it appear to subtend 1800 arcseconds. So accordingly, when I write my article I will just say that it only takes 40x magnification to make Jupiter appear the same size in the eyepiece as the full Moon does to the naked eye.

So far, so good but to my way of thinking that isn't quite the end of the story ....

We've also go to ask ourselves, why did Jupiter appear so small in Steve's eyepiece, when he was lead to believe that it would be larger?

What we've got to do now is consider how big Jupiter appeared in the eyepiece's True Field of View. As you'll appreciate this is a slightly different question and I hope will lead us to a clearer appreciation of what is going on.

First, we need to know what is your scope's focal length and what are the eyepieces' Apparent Field of View. I think Celestron's 70mm frac has a focal length of 400mm and for the sake of argument, we'll assume that the supplied eyepieces have an Apparent Field of View of a Celestron Plossl, something like 50º.

If this is so, we can work out the eyepiece's True Field of View (TFoV) by dividing the scope's focal length by the eyepiece's focal length giving us Magnification and then dividing the eyepiece's Apparent Field of View (AFoV) by the Magnification number. If we do this, we end up with something like this:

Eyepiece       Magnification       AFoV       TFoV       Arcseconds of TFoV

20mm             35x                         50º            1.43º       5148

10mm             70x                         50º            0.71º       2556

4mm               175x                       50º            0.28º       1008

Equipped with this information and knowing that Jupiter is about 45 arcseconds in apparent size, we now know that depending on the eyepiece used, Jupiter will cover 0.0087, 0.017 and 0.044 of the eyepieces' field of view (45/arcsec TFOV). In all cases, we're not talking big Multiplying these figures by 100 to arrive to some kind of percentage, we can see Jupiter will cover 0.87%, 1.7% and 4.4% of the diameter of what you see in the eyepiece.

To get some idea of what you can expect to see, if you mentally divide the field of view into 10 equally sized stripes or rings, then you'll appreciate that with the 20mm and 10mm Jupiter will barely cover a smidge of one of those imaginary divisions and with the 4mm should just about cover half of one of those divisions.

Needless to say, I hope I've got the figures about right or at least the right end of the argument, so you can at least appreciate the principle going on. This is a rather complex issue and the answer doesn't lie in simply throwing more magnification at Jupiter.

Typically, with something like a 3", some of Jupiter's features can be viewed probably at a sweet spot of around 90x but don't expect eye-piece filling views and don't expect to be able to garner too much detail. For that, you'll probably need a bigger scope in terms of aperture. Nevertheless, regardless of aperture, you have to work at observing to be able to tweak out the given subtleties which are often compromised by the 3 Cs: collimation, cooling and conditions (atmospheric and your own).

To get some idea of what a 3" scope with decent optics can expect from Jupiter, here's a little sketch I made with an APO 76mm:

[cotterless45]

That's neat. I use fruit and veg . Mostly pea size is ok for our seeing / transparency. On a good night it can be a marrow fat pea.

The best I've had is a peeled lychee. Try tangerine and it'll be too much and you definitely don't want an avocado or a banana !

Nick.

[-Aldebaran-]

O.k I've come to the conclusion after reading many threads and stickies by Qualia (should be re-named QUALIfied) he should re-write any and all references /articles relating to the world of astronomy!!

[ronin]

Jupiter is "on average" about 40 arcsec in size, so 40x alters this to 1600 arc sec which is 0.44 degree, which is close to the angle subtended by the moon.

Besides what you look through and how it sits in the eyepiece field the brain has a habit of making brighter things bigger, so the moon appears bigger then it actually is. Head outside at somewhere dark and look at Andromeda, M31, it is some 6x bigger then the moon, just your brain says "No way!". Many will say about the same maybe half as much again.

Remember the Orion nebula from a couple of months ago? That is twice the size of the moon, and possibly a bit more. How many people ask where is M42, no-one asks where is the moon, but both are "bright" and the one everyone knows and finds is half the size of the other.

[Sirius Starwatcher]

That's neat. I use fruit and veg . Mostly pea size is ok for our seeing / transparency. On a good night it can be a marrow fat pea.

The best I've had is a peeled lychee. Try tangerine and it'll be too much and you definitely don't want an avocado or a banana !

Nick.

Can you tell me where the banana fits , in all this !

[RLWP]

Can you tell me where the banana fits , in all this !

It is the standard reference for a quarter moon, of course

Richard

[Dr_Ju_ju]

Following on from Qualia's insight, to get a reasonable view of whet to expect use this site http://www.12dstring.me.uk/fov.htm & dial in the appropriate details.....

[steven_w82]

That's neat. I use fruit and veg . Mostly pea size is ok for our seeing / transparency. On a good night it can be a marrow fat pea.

The best I've had is a peeled lychee. Try tangerine and it'll be too much and you definitely don't want an avocado or a banana !

Nick.

Haha I love this.  This analogy is a litle better for my little pea brain

[steven_w82]

Warning: Math is not my strong point, so what I write may be completely wrong, but however tentative, let's give this a go

I imagine what the writer of the article has done is to say to himself: on the one hand, the full Moon subtends a half-degree of sky (1800 arcseconds). However, Jupiter only subtends about 45 arcseconds of sky. To go from 45 arcseconds to 1800 arcseconds, all I have to do is divide the latter by the former and I find that I only need to magnify Jupiter by 40x to make it appear to subtend 1800 arcseconds. So accordingly, when I write my article I will just say that it only takes 40x magnification to make Jupiter appear the same size in the eyepiece as the full Moon does to the naked eye.

So far, so good but to my way of thinking that isn't quite the end of the story ....

We've also go to ask ourselves, why did Jupiter appear so small in Steve's eyepiece, when he was lead to believe that it would be larger?

What we've got to do now is consider how big Jupiter appeared in the eyepiece's True Field of View. As you'll appreciate this is a slightly different question and I hope will lead us to a clearer appreciation of what is going on.

First, we need to know what is your scope's focal length and what are the eyepieces' Apparent Field of View. I think Celestron's 70mm frac has a focal length of 400mm and for the sake of argument, we'll assume that the supplied eyepieces have an Apparent Field of View of a Celestron Plossl, something like 50º.

If this is so, we can work out the eyepiece's True Field of View (TFoV) by dividing the scope's focal length by the eyepiece's focal length giving us Magnification and then dividing the eyepiece's Apparent Field of View (AFoV) by the Magnification number. If we do this, we end up with something like this:

Eyepiece       Magnification       AFoV       TFoV       Arcseconds of TFoV

20mm             35x                         50º            1.43º       5148

10mm             70x                         50º            0.71º       2556

4mm               175x                       50º            0.28º       1008

Equipped with this information and knowing that Jupiter is about 45 arcseconds in apparent size, we now know that depending on the eyepiece used, Jupiter will cover 0.0087, 0.017 and 0.044 of the eyepieces' field of view (45/arcsec TFOV). In all cases, we're not talking big Multiplying these figures by 100 to arrive to some kind of percentage, we can see Jupiter will cover 0.87%, 1.7% and 4.4% of the diameter of what you see in the eyepiece.

To get some idea of what you can expect to see, if you mentally divide the field of view into 10 equally sized stripes or rings, then you'll appreciate that with the 20mm and 10mm Jupiter will barely cover a smidge of one of those imaginary divisions and with the 4mm should just about cover half of one of those divisions.

Needless to say, I hope I've got the figures about right or at least the right end of the argument, so you can at least appreciate the principle going on. This is a rather complex issue and the answer doesn't lie in simply throwing more magnification at Jupiter.

Typically, with something like a 3", some of Jupiter's features can be viewed probably at a sweet spot of around 90x but don't expect eye-piece filling views and don't expect to be able to garner too much detail. For that, you'll probably need a bigger scope in terms of aperture. Nevertheless, regardless of aperture, you have to work at observing to be able to tweak out the given subtleties which are often compromised by the 3 Cs: collimation, cooling and conditions (atmospheric and your own).

To get some idea of what a 3" scope with decent optics can expect from Jupiter, here's a little sketch I made with an APO 76mm:

Qualia, thanks for taking the time to reply!  Your post was very informative, although I don't understand it and your maths is a wee bit better than mine  

The scope came with a x3 Barlow, which looks and feels cheap.  I assume that a x3 barlow is complete overkill for this?  Should I be getting a decent x2 instead?

[Qualia]

Steven, it's certainly not exact but broadly speaking there are a number of reasons why you can't just up the magnification on Jupiter ad infinitum.

The first is to do with the actual telescope being used. The brightness of a given object is dependent on the aperture of the telescope and magnification. The more aperture you have, the more light you have, so the more you can magnify before the image becomes too dim and detail just becomes impossible to see.

The second is also to do with the telescope being used. What fine detail you can hopefully tweak from a given object, what is termed as resolution, is also a function of the telescope's aperture. In theory, but often contradicted in practice, there is a theoretical upper limit to how much a telescope can magnify before the image becomes too faint and too blurry. 

Typically this limit is reeled off with a pat equation: maximum magnification = aperture in inches x 50. So, my 10" should - in theory - have a resolving limit of some 500x; my little 3", some 150x. However, this is only a theoretical limit and does not work in practice.

The primary limiting factor to obtaining maximum magnification is the Earth's atmosphere. Because we need to peer through the atmosphere to see anything in space, it follows that the more we magnify Jupiter, the more we magnify the atmosphere. Granting that most analogies are sloppy, you can, however, imagine the atmosphere like a big swimming pool; sometimes it is settled and calm, other times it is turbulent and choppy. 

The steadiness of the atmosphere is called the Seeing conditions. When the seeing is good, the atmosphere is steady and Jupiter will look sharp; when the seeing is poor, the atmosphere is turbulent and Jupiter is blurry. 

For something like Jupiter, I find my average-maximum-useful-magnification to be between 140x and 180x. When using the smaller 76mm, I find that it is running out of puff at about 140x.

Moving on....

A 3" f/5.7 scope is not an ideal planetary scope. Being of such a short focal length it relies on very short focal length (FL) eyepieces to gain significant magnification. A longer FL telescope allows one to use longer FL eyepieces, which in turn would give one more eye relief making viewing a more comfortable possibility. Moreover, it is limited by aperture. As such, due to its short focal length - enabling lower magnification for a given eyepiece, and limited aperture - preventing one from resolving too much detail, it is more suited for observing expansive views like open clusters, star fields and the such.

As such, just looking at the numbers, if you want to do some serious planetary viewing, I feel you will be pushing your system and eyes with the 3". This isn't to say that your scope can't look at planets but whatever our telescope, I think it is always helpful to have realistic and reasonable expectations of what is possible. That way, we avoid disappointment and deception.

In the long term, rather than buying more eyepieces for the 3", something like a 6" Dob might serve your better, or again, if you can save just a little more, something like an 8". With these scopes you can start getting some really interesting planetary observations under your belt and move into viewing deep space objects like galaxies, globulars and planetary nebulae with significant success.

[Moonshane]

if you hold a pencil with a rubber on the end at arm's length then I believe that the rubber can 'block out' the moon. it seems crazy but it's apparently true. the moon is actually a lot smaller in the sky than most people think.

[cotterless45]

"Is it large, but looks small because it's further away ?"

On the fruit scale , we're talking water melon and serious big banana here !

Nicko.

[Astro Imp]

if you hold a pencil with a rubber on the end at arm's length then I believe that the rubber can 'block out' the moon. it seems crazy but it's apparently true. the moon is actually a lot smaller in the sky than most people think.

For those of you with a Telrad, first look at the moon(naked eye) then look at the smallest Telrad circle. Now ask yourself, will the moon fit in the circle? Most of us will say the moon is way too large. Now aim the Telrad at the moon. Surprisingly the moon is an almost perfect fit.

[Stu]

"Is it large, but looks small because it's further away ?"

On the fruit scale , we're talking water melon and serious big banana here !

Nicko.

I think we are heading for a Father Ted sketch again

[RLWP]

I think we are heading for a Father Ted sketch again

Is it a small sketch, or just far away?

Richard

[jabeoo1]

if you hold a pencil with a rubber on the end at arm's length then I believe that the rubber can 'block out' the moon. it seems crazy but it's apparently true. the moon is actually a lot smaller in the sky than most people think.

This is why the subject of Astronomy continually fascinates, the simplest of principles are virtually impossible to ingest, meaning things like Ly distances, Pulsars, the massive array of stars, Globular clusters, galaxies etc etc are off the brain-scale.  I need to rethink this stuff nearly all the time to make progress into it.  Bonkers. 

[steven_w82]

In the long term, rather than buying more eyepieces for the 3", something like a 6" Dob might serve your better, or again, if you can save just a little more, something like an 8". With these scopes you can start getting some really interesting planetary observations under your belt and move into viewing deep space objects like galaxies, globulars and planetary nebulae with significant success.

I agree, I bought this little scope because it was cheap and I just wanted something a little more than binoculars!  I do plan on getting something with a little more oomph in the future, but the little 3" will do for now I think!

[CoolHand142]

Great explanation Q

[pevsfreedom]

Is it possible to view the 'other' moons of Jupiter, I mean besides the 4 galileo moons? What size/mag is required for this? Just curious, I can't see them at 250x in my 10" dob, was wondering if anyone can see them from Earth. 

[pevsfreedom]

This is why the subject of Astronomy continually fascinates, the simplest of principles are virtually impossible to ingest, meaning things like Ly distances, Pulsars, the massive array of stars, Globular clusters, galaxies etc etc are off the brain-scale.  I need to rethink this stuff nearly all the time to make progress into it.  Bonkers. 

Always seemed to me like 'space' is just something the human mind just hasn't evolved to tackle, which is why we all love to sit and ponder. I've always wondered if space is more like a 'long lost friend' which is why it's so interesting, or if it's just so foreign and thus interesting. Either way, it literally bottles the mind. 

[-Aldebaran-]

Is it possible to view the 'other' moons of Jupiter, I mean besides the 4 galileo moons? What size/mag is required for this? Just curious, I can't see them at 250x in my 10" dob, was wondering if anyone can see them from Earth.

considering that the largest four moons that you do see are between 3000 and 5000 km wide and the next largest is around 250km, they are pretty much just asteroids picked up by Jupiter's gravity and are much too small to be picked up from any size amateur telescope.

[John]

Is it possible to view the 'other' moons of Jupiter, I mean besides the 4 galileo moons? What size/mag is required for this? Just curious, I can't see them at 250x in my 10" dob, was wondering if anyone can see them from Earth. 

After the 4 Galilean moons the next brightest are  Amalthea ( magnitude 14.1) and Himalia (magnitude 14.6). In theory they should be observable with a 12" aperture scope however the proximitiy of Jupiter and it's glare increase the challenge. I've not managed to see them as yet with my 12" scope. I have read of someone using a 16" spotting Amalthea under superb viewing conditions when it was at it's furthest from Jupiters disk.