Scope maximum magnitude HELP!

[GazofCorra]

This is my first post so hi to you all, been reading posts as a guest for a while and finally took the plunge.

I class myself as a complete novice even though i know a small amount of info from reading books and web content, i have just purchased a pair of binoculars helios naturesport plus 10x50WA for sky viewing and i am expecting my first telescope later this week, which is a Meade ETX80 AT which comes with 2 meade 4000 super plossl EP's 26mm & 9.7mm.

From my reading my books it advises that you have a few EP's and quality ones at that, now here is what i understand....you divide the focal length of the scope (400mm ETX80) by the size of the EP (say 26mm) and it will give the the magnification 400/26 = 15X..... Now the ETX80 has a 2X barlow fitted which doubles the mag of the 26mm EP to 30X (13mm equivalent) which is ok as a low power viewer, my other EP is the 9.7mm which relates to 41x & 82x (barlow) 5mm equivalent....Now i want a 3rd EP that is more high power and was thinking of around a 5.5mm which would be around 72x & 144 with the barlow which is ample for my scope.

I think that the maximum magnitude of most scopes is worked out by 2x mag for each 1mm of aperture, so mine is 80mm x 2 = 160x maximum another thing is the speed of the scope which is worked out by dividing focal length by aperture, so my scope is 400mm (focal length) by 80mm (aperture) 400/80 = 5 so my scope speed is F5, which means it is a fast scope and all i know about scope speed is that it is said that for astrophotography a fast scope is better.....Well i am not using it for photography just normal observation.

Finally and to the point of the subject this post is supposed to be about is....I have read that there can be some downsides to using a short length EP (say a 5 or 4mm) on fast scopes with a small aperture and also that eye relief is a problem.....It is this that i do not understand and would appreciate some clarification (laymen's terms plz)

[Neilius]

Welcome to the forum, Gaz.

In answer to your question (an if I have understood this correctly), eye relief is to do with the size of your pupil. With eyepieces like 4mm, it becomes increasingly difficult to see through them - the less mm of eyepiece, the harder it is for the eye to see through it.

I think thats what eye relief means, someone will correct me if I'm wrong and then I'll have learned something too!

[Orion_the_Hunter]

I think thats what eye relief means, someone will correct me if I'm wrong and then I'll have learned something too!

Eye relief is the distance your eye should be away from the eyepiece. Eg people who wear glasses need a long eye relief, 20mm say, as their eye will be further away from the lense in the eyepiece.

I hope I'm not wrong now that I've corrected Neilius;)

[John]

Eye relief is the distance your eye should be away from the eyepiece. Eg people who wear glasses need a long eye relief, 20mm say, as their eye will be further away from the lense in the eyepiece.

I hope I'm not wrong now that I've corrected Neilius;)

I think you've got it

Short focal length eyepieces like plossls and orthoscopics have eye relief that is only a few mm so your eye has to get very close to the top lens. Recent designs like Tele Vue Radians and Baader Hyperions and the cheaper TMB planetary clones manage to allow short focal lengths with long eye relief which makes them more comfortable to use for many.

By the way, I'm not sure that an 80mm F/5 refractor will show good images at 160x in the "real world" of observing. You will probably find 100x - 120x the maximum that gives a sharp image and sometimes less than that. This is due to the UK seeing conditions not being optimum very often (almost never !).

[GazofCorra]

In answer to your question (and if I have understood this correctly), eye relief is to do with the size of your pupil. With eyepieces like 4mm, it becomes increasingly difficult to see through them - the less mm of eyepiece, the harder it is for the eye to see through it.

I think thats what eye relief means, someone will correct me if I'm wrong and then I'll have learned something too!

I understand where you are coming from but as i used binoculars for years here is what i know about them, eye relief has nothing to do with the pupil, what you are referring to is called "exit pupil" which is the diameter of the beam of light leaving the eyepiece and is worked out by dividing the aperture by the magnification of the binoculars, a 10x50 pair of binos have a exit pupil of 5mm and a 7x42 pair have a exit pupil of 6mm, now a dark adapted human eye opens to 7mm but changes with our age, if you are over 30 the pupil opens to about 6mm in the dark and by the age of 50 it is around 4 or 5mm, so the light from binoculars with a 7mm exit pupil is simply wasted. Eye relief is the distance your eyes must be from the eyepiece to see the whole field and on binoculars a eye relief of less than 9mm makes for uncomfortable viewing.

I do not know much about telescopes like a fast/slow scope or magnitude for viewing or the association of eyepieces and there magnitude, eye relief etc, that is why i asked the question

[John]

....I do not know much about telescopes like a fast/slow scope or magnitude for viewing or the association of eyepieces and there magnitude, eye relief etc, that is why i asked the question

The same principle on exit pupil size that you described for binoculars applies to scopes / eyepieces as well

There are also optimum exit pupils for planetary and deep sky object detail which are often debated. There are clearly some "sweet spots" though which probaly accounts for why some eyepiece focal lenghts (eg: 13mm) are considered particularly useful.

[GazofCorra]

By the way, I'm not sure that an 80mm F/5 refractor will show good images at 160x in the "real world" of observing. You will probably find 100x - 120x the maximum that gives a sharp image and sometimes less than that. This is due to the UK seeing conditions not being optimum very often (almost never !).

Thanks for the info as this is what i was looking to find out as i have not used a telescope so wanted to know what sort of power is useful especially here in the UK as a book i have says you will want a high power eyepiece of around 150x to 180x for use on planets if your scope can handle this magnitude.

BTW thankyou to those who replied to my post as all info is appreciated and will help others newbies understand (eye relief, exit pupil and scope magnification etc).

[GazofCorra]

The same principle on exit pupil size that you described for binoculars applies to scopes / eyepieces as well

There are also optimum exit pupils for planetary and deep sky object detail which are often debated. There are clearly some "sweet spots" though which probaly accounts for why some eyepiece focal lenghts (eg: 13mm) are considered particularly useful.

Many thanks for clearing that up for me and you have answered my next post already which was what eyepieces are useful (i know focal length of your scope matters) but wondered what people regard as useful EP's and which ones get used the most, also when and why would you use a small EP (say under 9mm) and how low is it wise to go?

[acey]

I have a scope like yours (80mm f5), mine is Celestron badged but was probably made in the same Chinese factory as the one you're getting. Very nice.

The rule of thumb "2 x aperture in mm" is very old - it apparently goes back to French telescope makers of the 19th century. Some scopes can go higher than that, some less. It's only a rule of thumb, not a law of physics.

A short-focus refractor is best suited to low power deep-sky work - the kind of thing you would do with binoculars. I use mine as a travel scope and I've had some wonderful views of Milky Way star fields etc. For me the upper magnification is limited by chromatic aberration (the false colour you inevitably get with a scope of this type) and stability of mount (I just use an ordinary photo tripod). An 8mm plossl with 2x barlow (to give x100) is as high as I go, though with a sufficiently stable mount you might manage higher. I've used it to view planets but it's not ideally suited to that. During the very close Mars opposition some years ago I was able to see the polar cap.

My advice would be first of all to try the scope with the supplied eyepieces, and if you feel you need to up the power then get a barlow.

[brianb]

The rule of thumb "2 x aperture in mm" is very old - it apparently goes back to French telescope makers of the 19th century. Some scopes can go higher than that, some less. It's only a rule of thumb, not a law of physics.

And it depends on the acuity of your eye. If you have "normal" eyesight (corrected by spectacles / contact lenses makes no difference) you really only need about 25x per inch (1x per mm) to see all the detail the objective can resolve, going over 30x per inch (1.2x per mm) reduces the image brightness (which can sometimes be useful, though IMO a neutral density filter is to be preferred) and contrast (& low contrast is an issue for planetary viewing).

Beginners always try to push the magnification too far. Don't waste money on very high power eyepieces, they simply aren't necessary. Training your eye helps a lot to see fine detail; there's no better training than using the scope, and you should expect serious improvement over a timescale of a year or two.

[narrowbandpaul]

welcome to sgl.

magnitude is the measure of how bright an object appears.

Back in greek times, it was surmised that an object of 6th magnitude was 100x fainter than an object of 1st magnitude. So a low magnitude is brighter.

In the 1800's (i think) Pogson set this down in a mathematically rigorous way. Using the above greek definition, and the fact that the eye has a logarithmic response we deduced what is known as pogsons equation

magnitude=-2.5logF

where F is the flux from the star in W/m2.

the 2.5 is derived from the fact that 5 magnitudes correspond to a 100x change in brightness (or flux). ie 2.5 ~ (100)^1/5

usually pogsons relation is expressed in relation to another star. ie

m1-m2=-2.5log(F1/F2)

the standard reference star in astronomy is vega, and has a magnitude of 0 in both the B and V bands (blue filter and visual filter), so setting m2=0

m1=-2.5log(F1/Fv)

a telescope allows you to see fainter objects because the aperture is bigger.

the approximate limiting magnitude (will depend on atmospheric extinction) for a point source is:

m_tel = 5log(D_tel/D_eye)+m_eye

where m_tel is the limiting mag with a telescope of aperture D_tel. m_eye is the naked eye limiting magnitude with no telescope and D_eye is the aperture of a fully dark adapted pupil.

typically m_eye is about 5 from a darkish site, and D_eye is around 6-7mm for an average person.

magnification, is the increase in the apparent size of an object. It is simply given by f_tel/f_eyepiece

hope this helps

paul

[pvaz]

I agree with all said about the maximum useful magnification but I must confess I personally find details hard to notice under 150x and most of my planetary observation is done at 170x or 240x. I only drop down to 120x if seeing conditions won't allow for more, but when that happens I usually look for other targets to observe.

[brianb]

m_tel = 5log(D_tel/D_eye)+m_eye

You forget:

1) the decrease due to light losses in the scope: with modern coatings this is not much, but still typically around 0.2 mags

2) the increase that occurs when you increase magnification because, up to a point determined by the resolution of the objective and the quality of the atmospheric seeing, a star remains a bright point but the background is darkened by magnification. This increase can be quite significant, up to 1.5 - 2 magnitudes when using an "optimum" magnification of around 20x per inch (0.8x per mm) of aperture.

Sidgwick (Amateur Astronomer's Handbook) gives a practical limiting magnitude of 10.5 + 5 log D (inches) which gives e.g. a limiting magnitude of 15.7 for my 11" SCT. This assumes telescope efficiency of around 50% which was typical when the book was written. With the aid of modern coatings and therefore a lower light loss in the scope, I have been able, in good observing conditions (rare), to glimpse stars of magnitude 16.1 (10.9 + 5 log D) But I should add that this requires a dark transparent sky, steady seeing, the object near the zenith, at least an hour's dark adaptation and many years' experience of searching for objects near the limit ov vision. The faintest stars that would be steadily visible to direct vision would be closer to the old formula 9 + 5 log D.

[narrowbandpaul]

how disgraceful of me not to account for 0.2 mags of light loss.

I did use the term 'approximately', as a caveat.

still thanks for the additional info, always nice to know a more accurate formula. Im not a visual observer so I always think in SNR and flux.

paul

[GazofCorra]

Thanks guys all useful infomation i am sure, but as a novice "NarrowBandPaul" you might as well have written that in chinese as i have no idea what you are on about , but maybe in time i will have a better understanding (doubt it lol).

Acey as you have a very similar scope to my own, it is good to hear that over 100x is not so good and i will stick to this, apparently my Meade ETX80 has a 2x barlow built in so my 9.7mm EP will give me 41x & 82X which will do, as i intend to use it as a travel scope as we have a caravan which we go away in and it will be a great scope to take away with me.

Jahmanson your explanations i understand and the same goes brianb to a point....So thank you guys

later i will buy a scope to use mainly at home and keep my ETX80 as a travel scope.

Got a lot to learn and taking in new stuff gets harder the older you get

[acey]

the 2.5 is derived from the fact that 5 magnitudes correspond to a 100x change in brightness (or flux). ie 2.5 ~ (100)^1/5

Correct, but for the wrong reason. You've got Pogson's formula

m1-m2=-2.5log(F1/F2)

Now suppose m1-m2 = -5, i.e a difference of 5 magnitudes. Then

log(F1/F2) = 2

and the brightness ratio is F1/F2 = 10^2 = 100.

So the reason for the 2.5 is not that it is close to the 5th root of 100, but that it is exactly half of 5.

[John]

...Got a lot to learn and taking in new stuff gets harder the older you get

Yep - thats what I find as well

Get your scope under a dark sky when away in your caravan and it will show lovely low power views - you will be surprised how many deep sky objects you can spot - many as misty patches but still amazing when you think how far they are away

[Ant]

Thanks guys all useful infomation i am sure, but as a novice "NarrowBandPaul" you might as well have written that in chinese as i have no idea what you are on about

I do not class myself as a novice - but it's still Chinese to me

[RobH]

I do not class myself as a novice - but it's still Chinese to me

I actually find Chinese easier I think, plus, it tastes better :D

[ronin]

I would think first of an eyepiece of say 6mm for the scope, 67.6x magnification. I have a similar scope and a 6mm is fairly decent. If you get something like a TMB planetary clone then you get decent eye relief.

If that works fine in your scope, then perhaps another in the same range at 5mm or 4mm. I think that 4mm is as small as would make general sense.

As to magnification in general. Everyone quotes 2xdia in mm. But there are several scopes in the refrator options. Yours is an achro, then come ED doublets, then apo triplets. Magically everyone says 2xdia. Why the same statement when an achro is relatively cheap but an 80mm apo triplet is substantially more. If they performed the same no-one would buy a triplet apo. There has to be a difference when you think about it.

The point being don't necessarily expect what you read.

If the ETX will get to 120x (3.3mm eyepiece or equivilent) then that should be OK for Saturn and as Jupiter is larger (80x is OK for Jupiter) you should then be covered for the planets.

[Neilius]

Thanks for the clear up guys!

[GazofCorra]

I suppose the mag of the EP/scope does not matter to much, the clarity of what is being viewed is possibly the most important thing and i guess the more light a scope can gather the more detail that can be seen and then it will be better if good quality EP's are used and when you do need to pump the power up and use small EP's with good eye relief and a good field of view.

I was under the assumption that to see an object that is far away you need a lot of power (magnification) to make it appear bigger to the viewer, like when you see a ship out at sea it appears as a tiny dot, then you use your binoculars and it now looks the size of a bumble bee and you can notice small details, you then get your spotting scope out and it now appears not only larger than when using your binoculars but a lot more clearer....Well you get what i am on about.... But when you are talking about a star or planet or a galaxy etc which is light years away i doubt any scope out there can make it look as if it is just at arm's length away.

:iamwithstupid:By writing this post i actually think my brain has just worked out something significant......We are not talking about a ship out at sea or even a bird up on a ledge...We are talking about an object that only appears to us a speck of light, so all we can hope to achieve is to gather as much of that light as is possible and then have it display as much of it's detail as we can get and by using EP's that magnify what we are seeing somewhat, that is all we can hope for.

[John]

Stars will never appear as more than a pinpoint in amateur scopes, no matter how much you magnify them. As you say they are light years away. Planets though are much closer (still millions of miles away but not light years) so magnification does make them appear as disks rather than pinpoints, reasonably large disks in the case of Jupiter, Saturn, Venus and occasionally, Mars. Much smaller disks in the case of the other planets.

The more magnification, the bigger the planetary disk appears however there are limits, defined by the seeing conditions and scope aperture, to how much magnification can be applied while retaining a clear, sharp view. You will see this discussed a lot on this forum !.

[brianb]

Stars will never appear as more than a pinpoint in amateur scopes, no matter how much you magnify them.

Well that's not strictly true ... you won't resolve their discs but a star will always appear as a blob at high magnification, if the seeing is poor it will just be hard to focus but if the seeing is really steady you will see a small disc surrounded by diffraction rings - the disc of course being due to the wave nature of light, not the disc of the star itself.

When the seeing is really bad the star image will be bloated even at low magnification, I've known it to be hard to focus an 80mm scope at x24.