Magnification

[alan potts]

I have a few scopes and a large selection of quality eyepieces.

I am of the belief that many members use too much magnification. I was using a power of X140 last night on Jupiter and everything was razor sharp. when I increase to X200 the image is still there but not as sharp as before. Now the scope was only a 6 inch GSO which is not maybe the best visual scope but I am happy with it.

Even on my APM 115mm whose quality is beyond question I find X200 is about as high as is good for it, I find I do not see any more, if anything I see less. Most of the time I stay below X160

Even on my LX I rarely go above X200 which is a good deal less than it can handle, this is not because of the seeing quality, I just feel I see more in a sharp small image than a low contrast bigger one.

Anyone hold a simlar view.

Alan.

[chiltonstar]

IMHO it depends on an individual's eyes and what he/she is looking at...... Certainly, for me x150 is the best mag for Jupiter, but not for Saturn, and double stars may need much more mag to show them, even beyond the Dawe;s limit.

Chris

[Stu]

I agree that it varies depending upon the target. I generally stick below x200 on Jupiter, I used to throw more at it but have learned that there's nothing much to be gained by going higher - x180 normally works well as a maximum regardless of scope, often less. Saturn definitely takes more, best views I've ever had were on a stunning night with the mak and 13mm ethos at x307 but that doesn't happen very often. Mars I also find needs, and can take more, x250 if the skies are good and then it shows some good detail.

Globulars in the mak are also nice at high power if the seeing is good.

I totally agree that it's a matter of finding the right mag for the target and the conditions, less is often more as you say.

Stu

[Ajohn]

I've posted a method of calculating magnifications that are suitable for various purposes and your numbers tie in with it. Not surprising really as they are correct. This time I will copy paste it and add a little more info.

To use his figures you need to work out the exit pupils size the eyepiece gives. That's just it's diameter in mm divided by the magnification. This is his list.

4.2mm and 3.1mm wide field.

2.0mm and 1.3mm coloured doubles and variable stars etc

A series of 5 eyepieces for general use and optimum views of planets according to conditions. The exit pupils of 0.63,0.67,0.75,0.83 and 1mm

0.5mm and 0.38mm High power where the diffraction ring round stars needs to be seen

This might be thought of as a professionals idea of the ideal eyepiece set. As the general use range is too extreme for planetary work he then goes on to say that generally a magnification of just over the diameter of the telescope in mm is optimum giving an exit pupil of 0.8 to 0.9mm and that several eyepieces around that allow optimum views to be found according to seeing conditions. These are all 50 degree plossl's and aimed at getting the best out of a 1/10 wave front error max newtonian. He doesn't rate p/v figures as they don't ensure smooth optics. He wouldn't rate the views from a newtonian at exit pupils much over 4mm even at F6 either. These figures account for a number of things - eye resolution for one, seeing conditions and unfortunately optical quality plus the detail that the scope can resolve which is dependent on diameter. The later one is why it makes sense to choose eyepieces based on exit pupils. Bigger scopes can resolve finer detail so can take more magnification - if seeing allows

So taking say a 6in scope the diameter in mms is 150 so the optimum magnification for planetary work will be around 150x which will give a 1mm exit pupil. ei 150dia/1mm exit pupil = 150x. The focal length of the eyepiece needed is then focal length / magnification. This should work out even in rather poor seeing conditions. In practice I have found that even an 8in scope wont always work well at this level of magnification which in the case of an 8in scope is 203mm/1mm = 200x, there is no point in being exact. An eyepiece to give a slightly larger exit pupil is worth having about. He lists a close spread range of exit pupils around this level of magnification so that one can be chosen to give the best view possible in this case in superior conditions.

The smaller exit pupils that give higher magnifications are for splitting double stars and star testing type alignment or evaluation. If you use these on an 8in scope your likely to see the effects of atmospheric turbulence and even more so as scopes get larger. The best way to describe mild cases of that is that the image will fluctuate. The diffraction rings may even come and go or wobble about or refuse to stay in focus. This gets more and more likely as the scope gets bigger unless seeing conditions are perfect.

The 1st range of larger exit pupils are aimed at giving contrast and light levels where the eye can detect colour in stars. Larger scopes may need these for planetary work as well. The problem with larger scopes on planetary work is that resolution increases but at the expense of increasing the effects of atmospheric effects as well. There is an argument in this area that a typical urban telescope user can't expect to achieve the maximum planetary magnification of even a 10in scope. In the early part of the year in the UK when conditions are cold and relatively stable and the sky is clear of water vapour I personally thing that people can only problem is that winters don't seem to be like that so much these days.

Things get different at the next set of larger exit pupil sizes that most would extend to 6mm. This is the area where big light buckets get useful probably right up to the size that people are likely to buy. Magnifications are low, contrast is high and conditions don't matter so much.

Refractors can inherently produce more contrast than the normal range of reflectors and cat scope due to the lack of a central obstruction. That means that a good refractor can be used at higher magnification levels. It's pretty easy to see if a refractor is "good" just use it terrestrially on a dead clear day and keep increasing the magnification. The contrast will decrease as the magnification goes up but it should still be possible to get a clean focus. What tends to happen is that the aberrations in the image are being magnified as well to such an extent that a clean fairly easily focused image can't be found. The excessive level of magnification wont reveal any more detail. It will just reduce contrast even further. So called empty magnification. The same test can be done on any type of scope. The results with exit pupils below 0.5mm can be disappointing but some scopes will hold up and even go further and that might even mean using a good quality barlow.

If anybody wants to know more about contrast and resolution there is plenty of info on MTF curves on the web. What usually isn't clear is that one curve applies to any scope. Contrast starts at 1, what goes in comes out, and goes down to zero at a point that can be calculated from the F number or NA. At Rayleigh's limit approximately 1/13 of the contrast going in comes out. Not much in respect to work on planets etc as the contrast is rather low to begin with. A lot of the information which is aimed at photography mentions MTF50. That's where contrast out is 1/2 what is going in. The bottom scale on the graphs is in units of resolution. It's also used to test photographic lenses but that aspect doesn't really relate to MTF curves as photographic lenses aren't diffraction limited.

John

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[Ajohn]

Seeing the comment about eye conditions larger exit pupils are more likely to cause problems than small. A lot depends on the defect in the eye but it's generally the larger exit pupils that cause problems as more of the lens in the eye's pupil is used.

John

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[Marki]

Very nice views of Jupiter in the range of x185 to x200 last night in both my 4" f10 frac and my 4" f13 frac. Went mushy above that for me when I went to x225. I find I very very rarely get above x250 in good seeing in any of my scopes (including an 8" sct) - just isn't worth it, except maybe on the moon, or pushing a tight split on a very clear night.

[alan potts]

Ajojn

very nice piece of work and I have to say I agree with a great deal About the only part I would not go with is the wide field. I have tested all sorts of eyepieces with up to 7.00mm EP and higher and I can't say that what I see disappionts me at all, I guess though I cannot see all the light and it is not as good as it could be if I were an owl. Thanks for posting it.

Marki,

I would think the moon is the execption to most rules. I find on my 12 inch I have to use at least X200 or I can't see for the rest of the night. I really should buy a filter.

As for the double stars and I stress it could just be me, I have never found a mass of magnification ever works. Through the summer I tried everything on Antares and could not split that, which I would have thought a 12 inch could do. I am talking well over 50 clear night and early mornings.

Thanks again,

Alan.

[perrin6]

At Mags of about 190X on my scopes, detail seems to get lost in the glare of bright planets and switching to about 285 X seems to improve detail perception (don't have eyepiece between 200 and 285X) . Perhaps it is my eyes or dirty optics, I just don't know. For splitting close doubles I use 285 - 420 X.

[twotter]

When it comes to mag I would agree that, in the main, less is more.

[scarp15]

I could only manage to throw 156x in my C8 at Jupiter last night and the better views came in at 127x. The previous evening, my daughter took a look at Jupiter and preferred the view in my 20mm Nagler at 102x, however the more expansive view and sharper gained contrast at a lower power, does sometimes make the image more pleasing.

[acey]

Depends on target, sky, scope and eye. If sky is limiting factor then the crucial parameter is magnification, otherwise it's more usually exit pupil (aperture divided by magnification). It's true that a large exit pupil will exacerbate optical limitations in the eye - that's why we've evolved to have pupils that are around 2-5mm diameter in daylight. For night vision we sacrifice optical quality in order to get increased light (i.e. our pupils dilate). The implication is that in order to optimise the effect of our own eye on the view through a telescope we should probably use exit pupil no larger than about 5mm and in most cases smaller. Laboratory studies done many years ago showed that visual acuity at low light levels is greatest for pupil size just under 3mm. For a six-inch (150mm) scope this would correspond to magnification x50. Upper bound is generally set by diffraction in the eye (exit pupil 0.5-1mm) which for a 150mm scope would be somewhere in the range x150 - x300.

[Ajohn]

The person who wrote down those suggestions is Texereau a French telescope type optician. It seems he made his living making larger telescopes in France and gained a reputation for improving others by refiguring the the mirrors. He was something of a perfectionist. At larger exit pupils where the magnification is low the viewed field is bigger and newtonian scopes in particular start to show increasing amounts of aberrations. Stars become radial fans rather than pin pricks of light. It gets worse as the F ratio gets faster. All I was pointing out is that he wouldn't be happy about that. He would also probably be basing his limit on the size of 2ndry mirror he recommends. That's 1/5 of the scopes diameter. He suggests having 2 sizes if wide fields are really needed and gives some sensible limits for film photography.

I sometimes use a 75mm, it's probably longer, on all of my scopes since I have had it. The exit pupil is huge. Much to big for the pupil of my eye so I sort of look around the exit pupil. Can be handy at times. Otherwise I've generally stuck to getting close to 6mm for minimum magnification and providing it is fairly dark all is ok. As 6mm has been recommended as a maximum for a long long time I have to wonder about 7 but can't say that I have ever knowingly tried it. I had drops put in my eye for an examination once and my pupils looked like they opened up to over 7 but vision suffered. I wonder if this is another reason for the older limit of 6,

This info and other things are all in a book he wrote initially in French. It was later translated to English and had a bit of an impact on ATM. Much to my surprise it can be downloaded for free - it's worth a read. There is a lot of good solid info in it on things other than making a telescope and even that part is of interest. One aspect of testing for a sphere works and I have never seen it mentioned anywhere else. He describes a test set up that is accurate and fairy easy to make as well.

https://ia600706.us....eATelescope.pdf

He seems to be responsible for what was a little known eyepiece becoming popular. Plossl's He seems to have persuaded a French firm called Clave to make all of the focal lengths I mentioned available. They crop up from time to time used and last time I saw one still fetch high prices despite their age.

Can't split the double - as he points out it gets harder and harder as the size goes up all down to atmospheric conditions. That's without accounting for the optics needing to be rigidly held in place and being of rather high quality and not sagging due to their own weight. It takes extremely complicated mirror cells to get round that problem as mirrors get thinner and thinner - not so much of a problem when scopes are used well short of their maximum capabilities as is often the case with dob's for instance.

He also wrote a 2nd version of the book. Same content plus information on making a pure cassergrain similar to the one he used himself at home through a hole in his roof. This one isn't so good in my view as the 2ndry mirror on these is a bit of a problem how ever it's made.

John

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[Ajohn]

Depends on target, sky, scope and eye. If sky is limiting factor then the crucial parameter is magnification, otherwise it's more usually exit pupil (aperture divided by magnification). It's true that a large exit pupil will exacerbate optical limitations in the eye - that's why we've evolved to have pupils that are around 2-5mm diameter in daylight. For night vision we sacrifice optical quality in order to get increased light (i.e. our pupils dilate). The implication is that in order to optimise the effect of our own eye on the view through a telescope we should probably use exit pupil no larger than about 5mm and in most cases smaller. Laboratory studies done many years ago showed that visual acuity at low light levels is greatest for pupil size just under 3mm. For a six-inch (150mm) scope this would correspond to magnification x50. Upper bound is generally set by diffraction in the eye (exit pupil 0.5-1mm) which for a 150mm scope would be somewhere in the range x150 - x300.

I'm surprised some one mentions exit pupil size and magnification as if they are separate entities - each of them determines the other. That's the whole point of my post. Exit pupils though are more flexible as they account for the size of the scope. Perhaps my post was too long to read.

John

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[Luke]

I do find I can see more detail above 200x if the conditions are good enough. Last night, 200x was too blurry on Jupiter for me.

I've only twice had sharp views of Jupiter so far at 300x, but they were hands down the best views I have ever had of it, and I had to wait as the planet drifted in and out of focus. Once in a 12 inch dob at Kelling, where my jaw dropped, it was like looking at a clear photo. And once at home in a 16 inch dob.

I guess it depends on your eyes and equipment. 300x is so far the limit for me. The missus seems to prefer lower mag than me.

[freeflyjay]

I have a 10" dob and was looking a jupiter a few weeks back (before the cloud set in for what seems like forever) with a meade 4000 zoom barlowed x2 and could go right upto 300x mag and it was crystal clear only the 2nd time i`d looked a jupiter and was testing the newly aquired meade, grs and banding were awesome it was truely like looking at a photograph, never thought it could be seen so clear, tried with a x3 barlow but max i could push it too was around 350x mag, but too be honest 300x was probably the best view.

[acey]

For dob users a further limitation is hand-tracking. With my 12" f4.9 I go up to x375 (exit pupil 0.8mm), with a 4mm Nirvana. I'm sure my scope and eye could handle higher, and there must be nights when the atmosphere would allow it, but I'd have trouble keeping up with the object in view.

[Ajohn]

The figures on exit pupils I gave sound like they are already working for a number of people with different sized scopes and illustrating that some times conditions wont be good enough.

It's simple for planetary and that sort of use. Apart from the broken corrector plate which I hope will get sorted out I would be thinking about that for a 9 1/4 in scope. That is 235mm so I would be thinking of magnifications around 235x for a 1mm exit pupil. I would suspect 300x would be too much but may be ok if the scope and conditions are great and that I might need something or a few eyepieces to give a figure somewhat under 200, say 180. The trouble with Texereau's list is that the chances of getting a full set in this range of magnifications are small on any scope. If I could only get one eyepiece I would go for something a bit under 235x say 200 or lower to be safe.

At the other end of the scale I would be looking for a 6mm exit pupil so 235/6= a bit under 40x. F10 scope so the focal length is 2350/40 = about 59mm that might make me think I also need a 2in visual back for it.

For star testing via diffraction rings and double stars something like a magnification of twice the diameter in mms, exit pupil of 0.5mm is getting into the right region - 460 say 500x on a 9 1/4 scope. That should be ok to use and maybe even further than that for star testing via diffraction rings. The is there because life in this area gets more difficult as the size goes up. And even worse as the F ratio gets faster as collimation needs to be held to ever decreasing limits. When star testing is mentioned it usually relates to in and out of focus images not in focus diffraction rings.

John

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[Josh Wilson]

The highest mag I've ever used was 400x. Somehow, my skies and scope hold up to this power. But with some objects that I've viewed, I notice that they can be seen more easily or better at higher powers. I spotted the Black Eye Galaxy (M64) at 125x, but at 50x, it was almost gone. There are many other objects like this.

We (astronomers), owe ourselves to magnification because without it, astronomy wouldn't be very fun. I agree that many astronomers take this for granted, but for the cause of curiosity.

[acey]

It's certainly true that for small DSOs you need to raise the magnification. This is because small, faint objects are indistinguishable from point sources and hence look star-like rather than nebulous. For discovering objects with his 18.7" reflector (about 2,500 of them, mostly galaxies) Herschel used x157 (exit pupil 3mm), going higher in some cases in order to confirm the object was non-stellar. With the 12" f4.9 I use x47 (exit pupil 6.5mm) for finding the field, and most of the DSOs I seek are invisible at that power. I then use a 24-8mm zoom, and find that at lowest power most are still invisible, but as I raise the power the object will pop into view as a small fuzzy, once its angular size has been sufficiently enlarged. The fainter the target, the larger it needs to be made in order to be distinguishable from a star. For very faint NGC objects I often need to go to highest zoom power (x187.5, exit pupil 0.6mm) before the object becomes visible, and sometimes even higher, using other eyepieces.

It's often said that lowest power is best for DSOs, but that's only really true for observers at light-polluted sites seeking to get a glimpse of large, bright objects such as Messiers. At a dark site, viewing "typical" NGCs, higher power is nearly always better, because the objects themselves are of small angular size and most are of reasonable surface brightness, able to withstand the sort of magnification that brings out fine details.

[Ajohn]

That doesn't stack up acey. A 12in scope needs a magnification of 508x for a 0.6mm exit pupil. 180 odd gives and exit pupil of 1.6mm which funnily enough is in the middle of Texereau's coloured doubles etc and that implies relatively high light levels coming out of the eyepiece and as a result a fair amount of contrast. It actually better contrast wise than what he suggests for planets. It needs to be too. People may find they prefer the look of planets at that level of mag as well down to the seeing and unfortunately in some cases probably the scope.

Josh using 125x on an 8in scope is also us using a 1.6mm exit pupil on small feint fuzzies.

All this is saying is that there must be sufficient magnification to see them. I think these numbers still fit in with what he is trying to suggest. I suspect we would have to hold a seance to get him to add small feint fuzzies to the book though.

It's also interesting to note that Josh's scope fits in with his idea of a capable scope that is easy to deal with and will give good views of all sorts of things 8in. It's also at the the short end of the F ratio's he suggests F6. That's a pretty decent F ratio for all scopes really for a variety of reasons and even heads in the right direction for photography without making it virtually useless for general use without it becoming a rather specialised wide field scope. I sold one of those recently. F4.2 A dob too. These can be great for wide field low mag star fields given a decent eyepiece but that's about it really but the aberrations on a F ratio that short are rather high going across the field of view. F6 is a lot better in that respect and F8 would be even better as far as maximum field coverage goes - he even explains that in his book and suggests a solution. That is available for download for free too. The solution is really aimed at photography though and not so good for visual use - that again makes F6 a pretty good option.

John

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[acey]

That doesn't stack up acey. A 12in scope needs a magnification of 508x for a 0.6mm exit pupil. 180 odd gives and exit pupil of 1.6mm

You're right, I typed a wrong number in the calculator - exit pupil for an 8mm eyepiece in an f4.9 is indeed 8/4.9 = 1.6mm - thanks for the correction.

With regard to optimum magnification for double-star observation, a classic 1913 paper by T. Lewis, "On The Magnifying Power Used By Double Star Observers", is well worth a look. He surveyed a range of leading visual observers and came up with the empirical formula M = 140*sqrt(A) where M is optimum magnification and A is aperture in inches.

http://articles.adsabs.harvard.edu//full/1913Obs....36..423L/0000423.000.html

[Ajohn]

Have to laugh Acey for a 6in scope that comes out at around a 0.5mm exit pupil less in fact. 0,444mm

8in a bit bigger 0.513mm

12in even bigger 0.626mm

An example of Texereau's point about life getting more difficult as the size goes up. I suppose few have downloaded it but towards the end of the book there are some pictures of what the atmosphere might do to a wavefront - the errors it introduces.

There is another aspect as well - weight of the mount needed. The figures he mentions sound crazy but there is a paper some where that I will try to find later that states just how accurate the mirror and 2ndry alignment needs to be. It's on a something or the other...observatory site.

Actually while using a sum that assumes problems as the sizes go up makes sense I don't see how it can work out in practice. Only a guide like Texereau's numbers. For instance if some one happened to live on top of a mountain next to one of the great telescopes they could use much smaller exit pupils than some one who has to observe over a land fill site. The later would generate thermals that are bound to upset observing - as do cooling pavements in urban areas that are likely to lag well behind the rate the atmosphere temperature changes.

The paper I mentioned is in publications on brayebrookobservatory.org but they seem to be off line at the moment.

John

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[alan potts]

Ajohn,

I read you posts twice, I have to say there were a little difficult for me to understand but the second read got the brain out of first gear. I found them very interesting indeed ,thanks for taking the time to write them. Good point you made about scopes and mirrors, I was having a moan about focuser quality the other day, seems to me that with the exception of my APM every scope needs a change of focuser to bring it up to doing it's job in some cases. Now I find the mirror cells don't support the mirrors.

The point I made about the focuser which I know is off topic, but it annoys me when I put say a 35mm Panoptic in the focuser of my 190mm Mac/newt and everytime it goes close to upright it gives up and collapses, seem to me most are only good enough to hold a tpye 6 Nagler,

No wonder there is a thread on how to remove it and put a Moonlite in its place.

Alan.

[acey]

Interesting quote from Lewis's paper:

"Thus in many textbooks it [i.e maximum useful magnification] is dismissed with an allowance of 50 or 60 per inch of aperture, a dictum born probably in the era of small telescopes and dying hard. In practice we find Dr Maw using powers of 300 and 400 on both his 6- and 8-inch telescopes, and Comas Sola 350 on his 6-inch. We should hence expect observers using a power of 1800 with the 36-inch Lick refractor... whereas the powers in general use do not exceed 1000 to 1500 at Lick."

http://articles.adsabs.harvard.edu//full/1913Obs....36..423L/0000425.000.html

Lewis's purpose was to dispute the 19th-century rule of thumb (still widely promulgated) that the minimum useable exit pupil was 1/50th or 1/60th inch, i.e. maximum useable magnification was 50 or 60 times aperture in inches. His study of "thousands of observations all over the world by various observers" resulted in the empirical formula he proposed - which has no theoretical basis, and is something that observers can take or leave as they choose.

The limiting magnitude of the 60-inch telescope at Mount Wilson was investigated by the observatory director, Ira Bowen, in 1947. His paper is also very interesting.

http://adsabs.harvard.edu/full/1947PASP...59..253B

Bowen found that for him the smallest useable exit pupil was 0.07 inch or 1.8mm (magnification about x850). This could be attributed both to the atmosphere and to his own eyes. Optimum magnification has also been studied theoretically by Garstang, leading to yet different conclusions:

http://articles.adsabs.harvard.edu//full/1999JRASC..93...80G/0000080.000.html

All of these theoretical and empirical studies have their own strengths and weaknesses, and there can be no doubt that optimum and maximum magnification will remain an endless source of conversation for amateur astronomers. But what it all comes down to is the particular sky, scope, eye and target in question, and since everyone's eye is different there will never be a rule or formula that works for everyone. We all just start low and work up until there is no further benefit to be gained.

[ronin]

This has not only gone off topic it is close to off the planet and whizzing past Voyager shortly after leaving Earth.

Should answers to the original post be Yes/No ? :grin: :grin: