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How to use higher Magnification than the rated maximum of a telescope?


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I am having newtonian 130mm F5 OTA, for which the rated maximum magnificaton possible is 260x. With this seeing a solar system bodies are not that easy. As per the stellarium simulation, iIf I use 5x barlow with 2mm eyepiece I can see it very nicely. Is it a good idea to do so? What will happen if I use higher magnification than the rated maximum?

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The practical maximum magnification of a telescope is defined by it's Dawes limit , which is about 2x per mm of aperture. Therefore a 130mm aperture telescope has a practical maximum magnification of 260x. Magnifications greater than that can be obtained by the resolving power of the telescope but the results will be disappointing. You will not see any greater detail than at 260x because the optics are not capable of it. And definitely not at 1625x ((650x5)/2)

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The image does become larger, but also blurred and much dimmer. What is still useful to you as observer depends on the contrast and brightness of the object (moon and Mars readily take more magnification than Jupiter, I find), and on your own visual acuity. I have a visual acuity of around 1.7 (well above average), and therefore the blurring becomes apparent at lower magnification than if you have more average visual acuity. Having said that, using a 5x Barlow and 2mm lens in a 130mm F/5 yields 1,625x magnification, which will certainly show the blurring effect, and the image will become very dim indeed. I once pushed my Celestron C8 to slightly  more than the official maximum (406x) to about 450x on Mars, and that did sort of work, but showed no more detail than at 290x, I felt. I think the Stellarium simulator just adds magnification, and doesn't take the resolution of the scope into account

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Most maximum magnifications quoted are not practical to achieve.  I have a 8" scope (much larger than yours) and I can just about achieve x240 with a high quality 5mm EP, but the seeing has to be exceptional and the object being looked at conducive to be looked at (in most cases this means bright enough).  Thus, it works best on things like Jupiter, the moon and double stars.  You will find it is better if things are smaller and brighter (i.e. less magnification).  Estimates from things like Stellarium are not adjusted for real life situations.   I would say you need to throw the x5 Barlow away unless you want to use it with something like a 30mm EP.

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You can use higher than rated magnification on small bright deep-sky objects like planetary nebulas or compact galaxies but not on planets, Sun an Moon. Nebular targets don't have small details visible to the eye, only large-scale features, so you can't lose these small details you lose with planets.

However that doesn't work on globular clusters because the tightly packed stars are like small planetary detail. I'm not sure how high power can be increased but 4x per millimeter seems ok; that would be 520x with a 130mm scope. Everyone is free to experiment, and I'd like to see the findings. Tracking will be tricky if your scope is not motorized, and your eyepiece don't show a wide field, though.

Edited by Ben the Ignorant
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To understand what "empty magnification" means you can take this photo of Saturn which is pretty more or less what you see through your scope.

p018q087.jpg.3c4693ac76573f81b96d4224604b2b64.jpg

Now, you can save this image on your p.c. and then try to view it zooming in so that the planet fills your screen. Not really a pretty sight? It's actually worst because it's very blurry.

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1 hour ago, Ben the Ignorant said:

You can use higher than rated magnification on small bright deep-sky objects like planetary nebulas or compact galaxies but not on planets, Sun an Moon. Nebular targets don't have small details visible to the eye, only large-scale features, so you can't lose these small details you lose with planets.

However that doesn't work on globular clusters because the tightly packed stars are like small planetary detail. I'm not sure how high power can be increased but 4x per millimeter seems ok; that would be 520x with a 130mm scope. Everyone is free to experiment, and I'd like to see the findings. Tracking will be tricky if your scope is not motorized, and your eyepiece don't show a wide field, though.

Sorry, not correct. Planetary nebulae and galaxies are particularly unsuitable for higher-than-rated magnification. They have far lower surface brightness, and far lower contrast than planets or the moon, so the loss of brightness you incur when going to high magnification means you lose much more than when observing high surface brightness, and high contrast features on moon and planets.  Planetary nebulae can in some cases handle higher magnifications, because but only those with comparatively high surface brightness. I have used 245x on the Eskimo Nebula with my 8" scope, and that worked surprisingly well. On the Helix, which is huge and has low surface brightness, I used just 65.5x in my 8" scope for my best view. Anything higher and it seemed to fade away. On galaxies, higher magnification does not work well at all, and an exit pupil of around 2mm (one quarter of the maximum rated magnification) is generally advised.

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31 minutes ago, michael.h.f.wilkinson said:
1 hour ago, Ben the Ignorant said:

You can use higher than rated magnification on small bright deep-sky objects like planetary nebulas or compact galaxies but not on planets, Sun an Moon. Nebular targets don't have small details visible to the eye, only large-scale features, so you can't lose these small details you lose with planets.

However that doesn't work on globular clusters because the tightly packed stars are like small planetary detail. I'm not sure how high power can be increased but 4x per millimeter seems ok; that would be 520x with a 130mm scope. Everyone is free to experiment, and I'd like to see the findings. Tracking will be tricky if your scope is not motorized, and your eyepiece don't show a wide field, though.

Sorry, not correct. Planetary nebulae and galaxies are particularly unsuitable for higher-than-rated magnification. They have far lower surface brightness, and far lower contrast than planets or the moon, so the loss of brightness you incur when going to high magnification means you lose much more than when observing high surface brightness, and high contrast features on moon and planets.  Planetary nebulae can in some cases handle higher magnifications, because but only those with comparatively high surface brightness. I have used 245x on the Eskimo Nebula with my 8" scope, and that worked surprisingly well. On the Helix, which is huge and has low surface brightness, I used just 65.5x in my 8" scope for my best view. Anything higher and it seemed to fade away. On galaxies, higher magnification does not work well at all, and an exit pupil of around 2mm (one quarter of the maximum rated magnification) is generally advised.

Oooo....a difference of opinion.  @Michael.h.f.wilkinson your thoughts seem to gel with what I have seen.  I still think I am a beginner so I tend to report what I see (as I experience what I see whereas sometimes I still only guess with the science).  My experience with my Pentax 5mm is that unless the object is nicely bright I tend to lose them in the EP thus as many DSO's are little more than faint dim grey wisps anyway I find I tend to lose them in the 5mm.  The brighter planets seem to fair better - Saturn, Jupiter thing like bits of the moon, the edge of the sun (with appropriate filters), so that's what I see and that's what I reported, when I read @Ben the Ignorant I thought maybe I was incorrect, but you also seem to agree with me.  At the very least it tells the OP that very high magnification will not work on all objects we now just have to agree on what objects will be better.  I find my high mag.  Pentax 5mm seems  at it's best on double stars, but even then the seeing conditions have to be exceptional.

Edited by JOC
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There is also another aspect of high magnification that will also reduce the image clarity and that is the speed at which the object races through your eyepiece field of view and vibration effects unless you have a sturdy tracking mount. I would think you will see more with lower magnifications that allow you to study the object over a period of time.

Alan

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To sum up from my own experience:

Starters to observing always want bigger and higher magnification, Hubble like images to the eye,
sadly that is not reality, smaller and sharper is best, you need to hone your observing skills.

Lower magnification often produces nicer to the eye observing.

The conditions on the day also have a massive impact, sometimes, but not often, 
higher magnification may be possible, but not often.

Not unless you are in one heck of an observing location and its perfect conditions.
 

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2 hours ago, mnanand said:

I am having newtonian 130mm F5 OTA, for which the rated maximum magnificaton possible is 260x. With this seeing a solar system bodies are not that easy. As per the stellarium simulation, iIf I use 5x barlow with 2mm eyepiece I can see it very nicely. Is it a good idea to do so? What will happen if I use higher magnification than the rated maximum?

Despite being no expert with astro maths this sounds extreme  magnification for the 130mm F5 telescope.

I think you do 5 * 130 to get F/L which is 650mm

650/260 (the maximum rated mag) suggests that a 2.5mm EP is the largest most magnifying EP which should be used.  However I know, in the UK, at least that my 5mm in my F5 approx. 200mm mirror telescope is about all it can take.  To be honest I'd be very surprised if you got a huge amount of sense out of something much larger in mag. than perhaps 10mm-8mm EP.  In fact, depending on where you bought it, you might find that even the mag. from the lowest number EP you have isn't really practical (sometimes telescopes are sold with over optimistic kit).  Unless conditions in the UK are quite good I often find that I stop at around 8mm.

 

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NB.  Have you looked at this thread yet:

Even if you don't read all the text at the least scroll down and look at the pictures and remember that the small ones are through a scope even larger than yours.

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When observing planets, much does indeed depend on the seeing conditions. With my F/10 8" SCT I generally start out with the 14mm Delos (145x), which is generally good, except on nights of really bad seeing, and move up to the 10mm Pentax XW (203x) which is often OK, but you may have to wait for moments of good seeing. On good nights, the Delos 8mm can be used (254x), and on really good night I use the XW 7 mm (290x), but again, you need to wait for moments of good seeing. I also have a Pentax XW5 (406x) which I have occasionally used on the moon, but in practice I only use it regularly in my 80mm F/6 scope, where it produces a very usable 96x. I am sometimes tempted when a second-hand XW3.5 turns up, which would yield 137x in the 80mm F/6 triplet, but as I do not use that for planetary viewing much, I would be better off saving money for that big, fast Dob I am longing to get.  Once I have that, I will have an excuse to get that XW 3.5 ;)

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2 hours ago, Ben the Ignorant said:

You can use higher than rated magnification on small bright deep-sky objects like planetary nebulas or compact galaxies

I said extreme powers are ok only on the small and bright nebulas and galaxies. The reduction in surface brightness is comparable to that of a light pollution filter.

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3 hours ago, mnanand said:

I am having newtonian 130mm F5 OTA, for which the rated maximum magnificaton possible is 260x. With this seeing a solar system bodies are not that easy. As per the stellarium simulation, iIf I use 5x barlow with 2mm eyepiece I can see it very nicely. Is it a good idea to do so? What will happen if I use higher magnification than the rated maximum?

A smaller sharper image is better than a larger blurred image.

As a guideline - when the planets are high is the sky I use the following magnifications in my 5" refractor :-

Saturn x200

Jupiter x180

Mars x250

Hope that helps

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I’ll start by saying that I’m not a big visual observer, but I do dabble and I always take a look at Jupiter, Saturn and Mars when they are around. I have classic C14, so a 14” aperture (with central obstruction) and  native focal length of nearly 4m.

My normal viewing magnification range for the planets is around x200-x325. My favourite is with a 12mm TV  Nagler (3905/12=x325), but I often use a 25mm Meade with x2 Powermate (3905x2/25=x312). On nights of poor seeing I use my ES68/40mm with the x2 Powermate (3905x2/40=x195). If seeing is exceptional then I have used the 12mm with the x2PM for a magnification around x650, but that is extremely rare and only worked one night on Jupiter this past year and even then it was questionable whether I got anything more than viewing at x325. I do get away with x650 on the Moon, when the seeing is good and the Moon is riding high; I have even got up to x1000 with a 4mm TV (just because I could), but really that was not a great combo.

When viewing DSOs I never get above x325 and most often have the best visual experience at around x200.

As others have commented a smaller sharper image is almost always better than a larger blurred image.

Also be aware that Jupiter and Saturn will remain very low, i.e., close to the horizon, for the next few years, so in addition to the resulting poor seeing, they will also suffer from atmospheric dispersion which appears even worse at high magnifications, unless you introduce an atmospheric dispersion corrector (ADC) into your optical train.

Good luck, Geof

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1 hour ago, Ben the Ignorant said:

I said extreme powers are ok only on the small and bright nebulas and galaxies. The reduction in surface brightness is comparable to that of a light pollution filter.

But as I point out even small, bright galaxies have lower surface brightness than planets, and almost all planetary nebulae (apart from one or two near stellar ones, like Jonkheere 320 an d 900 in Orion and Monoceros respectively) have distinctly lower surface brightness than planets. The effect of going from 130x to 260x means losing two full stops in terms of surface brightness (a factor of four), and going to 520x means a reduction of 16x with respect to 130x. These numbers correspond to 25% and 6.25% transmission filters, which is more like a moon filter than any LPR filter I have ever used. LPR filters have transmission curves like the one found here

https://www.firstlightoptics.com/light-pollution-reduction/idas-p2-light-pollution-suppression-filter.html

 

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Of course extreme powers make things blurry and fainter, but they also make them much bigger on the retina, which can render them easier to detect. Galaxy M94 in Canes is quite tight and dense, it takes very high power well, NGC 7662 in Andromeda is another example of very high surface brightness. The Sombrero galaxy, M104, and the spindle galaxy, NGC 3115 are pretty strong, too. NGC 2903 in Leo is not as obvious but has interesting mottling that can show with the right combination of large aperture (not necessarily the 130 mnanand is using), good transparency and strong magnification.

The detail (obviously larger than planetary detail) that can't be seen at lower power can sometimes be seen at "off-limit" magnifications. All it takes is some personal experimentation to exploit this possibility when conditions and gear allow it.

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14 minutes ago, Ben the Ignorant said:

Of course extreme powers make things blurry and fainter, but they also make them much bigger on the retina, which can render them easier to detect. Galaxy M94 in Canes is quite tight and dense, it takes very high power well, NGC 7662 in Andromeda is another example of very high surface brightness. The Sombrero galaxy, M104, and the spindle galaxy, NGC 3115 are pretty strong, too. NGC 2903 in Leo is not as obvious but has interesting mottling that can show with the right combination of large aperture (not necessarily the 130 mnanand is using), good transparency and strong magnification.

The detail (obviously larger than planetary detail) that can't be seen at lower power can sometimes be seen at "off-limit" magnifications. All it takes is some personal experimentation to exploit this possibility when conditions and gear allow it.

I assume the OP has posted in "Getting started equipment help and advice" for a reason............

Do you think your posts encouraging a newcomer to the hobby to start out his observing experience by ignoring Dawes limit is the best advice? 

 

 

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Another aspect to consider when striving for higher powers is how well the telescope is collimated.  A telescope has to work harder as one proceeds to go higher in power.  The more accurate, the more bang-on, the collimation, the sharper the images will be at the higher powers.  Also, how well was the parabola of the primary-mirror figured at the factory?  That plays a part, too.  The level of quality of the eyepieces can be important as well.  An f/5 parabola can be a bit picky as to the eyepieces introduced to it.  Plossls are the barest minimum to consider.  

What eyepieces do you have?

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If you have only the eyepieces that came with the telescope kit, this would be a very good value with which to upgrade...

http://www.tejraj.com/gso-eyepiece-kit.html

Also, a 5x barlow is generally used when imaging.  For visual use, a 3x barlow is practically the maximum to be employed.

Edited by Alan64
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FWIW, as it's a sample of one, I used a 3.2mm BST Starguider with my 150PDS, which  gives 234x magnification. The image of Jupiter was intermittently blurry and pin-sharp with fantastic detail, so no regrets.

However, I had been advised not to get that eyepiece as it would be 'useless' due to it giving too much magnification - presumably for UK skies. This turned out not to be the case, and it is within the 2x-aperture-mm rule mentioned here.

My take from all of this is to try it and see. There are lots of variables - and almost as many opinions.

If you have the option to return kit if it doesn't work out, give it a try. Maybe join an astronomy club and borrow kit to see for yourself how far you can push things?

I guess that as you gain observing experience and more eye for detail you'll find your own limit in any case.

 

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