Magnification on Jupiter, yes, yes or no, no?

[John Baars]

Novice amateurs are often lured in advertisements by the astronomical magnifications offered by the small instrument.
That this is not always sunshine, they do not yet know. When I started, the manufacturer promised 450X magnification with my 60mm refractor. On Jupiter, 100X turned out to be the maximum for a good image.

By comparison, my current 120mm Apochromatic should reach 900X in that same faulty reasoning from above. In practice, in the field, on Jupiter the optimum is between 120 and 180X with the 120mm and more often an outlier down than up. For the record, 180X is rarely achieved.


A refractor under excellent conditions can handle magnification twice its aperture in millimeters. A mirror telescope because of the central obstruction a little less. Above that, little or no detail becomes visible. However, the doubling of magnification as a maximum is not set in stone. Exceptions confirm the rule. Usually on a planet like Jupiter, less is better.
Still, reports of huge magnifications on Jupiter with colossal scopes reach us. Without exception, these appear to be top instruments at top viewing conditions and top transparency. If one of these elements is not "top", everything is spoiled. So it is far from realistic from our backyard.


Let's see.

Doubling the magnification produces :
- An image that is 4X less bright.
- An image that is 4X less rich in contrast.
- an image hampered by the protein strings in our own eye fluid, the floaters.
- an image that appears a lot more turbulent due to the seeing magnified along with it.

Magnifying even higher does not yield more details, we call that empty magnification.

Sometimes high magnification with calm seeing brings out small details better, but you have to wait for those moments. Most of the time, however, it gets worse.

So there are a lot of good reasons not to choose high magnification on Jupiter.

Just some pictures that I have edited to the best of my ability.

1. A sketch of Jupiter some years ago at approximately 100X. Quite a lot of details are visible.

2. The same, but at almost double magnification, i.e. 200X. Some details are already fading, a few are becoming slightly more visible. I would never go higher than this. A picture like this would immediately prompt me to lower the magnification.

3. Another doubling. Much fuzzier , no more sharp details. This is the "ad magnification," of the 60 mm instrument from the beginning of the story.
The image is remarkably larger, but not sharper or more detailed. In fact, you see even less. This is what we call an "empty magnification".

Let's be honest, which of the three Jupiters would you like to see in your eyepiece?

[helicon]

Thanks for the very helpful post John. The first scope I received at around age 6 or so was a 40mm x 400x drawtube refractor. I was slightly disappointed that what I could see, mainly the moon and Jupiter, did not match the colorful photos of nebulae and galaxies on the box!

[Makuser]

Hi John. Thanks for the helpful guide on sensible magnification. So many folks (especially at Christmas time) see the telescope boxes with a Space Telescope image near the store checkout lane, and they usually include a cheap wobbly mount. Mathematically, the included cheap short focal length eyepieces along with a cheap Barlow lens will provide the ridiculous advertised magnifications. But what you get is an unusable dark blurry blob image on a shaky mount. Sadly, these telescopes soon end up in a garage sale or pawn shop and what could have otherwise been a long and fascinating interest in the astronomy hobby is cut short. Thanks again John for your informative report and the nice illustrative sketches.

[Bigzmey]

Well explained John! A picture worth a thousand words. Matches my experience to the letter.

On a typical night here best views of planets are achieved at 100-150x and using scopes with aperture above 100mm refractor is counterproductive because at that powers they intensify the glare, but don't provide extra details on top what can be seen with 100mm.

[John Baars]

Thanks for the very helpful post John. The first scope I received at around age 6 or so was a 40mm x 400x drawtube refractor. I was slightly disappointed that what I could see, mainly the moon and Jupiter, did not match the colorful photos of nebulae and galaxies on the box!

Thanks!
Fortunately, you are among those who were able to overcome that.

Hi John. Thanks for the helpful guide on sensible magnification. So many folks (especially at Christmas time) see the telescope boxes with a Space Telescope image near the store checkout lane, and they usually include a cheap wobbly mount. Mathematically, the included cheap short focal length eyepieces along with a cheap Barlow lens will provide the ridiculous advertised magnifications. But what you get is an unusable dark blurry blob image on a shaky mount. Sadly, these telescopes soon end up in a garage sale or pawn shop and what could have otherwise been a long and fascinating interest in the astronomy hobby is cut short. Thanks again John for your informative report and the nice illustrative sketches.

Thank you. I hope we as an astronomical forum can function a sort of Christmas-present warning. I thought it was about time to visualize it too.

Well explained John! A picture worth a thousand words. Matches my experience to the letter.

On a typical night here best views of planets are achieved at 100-150x and using scopes with aperture above 100mm refractor is counterproductive because at that powers they intensify the glare, but don't provide extra details on top what can be seen with 100mm.

Thanks.
Like I said above, it was about time.
For us, as experienced amateurs, it seems normal. However, I still sometimes read sentences like: "at 300 + Jupiter still looked razor sharp". I suppose those were not earth-based observations, made with alien instruments. I agree, for city dwellers good f/10-f/15 achro refractors and apo's in the 100-150mm range seem excellent performers on planets.

[Razz]

I'm still new at this and I will admit that, before I got my first scope, I was always of the mindset that the higher the magnification the better. More magnification means a closer view. Closer view means better detail. I never knew about a scope's limitations, seeing issues being magnified as well, along with the fact that even the slightest movement of the scope is a huge deal with higher magnification. Thanks to posts like this I became aware of all these things. It really helped me tone down my expectations so I didn't get frustrated and give up. So, thank you John and all the others that post info like this. We beginners appreciate it.

[John Baars]

(...) I never knew about a scope's limitations, seeing issues being magnified as well, along with the fact that even the slightest movement of the scope is a huge deal with higher magnification. (...)

Thank you.
Thanks for reminding me. The mount and tripod used by many manufacturers of those low cost Christmass-present toy-telescopes is almost always inadequate. Often the closing item on the budget. Very shaky, to say the least. When magnification is doubled, the visibility of those shakes too. Adequate focussing becomes a true issue. One can't focus on fine details when they are troubled by heavy trembling.

[Lady Fraktor]

A excellent write up as always John!
I must admit that I had exceptional seeing and transparency last night so was exceeding the 100x mag limit by a small bit
Jupiter, lunar and globular were simply stunning!

In average seeing/ transparency no magic glass will benefit either.
I was out for 2 nights with 3 telescopes, OK-4, FPL-53, Fluorite and they all presented quite similar amount of details.
Atmospheric limits kept them all on par

[notFritzArgelander]

Nice illustration of the effects of over magnification!

I'd like to add a little bit about the effects of central obstruction. With a perfectly collimated scope the central obstruction depresses contrast at low spatial frequencies and enhances contrast slightly above a perfect unobstructed optic at the high spatial frequencies needed for planetary detail . See figure 104 at this link: https://www.telescope-optics.net/obstruction.htm

The problem with apparent loss of contrast in centrally obstructed optics arises more from bad collimation practice. The superb planetary photographs from centrally obstructed optics are produced by folks who collimate early and often.

So the loss of contrast with centrally obstructions is mostly due to poor collimation hygiene.

[Star Dad]

Nice write up. I would add that a more expensive scope does not make a better one. My case in point is my $300 Newt at 1000mm fl yields an image at 111x the same quality as my club's $2700 Maksutov-Cassegrain at 2370mm fl. If seeing is excellent I will bring out the Mak - in the hopes it will be a superior image. But in my locale (Eastern CT) I have yet to have a night this year with a 5 seeing rating (according to MeteoBlue). I warn people not to buy such a scope unless they are planning to move to where clear skies are in abundance. I also warn them of the lure of those 60mm f10 refractors big box stores sell. I constantly have to explain that more power doesn't necessarily mean a better image... and most deep sky stuff rarely uses anything above 50x. I find most folks are just happy to see the rings (and Cassini division) of Saturn and the storms on Jupiter all at 111x. I bought a 5mm lens (200x) and rarely use it and a 3mm lens (333x) which was a total waste of money - except on the moon.

[John Baars]

Nice illustration of the effects of over magnification!

I'd like to add a little bit about the effects of central obstruction. With a perfectly collimated scope the central obstruction depresses contrast at low spatial frequencies and enhances contrast slightly above a perfect unobstructed optic at the high spatial frequencies needed for planetary detail . See figure 104 at this link: https://www.telescope-optics.net/obstruction.htm

The problem with apparent loss of contrast in centrally obstructed optics arises more from bad collimation practice. The superb planetary photographs from centrally obstructed optics are produced by folks who collimate early and often.

So the loss of contrast with centrally obstructions is mostly due to poor collimation hygiene.

Quite so, I am aware of those optical test results.
On top of that maybe I should add that in most popular obstructed systems, like Newtonians, SCT´s and Maksutovs the wavefront passes the turbulent air inside the tube twice, trice respectively. The first time even alongside the cold tube wall. In refactors only once, away from the tube wall. In the field this means a headstart for the refractor. All in all it means that in field comparison situations with two telescopes of equal aperture and quality, I have never seen an obstructed telescope come out on top of an unobstructed one. When in equilibrium( if reached) they hardly differ, I must admit.

[John Baars]

(...)
I must admit that I had exceptional seeing and transparency last night so was exceeding the 100x mag limit by a small bit
Jupiter, lunar and globular were simply stunning!

In average seeing/ transparency no magic glass will benefit either.
(...)
Atmospheric limits kept them all on par

I may hope , you could magnify well over 200 or even as far as 300, given the circumstances!
Under average circumstances they all sing the same song, I am afraid.

[Bigzmey]

In spring of 2016 Jupiter was in Leo, very high in the sky during evenings. There was something in the air that spring, the seeing was fantastic and I was able to push to above 400x with my 8" SCT. The level of fine details was unreal. So, large reflectors are capable of delivery of great planetary views, but conditions need to be right. 6 year later I am still waiting/hoping for another spring like that. The things however seem to be going the other way in terms of seeing.

[gregl]

In spring of 2016 Jupiter was in Leo, very high in the sky during evenings. There was something in the air that spring, the seeing was fantastic and I was able to push to above 400x with my 8" SCT. The level of fine details was unreal. So, large reflectors are capable of delivery of great planetary views, but conditions need to be right. 6 year later I am still waiting/hoping for another spring like that. The things however seem to be going the other way in terms of seeing.

Those days are rare. Back in 2014 I was up at our observing site at 8000 ft. altitude, where the word "dark" really means something. The atmosphere was rock solid, and the old timers who had been going up there for many years said it was the best night they had ever seen up there. Through my 8-inch SCT I saw the ice cap on Mars and some mottling of the surface. I was able to run through all my eyepieces and, as I recorded in my log, "This seems to be a night for the 9mm ortho at 225X. Everything seems to be popping in nice." I even took the scope to an outrageous 338x through a $12 plossl and noted only a slight loss of sharpness. From my log of that night, at 338x, "The white spot was still there, looking like a tiny dot of whipped cream applied by a stingy restaurant manager to a scoop of caramel crunch ice cream."

Turning to Saturn, I got the Encke gap, which I've never bagged since. My log reads, "This view of Saturn is so phenomenal I could spend the whole evening looking at that."

I concluded, "Observing is a bit of a gamble.... But this night was one that started out exceptional and remained that way until we were too exhausted to continue. And considering all the variables involved, I may never see Saturn like that again, but I won't forget what it can look like."

And so it is, I've never had a night like that since.

[Lady Fraktor]

(...)
I must admit that I had exceptional seeing and transparency last night so was exceeding the 100x mag limit by a small bit
Jupiter, lunar and globular were simply stunning!

In average seeing/ transparency no magic glass will benefit either.
(...)
Atmospheric limits kept them all on par

I may hope , you could magnify well over 200 or even as far as 300, given the circumstances!
Under average circumstances they all sing the same song, I am afraid.

I will have to write something up, I was doing comparative views with the 80mm, 105mm and 115mm.
For the 3rd night I added the TAL 100RS to the mix and was a solid performer against the competition.
I will say that when I finished viewing on Wednesday night I was using a Vixen HR 2.4mm in the 105 and the views where just stunning.
Viewing the lunar surface was like looking at a high resolution image.

[PeterD]

John, that is a very good write up. I am glad I am not the only one who uses modest magnification on Jupiter.
I went through my notes yesterday evening it is mostly 111x I observe the Jupiter with. When the conditions are good I go up to 166x and I have only one record when 250x was enjoyable. I observe with a smallish 80mm to 100mm refractors only.

[John Baars]

(...)
I will say that when I finished viewing on Wednesday night I was using a Vixen HR 2.4mm in the 105 and the views where just stunning.
Viewing the lunar surface was like looking at a high resolution image.

The lunar surface and in particular high contrast craters and shadows are always very enjoyable with high magnifications.

[John Baars]

John, that is a very good write up. I am glad I am not the only one who uses modest magnification on Jupiter.
I went through my notes yesterday evening it is mostly 111x I observe the Jupiter with. When the conditions are good I go up to 166x and I have only one record when 250x was enjoyable. I observe with a smallish 80mm to 100mm refractors only.

Thanks,
Yes, Many people start out on Jupiter with the highest possible magnifications. Pity. So much fun details are lost.

[PeterD]

The lunar surface and in particular high contrast craters and shadows are always very enjoyable with high magnifications.

Yes, 4mm ortho that gives 250x with my current scope is my main moon eyepiece. I sometimes even go further withe a barlow but not very often.

Yes, Many people start out on Jupiter with the highest possible magnifications. Pity. So much fun details are lost.

I think the human brain is very efficient at magnifying what otherwise appears rather smallish in the eyepiece.
I was looking at the Jupiter with my 6 years old the other evening. She has drawn a picture of what she has seen about half an hour later. There was Jupiter and 3 moons. Jupiter itself covered perhaps 1/4 of an A4 sheet. I asked her if was really that big in the eyepiece. Yes, she replied with no hesitation
The TFOV with the scope/eyepiece combination was about 0.4deg. Jupiter was a tiny disc really.

[davesellars]

I've managed to get in quite a bit of planetary observing this summer and early Autumn with both cyclops and binoviewers. Both my 102ED and 80ED can easily manage 50x per inch without any real loss of sharpness however the 12" dob's limit I would put at somewhere around 25x per inch before I start to see loss of sharpness (at 300x). Obviously, these figures are limited to very good seeing conditions and realistically, much less is achieved due to worse seeing.

From my observations of Mars or Saturn, I've found that when seeing conditions are suitable I can really use a very high magnfication not to the detriment to the contrast of the planet. Similar to observing the Moon...

From my observations of Jupiter using 120x with binoviewers has generally yielded more detail and contrast that at 180x (using the 102ED), however at times I have managed to get a very good view at 180x but this has been rare and seems to need good transparency as well as very good seeing. Contrast and detail is very easily lost with Jupiter and I concurr with your findings that much more is visible by dropping the magnification. I wil say though, that at least 50% more detail is visible with binoviewers than with cyclops with colour also more visible.