Highest Useful Magnification in Your 80mm Refractor?

[Renato1 (Renato)]

Hi Everyone,
I've been trying to measure the highest useful magnification in my old 80mm ED f/6.25 refractor. The other night - cold, clear, no stars twinkling - I thought I'd figured out the bounds. With a 3mm eyepiece at 167X I had a very good image of Jupiter, but with a 2.5mm eyepiece at 200X - I had to stare hard to just make out the bands, though occasionally I'd see some faint detail. Not much contrast at 200X.

Then next night, I dug out my old University Optics achromat refractor and repeated the exercise. Again, good image at 167X, but a far worse image than in the ED telescope at 200X - I could barely make out a hint of detail

And thus I thought,
"Mission Accomplished - at 200 times, the scope is useless".
At which point some cloud went in front of Jupiter, and the 200X image in my achromat was suddenly better than in my ED refractor, with bands easily visible.

Of course, it's been cloudy or raining ever since, and I haven't gone back with Barlows or Powermate and Polarising and ND filters to see just how high a useful power I can get from the 80mm diameter.

So, I'm very curious. How far, magnification-wise, have you pushed your 80mm refractor on Jupiter before deciding the exercise had become pointless?
Regards,
Renato

[Ukastronomer (Jeremy)]

Isn't it usual that it is 10x for every inch of aperture, depending on the quality of the scope hence there is no definitive answer.

My 120 triplet will go 15x or more per inch on good seeing whilst an acromat 8-10

I see no reason to "push" it, for what reason if the image is degraded

Too many factors you can't account for including peoples eye site and eyepiece quality on given scope, Experienced planetary observers use 20x to 30x per inch of aperture to see the most planetary detail.

https://www.skyandtelescope.com/astr...magnification/

[Wavytone]

Not true, though a starting point for “average optics”, “average seeing” and “average eyesight” :

- x2 per mm of aperture for refractors,
- x1.5 per mm for newtonians,
- x1.2 per mm for SCTs.

The useful maximum varies according to:
- the type of object observed, and it’s brightness;
- the type of scope;
- the quality of the optics,
- the observers eyesight, and
- the seeing conditions !


The brightness of the object matters a lot because the distribution of rods and cones in your retina is not uniform. If it’s bright enough that the cones respond (colour) the spacing of the cones in the macula (centre spot in the retina you use for reading) is extremely dense, but but the spacing drips off dramatically outside the macula. The spacing of the rods (night vision) is the inverse, ie few in the macula and more outside the macula - hence the trick of averted vision for faint objects. This also comes into play when deciding what magnification is optimum for a particular object.

Regarding faint extended objects like DSOs there is a sweet spot at about 1X per mm where things appear reasonably bright and sharp and increasing the magnification beyond that just makes them harder to see and no more detail is to be had.

But on bright double stars, with good eyesight, to resolve Dawes criterion on double stars Dawes noted that 3X per mm is required. This is interesting because even with modern optics few refractors can usefully go to such extreme power. On your 120mm triplet that means 80X per inch - far more than you suggested.

The other aspect is the quality of the optics; for example on double stars, the moon and planets an Intes M715 and my MK91 can run to 2.5X per mm of aperture. Yet if you try that in the average SCT the image breaks down into a blurry mess at half that.

Similarly short achromatic refractors will run out around 1.5X per mm, beyond that everything is a blurry mess. The reason why achro refractors were f/15 to f/20 was to reduce aberrations to achieve the best possible performance, and to provide a focal length long enough to achieve 3X per mm with comfortable quality eyepieces.

[Renato1 (Renato)]

Originally Posted by Ukastronomer

Isn't it usual that it is 10x for every inch of aperture, depending on the quality of the scope hence there is no definitive answer.

My 120 triplet will go 15x or more per inch on good seeing whilst an acromat 8-10

I see no reason to "push" it, for what reason if the image is degraded

Too many factors you can't account for including peoples eye site and eyepiece quality on given scope, Experienced planetary observers use 20x to 30x per inch of aperture to see the most planetary detail.

https://www.skyandtelescope.com/astr...magnification/

Thanks for that, but I think you mean 10X per centimeter rather than inch.

20X per centimeter I easily get in the 80mm telescopes, but what I'm exploring is how far towards the 30X per centimeter I can get out of them.

Why am I pushing in such a tiny telescope?
Because I'm just making my way back to viewing after not looking through a telescope for two years - after a back operation, followed by recovery, followed by feeling like I was about to lose my footing whenever I went out in the dark and looked up. I still can't sit down for very long, so I have to do quick sessions with a lightweight telescope. My 100mm achromat refractor is pretty good on DSOs, but inferior to my two 80mm refractors on planets.

Wouldn't your 120mm triplet easily do somewhere between 20 and 30X per centimeter - 240X to 360X on the planets? Or more maybe?
Cheers,
Renato

[Renato1 (Renato)]

Originally Posted by Wavytone

Not true, though a starting point for “average optics”, “average seeing” and “average eyesight” :

- x2 per mm of aperture for refractors,
- x1.5 per mm for newtonians,
- x1.2 per mm for SCTs.

The useful maximum varies according to:
- the type of object observed, and it’s brightness;
- the type of scope;
- the quality of the optics,
- the observers eyesight, and
- the seeing conditions !


The brightness of the object matters a lot because the distribution of rods and cones in your retina is not uniform. If it’s bright enough that the cones respond (colour) the spacing of the cones in the macula (centre spot in the retina you use for reading) is extremely dense, but but the spacing drips off dramatically outside the macula. The spacing of the rods (night vision) is the inverse, ie few in the macula and more outside the macula - hence the trick of averted vision for faint objects. This also comes into play when deciding what magnification is optimum for a particular object.

Regarding faint extended objects like DSOs there is a sweet spot at about 1X per mm where things appear reasonably bright and sharp and increasing the magnification beyond that just makes them harder to see and no more detail is to be had.

But on bright double stars, with good eyesight, to resolve Dawes criterion on double stars Dawes noted that 3X per mm is required. This is interesting because even with modern optics few refractors can usefully go to such extreme power. On your 120mm triplet that means 80X per inch - far more than you suggested.

The other aspect is the quality of the optics; for example on double stars, the moon and planets an Intes M715 and my MK91 can run to 2.5X per mm of aperture. Yet if you try that in the average SCT the image breaks down into a blurry mess at half that.

Similarly short achromatic refractors will run out around 1.5X per mm, beyond that everything is a blurry mess. The reason why achro refractors were f/15 to f/20 was to reduce aberrations to achieve the best possible performance, and to provide a focal length long enough to achieve 3X per mm with comfortable quality eyepieces.

Thanks Nick,
I can't argue about the 1X per mm for fainter DSOs, which equates to a 2mm exit pupil - I've tested above and below that spot for years and, with the exception of faint open clusters where a 3mm exit pupil is sometimes better, it's pretty pointless deviating from that spot.

I seem to be pretty lucky with my short achromat 80mm which easily does better than 1.5X per mm - but when I bought it, University Optics claimed it could rival more expensive refractors because of the special glass they used. Though now that you've mentioned that 1.5mm figure I'll go and check whether my 100mm achromat gets close to it, though I think it will struggles and need filters.

The 1.2X per mm in an SCT I think is generally correct, though about once every two or three weeks things being steady enough to take my 8" SCT to 2X per millimeter.

The 2.5X per millimeter in your Intes is what my 80mm achromat surprisingly achieved when the cloud got in the way of Jupiter. Now I'll have to see if with filters I can get the same to happen again in both 80mm refractors, and if so, push ahead to a higher magnification.

The other factor, in addition to those you mentioned, is the gunky stuff one has running around inside one's eyes that shows up in the eyepiece at high power. Whenever I sit down and look through the refractors at very high power, I say "Yuck" from how the stuff interferes with the image. But then, after a few minutes, the effect isn't so bad. I'm not sure if they've gone away - or if my brain is filtering them out.
Regards,
Renato

[Renato1 (Renato)]

Well, in the early hours of the morning, with clear skies and cold weather and Jupiter overhead, using my 2.5mm TMB eyepiece, my little 80mm ED refractor delivered nice enough views of Jupiter and Saturn at 200X.

Which means there is still work to be done to find the actual highest magnification I can push that telescope to.
Regards,
Renato

[DeWynter (ILYA)]

Originally Posted by Renato1

Well, in the early hours of the morning, with clear skies and cold weather and Jupiter overhead, using my 2.5mm TMB eyepiece, my little 80mm ED refractor delivered nice enough views of Jupiter and Saturn at 200X.

Which means there is still work to be done to find the actual highest magnification I can push that telescope to.

"Nice view" doesn't mean resolving more details. You can increase magnification, the view in EP would be bigger, but it doesn't mean that you will resolve more faint details on the surface of the Jupiter. It could be just opposite. Sometimes you lowering the magnification and you see more. Smaller view, but more details.

Wavytone was correct saying - it's also depends on target. I've got 100ED and I found that for Moon I can set anything up to 3D (D is the diameter of aperture in mm's so it's x300) and I'll be fine. Amazing views. But Moon is very bright and contrast.

For Jupiter, I found that magnifications between x130x to just over x180 (so it's between 1.3D and 1.8D) work really well. Going over x180 on most days is useless - no increase in details resolution even with ED refractor.

Saturn on the other hand can tolerate a bit higher resolution - up to 2.2D (x220).

0.5D - 0.7D is best for DSO as it gives nice 2mm - 1.5mm exit pupil for DSO's. To resolve stars in globulars you can go up to 1D.

[Renato1 (Renato)]

Originally Posted by DeWynter

"Nice view" doesn't mean resolving more details. You can increase magnification, the view in EP would be bigger, but it doesn't mean that you will resolve more faint details on the surface of the Jupiter. It could be just opposite. Sometimes you lowering the magnification and you see more. Smaller view, but more details.

Wavytone was correct saying - it's also depends on target. I've got 100ED and I found that for Moon I can set anything up to 3D (D is the diameter of aperture in mm's so it's x300) and I'll be fine. Amazing views. But Moon is very bright and contrast.

For Jupiter, I found that magnifications between x130x to just over x180 (so it's between 1.3D and 1.8D) work really well. Going over x180 on most days is useless - no increase in details resolution even with ED refractor.

Saturn on the other hand can tolerate a bit higher resolution - up to 2.2D (x220).

0.5D - 0.7D is best for DSO as it gives nice 2mm - 1.5mm exit pupil for DSO's. To resolve stars in globulars you can go up to 1D.

Thanks for your perspective. But if you get good views at 300X on the moon with your 100mm ED, I suspect that you will be able to get at or near that on Jupiter under the right conditions.

Like you I couldn't get good views on Jupiter past around 167X most times I tried it. But I was viewing at an angle of 45 to 60 degrees - where there were bound to be some houses along that line of sight putting out heat.

But at 2.30 to 3am this morning, Jupiter was probably 80 degrees up in the sky, Thus I was looking over less properties, and most people had gone to bed and turned off their heaters, and the atmosphere was steady.

Could it be that the moon is more often in a better position than Jupiter is, for you to get your nice 300X view?

I agree with you about the DSOs. A 2mm exit pupil, or slightly higher magnification in light polluted areas, gives the best view. I was making out a few little points of light in globular M22 with a 1mm exit pupil this morning.
Regards,
Renato

[Wavytone]

Originally Posted by Renato1

How far, magnification-wise, have you pushed your 80mm refractor

I’d say you are using less-than perfect optics.

Firstly, real magnification tests are usually done using double stars and for these, to stand any chance at the Dawes criterion the scope must be at about X3 per mm of aperture, or a tad more, in order to distinguish the difference between the Airy disk of a single star vs a double. Note in this the disks are overlapping- not separated.

This is a really tough test and to be honest I’ve seen very few modern scopes that can usefully reach Dawes limit; most cannot. It also means some of the antique old refractors from 150 years ago really did have superlative objectives - while they were doublets, at f/15 or more some really were extremely good with a superb polish on the surfaces to avoid scattered light, which plagues open-tube reflectors.

For your 80mm that suggests 240-260X so at 200X you are getting close to the useful upper limit.

But as for your achro... well you can join the dots.

I have a 70mm quad APO which I’ve used at 200X on Jupiter, Saturn and Uranus. It does these surprisingly well for such a small aperture.

Extreme magnification is also the reason why I have my current scope.

[Wavytone]

Originally Posted by Renato1

How far, magnification-wise, have you pushed your 80mm refractor

I’d say you are using less-than perfect optics.

Firstly, real magnification tests are usually done using double stars and for these, to stand any chance at the Dawes criterion the scope must be working at about X3 per mm of aperture, or a tad more, in order to distinguish the difference between the Airy disk of a single star vs a double. Note in this the disks are overlapping- not separated.

This is a really tough test and to be honest I’ve seen very few modern scopes that can usefully work at Dawes limit. It also means some of the antique old refractors from 150 years ago really did have superlative objectives - while they were doublets, at f/15 or more some really were extremely good and the surface polish must have ben superb to avoid scattered light.

For your 80mm that suggests 240-260X so at 200X you are getting close to the useful upper limit.

But as for your achro... well you can join the dots. FWIW a while back one of the members of my club set up a classic Vixen 60mm f/15 achro refractor and it was simply perfect at high power - an excellent reminder of how good a long achro could be. On Jupiter I’d expect it matches your ED 80mm.

I have a 70mm quad APO which I’ve used at 160-200X on Jupiter, Saturn and Uranus. It does these surprisingly well for such a small aperture.

Extreme magnification is also the reason why I have my current scope.

[Renato1 (Renato)]

Originally Posted by Wavytone

I’d say you are using less-than perfect optics.

Firstly, real magnification tests are usually done using double stars and for these, to stand any chance at the Dawes criterion the scope must be working at about X3 per mm of aperture, or a tad more, in order to distinguish the difference between the Airy disk of a single star vs a double. Note in this the disks are overlapping- not separated.

This is a really tough test and to be honest I’ve seen very few modern scopes that can usefully resold at Dawes limit. It also means some of the antique old refractors from 150 years ago did have superlative objectives - while they were doublets, at f/15 or more some really were extremely good.

For your 80mm that suggests 240-260X so at 200X you are getting close to the useful upper limit.

But as for your achro... well you can join the dots. FWIW a while back one of the members of my club set up a classic Vixen 60mm f/15 achro refractor and it was simply perfect at high power - an excellent reminder of how good a long achro could be. On Jupiter I’d expect it matches your ED 80mm.

I have a 70mm quad APO which I’ve used at 160-200X on Jupiter, Saturn and Uranus. It does these surprisingly well for such a small aperture.

Extreme magnification is also the reason why I have my current scope.

Thanks for your input. That's a mighty fine 70mm you have.

My brain was a tad befuddled by the cold last night when I got the 200X image looking good, or I would have gone and dug out my 2.5X Powermate and taken the power to 250X with a 5mm eyepiece to see what image that combination delivered.

If it stays clear I'll try that tonight, but depending on results, will have other eyepieces handy to go higher or lower power around your suggested range.
Regards,
Renato

[pgc hunter]

Jupiter is a tricky one when it comes to magnification. Jupiter, despite its size and brightness, does not handle magnification well. Its surface features are low contrast and are not "hard edged", that is, not well defined, and are well delicate at best. Increasing magnification actually spreads out the meager light and contrast of those features, resulting in what appears as a fuzzy low contrast image.

Last night I had some good seeing and was using my Vixen 130mm newtonian. The disk of Jupiter and its moons were crisply defined at 232x, infact, the moons were clearly seen as disks. The surface features of the planet, however, while there, were quite delicate and took minutes of viewing to tease them out properly. You need to take the time when viewing Jupiter, when you persevere, you will be rewarded. The planet is also very bright, and its sheer brightness serves to "oversaturate" the low contrast, delicate cloud features. It is often said that dark adaption is *bad* for Jupiter. On a more average night, about 130x provides the best view in my 130mm, while a better night allows 180x, and that perfect night can see 200x or more.

Saturn and Mars on the other hand, they handle magnification far better. Last night on Saturn, 232x was a piece of cake, crisp globe, crisp rings, crepe ring, Cassini Division, colour, it was a sight. probably could've gone 300x if I waited for it to rise higher. The features on Saturn and Mars are far higher contrast than those on Jupiter, hence they can handle magnification better.

[Renato1 (Renato)]

Originally Posted by pgc hunter

Jupiter is a tricky one when it comes to magnification. Jupiter, despite its size and brightness, does not handle magnification well. Its surface features are low contrast and are not "hard edged", that is, not well defined, and are well delicate at best. Increasing magnification actually spreads out the meager light and contrast of those features, resulting in what appears as a fuzzy low contrast image.

Last night I had some good seeing and was using my Vixen 130mm newtonian. The disk of Jupiter and its moons were crisply defined at 232x, infact, the moons were clearly seen as disks. The surface features of the planet, however, while there, were quite delicate and took minutes of viewing to tease them out properly. You need to take the time when viewing Jupiter, when you persevere, you will be rewarded. The planet is also very bright, and its sheer brightness serves to "oversaturate" the low contrast, delicate cloud features. It is often said that dark adaption is *bad* for Jupiter. On a more average night, about 130x provides the best view in my 130mm, while a better night allows 180x, and that perfect night can see 200x or more.

Saturn and Mars on the other hand, they handle magnification far better. Last night on Saturn, 232x was a piece of cake, crisp globe, crisp rings, crepe ring, Cassini Division, colour, it was a sight. probably could've gone 300x if I waited for it to rise higher. The features on Saturn and Mars are far higher contrast than those on Jupiter, hence they can handle magnification better.

Thanks. Hard to believe but one of the best views I ever had of Jupiter was the night of the Great Comet Crash back in the 90s - through wispy clouds, using a cheap K-Mart Focal 114mm reflector which I kept at my parent's place, and which had an actual spider living on the spider vane. I made the mistake of removing and cleaning the mirrors, and never got it working that well again.

Oddly enough, last night at 200X in my 80mm refractor, I thought the view of Jupiter showed more detail on the planet's body than it did on Saturn at the same power.

Interesting your perspective on Mars, no matter which big or small telescope I use, I always have a tough time with it.
Regards,
Renato

[pgc hunter]

Jupiter certainly will show more detail than Saturn, simply because there is more detail But that detail is all low contrast gas on a very bright background.

Mars needs very good seeing to get the most out of it, simply because it is tiny. Any turbulence will destroy that little disk. But when the seeing is good, crank up the power!

[Renato1 (Renato)]

I took the 80mm refractor up to 250X in the early hours of yesterday morning. Image of Jupiter wasn't crash hot. Then I left the telescope there for three quarters of an hour to get it nice and cold. The image became pretty sharp, but detail wasn't that good. I applied a polarising filter which resulted in a bit more detail being seen on the surface - but still not as much as at 200X a week ago.

Next time get a good night. I'll try 217X and 227X and see what image that delivers.
Regards,
Renato