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Aperture vs focal ratio


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Nearly all of my life, I've heard two things.

One is that aperture is King . You always want as much Aperture as you can afford.

Two is that longer focal ratios are always better or generally better for lunar and planetary observation.

Here's the issue though. As aperture increases, the focal ratio goes down to keep the size of the telescope from becoming an unwieldy beast. Most of my best views of the moon and planets have been from mediam to large (8" to 12")apertures with ff4.5 to f/6.0 focal ratios. So, is a longer focal ratio always better for lunar and planetary or is aperture still king? 

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Two different things really.

Aperture - it dictates possible level of detail that can be observed. Due to nature of light and diffraction effects - aperture limits level of detail that can be resolved. Larger aperture is able to resolve finer detail.

Focal ratio acts in couple of ways to improve high power views:

- slower scopes often have shallower curves that are easier to make. Any optical aberrations will be therefore smaller on slower scopes for same level of manufacturing quality. Other aberrations also get smaller with slower scopes - like in reflecting scopes - spherical primary will impact less spherical aberration if scope is slow, or in parabolic - there is less coma if telescope is slow

- much more easy to get high powers with regular eyepieces. 6mm plossl or ortho will be very uncomfortable for viewing, but 18mm versions will be quite comfortable. They will yield same magnification on F/5 and F/15 systems respectively.

- slower scopes make simple eyepieces perform better

 

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  Although its often stated that aperture is King, and that longer focal lengths are better for lunar and planetary, there are other factors that often come into play. Atmospheric seeing is one such factor, where smaller apertures appear to be unaffected while larger apertures can be hindered greatly, so things are not as clear cut as they may first seem. However, where a 6" scope may be unaffected by seeing while a 10" may be hindered by it, it doesn't necessarily mean the 10" will perform like a 6". Very often the smaller aperture will actually outperform the larger scope. That is one reason many astronomers have more than one telescope, so they can bring out the gun that suits the situation and subject being studied.

  Longer focal length scopes can show less aberrations than their shorter equivalents, but its good to remember that advancements in optical design of both telescopes and eyepieces has largely overcome such differences. Today a good quality shorter focal length lens or mirror when matched with equally well designed eyepieces, Barlow lenses or powermates, can match or even surpass their longer siblings in performance. And shorter scopes are often easier to use, cheaper to mount and easier to store or house, and because of this they may be used more often than a cumbersome Goliath.

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17 hours ago, cpsTN said:

Nearly all of my life, I've heard two things.

One is that aperture is King . You always want as much Aperture as you can afford.

Two is that longer focal ratios are always better or generally better for lunar and planetary observation.

Here's the issue though. As aperture increases, the focal ratio goes down to keep the size of the telescope from becoming an unwieldy beast. Most of my best views of the moon and planets have been from mediam to large (8" to 12")apertures with ff4.5 to f/6.0 focal ratios. So, is a longer focal ratio always better for lunar and planetary or is aperture still king? 

I’m a refractor nut so tend to enjoy the stable, if less detailed views in a good 4” or 5” refactor, but will be the first to admit that if skies permit, greater aperture with show much more detail.

Some real world experiences:

Tak FC100DC f7.4 side by side with an 8” f10 Edge in seeing varying from below average to good, viewing Jupiter. The Tak gave a good stable view most of the time, detail was good and the GRS showed as a fairly pale colour. In the 8”, the view was much more variable, going from mush to good sharp detail repeatedly as the seeing changed. When the seeing was good, more detail was visible and the view was more colourful, GRS a more vivid orange colour as I recall. I actually preferred viewing the refractor image because it was more stable and easy to concentrate on, but the 8” showed more when the seeing steadied.

 

8” f8 1/10th wave dob vs Vixen FL102S f8.8 on Mars. I set these two up when Mars was near opposition and the seeing was excellent. This turned out to be no contest, I spent virtually all my time viewing through the 8” because the detail was much better in it, and the views stable because the seeing was so good. Nothing to do with focal ratio (virtually the same), all to do with aperture. Report here:

400mm f4 dob vs aperture masked 170mm f9.4 dob

I experienced this when observing with @Moonshane (let’s see if I can tempt him out of retirement with the tag 😉. Miss your presence on here Sir!). The conditions were pretty poor, but the two configurations behaved in line with expectations; larger aperture gave more detailed and colourful (more saturated) views which were much more susceptible to the variable seeing. When seeing was good the views were better, when poor they were worse. Conversely the 170mm was more stable but a little less detailed and with more muted colours.

I’ve referenced this numerous times on the forum but one example here;

As others have mentioned, longer focal ratio obstructed scopes have the benefit of being easier on eyepieces, and should have better contrast due to the smaller central obstruction, yet a larger aperture, faster scope is still likely to outperform it if seeing conditions allow. It can come down to preference; I prefer a more stable view as I find it more relaxing and enjoyable to observe rather than fighting an unstable one, even if it does ultimately show more detail. Your observing location makes a big difference; if we all lived in the Atacama desert I suspect many of us would have 40” f2.5 dobs 😉🤣.

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I knew what people were going to say generally speaking to this topic. This came to my mind that the other day when someone said they moved from a 6in f/5 Newt (I believe) to a 4in Mak and said they could see more detail and more clarity on the moon. But I was thinking you shouldn't be able to see more detail and more clarity as a general rule when you move down in aperture, assuming equal quality of optics, especially when you're moving down 2in in aperture. When you go from a 6in telescope to a 4in telescope, you were losing tons of brightness at equivalent magnifications. ???

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16 minutes ago, cpsTN said:

I knew what people were going to say generally speaking to this topic. This came to my mind that the other day when someone said they moved from a 6in f/5 Newt (I believe) to a 4in Mak and said they could see more detail and more clarity on the moon. But I was thinking you shouldn't be able to see more detail and more clarity as a general rule when you move down in aperture, assuming equal quality of optics, especially when you're moving down 2in in aperture. When you go from a 6in telescope to a 4in telescope, you were losing tons of brightness at equivalent magnifications. ???

If the newtonian was out of collimation (F/5 has a small collimation "sweet spot") and the mak in good collimation (much more likely) that might well make the difference.

 

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5 hours ago, cpsTN said:

A 6-inch aperture collects more than double the amount of light than a 4-inch aperture will. The 6-inch would have to be twisted into a pretzel to perform that poorly. 

Not where resolution of lunar and planetary detail is concerned.

While the light gathering power of the additional aperture is significantly more with the 6 inch, the resolving power increase is somewhat less. With the collimation "sweet spot" of an F/5 scope being quite small compared with the tolerance of a 4 inch mak (which usally around F/12) plus the diffraction efects of the secondary support system of the newt, I can quite see why a slightly miscollimated F/5 6 inch newtonian could be outperformed on lunar sharpness by a 4 inch mak in good optical shape.

For observing deep sky objects, where aperture is more important, the 6 inch scope would have an advantage of course.

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6 hours ago, cpsTN said:

A 6-inch aperture collects more than double the amount of light than a 4-inch aperture will. The 6-inch would have to be twisted into a pretzel to perform that poorly. 

There have been times when I’ve seen a lot more detail with a 4” apo than a 6” f5 newt. It comes down to conditions, the scope itself, cooling, seeing, target, blah blah blah.

Sometimes Jupiter in a larger scope can just appear too bright which washes out any detail. If the conditions don’t support getting the mag up then you don’t see anything. A good 4” apo refractor, or Mak, will often show more under these circumstances. It just varies so much. I’ve had great planetary views through a 6” f8 newt, and bad ones through a 12” f4.8 dob (as well as good ones!)

Perhaps at small apertures the focal ratio has more impact in obstructed scopes, but by the time you get up to 12” and above there is so much aperture/resolution available it overcomes any inherent benefit of the longer focal ratio.

I will say that my 8” f8 gives wonderful planetary views, enough aperture to give great detail, but small enough to cut through seeing conditions well still. The small central obstruction helps with contrast too, lovely scope.

One of the biggest challenges I find, and one reason I don’t spend forever comparing different kit, is that our U.K. conditions are just so changeable it is hard to know if your comparisons are valid or not.

 

 

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6 hours ago, cpsTN said:

A 6-inch aperture collects more than double the amount of light than a 4-inch aperture will. The 6-inch would have to be twisted into a pretzel to perform that poorly. 

I've actually owned a 4" Mak and a 6" f5 newt at the same time and observed with them together. The Mak will not show "more detail" when lunar observing. 

At higher magnifications the view through the maksutov was more stable, with better contrast and easier on the eye, but the newt did resolve finer details.

During Mars opposition 2020 I had them both out and the newtonian hands down showed more albedo effects and the southern ice cap was better resolved. The only benefit the mak had was a lack of diffraction spikes on a bright Mars. Essentially, observing is easier with the Mak but you don't see quite as much...

747738776_IMG_20200920_2029387932.thumb.jpg.eaafacccda62cefff8f6353f2a19e264.jpg

The newtonian was perfectly collimated. 😉

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