Images from ground-based telescopes are invariably distorted by the

Choice (A) only talks about two specific telescopes, and that is not enough data to make general statements about the practicality of each technology. Also, just because a space telescope is behind schedule and over budget does not necessarily mean that the technology is impractical or unaffordable. The planners may have underestimated the costs, but that doesn't tell us how the actual costs compare to the actual costs of ground-telescopes. Finally, space telescopes might become more cost-efficient and practical as the technology is developed. Thus, even if space telescopes are less practical now, that does not imply that they won't eventually replace ground-telescopes as costs go down.

Choice (D) just tells us the best way to use ground-based telescopes. The arrangement described might be better than any other EARTH-BASED arrangement, but if a space telescope is even better than the BEST ground-based setup, then the ground-based telescopes will likely become obsolete.

Sorry experts I just don't understand how E is the correct answer:

It just says that for more detailed studies astronomers should use telescopes with more light-gathering capacity than space telescopes can provide......... NO ONE SAYS THAT GROUND-BASED TELESCOPES CAN PROVIDE THAT CAPACITY, CHOOSING ANSWER E I HAVE TO MAKE AN IMPORTANT ASSUMPTION THAT IS NOT SPECIFIED ANYWHERE IN THE TEXT....

Let's put some of the information given in the stimulus and in option E down:

1. Astronomers rely on DSA. This means that astronomers are already using DSA to determine the chemical composition and evolutionary history of stars.
2. Space telescopes cannot provide DSA; DSA can be provided only by telescopes with more light-gathering capacity than space telescopes can provide. So DSA needs non-space telescopes.

Because astronomers are already using DSA, and space telescopes can't do DSA, astronomers must be using telescopes that are not space telescopes. Such telescopes must be terrestrial (the opposite of space). Hence E is quite solid as an answer choice.

More generally, you'll find that the correct option does very often involve a "stretch". The GMAT doesn't say how much of a stretch is too much, but a little bit is okay. It'd be very hard to make a compact CR question if the question makers could not take certain things for granted. In this case, as soon as we see that E gives us a weakness (of space telescopes), we should be seriously considering it as a "candidate" option.

AjiteshArun, thanks for the great response!

The other key to this question is that we are looking for something that "would cast the most doubt on the conclusion drawn above."

So we just need something that would make us question the author's logic/conclusion. The author says that space telescopes should provide superbly detailed images. Therefore, ground telescopes will soon become obsolete. In reaching the conclusion, the author is only considering image quality.

Choice (E) says, "You (author) might be right about image quality, but you haven't considered DSA." So even if we don't know that ground telescopes have the capacity needed for DSA, we already have a reason to doubt the author's logic. The author was ONLY considering image quality and failed to consider DSA.

As explained perfectly by AjiteshArun, it is very reasonable to suspect that ground telescopes have such capacity. However, even without knowing that for sure, we've already poked a hole in the author's argument. That doesn't necessarily PROVE that the author is wrong, but it certainly casts doubt on the argument/conclusion.

None of the other answer choices affect the author's argument, so (E) is definitely the best answer.

AjiteshArun

GMATNinja

i have gone through your explanation regarding option e. i am still not convinced why e is correct and b is not as, as per me, both b and e weaken the argument.

the argument is precisely about two types of telescopes ie type X and type Y. The author tells us something bad(in terms of image quality) about type X and tells us something good(in terms of image quality) about type Y. The author draws a conclusion regarding type X ie that type X will become obsolete due to bad image quality. ........... Clearly the author has assumed something good about type Y ie. type Y will not become obsolete. So to weaken the assumption, we need an option that tells us something bad about the type Y telescope. Option E clearly does that.

Option B clearly indicates that Type X will not / should not become obsolete as type X's position can be adjusted(if kept on mountain summits) to get a decent image quality. Also the fact that type X, if kept on mountain summit, won't be influenced by atmospheric pressure clearly makes answer choice B an interesting one.

All in all, option B directly attacks the entire argument and option E clearly weakens the assumption. You and I both know that both 'attacking/weakening the entire argument' and 'weakening the assumption' are correct when it comes to weaken questions. how do i choose between the two??

kindly help..

I look forward to hearing from you.

Hi aditliverpoolfc,

You're thinking along the right lines. Just a couple of quick points:

Your reasoning is spot on. According to E, ground-based telescopes have more light-gathering capacity than space telescopes do. So ground-based telescopes make "detailed spectral analyses" possible, even though the pictures they generate are affected by atmospheric distortion.

For example, if someone argues that mobile phone screens always have higher DPI than desktop monitors and will therefore make desktop monitors obsolete, we can weaken that by saying that working on large spreadsheets and documents is easier on desktop monitors than it is on phone screens. That is, monitors may not be as sharp as phone screens, but more content can fit on a monitor than on a phone screen.

The first sentence says:
Images from ground-based telescopes are invariably distorted by the Earth's atmosphere.

Option B says:
Ground-based telescopes located on mountain summits are not subject to the kinds of atmospheric distortion which, at low altitudes, make stars appear to twinkle.

It's important to recognize that this option does not mean that ground-based telescopes located on mountain summits are not affected by atmospheric distortion at all. It just means that such telescopes are not affected by some (specific) types of atmospheric distortion (the ones that make stars appear to twinkle).

It may help if we replace the which with a that (GMAT sentence correction to the rescue ):

Ground-based telescopes located on mountain summits are not subject to the kinds of atmospheric distortion that make stars appear to twinkle.

Hi GMATNinja could help clear my small doubt?

The premise does not state that the ground-based telescopes are telescopes with more light-gathering capacity.
Although, option E states that space telescopes might not be reliable but how can this cast a doubt on the obsolescence of the ground-based telescope?
There might be other telescopes with light-gathering capacity which are not ground based....

Thank you,
Dablu

gurudabl If space-based telescopes can't do this job, then their superior quality doesn't guarantee that they will take over for all tasks. There's no indication that any other sort of telescope exists beyond ground-based and space-based, but even if such an option showed up (balloon-based?), we wouldn't know that these would be appropriate, either. All we need to know is that space-based telescopes cannot do the job, so it's premature to say that ground-based telescopes are obsolete without further information.

My problem with this question is that E does not indicate that ground-based telescopes provide the light-capturing capacity that is necessary. Just because space telescopes do not provide such a capacity, does not mean that ground-based telescopes do. For E to break a hole in the argument that ground-based telescopes will become obsolete soon for advanced astronomical research, we MUST know that they offer such capacity. Otherwise, both telescopes might as well be obsolete for such an objective. I don't necessarily agree with GMATNinja 's reasoning above.

I chose A as the least worst answer. The only thing that it did do is explain that, at least soon (which is technically in the eye of the beholder), ground-based telescopes won't be obsolete, regardless of their use.

Check out this post by AjiteshArun. The answer choice tells us that astronomers CURRENTLY rely on detailed spectral analyses. If the astronomers currently rely on this capability, we know that the capability must currently exist. In addition, we know that it can't possibly exist in space telescopes, because it "require[s] telescopes with more light-gathering capacity than space telescopes can provide."

In order to DISPROVE the conclusion, we would absolutely need to know that ground-based telescopes can be used for detailed spectral analyses. Luckily, we are only trying to CAST DOUBT on the conclusion, so we don't need to worry about telescopes suspended on hoverboards or, as explained in this post by DmitryFarber, carried by balloons.

Remember, the author's rationale is that ground-based telescopes will become obsolete BECAUSE orbiting space telescopes will provide better image quality. But choice (E) tells us that ground-based telescopes MIGHT have another function that orbiting space telescopes cannot provide. That's a very good reason to suspect that ground-based telescopes might NOT become obsolete, even if the images from ground-based telescopes are no longer used.

In case it helps, this post attempts to explain why choice (A) doesn't work.

Hopefully that makes things a bit more clear! Or less distorted?

Hi GMATNinja,

But we need to assume here that ground-based telescopes have more light-gathering capacity than the other ones. So how can we say this answer is correct?

Remember that we must take the premises (and any answer choice we're looking at) as facts. So if the argument tells us that ground telescopes are invariably going to produce distorted images, then that's that. B and C suggest some ways that we might avoid some of this distortion, but that doesn't change the fact that the images will be distorted. So we're left with this: If ground images are distorted, but space images will not be, then why would we still want ground telescopes? What would prevent them from going obsolete? We need to see that either space telescopes will fail in some important way or ground telescopes will still fill a need that space telescopes cannot.

E tells us that there's something very important that space telescopes cannot do. We can actually infer (without assumption) that ground telescopes can fulfill this function, because if astronomers rely on this data, they must already be getting it! So unless there's some third kind of telescope (an aquatic telescope?!?), we must be getting that data from ground telescopes. But honestly, even if we didn't make that inference, we could still pick E. It's showing that space telescopes can't do everything, so that even if ground telescopes weren't currently filling this need, we might keep developing them for that purpose.

B and C both fail because they both compare some ground-based telescopes to other ground-based telescopes. They don't give us any reason to say that space telescopes, which have no distortion, will not take over. Compare this to an assessment of two medicines: X and Y. If X always has side effects, while Y does not, then the fact that we might avoid some of the side effects of X doesn't do anything to dissuade us from picking Y.

If the question asked, "which of the following PROVES that the conclusion above is utter garbage?" then we'd have a big problem choosing (E).

However, that's not what the question asks -- instead, we just have to cast doubt on the conclusion.

Imagine you're sitting in a meeting, and your colleague Matt says that ground-based telescopes will become obsolete because space-based telescopes are so great.

Then, another colleague, Harry, chimes in with the information in (E). He says that actually, space-based telescopes can't do a really important task!

Now, Harry hasn't proven that Matt is completely wrong. However, he has cast doubt on Matt's argument -- after learning the information in (E), you'd have misgivings about Matt's conclusion. If space-based telescopes can't do a certain task, then perhaps ground-based telescopes won't become obsolete.

So you're right that we don't know FOR SURE that ground-based telescopes have more light-gathering capacity -- but just the possibility that ground-based telescopes are better in some ways casts doubt on the conclusion that they will become obsolete.

I hope that helps!

[quote="CrackVerbalGMAT"]
I chose option E but then went back to Option D. Option E nowhere says that ground based telescopes have more light gathering capacity, it just says that space based telescopes do not. Option D on the other hand, says that it is a ground based telescope, which permits the best eart based observation of an area which is a prime target for astronomical research. The only problem was that it says earth-based. But option 5 doesn't even mention that they're talking about ground based telescope. There could easily be a third kind of telescope. Isn't this too big of a logical leap?

Posted from my mobile device

Hi ashishsamant2311,

Whether this is too big of a logical leap or not is something that only you can decide. I can give you my opinion though. I believe it is not.

If images from ground-based telescopes are invariably distorted by the Earth's atmosphere, it is reasonable to assume that all terrestrial telescopes will face the same problem (because of the nature of the Earth's atmosphere). It is not reasonable to assume that there is another (terrestrial) alternative that can get around the problems created by the Earth's atmosphere.

If you chose D because you didn't like E, maybe this post will be helpful.

As we said in our previous post, the question does not ask us to PROVE the conclusion wrong. Rather, it asks that we CAST DOUBT on the conclusion.

With (E), perhaps we can’t assume that ground-based telescopes have more light-gathering capacity than space telescopes. But (E) does introduce the possibility that space telescopes have disadvantages that COULD make them less appealing than ground-based telescopes. So, it does give us SOME reason to doubt the conclusion.

On the other hand, (D) provides no reason to doubt the author’s conclusion. (D) only tells us that, when observing the center of the Milky Way, the BEST place to put our “earth-based” telescopes is at high altitudes near the equator. But these images will still be distorted by the atmosphere, and we are given no reason to believe that the superbly detailed images of space telescopes would not provide superior data. For that reason, (D) does not cast doubt on the author’s conclusion, and (E) is correct.

This question is an excellent example of where eliminating wrong answers, rather than trying to find a right answer, can go a long way. Sure, (E) doesn’t completely PROVE the conclusion wrong. But it does CAST DOUBT on that conclusion. We may not always be able to find the perfect answer, but (E) is certainly the “least wrong answer.”

I hope that helps!

Passage analysis


Images from ground-based telescopes are invariably distorted by the Earth’s atmosphere.

The earth’s atmosphere mis-shapes the images taken by the telescopes used from the ground.


Orbiting space telescopes, however, operating above Earth’s atmosphere, should provide superbly detailed images.

The space telescopes work from above the earth’s atmosphere.
Therefore, they are expected to provide finely detailed images.
We can infer that the images taken by these telescopes will not be distorted.

Therefore, ground-based telescopes will soon become obsolete for advanced astronomical research purposes.

Hence, ground-based telescopes will soon be stopped being used for advanced astronomical researches.


Conclusion

Ground based Telescopes will no longer be used for advanced astronomical research.



Pre-thinking

Weaken Framework

Now per our understanding of the passage, let’s first write down the weaken framework:

What new information will make us believe less in the conclusion


Ground based Telescopes will no longer be used for advanced astronomical research.

Given that

The earth’s atmosphere causes changes in the shape of the images taken by ground-based telescopes
Orbiting space telescopes function from above the atmosphere.
They are expected to provide sharply detailed images.


Thought process



The author’s conclusion is completely based on one specific disadvantage of the ground- based telescopes.



And based on that the author chooses to ignore any advantage or purpose served by ground-based telescopes in advanced astronomical research.



Weakener

So, if any option indicates that the ground-based telescopes offer a certain advantage necessary

to advanced astronomical research, over and above the advantage provided by space based

telescopes, then it will weaken the conclusion.

With this understanding, it becomes very easy to identify option E as the correct choice.