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blusky

1st Planetary scope: Refractor vs Dobsonian

Best planetary scope?  

1 member has voted

  1. 1. Best planetary scope?

    • Refractor 5"
      16
    • Dobson 8"
      25
    • Newtonian 6"
      3


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great_bear    111
Many myths also stem from comparisons between say a TEC 140 and some cheap 1/4 PV 10 Newtonian

Agreed - you rarely see best-of-breed comparisons discussed - especially since well-made Newtonians are so under-appreciated.

There's a 6", F11, 1/12th Wave P.V. Newt that's languishing in the For Sale section here on SGL - an absolute bargain. Yet it seems to have attracted little interest.

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umadog    358

Indeed, this is the point. The reason that beginners should be steered toward f/6 Newts is because this is a cheap way of getting good images. The mirrors are easier to make well and the eyepieces need not be expensive. Coma correctors aren't necessary. Large (and fast) can be better, but quality is a lottery if you're buying Synta or GSO, although those manufactures can produce some nice stuff. If you go home-grown you have a much better chance of a good mirror but you're paying a lot more for it. In the end a lot of this comes down to economics not optics. Finally, there's the hassle factor. To get the best out of a refractor you don't have to do anything very special. To get a good view out of a Newt (particularly a big one) you have to plan ahead with cooling and collimation.

Edited by umadog

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Agreed - you rarely see best-of-breed comparisons discussed - especially since well-made Newtonians are so under-appreciated.

There's a 6", F11, 1/12th Wave P.V. Newt that's languishing in the For Sale section here on SGL - an absolute bargain. Yet it seems to have attracted little interest.

I used to have a 6" F/8 with similar specs. Very nice scope indeed. The C8 still had it beaten, so I sold it.

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great_bear    111

I like Astro-Physics' scope designer Roland Christen's take on it here <click> (very short, but well worth a quick read)

Note that when it comes to refractors, Roland says:

"Refractor, 7" to 9" Apo, F8 to F11"

- so even though he is in the business of designing and selling APO telescopes, even Mr Christen himself does not try to convince us that today's small APOs are up to the job! :)

Edited by great_bear

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John    18,851
.....- so even though he is in the business of designing and selling APO telescopes, even Mr Christen himself does not try to convince us that today's small APOs are up to the job! :headbang:

So by implication, are all those of use who have them and enjoy using them wasting our time then :)

Up to what job ? - it's a hobby - we do it for enjoyment don't we ;)

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So by implication, are all those of use who have them and enjoy using them wasting our time then :)

Up to what job ? - it's a hobby - we do it for enjoyment don't we ;)

I read as a statement that bigger, quality scopes do better on planets than smaller, quality scopes, more-or-less regardless of the design. A small APO, however good at DSO photography, is not as good as a big, quality Newtonian, especially if it is not too fast (or if it is fast, it must be well-collimated), for planetary work.

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great_bear    111
So by implication, are all those of use who have them and enjoy using them wasting our time then :)

Up to what job ? - it's a hobby - we do it for enjoyment don't we ;)

Why, the job of being a planetary scope for serious study of course! :headbang:

(the subject of this thread)

Now, people may treasure their small APOs for any number of reasons, and that's all well and good - but that's not necesarilly helpful to the observer looking to select the optimum instrument for planetary observations.

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John    18,851
Why, the job of being a planetary scope for serious study of course! ;)

(the subject of this thread)

Now, people may treasure their small APOs for any number of reasons, and that's all well and good - but that's not necesarilly helpful to the observer looking to select the optimum instrument for planetary observations.

Fair enough.

Maybe my ED120 is not a serious planetary scope then :headbang:

But it still gives me darn fine views of them :)

Edited by John

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Fair enough.

Maybe my ED120 is not a serious planetary scope then :headbang:

But it still gives me darn fine views of them :)

It is a serious scope, just not quite as serious as an 8" APO, or better still, the 530mm APO made by APM (that's aperture not focal length ;)).

Really serious observers don't really want to muck about with scopes, they want a spaceship :p

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angusb1    49
Fair enough.

Maybe my ED120 is not a serious planetary scope then ;)

But it still gives me darn fine views of them :)

Do you prefer it for planetary over your 10" reflector?

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John    18,851

If I bought a 530mm APO I'd need the spaceship as well - to escape my wife's wrath :)

Joking apart, I think my original recommendation to the OP was to go for an 8" F/6 dobsonian - they are pretty good at viewing many types of object come to think of it ;)

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John    18,851
Do you prefer it for planetary over your 10" reflector?

Yes, as it happens, I do ;)

But probably not for any logical reasons - I just like the way refractors, especially ED refractors, present the objects you view :)

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angusb1    49

Funny thing isn't it, there's something about the view through an apochromat that is very pleasing. Best answer is to have an apo and a large reflector :)

I think the question the OP should be asking is not which scope to buy for planetary viewing, but which scope to buy for planetary viewing first.

And then there's SCTs......

Edited by angusb1

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great_bear    111
I think the question the OP should be asking is not which scope to buy for planetary viewing, but which scope to buy for planetary viewing first.

Like I always say; when choosing your first scope, plan to end up with two - you will anyway! :)

Edited by great_bear

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great_bear    111

I don't give much credance to Gary Seronik's opinion. A couple of comments from me here:

S&T(GS): "I call this assertion a myth for two reasons. First, my own observations do not bear it out."

Wow!! How flimsy is that! What a totally self-centric viewpoint! If he doesn't observe the phenomena, why not investigate those who do? There's plenty of people who do observe this - maybe he's doing something different? This reasoning is philosophically unsound, being classed as "Argument from self-knowing" i.e. (from Wikipedia) "Arguments from self-knowing take the form: If P were true then I would know it; in fact I do not know it; therefore P cannot be true."

S&T(GS): "Second, no one has proposed a plausible mechanism for it."

Oh dear... That's a shameful thing for anyone with a scientific mind to say, it's "Argument from ignorance".

As Carl Sagan says in The Fine Art of Baloney Detection (chapter from The Demon-Haunted World) "This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence"

S&T(GS): "my principal telescope was an optically good 12½-inch f/5 reflector. I built a 5-inch-diameter off-axis mask that could quickly be placed at the front of its tube[...] Not once did the reduced-aperture view show greater detail than the full-aperture view."

Maybe so - but it's not quite the same as actually having large and small telescopes side-by-side and directly comparing the views is it?Maybe the alledgedly better views in the smaller telescope were due to another reason - such as reduced turbulence within the tube due to its different geometry?

He'd be doing astronomy a far better service by investigating why people are reporting such a phenomena rather than effectively saying "I've not seen it, so this magazine officially declares it a myth."

I'm personally not asserting an opinion either way - but what I am saying is that Gary Seronik's article has no credibility on this issue since it makes no philosophically sound assertions.

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Just a few points:

If as a scientist, I cannot reproduce observations of others, which concerns a claim running counter to scientific theory, and I cannot find a mechanism to support this (assuming a thorough search or prior knowledge), how can I do anything other than reject the claim?

Using a stopped-down version of the same scope has the advantage of limiting the difference in views just to the aperture effect, to the exclusion of others. If, as optical theory would expect, the full aperture view is better, we must conclude that, all other things being equal, aperture reduction leads to image degradation.

I think Sagan also said "Extraordinary claims require extraordinary evidence"

Again, optical quality is very important, but a 5" scope cannot resolve things that a similar quality 8" can.

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angusb1    49
If as a scientist, I cannot reproduce observations of others, which concerns a claim running counter to scientific theory, and I cannot find a mechanism to support this (assuming a thorough search or prior knowledge), how can I do anything other than reject the claim?

You can't reject the claim unless you can falsify it. If the claim is not falsifiable in the first place it is a worthless statement. If someone asserts that a small aperture scope can show better views or more detail than a scope of larger aperture they need to specify under what conditions this can happen. That way a test of the scopes under the specified conditions can be carried out. Otherwise you will never be able to say that the claim is false because there could always be some set of conditions that haven't been tested yet where it could be true.

Personally I think the whole question is a very subjective one with regard to the enjoyment of the viewing experience. After all we're talking about an experience which one person considers more pleasant than another experience. The question of resolution is more objective and could be tested.

Edited by angusb1
"what these conditions are" to "under what conditions this can happen"

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dweller25    1,035

Good thread this .... but the original poll suggests the answer to the OP's question is an 8" Newt. My experience despite owning a very good 5" Flourite APO backs that up.

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great_bear    111
If as a scientist, I cannot reproduce observations of others, which concerns a claim running counter to scientific theory...

But there's nothing asserted here that's counter to scientific theory here.

No-one's deputing optical science - that larger aperture has higher resolution - the contention is that smaller telescopes are observed to be less susceptible to poor seeing than large ones are and under such conditions produce better images.

Using a stopped-down version of the same scope has the advantage of limiting the difference in views just to the aperture effect, to the exclusion of others.

But you're missing the key point: People don't say "smaller aperture (alone) is less susceptible", they say "smaller aperture telescopes are less susceptible". So to reproduce the phenomena as described you need a smaller telescope not merely reduce the aperture on a big one. Doing otherwise means you're already assuming the cause (e.g. atmosphere, when it may be internal tube currents for example).

The first step is reliably reproducing the observation. Next would be quantifying it, then some hypotheses, and then finally, some test measures to prove which hypothesis (if any) is correct.

But - as I'm sure you'll agree - the amateur astronomy community seems to have a bit of an aversion to testing & reproducing such observations in an objective manner, prefering instead to trade pulled-from-the-air theories about what "might" be going on... :)

Edited by great_bear

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umadog    358

People may not say "smaller aperture alone", but this is implied. It's implied because the argument being made is that the purported reduction in image quality is due to seeing affecting large scope more than small scopes. Clearly everything else must be equal for one to be able to make this judgement. As you say, quite possibly this is not the case.

Observing a better image in a small compared to large scope doesn't tell you why the difference is there. This is why this is an unsuitable test. Stopping down is a more controlled experiment since everything else is held constant. Sure, the large scope may, for various reasons, produce a worse image than a small scope overall. This isn't relevant, however. What matters is whether stopping down the large instrument provides a relative improvement in the views. This would be evidence for the seeing hypothesis. It doesn't really matter if the views are now the same as an equivalently sized smaller scope. That's a different question.

There are caveats to stopping down. Firstly, it assumes that the mirror doesn't have a turned edge or any other local defects which the mask occludes. Secondly, it assumes that seeing is the limiting factor. If the scope isn't cooled down or has horrible optics then these may mask a "seeing benefit" should one actually be occurring. The test scope would therefore have to possess excellent optics, be in excellent collimation, and well cooled down. In practice, knowing these things to be the case is possible so the experiment is feasible. The test can even be done objectively: simply image the PSF of a star using a webcam. The movie sequences can be analysed and compared.

Edited by umadog

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great_bear    111
Observing a better image in a small compared to large scope doesn't tell you why the difference is there. This is why this is an unsuitable test. Stopping down is a more controlled experiment since everything else is held constant.

True, but it's less helpful - because people can argue (quite rightly) that you're not replicating the phenomena under the same conditions which they're reporting it under.

Better still, is to sort it out once and for all by having large and small telescope side-by-side, under the kind of "seeing" conditions commonly reported, and then after confirming such a difference, investigate further by stopping down the big scope to match the smaller and see whether the two scopes now show identical images. You'd want to have the stopped-down large scope match the F-Ratio of the smaller scope - since the dreaded "F-Ratio Conjecture" hot potato will make an appearance at some point in the conversation :)

My own standpoint on this - for what it's worth - is that the statement "Smaller scopes perform better in bad seeing than large ones" is inherently flawed anyhow, since how do you know that the observer making such a statement is technically able to detect the difference between actual bad seeing, and poor thermal management in the larger scope?

I agree that stopping down presents a strong case for detecting the latter - but only if you've got a small telescope of equivilent aperture and focal-ratio to act as a reference.

Edited by great_bear

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umadog    358

I do see what your point, it may help to have a small scope there too as a sanity check. But the question is what the smaller scope is a reference for.

If the views in the big scope don't match the small one after stopping down then what that tells you is that there's something "wrong" with the bigger scope. It doesn't speak to the effect of aperture on seeing, which is what we're trying to test. Only an improvement in the views through the bigger scope will do that. The improvement doesn't have to match the smaller scope. This is why it doesn't make a good reference for this test.

The boundary layer can be seen through the eyepiece if you defocus whilst looking a bright star. There's a pretty obvious difference between bad seeing and a boundary layer. More subtle tube currents may be harder to detect, however.

Really nailing this one with convincing experiments would probably be quite hard, I must say. More effort than it's worth. At the end of the day, we all know which scope in our arsenal gives the best views for particular objects and we just choose appropriately. We may not know exactly why one thing works well and another does not.

Edited by umadog

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Moonshane    10,782

maybe I should do a head to head with my 6" f11 and 16" f4 stopped down to 6.7! f11.

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great_bear    111
More effort than it's worth. At the end of the day, we all know which scope in our arsenal gives the best views for particular objects and we just choose appropriately. We may not know exactly why one thing works well and another does not.

Sorry, but that's not good enough! :)

We need to find out because:

  • You can't be a master of your tools if you don't know why one tool sometimes outperforms another
  • We owe a debt to our forebears to advance telescope science for the next generation of observers
  • Forum polls like this deserve a quality answer
maybe I should do a head to head with my 6" f11 and 16" f4 stopped down to 6.7! f11.

Definitely! - and see if you can do a 6" F5 at the same time for reference! ;)

Edited by great_bear

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