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Where does "serious aperture" begin?

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#26 auriga

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Posted 05 March 2010 - 01:06 PM

Undoubtedly, the most meaningful definition of "serious aperture" relates to the optical resolution reach of the scope - what is it capable of resolving with articulate sharpness and sufficient brightness, without unacceptable aberrations.

Nonetheless, a meaningful (nonoptical) informal correlation with "serious aperture" used to be the size where it is no longer possible for average-height people to observe near the zenith without aid of a stepladder. That's because it used to be forbiddingly difficult and expensive to obtain consistently high-quality mirrors below about f/4.5. The dividing line fell between 15-inch f/4.5 scopes at the upper end of "don't need a ladder", and 18-inch f/4.5 scopes at the lower end of "need at least a two-step stool", with 16-inch scopes in ambiguous territory, depending on your height. Beyond 18", and you used to need a regular multi-step ladder at least four to five feet high for an f/4.5 20" scope.

Now, however with consistent high-quality sub-f/4.5 mirrors becoming available from a limited, but sufficient (to meet current premium-price demand) number of sources - f/4.2, f/4.0, f/3.7 and even f/3.3 mirrors...this "stepladder" heuristic is becoming badly frayed (at least at the highest premium end of the spectrum), and 20-inch scopes have replaced 15-inch scopes as the upper bound of "stand on the ground". Another informal (nonoptical) heuristic measure these new very fast f-ratio larger-aperture scopes have recently frayed is the distinction between scopes small enough to be easily portable in an average-sized sedan, vs ones requiring a more ample vehicle, which used to correlate roughly (though imperfectly) with the "stepladder" heuristic. (Maybe it still does, though shifted along with the stepladder heuristic toward 20" instead of 12" or 15").

One informal optical measure is how overtly the aperture is capable of showing spiral structure in how many galaxies under say, mag 6.0 skies. This can start with as little as 10" or especially 12", but occurs much more readily with 18" or 20". I've heard lots of people comment that good 20" scopes is where significantly more detail begins coming out, just as 10"-12" seems to be another similar lower-level quantum boundary.


Chris,
This is like the rest of your posts, astute, well-reasoned, and based on considerable experience. Excellent.
Bill Meyers
 

#27 Dain

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Posted 05 March 2010 - 01:44 PM

It begins with your own two eyes.
 

#28 dave b

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Posted 05 March 2010 - 02:33 PM

i think this thread is getting off on the same tangent that most "big dob" threads tend to do - talking about price, weight, ladders, backache, it won't fit in your bad floormat toyota......

but what i think the OP was asking (and he even asked everyone to refrain from any joke or cute answers) was at what size does "serious aperture" begin at. not at what size does it cost too much, weigh too much, require a ladder too much, require a pick up or SUV too much........
 

#29 hapo

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Posted 05 March 2010 - 02:41 PM

17-20" minimum in a cassegrain format, to avoid ladders, backache, mounted in a permanent observatory so no need to weigh it to vans.
 

#30 Jaxdialation

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Posted 05 March 2010 - 02:42 PM

Read his post again Dave. He actually opened the door to these comments. I haven't noticed him (or a moderator) complaining. So let us return to the thread.
 

#31 Mike B

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Posted 05 March 2010 - 03:07 PM

Since Dobs are now available in basically 2-inch increments from 4" well into the 30's, i think its fair to say its a continuum... but not necessarily a smooth one. There are MANY thresholds as one proceeds up the aperture scale- thresholds both of constraint (weight, storage, back-strain, ladders, floormats...) and of performance (what you'll see & how it looks).

To discuss one without the other would be misleading to many readers not having the experience... and there will likely be as many newbs as adepts reading thru this- especially "lurkers" trying to educate themselves in these matters. Fortunately, this thread has covered BOTH threshold aspects, and done so quite well!

If i may be so bold- what i've distilled from this thread on the performance aspect is that there are a couple of optical/visual thresholds going up the scale. The first seems to be around 12" of aperture, and the next seems to be around 22".

However, there's a huge physical constraint threshold right around the 14-16" range, one i can address because that's right where my upgraditis came to a halt! And it did so precisely because of those constraints, not even so much for costs.

So if ~22" is the threshold where "serious aperture" begins, anyone wishing to venture there will need to plow thru the physical constraints therewith... pay the price of admission, as they say.

But then i'm pretty "serious" about my 15" of aperture, as i suspect the owners of 12-18" scopes typically are of theirs. So perhaps we're approaching this matter all wrong? Where, then, is the point of scope & aperture committment necessary to "hook" a person for "serious" observing?

I have a good friend with an AP155 refractor- the OTA itself costing ten grand! Then there's its mount... :foreheadslap: Would anyone dare assert that "piddly li'l" 6" refractor isn't "serious" aperture? Not me.

So i suspect there's several ways of approaching the subject.

The OP began with:

What do you consider to be the minimum aperture that could be classified as a "light bucket"?

That probably refers to the first "threshold"... 12". That's plenty for serious enjoyment in viewing!
:grin:
 

#32 Starman1

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Posted 05 March 2010 - 03:18 PM

For years I thought an 8" scope was a "lifetime" scope. Probably around 15000 DSOs are reachable, and pretty much all star clusters. You could spend a lifetime with one and become quite an accomplished observer.
But my interests shifted more to galaxies so I moved up "a magnitude" to a 12.5". And while I certainly can see more galaxies and details therein, the biggest difference in appearance came with the mundane, easily visible, brighter objects.
I've seen (and it wasn't possible in an 8"):
--individual stars in M31 (NGC206 stars)
--stars across the face of M14
--tons of H-II regions in most of the nearer galaxies
--white swirls inside the GRS on Jupiter
--brightness variations on Ganymede
--differential colors in the Galilean moons
--the Keeler Gap in Saturn's rings
--the outer spiral arms of M81 and NGC7331
--to-the-core resolution on M15
--red giants in M13
--dark lanes in tons of edge-on galaxies
--M17 and M16 as part of the same nebula
--wonderful striations across the face of NGC6888
--B33 (Horsehead), both with and without a filter
--galaxies in some faint Abell Galaxy clusters
--several Abell planetaries

I sit when I observe except at the zenith.

The next logical step (to gain a magnitude): 20"
But it's too big to easily carry by one person and transport in a small, high-mileage, car. I regularly observe at dark sites frequented by others with larger scopes, and I've learned that, by and large, most big scope observers don't go after targets any fainter than I do.
And I hate standing or using a ladder to observe.
A 20" f/3 would work, but the issue of lifting the scope would still remain.

So, for me, though I'm tempted by larger apertures, MY serious aperture is 12.5". I guess the key is, if you observe a lot of things, and use the scope quite a bit on a variety of targets, that constitutes serious observing. And, no matter what aperture is used, by extrapolation that's serious aperture.
 

#33 Carl Wright

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Posted 05 March 2010 - 03:22 PM

My 22" is at the cleaners. Going out with my 10" tonight. Bummer :bawling: :bawling: :bawling:
 

#34 dave b

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Posted 05 March 2010 - 03:39 PM

Read his post again Dave. He actually opened the door to these comments. I haven't noticed him (or a moderator) complaining. So let us return to the thread.


john, your right, i totally missed the part where he said he wanted to hear about people's real or imagined physical and monetary limitations on the scope they choose.

i had thought he wanted to know at what size a scope reached that magical threshold where almost everything seems to be reachable, where the entire star party challenge list is findable, where binoviewers seem to not cause any light loss, where blinking nebula decide to stop blinking, where you can show your wife many a galaxy and she does not just shrug indifferently.

sorry about that....
 

#35 FirstSight

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Posted 05 March 2010 - 03:41 PM

The next logical step [above 12.5](to gain a magnitude): 20" But it's too big to easily carry by one person and transport in a small, high-mileage, car.


Mike Lockwood had his 20" f/3 set up literally two steps outside my tent throughout Winter Star Party, and (correct me if I'm wrong Mike) he often transports it in a Suburu Outback. I watched him set it up on Monday, and tear it down Friday morning, and he did it all by himself rather easily. The thinner f/3 mirror is significantly lighter than most big-aperture mirrors used up until recently.

Now there are many good reasons besides easy manageability/transport to choose to stop at 12.5" rather than 20" (not the least that the new ultra-fast f-ratio "big" scopes from Starmaster and Webster are much more expensive than premium-quality 12"-range scopes)...but my impressions of what is "manageable" were dramatically changed a few weeks ago by the privilege of close proximity to Mike and his scopes at WSP. I previously thought 18" or maybe even 15" was the max for practical useability. Now, the only inhibition (albeit a big one) from purchasing one of Starmaster/Lockwood's f/3.3 20" scopes is...[drum roll...] coming up with the money within a doable time frame (I will never go into debt to buy anything astronomical...accumulated cash only is the path for me).
 

#36 dave b

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Posted 05 March 2010 - 03:49 PM

i dont know if you guys have the old, free video tape from obsession, but one of the segments is a 110 pound woman setting up a 20" telescope all by herself.

like chris above says, if you are not constrained by money and a subcompact vehicle, you can certainly set up a 20-40" telescope all by yourself.
 

#37 Starmix

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Posted 05 March 2010 - 03:58 PM

As a follow up question, I would like to ask....Where does serious aperture PRACTICALLY end. Meaning at what point does the laws of diminishing returns set in for the amateur astronomer, to where that extra ten inches does nothing more without, a change of location(ie...to an isolated mountain)or better technology(serious observatory with climate control).
 

#38 jdownie

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Posted 05 March 2010 - 04:26 PM

As a follow up question, I would like to ask....Where does serious aperture PRACTICALLY end. Meaning at what point does the laws of diminishing returns set in for the amateur astronomer, to where that extra ten inches does nothing more without, a change of location(ie...to an isolated mountain)or better technology(serious observatory with climate control).


Excellent question. If we use a one magnitude gain as a sensible threshold of going to a larger aperture we get:

8"
12.5"
18"
30"

Larger mirrors have multiple practical disadvantages, so the tradeoffs involve a set of curves, some going up, some going down.

One of the key issues is one's local observing conditions. In an area of poor to average seeing, the 18" and even, perhaps, the 12.5" are unlikely to give significantly more than the smaller scope. In a dark site with good seeing the relationships favor the larger, obviously. The intended targets are also important - there are good reasons that seasoned astronomers use 4 - 6" scopes for the planets and the moon.
 

#39 StarStructure Telescopes

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Posted 05 March 2010 - 04:30 PM

In an area of poor to average seeing, the 18" and even, perhaps, the 12.5" are unlikely to give significantly more than the smaller scope. In a dark site with good seeing the relationships favor the larger,



I think we've been here before.

Mike
 

#40 Karl_Bonner_1982

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Posted 05 March 2010 - 05:02 PM

....
So, if we take a 5mm pupil and compare it to a 102mm scope, as a ratio 5:102mm, the next step up is 102mm:2080mm, which is an 82 inch scope.

I conclude therefore that the next step up is 82 inches and that everything under that, particularly amateur level instruments from 5 to 30 inches, is just piddling around.

Incidentally to my way of thinking it is not coincidental, looking at things this way, that the apertures of the 19th century up to and including the Rosse telescope charted the sky but did not revolutionize our understanding of it. It was the hundred inch Hooker telescope that took things to the next level by resolving stars in other galaxies.


I never thought of it quite this way. We're used to thinking in ratios and geometric/exponential terms when it comes to going up the aperture scale, but it's easy to forget what happens at the bottom of the scale. The jump from 2" to 4" is every bit as dramatic as the jump from 10" to 20" or from 20" to 40". At the bottom it's not hard at all to double the light by increasing the aperture 41%, but when you get up into stepladder territory, you're talking thousands of $ and 100+ pounds to reach the next step.
 

#41 Starman1

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Posted 05 March 2010 - 05:02 PM

A 1 magnitude gain is a factor of 2.512 by area.
That translates to 8", 12.5", 20", 32" in one sequence
or 6", 10", 16", 25" in another.
It's not surprising that those are standard sizes.
 

#42 dave b

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Posted 05 March 2010 - 05:11 PM

a larger scope always "gives" more; try seeing the veil in a light polluted city with a 10" (cant see it at all), and then try with a 30" (yep, there she is).

but the law of diminishing returns money wise starts relatively quickly:

a 10" scope might cost $6 per square inch of aperture, a 22" might cost $30 per square inch and a 32" might cost $33. you got to have the coin to observe with the big boys.
 

#43 jdownie

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Posted 05 March 2010 - 05:37 PM

In an area of poor to average seeing, the 18" and even, perhaps, the 12.5" are unlikely to give significantly more than the smaller scope. In a dark site with good seeing the relationships favor the larger,



I think we've been here before.

Mike


Of course we`ve been here before.

What percentage of CN postings do you suppose relate to places to which we haven`t been?

John
 

#44 StarStructure Telescopes

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Posted 05 March 2010 - 05:44 PM

a larger scope always "gives" more; try seeing the veil in a light polluted city with a 10" (cant see it at all), and then try with a 30" (yep, there she is).




David,
Great analogy, can’t disagree.

However, that wouldn’t be one of those objects I would share with your wife. I’m pretty sure she would just shrug indifferently, followed by a good :ranting:

The Veil, IMO, is pretty much worthless in light polluted skies in a 30”. But yes, you can see it.

Mike
 

#45 StarStructure Telescopes

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Posted 05 March 2010 - 05:44 PM

Of course we`ve been here before.

What percentage of CN postings do you suppose relate to places to which we haven`t been?



Good point.
Mike
 

#46 nightstalker

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Posted 05 March 2010 - 05:47 PM

I'd reckon 12 " is where scopes start to diverge from
giving great views to seriously amazing ones . Its also a point where the landscape starts to change a little , refractors are for the most part are no longer affordable or practicle , the humble newt even starts to take on more of a physical presence in its storage and use.
 

#47 skyward_eyes

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Posted 05 March 2010 - 05:49 PM

The only thing that keeps me from going to a Meter class scope would be money! Id be happy with a 25" though.
 

#48 GeneT

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Posted 05 March 2010 - 06:17 PM

Serious aperture is the aperture you will use on a regular basis. I know people who kept moving up to larger telescopes, then quit using them because they were too much of a hassle. True, a 25 inch will show you everything. I bought a 12.5 inch Porta Ball. It is a nice, easy to grab and set up telescope. I got aperture fever. I bought an 18 inch Ultra Compact. It is easy to set up and transport for an 18 inch telescope. However,it is more hassle than my Porta Ball--but not so much hassle that I won't use it. Wisdom comes from knowing that I have reached my limit. My 18 inch mirror gathers more than twice as much light as my 12.5 inch. At the eye piece, that is a significant difference. The best option would be to live at a dark sky site, and set up a 25 inch telescope in a roll off roof observatory. You could leave everything all set up. The second best option would be to own such a place, but as a weekend get away home. In San Antonio where I live, we have Mag 3 skies, and trees in my front and back yards too high to do any viewing. Therefore, to view, I have to drive to a site. That is why my 18 inch is the largest telescope I will own. I want to use it--not have it sit in a garage.
 

#49 Dain

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Posted 05 March 2010 - 06:29 PM

Serious aperture is the aperture you will use on a regular basis. I know people who kept moving up to larger telescopes, then quit using them because they were too much of a hassle. True, a 25 inch will show you everything. I bought a 12.5 inch Porta Ball. It is a nice, easy to grab and set up telescope. I got aperture fever. I bought an 18 inch Ultra Compact. It is easy to set up and transport for an 18 inch telescope. However,it is more hassle than my Porta Ball--but not so much hassle that I won't use it. Wisdom comes from knowing that I have reached my limit. My 18 inch mirror gathers more than twice as much light as my 12.5 inch. At the eye piece, that is a significant difference. The best option would be to live at a dark sky site, and set up a 25 inch telescope in a roll off roof observatory. You could leave everything all set up. The second best option would be to own such a place, but as a weekend get away home. In San Antonio where I live, we have Mag 3 skies, and trees in my front and back yards too high to do any viewing. Therefore, to view, I have to drive to a site. That is why my 18 inch is the largest telescope I will own. I want to use it--not have it sit in a garage.


+10

My exact thoughts as well.
 

#50 nirvanix

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Posted 05 March 2010 - 08:20 PM

Well said.

Enthusiasm can overcome all obstacles I suppose. To be serious about aperature you have to seriously gauge your enthusiasm in relation to all other factors. My locale trumps my desire to go big, at least for now. Getting out to dark sky in bitterly frozen tundra can be thrilling, but not in a good sense. If I ever move to the countryside I'm sure that will change. In the meantime I cruise CN and enjoy tales of galaxy-busting by the folk with big scopes. :grin:
 


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