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Consistancy in Size Designation of Focal Length and Aperture?

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#1 Deepskyclusterstruck

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Posted 06 December 2018 - 02:18 PM

Is there a standard followed consistally in refering to whether a telescope aperture or for that matter focal length is small, medium, or large either as an industry standard or in the literature?

#2 Napp

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Posted 06 December 2018 - 02:27 PM

Small, medium and large are in the eye of the beholder.  At one time 8” was large.  It’s medium now possibly headed to small.  I just ordered a 16”.  It used to be the entry large now I wonder if it’s more the end of medium.  When you go to one of the major star parties you get an idea of large when you see the huge dob’s some folks have.  I think the largest amateur scope is now 70”.


Edited by Napp, 06 December 2018 - 02:27 PM.

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#3 havasman

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Posted 06 December 2018 - 02:42 PM

Is there a standard followed consistently in referring to whether a telescope aperture or for that matter focal length is small, medium, or large either as an industry standard or in the literature?

No, not really. And if there was, it wouldn't mean anything. It's subjective and variable between people and situations.


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#4 Redbetter

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Posted 06 December 2018 - 03:06 PM

I don't know that there is any standard.  Generally small aperture seems to run about 4" or less, medium roughly 5" to 8 or 9", with ~10" being the beginning of large.  The transition from large to very large or "monster" scopes is probably around 20".  Most of the quibbling is about which side of any imaginary boundary that a particular scope best fits.   Depending on context of the discussion and type of target, the ranges vary. 

 

Focal length is probably only relevant in the context of bulk/portability.  Part of what we think of as small, medium, large has to do with the difficulty of setting up or transporting rather than the aperture.   And with aperture there is also "clear aperture equivalence" to keep in mind--incorporating total system reflectance/transmission as well.  A 4" ED/apo refractor has nearly the same level of transmittance as a 5" SCT/Mak/Newt.  I have a 5" Mak with a focal length much longer than our 10" Dob, and even more so than the 4.33" refractor, but the 5" Mak performs much like the 110 ED (4.33") in terms of light grasp/planetary resolution but has lesser contrast on larger and planetary detail scale, so the refractor gets the nod.  The 10" blows both of them away by a large margin. 

 

In a physical sense, some of the lighter, shorter ratio scopes with 20+ inches of aperture don't seem all that large, a 20" f/5 is pushing the upper end of large, while a 25" f/5 is definitely in the very large/monster scale.  On the other hand the 5" f/15 Mak is so small that I think of it as a small scope while the 110 refractor is crossing over in to medium for me despite very similar views.  And an 8" SCT seems well ensconced at medium for me as would 6" SCT's, Maks, Dobs.  A 5" apo refractor set up is firmly in the medium camp.  A 6" ED/apo would seem to fit in medium.



#5 Conaxian

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Posted 06 December 2018 - 03:30 PM

Nowadays the large are really large and the scale is sliding upward.

In Burnham's Celestial handbook he referred to anything smaller than 5" as small, 5" to 8" as medium, and anything larger than 8" as large.  With my budget, his classifications still make sense.

 

Then again, there are 5" APOs that cost a lot more than my car.

Actually, the advances in automation and mass production and the easy credit available to about everyone may be what have brought about the really nice, yet reasonably priced scopes we can enjoy today.

 

In Burnham's day all the optics required a lot more hand work that cost money.

 

I'd say anything under 6" is small, 6 to 10" medium.


Edited by Conaxian, 06 December 2018 - 03:36 PM.


#6 TOMDEY

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Posted 06 December 2018 - 03:45 PM

Sort of fungibly Wonderlandish:

 

>When it's all you have, it's too small.

>When you try to store it in the closet, it's too big.

>When you display it to friends, it's too small, again.

>When you try to load it into the car, it's annoyingly Big.

>When you have to climb a ladder to the eyepiece, it's painfully Big.

>When you look into the eyepiece... it's Gratifyingly BIG!

 

Tom

 

~~

1950 med=4"  big=6"  large=8"  vy-large=12"  huge= 16" gargantuan=24"

 

2018 about 50% bigger than the aboves

 

There are no standards at all / therefore opinions regarding superlatives may vary...  Tom


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#7 amzking

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Posted 06 December 2018 - 08:26 PM

My 8" SCT is "large" because anything bigger and I wouldn't be able to carry it fully assembled into the yard.  lol.gif


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#8 Deepskyclusterstruck

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Posted 07 December 2018 - 01:52 AM

I don't know that there is any standard. Generally small aperture seems to run about 4" or less, medium roughly 5" to 8 or 9", with ~10" being the beginning of large. The transition from large to very large or "monster" scopes is probably around 20". Most of the quibbling is about which side of any imaginary boundary that a particular scope best fits. Depending on context of the discussion and type of target, the ranges vary.

Focal length is probably only relevant in the context of bulk/portability. Part of what we think of as small, medium, large has to do with the difficulty of setting up or transporting rather than the aperture. And with aperture there is also "clear aperture equivalence" to keep in mind--incorporating total system reflectance/transmission as well. A 4" ED/apo refractor has nearly the same level of transmittance as a 5" SCT/Mak/Newt. I have a 5" Mak with a focal length much longer than our 10" Dob, and even more so than the 4.33" refractor, but the 5" Mak performs much like the 110 ED (4.33") in terms of light grasp/planetary resolution but has lesser contrast on larger and planetary detail scale, so the refractor gets the nod. The 10" blows both of them away by a large margin.

In a physical sense, some of the lighter, shorter ratio scopes with 20+ inches of aperture don't seem all that large, a 20" f/5 is pushing the upper end of large, while a 25" f/5 is definitely in the very large/monster scale. On the other hand the 5" f/15 Mak is so small that I think of it as a small scope while the 110 refractor is crossing over in to medium for me despite very similar views. And an 8" SCT seems well ensconced at medium for me as would 6" SCT's, Maks, Dobs. A 5" apo refractor set up is firmly in the medium camp. A 6" ED/apo would seem to fit in medium.



#9 Deepskyclusterstruck

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Posted 07 December 2018 - 01:58 AM

Would the difference between Maks and ED refractors be true accross the spectrum of sizes do you think then? Would a 7 inch Mak forinstance be approximately equivalent to a 6 inch ED refractor in the ways you so well elucidated for the 5 to 4 comparison?

#10 Deepskyclusterstruck

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Posted 07 December 2018 - 02:03 AM

Also when you say total reflectance/transmission I do get the gist but not entirely? Can you clarify? As for less contrast on larger and planetary detail with the Mak would that be less the case if the comparison were between a 6 inch Mak and a 4 inch ED refractor? On an applied level I would like to be able to reproduce the advantages of an ED refractor or fully apochromatic one if I can with a larger Mak so I avoid bulkiness and extreme cost while still benefitting in the same manner. Maybe this is like wishing rain water were beer.

Edited by Deepskyclusterstruck, 07 December 2018 - 02:29 AM.

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#11 SeaBee1

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Posted 07 December 2018 - 08:49 AM

I'm in the camp that considers small/medium/large/monster as a subjective classification... one man's meat is another man's poison... one man's medium is another man's large. But I do think that there is some consensus on the outer ends... small is small is small and monster is monster is monster... to most people.

 

 

Also when you say total reflectance/transmission I do get the gist but not entirely? Can you clarify? As for less contrast on larger and planetary detail with the Mak would that be less the case if the comparison were between a 6 inch Mak and a 4 inch ED refractor? On an applied level I would like to be able to reproduce the advantages of an ED refractor or fully apochromatic one if I can with a larger Mak so I avoid bulkiness and extreme cost while still benefitting in the same manner. Maybe this is like wishing rain water were beer.

 

But here I think what you are searching for is not so much the distinction of small/medium/large, but what are the advantages of different scope types. What you need to know, basically, is that the two scope types you are comparing is obstructed aperture (the Mak) and unobstructed (the 'Frac). The common consensus is that in order to overcome the advantages of an unobstructed aperture, one must go up in size for the obstructed aperture to sort of level the playing field. At the sizes you are attempting to compare, the difference, IMHO, will be hard to detect for anyone but an expert observer. And, since I AM NOT an expert observer, I personally am not one to get hung up on this point.

 

At the aperture size you are considering, I think you will be far better served to look at the overall SYSTEM quality of any scope you might consider owning. Things like optical quality, mount stability, alt/az vs equatorial, goto vs manual, etc... and to really mess things up, how much are you willing to spend to get what you want? How willing are you to compromise on any one point? These are the things that are going to matter most.

 

Good hunting!

 

CB


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#12 Redbetter

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Posted 07 December 2018 - 03:23 PM

System transmission is reduced by both the central obstruction, and the mirrors/other optical surfaces.  Mak's & SCT's have losses for the front corrector/meniscus, the primary mirror, and the secondary mirror.  Celestron's XLT system results in 83.5% transmission in addition to the obstruction.  Then there is the diagonal.  A commercial Newt/Dob typically has mirrors that are each around 90-92% reflectance along with a smaller overall percent area obstruction.  Higher end dobs often have higher reflectance primary and secondaries, each in the 95% range.  A decent refractor with modern coatings (and adequately sized baffles) only loses a few percent at the center of the field.  A refractor loses a little more for the diagonal (as in the SCT/Mak) which anymore are often of the 99% variety.  Also, there is typically more overall scatter with mirrors compared to lenses, and that impacts the contrast somewhat in the refractor's favor.

 

The performance/aperture ratio depends on the target type as well as the telescope type/quality.  My 110ED is an older type with less than apochromatic correction visually and in some ways probably performs more like a 100mm ED, yet it still allows fainter moons to be detected near Saturn than the Mak does burdened by nearly 40% linear obstruction.  Somewhat finer planetary detail is apparent in the refractor, and this is compared to a Mak that holds up well at planetary magnification.  (On the other hand, both show considerably more planetary detail than a visually apochromatic 80 ED with better glass.)  Most DSO's are of lesser overall brightness and our eyes have difficulty detecting finer detail in low surface brightness objects, so the overall total light gathered is the primary factor.  However, even with DSO's there is some improvement in background contrast with a good refractor--this is apparent in some of the widest true field applications with diffuse nebula.



#13 Deepskyclusterstruck

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Posted 07 December 2018 - 07:10 PM

I think I followed you up to the last sentence. The improvement in background contrast with a refractor on DSO's. For every advantage you spoke to of the refractor(where such existed) my impression (and I hope I have this right) is that if the aperture in a Newt,Mak,or SCT is large enough in aperture it will catch up with or even exceed the refractor even in the very same areas in which the refractor would normaly surpass them if all were the same aperture. But with widest true field applications on nebula there seems to me, ( maybe my misunderstanding), that something qualitatively different is going on. My understanding(flawed or not) of what you are saying here is that when it comes to nebula the refractors superior performance is one that other telescope designs even when larger in aperture then the refractor compared with, would still not match its abilities in performance in regards to background contrast. Is this consistant with what you are saying even if the focus(sory for the pun) may be different in emphasis and scope(sory again)?

#14 Tony Flanders

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Posted 08 December 2018 - 07:38 AM

Is there a standard followed consistally in refering to whether a telescope aperture or for that matter focal length is small, medium, or large either as an industry standard or in the literature?

Absolutely not. But everyone will agree at the extremes. A 60-mm refractor is definitely small; a 24-inch Dob is definitely big.

 

In between is where you get into trouble. I still think of an 8-inch scope as big, since it was the biggest commercial size readily available when I was a child. But many people would call it medium-sized, and some might even call it small -- depending on the context.



#15 Deepskyclusterstruck

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Posted 08 December 2018 - 07:38 AM

By the way, though I deviated a little from my initial topic I did appreciate the responses both to my starting point and the detour I took. Thank you all!

Edited by Deepskyclusterstruck, 08 December 2018 - 07:39 AM.


#16 Deepskyclusterstruck

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Posted 08 December 2018 - 09:17 AM

I found that the 6SE though touted frequently as portable was a chore carying 100 yards which I guess is what got me thinking about sizes. An 8 inch Dob would push me beyond my limits I think and Im a more or less medium sized man of normal strength. An 8 inch of any telescope seems like it would be to heavy and then it occured to me that the compactness was realy more the issue then weight for me. I can carry alot more weight when its in a more condensed space. But then of course the length of the tripod to support the weight began to factor in and soon anything, even Cats seemed potentialy problematic even with their more compressed size. My primary interest had initialy been refractors but weight , length, and cost seemed to top off for me after 4 inches or so. I had read much that seemed to indicate maks were very close in optical performance to refractors atleast for some things and that larger aperture would compensate for whatever difference there might be in performance. I also knew if the size became too large I couldnt manage the 100 yards. I had been reasonably impressed with looking through the SCT and yet had read that Maks were somewhat better atleast for visual. But Macs are longer then SCTs of the same aperture at least and heavier due to the thicker correction plate. But I figured I didnt mind just a little more weight and length if the mount and tripod were collapsable enough to carry in a bag in one hand with the scope in the other. I started calculating as best I could with limited experience but actual numbers of weight of scope and mount, retracted length of tripod and so forth what would work for me and came up with a tentative idea of what seemed to fit my needs. Achromats seemed like for reasons of chromaric aberation and weight and cost to be out of my goals after 4 inches. But I still wanted the opticaly perceptabl qualities of one as best I could achieve within my limits(even if the design and means of producing that visual experience were different...ie telescope type. It seemed a 7 inch Mak with sturdy tripod if collapsable into a small enough bundle would still potentialy be doable for those 100 yards. Haevy yes but not too bulky. At 8 inches of aperture maybe I could manage a Schmidt though I was doubtful(still am) so I started using these as a sort of guideline. What people were calling small, medium, and large seemed so inflated with what seemed to make sense it kept occuring to me that there was no consistant standard but I wasnt sure and yet if there were one it might help me to know it and have a better idea what was meant when people used small, medium, and large with telescopes so Id know the lingo so to speak and know thereby what was being meant. Now I know there is no standard in the strict sense but I do have an interesting and useful impression of how others think about this. My thanks again for that! Im still unclear whether aperture size of a Mak if large enough will achieve all the same optical visualy experienced quality of a refractor but my sense in part from what I have read here is that it will and the differences in design might not be so important at smaller sizes anyway but do make a difference to an extent. I guess I want the best optical quality I can get in a transportable (hundred yard trip to the field) in skies that yield about 4.5 mag but will also excell under much darker skies on the rare occasions when I can get out to them. Lets say for the sake of pinning a number 1000 max for the OTC and no more then 350 for the mount.

Edited by Deepskyclusterstruck, 08 December 2018 - 09:49 AM.


#17 Napp

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Posted 08 December 2018 - 11:56 AM

How about a dob and a wagon?



#18 TOMDEY

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Posted 08 December 2018 - 12:09 PM

For "average" scopes in design-type classes... much of the above is correct. But it's also an historical thing:

 

Common observer's metrics:

 

>resolution

>contrast

>cost

>convenience

 

>WAY back, the common wisdom was that refractors were better than reflectors, because speculum mirrors had pathetic reflectivity.

>Then, silver-coated glass mirrors came on the scene and exceeded the uncoated, absorbing glass for throughput, mostly due to the low transmission, scatter and absorption experienced in glass. In smaller sizes, up to about 20-in, glass still was preferred. But "professional" apertures were approaching 48-in and refractors couldn't compete (at all) in that size range.

>Then, early/mid 1900's, refractors were touted as identically/inherently better inch-for-inch, especially in the then amateur range of 3-6 inch.

 

But, today, the physics (and available practice) kill nearly all of the type advantages. That is to say, properly-executed, it has reverted to just aperture... with only a polite nod to the central obstruction consideration. If a telescope of any type comprises aggressive baffling, enhanced coatings, superior polish, good wavefront, APO color-control, temperature control... under those "premium" boundary conditions... all that's left is the central obstruction consideration.

 

>it the obstruction is small-medium, the performance differential is insignificant/academic. Say, obstruction up to diam/area 20/4%, the diffractive and throughput affectives are dinky.

>That is satisfied by scopes such as Big Dobs and Hubble!

 

So, there is little inherent design-type edge  any more. On the other hand, really wonky metrics like [operational cost per data bit, or information throughput per dollar] Those things vary a LOT as a function of design type. And That drives nearly all of the professional data-collecting optical builds.  Tom



#19 stargazer193857

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Posted 08 December 2018 - 02:02 PM

What is large depends on how heavy it is. Also how long to set up and how big the ladder.

A throw together easy to transport 24" f2.6 could count as medium. A 16" that requires a ladder would count as large.

Edited by stargazer193857, 08 December 2018 - 02:03 PM.


#20 Tony Flanders

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Posted 08 December 2018 - 03:45 PM

For what it's worth, I think of "large" as referring to aperture, not to a scope's physical size. In fact I use these terms only when describing what one can expect to see through a telescope, and physical size is irrelevant for that purpose.

 

So, for instance, I consider an old-fashioned 4-inch f/15 refractor to be a small or medium scope due to its modest aperture, despite the fact that its tube is 5 feet long, and it requires a truly ginormous tripod or pier.

 

I do make some concession to telescope type -- 5 inches seems definitely small to me for a Newtonian, SCT, or Mak-Cas, but perhaps medium or even large for a refractor. That's partly because practical amateur refractors simply don't come in apertures that would be considered large for Newtonians, and partly because of the modest extra efficiency of the refractor design.



#21 Redbetter

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Posted 08 December 2018 - 06:31 PM

What is large depends on how heavy it is. Also how long to set up and how big the ladder.

A throw together easy to transport 24" f2.6 could count as medium. A 16" that requires a ladder would count as large.

No.  I don't see how a 24" is going to be classified as medium, no matter how short the ratio.  Weight, bulk, and what one can see in the eyepiece render would likely destroy such a narrative.  Nevermind the cost of 24" f/2.6.  It is as if someone was throwing about a slew of fanciful hypotheticals on classes of scopes that one hasn't observed with or lived with... 
 


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#22 Deepskyclusterstruck

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Posted 08 December 2018 - 08:40 PM

For "average" scopes in design-type classes... much of the above is correct. But it's also an historical thing:

Common observer's metrics:

>resolution
>contrast
>cost
>convenience

>WAY back, the common wisdom was that refractors were better than reflectors, because speculum mirrors had pathetic reflectivity.
>Then, silver-coated glass mirrors came on the scene and exceeded the uncoated, absorbing glass for throughput, mostly due to the low transmission, scatter and absorption experienced in glass. In smaller sizes, up to about 20-in, glass still was preferred. But "professional" apertures were approaching 48-in and refractors couldn't compete (at all) in that size range.
>Then, early/mid 1900's, refractors were touted as identically/inherently better inch-for-inch, especially in the then amateur range of 3-6 inch.

But, today, the physics (and available practice) kill nearly all of the type advantages. That is to say, properly-executed, it has reverted to just aperture... with only a polite nod to the central obstruction consideration. If a telescope of any type comprises aggressive baffling, enhanced coatings, superior polish, good wavefront, APO color-control, temperature control... under those "premium" boundary conditions... all that's left is the central obstruction consideration.

>it the obstruction is small-medium, the performance differential is insignificant/academic. Say, obstruction up to diam/area 20/4%, the diffractive and throughput affectives are dinky.
>That is satisfied by scopes such as Big Dobs and Hubble!

So, there is little inherent design-type edge any more. On the other hand, really wonky metrics like [operational cost per data bit, or information throughput per dollar] Those things vary a LOT as a function of design type. And That drives nearly all of the professional data-collecting optical builds. Tom



#23 Deepskyclusterstruck

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Posted 08 December 2018 - 08:42 PM

"data-collecting optical builds" is realy a thing? Are you pulling my leg?
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#24 TOMDEY

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Posted 08 December 2018 - 09:16 PM

"data-collecting optical builds" is realy a thing? Are you pulling my leg?

Hi, Deep! Ummm... nope!  I was referring to Imaging Satellites, which was what I did, before I retired.  I make up terms, when there seems to be nothing in the vernacular.  Uncle Sam seriously classifies Imaging Sats as nothing more or less than IMINT data-collecting platforms, where the data is Intelligence (quantifiable knowledge regarding the state of the cosmos or of the earth, and its inhabitants... flora, fauna, sapiens...)  And the metric that Uncle (aka we citizens) pay for is actionable bits per dollar.  That's how contracts and builds are awarded!  Whoever can provide the most utilitarian data per buck... wins the contract!  Here's a cartoon (my creation) that shows that action-loop (ostensible defense against attacking Martians!)  There really ARE people who worry over this stuff... so we can either sleep soundly at night... or enjoy looking at the stars!  Tom, guru optique

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#25 Deepskyclusterstruck

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Posted 09 December 2018 - 07:16 AM

I am impressed, amused, intrigued, and slightly spooked all at once. Id be interested in knowing more about your work but my lizard brain is still trying to determine how to ask my next question?


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