Telescope myths

[YKSE]

I read this one long time ago, I'd like think that atmospheric turbulence does have different impact on aperture size, Fried parameter is describing this effect.

As to narrow band or line fillters, I agree with John,  they remove unwanted spectrum of lights, enhance the right nebulas in any scopes, nothing s more far from truth than  claiming "removes 90% of light".

[pete_l]

I think the reality is, as you go to larger apertures you do see more detail but in poorer seeing conditions you have to wait longer for the moments of clarity, and they are more fleeting than with a smaller scope. Ultimately you do normally see more with the larger aperture but that isn't always my experience!

It's also possible that since the cost of a large aperture 'scope increases extremely rapidly with size, that the small aperture scopes are better quality for the price that people are willing to pay. But a large aperture instrument, with the same optical, materials, coatings and build quality would be unaffordable. Hence people buy "cheaper" large scopes and "costlier" small ones.

[mikeDnight]

My personal experiences are somewhat different to those already mentioned. Though I accept the obvious advantages of aperture with regard to lightgrasp and theoretical resolution, i have rarely ever seen a large aperture scope deliver consistantly sharp planetary detail. The finest view of Jupiter I've ever had was through an old orange C8, it was highly detailed and as tack sharp as any refractor. And thats just about it. Nearly all other views through such instruments have consistently proved to be soft and lacklustre by comparison. I've used many scopes from 3" to 30" and planetary views have nearly always been disappointing by comparison to a good refractor.

When it comes to studying dim objects such as galaxies and nebulae, aperture certainly comes into its own and even the best small refractors cannot compete visually. Lunar and planetary detail though demands fine optics, and although there are many good telescopes on the market, not all hàve the optical quality good enough to give top end planetary performance. On countless occasions I've taken the opportunity to use my scopes in side by side comparison observations with scopes of other designs and can count on one hand, and with fingers to spare, the times when my refractors have met with serious competition.

With regard to myths, I don't agree with Gary's view on seeing. I've seen a 5" outstripped by a 3" while looking at Jupiter, the 3" revealing more detail while the excellent 5" was more affected by the poor seeing. And I've seen the same 5" time and again wipe the floor with 12" and 16" SCTs as well as large Newts on the Moon, Mars, Jupiter and Saturn. I've seen a 4" ED split a binary down to 0.9" arc, way closer than its 1.1" arc theoretical resolution. And my own 4" shows stars down to 13.2 mag and probably fainter, far fainter than generally thought possible. I've seen many strange things over the years and all things considered, I think telescopes are an enigma, and those who are passionate about them are just plain weird.

Mike

[Roy Challen]

Myth- Huygens eyepieces are of abysmal quality.

Most, no, the vast majority of Huygens are rubbish. However, my Zeiss H-25, in my f/16 achro is probably the sharpest ep I have ever looked through. Very little in the way of any kind of aberration.

Huygens have a poor rep because they are cheap to make and all too often they are made cheaply and supplied with scopes that can't make the best use of them.

[Alien 13]

The biggest myth is the 3 inch frac equaling a 6 inch newt in overall image quality probably but resolution maybe not.

Alan

[mikeDnight]

The biggest myth is the 3 inch frac equaling a 6 inch newt in overall image quality probably but resolution maybe not.Alan

I think there may have been some truth in that myth when primary mirrors were made from rapidly tarnishable speculum, or when most mirrors were made by inexperienced amateurs over a kitchen sink. The almost inevitable turned down edge and poor figure, as well as a far from flat secondary is probably the reason why many Newts got a bad press, while the professionally made refractor got praised. I once owned a 6" F10 Newtonian which was one of the finest scopes I've ever used. I love the long focal length Newts, coma disappears and its planetary performance becomes equal to that of any top class apo.

Mike

[michael.h.f.wilkinson]

+1 for the slow Newtonian on planets. My old 6" F/8 was superb on planets. The CO was very small, the figure of the mirror great. The C8 still edged it out on detail, but not on contrast.

[niallk]

Scope myth(?!):

Cheap particle board dob bases don't last...

My experience:

My 250px base is still like new... after 6+ years of use - I always observe on inevitably dewy grass, often with drips of water on every surface at the end of a night. I often see the likes of the Skywatcher bases derided in that they will not last/soon disintegrate, but thankfully mine seems just fine. Seems like a very good design decision to give customers the best aperture at a great price point. I'm sure there are harsher climates than mine here in dampish Ireland, but all I can go on is my own experience. Getting many years of enjoyment (hopefully will continue beyond a decade) out of a €500 scope makes it exceptional value imho

[Alien 13]

I think there may have been some truth in that myth when primary mirrors were made from rapidly tarnishable speculum, or when most mirrors were made by inexperienced amateurs over a kitchen sink. The almost inevitable turned down edge and poor figure, as well as a far from flat secondary is probably the reason why many Newts got a bad press, while the professionally made refractor got praised. I once owned a 6" F10 Newtonian which was one of the finest scopes I've ever used. I love the long focal length Newts, coma disappears and its planetary performance becomes equal to that of any top class apo.

Mike

 

I do wonder how close they get when a lot of observers seem happy to view through a newt with an inch of dust/crud on the mirror which admittedly probably doesnt change the resolution but must impact on contrast.

Alan

[Macavity]

"Short-focus Newtonians require much larger secondary mirrors than long-focus models." 

A rather interesting one, and doubtless the plot is correct, so potentially useful!  

http://www.skyandtelescope.com/wp-content/uploads/FieldGraph.gif

Much as I love my 8" f/4 TS/GSO "video lightbucket" (I make no grander claims)

it can be disconcerting to discover it has a full blown "3" refractor" poking out at

right angles to the OTA! Not to mention the potential effect on scope balance! 

But the latter is rather easily solved... Just point the draw tube downwards and

parallel to the axis of the weight arm! (More convenient for visual checks too!)

Also, these things are constructed for use with a DSLR of ~45mm back-focus,

not to mention all the other junk filter wheels, focal reducer in-focus etc. etc.

To get the focus anywhere *near* the right place needs a largish secondary?

As he seemingly(?) implies, sometimes there are elements of wisdom in myth!

But I do sometimes feel Astronomy can be limited by "you shouldn't do that..." 

[ollypenrice]

It's also possible that since the cost of a large aperture 'scope increases extremely rapidly with size, that the small aperture scopes are better quality for the price that people are willing to pay. But a large aperture instrument, with the same optical, materials, coatings and build quality would be unaffordable. Hence people buy "cheaper" large scopes and "costlier" small ones.

I think this is very true. Only a handful of manufacturers aim to apply their highest standards to their largest scopes. Those who do cannot really expect to find a mass market but I'm sure we all wish them luck. Fortunately, in many walks of amateur astronomical life the combination of a big and basic scope with a smaller and more exquisite one will cover most of the bases. Not all, but most.

Olly

[Moonshane]

I think there may have been some truth in that myth when primary mirrors were made from rapidly tarnishable speculum, or when most mirrors were made by inexperienced amateurs over a kitchen sink. The almost inevitable turned down edge and poor figure, as well as a far from flat secondary is probably the reason why many Newts got a bad press, while the professionally made refractor got praised. I once owned a 6" F10 Newtonian which was one of the finest scopes I've ever used. I love the long focal length Newts, coma disappears and its planetary performance becomes equal to that of any top class apo.

Mike

yep, me too

[timwetherell]

yep, me too

How can that scope possibly perform as well as a refractor Mike, you look in the wrong end 

[Moonshane]

It performs better than the refractor in this thread

http://stargazerslounge.com/topic/220387-something-not-quite-right-with-my-new-vixen-frac/

[Dave Lloyd]

The scope you happen to have is adequate.

[HD-AP]

My personal experiences are somewhat different to those already mentioned. Though I accept the obvious advantages of aperture with regard to lightgrasp and theoretical resolution, i have rarely ever seen a large aperture scope deliver consistantly sharp planetary detail. The finest view of Jupiter I've ever had was through an old orange C8, it was highly detailed and as tack sharp as any refractor. And thats just about it. Nearly all other views through such instruments have consistently proved to be soft and lacklustre by comparison. I've used many scopes from 3" to 30" and planetary views have nearly always been disappointing by comparison to a good refractor.

When it comes to studying dim objects such as galaxies and nebulae, aperture certainly comes into its own and even the best small refractors cannot compete visually. Lunar and planetary detail though demands fine optics, and although there are many good telescopes on the market, not all hàve the optical quality good enough to give top end planetary performance. On countless occasions I've taken the opportunity to use my scopes in side by side comparison observations with scopes of other designs and can count on one hand, and with fingers to spare, the times when my refractors have met with serious competition.

With regard to myths, I don't agree with Gary's view on seeing. I've seen a 5" outstripped by a 3" while looking at Jupiter, the 3" revealing more detail while the excellent 5" was more affected by the poor seeing. And I've seen the same 5" time and again wipe the floor with 12" and 16" SCTs as well as large Newts on the Moon, Mars, Jupiter and Saturn. I've seen a 4" ED split a binary down to 0.9" arc, way closer than its 1.1" arc theoretical resolution. And my own 4" shows stars down to 13.2 mag and probably fainter, far fainter than generally thought possible. I've seen many strange things over the years and all things considered, I think telescopes are an enigma, and those who are passionate about them are just plain weird.

Mike

It's

all about three  things seeing  accuracy  and apeture of the optical system .i.e . total system thorughput accuracy  vs seeing .

Look through an astrophysics refractor or a sinden optics reflector and you will see a difference even in poor seeing conditions .

Optical quality cost's lots of money in term's of hour's required to produce  .Today i think that we are lucky that quality optics are availiable for a reasonable price ,but in the past as now the bottom line is quality cost's and can you tell the difference in average seeing conditions .for the most part probably not ,most of the time but when seeing allow's these premium optics deliver views that most will never see.  

Manufacturer's can seduce us with pretty looking high tech looking telescopes in many pretty colour's and sexy /carbon fibre materials ,but a telescope is not for looking at unless it is  an engeneering sculpture ,but for looking through !!

Some of my scopes are cosmetically challenged ,but when put up for side by side comparison will optically put these pretty looking scopes to shame ,which is all that count's ,as russell porter said the mirror's the thing ,but it's the quality of the optic's refractive or refective that count's !

But any telescope no matter how badly made can show you something  40 year's ago i made a 4 inch newtonian from a pyrex mirror blank purchased by mail order from charles frank ltd in 1972.

I ground figured and polished it to an f8 curve of one hidious wave ACCURACY .It produces good view's of the moon and planets which for me then as now which  is all that counts .

 I still have it to this day it's probably the most innaccurate telescope optic in the world ,but knowing how long and how hard it took to produce ,makes me appriciate the time skill and patience of the best optical worker's in this world today to produce what they do at so affordable a price .

But with premium optics you really do get what you paid for and rightly so..........

[Tomjo59]

Neil English wrote an article in the August edition of Astronomy Now (Optimising an eight-inch Newtonian for Visual use), where he concludes that an eight-inch newt at f6 hits a 'sweet spot' of decent aperture, performance and cost. This could probably be extended to a 100mm long-focus refractor too, if one were to be allowed just one scope. Astro imaging is a different matter of course.

Although more aperture would always be better in ideal observing conditions, we have to be realistic about the British climate!

I love my 10" Newtonian, but I wouldn't want anything bigger, given the aforementioned climate, convenience of carrying it out to the garden etc. I couldn't justify the extra cost just for one or two nights a year when conditions were perfect, so our UK climate may be a cure for aperture fever!

[John]

Neil English wrote an article in the August edition of Astronomy Now (Optimising an eight-inch Newtonian for Visual use), where he concludes that an eight-inch newt at f6 hits a 'sweet spot' of decent aperture, performance and cost. This could probably be extended to a 100mm long-focus refractor too, if one were to be allowed just one scope. Astro imaging is a different matter of course.

Although more aperture would always be better in ideal observing conditions, we have to be realistic about the British climate!

I love my 10" Newtonian, but I wouldn't want anything bigger, given the aforementioned climate, convenience of carrying it out to the garden etc. I couldn't justify the extra cost just for one or two nights a year when conditions were perfect, so our UK climate may be a cure for aperture fever!

I have the F/5.3 12" version of your Orion optics newtonian, also on a dob mount.

My skies are nothing special (not too far from yourself !) but I do find that I can routinely use 250x - 300x on the planets, and sometimes more on the moon. I changed to my current scope from a 10" F/4.8 Orion Optics and don't regret it one bit

I also have excellent 4" and 4.7" ED doublet refractors but the 12" dob gets out under the stars the most.