Minimum Useful Scope Aperture: Outdated concept ?

[John]

Having been in this hobby for quite a while (30+ years) I've read a lot of books on the subject over that time. Looking back at some of the titles that got me started such as the Observers Book of Astronomy, The Amateur Astronomer and The Amateur Astronomers Handbook I see that there are recommendations in there regarding the minimum aperture telescopes that are considered "useful" (whatever that means) for astronomy. Commonly for refractors, 80mm / 3 inches is often quoted and for reflectors (newtonians usually) 150mm / 6 inches seems to be where the "serious" observing starts. In my early days in the hobby I found this a little frustrating because all I could afford was a 2.4 inch refractor of the type sold in department stores and in "Mums catalogue"

Much as I enjoyed these wonderful books I can't help feeling that this particular advice is no longer really relevant today. Maybe there is less emphasis now on amateurs pursuing rigourious observing programmes and more on just getting out under the stars and enjoying yourself but it seems to me that many of us are regularly using quite small aperture scopes and having a rewarding time with them.

So if I was writing a book of the type that I mention above (I'm not you will be relieved to know !) I think I would take a much less prescriptive approach to what constitutes a minimum useful aperture to enjoy astronomy with. There are decent telescopes now with apertures from 50mm / 2 inches which have their place in providing enjoyment.  Of course it's also useful to have an idea of what the limitations of small apertures are, to avoid disappointment / frustration so I think I'd try and outline what those are as well as indicating what they are capable of.

Does the old advice on minimum apertures still have some merit though ?

I'm very interested to hear what others think

 

 

 

 

 

[almcl]

That made me reach for my 1962 edition of the Observer's Book of Astronomy! (Yes, still got it.)

I notice that while Sir Patrick condemned small refractors, he instead recommended binoculars as an alternative and having used a pair of 1960s manufacture 9 x 30 Ross binos the other night, I think that both telescopes and binoculars have come a long way in the intervening years.  Both in price per millimetre of aperture and in overall performance, today's equipment seems much superior and this has surely pushed back the boundaries of what is 'useful'.

[SIDO]

I think for visual astronomy a telescope were one can see things they have never seen before with enough resolution to define at least minimum details is where the lower bar really has always been regardless of the literature espoused by many publications over the decades, so then the debate where serious astro begins becomes again a question of aperture when in reality its a conglomeration of aperture, sky brightness, skill level, experience, scope quality and then add to all of that the level of commitment an individual apllies and how consistent there efforts may be as well each individual may interpret "Where serious Astronomy Begins" differently both at any one given point or at different points in their unique journey.

I would in summary conclude the point of serious astronomy has very little to do with aperture and more to do with the individual and conditions with aperture being important but just one of the many criteria that is so.

 

Edited March 1, 2019 by SIDO

[Demonperformer]

I wonder if it has anything to do with the increase in light pollution? Under a dark sky, you can see a lot more and under a heavy LP sky, even a small instrument, by darkening the background, simply enables you to simply "see" them.

This suggests a practical experiment for me: produce a 40mm mask for my 4" frac and take it out and compare what I can see with it "under the streetlights" compared with no instrumentation - a sort of "count the stars I can see in the Pleiades" sort of thing ...

Now, where's my "blue peter" kit?

[nightfisher]

Yes John, the old adage is maybe a little out of date, lot of people are very happy with 100mm newts and the 72mm ED scopes are gaining favour and even smaller scopes are being employed, i have owned large scopes myself, up to 200mm and now i am just happy with a 127, a nice combination of aperture and portability

[Mr Spock]

I used to have the ubiquitous 60mm refractor in my teens. It didn't stop the wonder, only fueled it. Plus it was very good for double stars.

[Stu]

Being the rebel I am, I've never really taken much notice of limits like this. I've been a long term proponent of the use of small but good quality apos because they allow much more regular observing and can be taken anywhere.

As others have said, much is to do with current light pollution issues and lifestyles. Were it not for these smaller scopes I would not have had many of my most enjoyable sessions.

There is the question as to when you are doing serious observing or something of scientific value. I'm afraid to say my observing has never been either of these things, it is done for enjoyment and is done to a competent level I would hope, but has never been about scientific achievement, therefore I can use whatever I like! I prefer apo refractors because to me the views have to have that lovely aesthetic appeal to them.

My most recent little whim is a 50mm ED spotting scope on a monopod, which is about as easy as it gets and is up and running in an instant to allow widefield sweeps of the sky and is great on asterisms and open clusters.

I have a little Tal Alcor 65mm newt with beautifully sharp optics and above this, my little TS72ED is a wonderful scope and can achieve an awful lot for such a small scope, including solar observing.

Above all, I just enjoy seeing what I can see in any optical instrument. To me it is just as rewarding to try to spot M37 under my skies with a 50mm scope as it is to try to spot a faint galaxy with a big dob under pristine skies.

[Size9Hex]

Really like this topic.

If "useful" means useful for getting you excited in seeing and learning what’s up there, the minimum aperture is your eyes and anything above that is a bonus! ?

I used 50mm binos for months before getting a scope and they revealed loads. It was this aperture that gave me my ongoing enthusiasm for the hobby. The step up from the naked eye was utterly enormous. As others say, this is dependent on the skies by at least somewhat dark. If the binos had been a 50mm scope with potential for high mag, then even better!

Edited March 1, 2019 by Size9Hex

[pete_l]

I think that one major reason why such small telescopes were suggested had more to do with price than usability.

In the 1960s optical gear in the UK was very, very, expensive. Most telescopes would have been hand-made and therefore of dubious optical quality. They also didn't have the modern glasses, design tools or eyepieces we take for granted. Even today, prices increase very quickly with aperture: it must have been even worse 50+ years ago.

I reckon that is why the recommendation for binoculars started. More due to cost than providing decent views (and it was only eyeballing, photography was virtually non-existent in the amateur sphere). Even so, they would start at more than twice the weekly pay of most people (roughly £10 a week in 1963, before tax). So £30-£50 for a pair of 10x50s was a luxury item - think £1,000 now! Even army surplus was pricey. There's an interesting article about binoculars here

 

Edited March 1, 2019 by pete_l

[Ricochet]

7 hours ago, John said:

Does the old advice on minimum apertures still have some merit though ?

How many telescopes were the authors of this old advice expecting their readers to be able to buy? These days lots of people own multiple scopes, but when scopes were relatively more expensive was it the norm that people would only own a single telescope?

Perhaps we should split the question into two:

1. What is the minimum aperture that a telescope has to be to for it to gain regular usage when you have larger options?
2. What is the minimum aperture that you would choose if you could only own one telescope?

I suspect that these answers would not be the same.

[Stu]

9 minutes ago, Ricochet said:

How many telescopes were the authors of this old advice expecting their readers to be able to buy? These days lots of people own multiple scopes, but when scopes were relatively more expensive was it the norm that people would only own a single telescope?

Perhaps we should split the question into two:

1. What is the minimum aperture that a telescope has to be to for it to gain regular usage when you have larger options?
2. What is the minimum aperture that you would choose if you could only own one telescope?

I suspect that these answers would not be the same.

In my instance the answer to those two questions is 50mm and 100mm respectively so you are correct, but the difference in my case is perhaps not as much as might be expected.

[JeremyS]

Indeed there is no minimum "useful" size limit. The 3-inch refractor/6-inch reflector concept was something Patrick More promoted. There was some sense in in though. I couldn't aspire to one when I was a lad, but I made great use of my 60 mm Tasco refractor.

Interestingly, in his "Starlight Nights", the great  observer Leslie Peltier relates that when he was going to join the AAVSO in 1918 it had a minimum size telescope requirement of 3-inch. He was disappointed as his was only 2-inches. But when he filled in the application form, his letter 2 had an extra curlicule, making it look like a 3. He was in! And he became one of the most prolific variable star observers as well as discovering many comets.

[JeremyS]

Is this serious?

 

[mikeDnight]

27 minutes ago, JeremyS said:

Is this serious?

 

If Leslie Peltier had this scope & eyepiece,  it would have definitely been put to good use! Perhaps not as ridiculous as it first appears in the right hands! 

[Macavity]

Another one brought up on SPM's Observer's Book"!
Three inch refractor... Six inch reflector... Notorious Pillar & Claw Mount! ?
(I did try out the One Dioptre spectacle lens idea too!) 

My pocket money would only stretch to one of the "dread" 30x30 scopes
sold in many seaside resorts! But it taught me quite a lot. I rapidly realised
it was *usable* if I stuck my microscopes e.p's (longer focal lengths!) in it... 
But with only £12.50 in P.O. savings, when I left school, I simply gave up! ?

Next 30 years (Uni... jobs... travel) I completely almost forgot Astronomy.
Until... By chance... I bought an Astro Mag in WHS! I was *amazed* I could
finally afford a REAL telescope (OK... an ST102 / EQ3-2 Combo anyway!) ?

And I did LOVE it... But I had supposed this 4-inch scope would be FAR
better that SPM's 3" (I suspect f/12) doublet achromat? lol. I knew about
"chromatic aberration" so I paired the ST102 with a MAK90. With that
I became aware that a smaller color-free / well figured scope had merit! ?

WITH HELP from Astronomy Forums (+ my experience as a "scientist")
I began to understand the underlying issues more clearly? It isn't really
just about SIZE (though it does matters! lol) but a whole host of stuff...  ?

My real "revelation" came with discovery of Video Astronomy.
Again that (like the Astro Mag) a completely random thing! ?

My (Astro) Life is mish-mash of chance / learning / Darwinian selection? ?
"Faint heart never won fair lady" sometimes "making the BEST of things"!

Edited March 1, 2019 by Macavity

[Alfian]

In terms of the cost and quality of what is availaible now compared to 30 years ago I think the goal posts have definitely moved. The SW 72ed mentioned is a good example. The discussion regarding "serious astromomy" and more casual (but keen!) observation just for pleasure  is also an interesting. I often think when tempted to acquire yet another piece of equipment, when I don't use what I have nearly enough, what Galileo was using for "serious" astronomy. It puts me to shame. 

[JeremyS]

9 hours ago, John said:

So if I was writing a book of the type that I mention above (I'm not you will be relieved to know !)

Actually, John, I think you'd be an excellent person to write a book given your experience with so many types of telescope and different observing targets. You really should think about it seriously.

[John]

Thanks Jeremy - I'm flattered

 

[mikeDnight]

Stating a limit on the minimum useful aperture serves only to place a mental block in the mind of an observer. Back when mirrors were made from rapidly tarnishing speculum metal, or home ground mirrors that were of questionable quality and silvered, small reflectors may have performed relatively poorly. Today I'd imagine there are relatively few "bad" reflectors in existance.  I've seen a 4.5" F11 Newtonian (generally considered little more than a toy), kick the socks off a 4" Vixen F9 fluorite apo while looking at Saturn. I've seen the same 4" Vixen fluorite outperform many very much larger reflectors, as regards image quality and definition, on lunar & planetary targets. Much of what I've seen seems to be counterintuitive. I've detected detail in Saturn's rings that should be beyond the capability of my telescopes, yet it's obvious. I've split at least one double star that was well beyond the theoretical resolution of the scope I was using, and I've seen faint nebulae in impressive detail, where you'd imagine only a large aperture would be successful. 

When practicing my lunar sketching, I tried to find a simple target to draw and chose the crater Werner. Situated in the southern uplands, it at first appeared to be an easy crater to draw. However, as I was sketching the crater floor looking for variations in shade etc, I noticed a very fine rille running from one of the central mountain peaks, up and over the southern terrace wall, and out into the rough terrain beyond. Thinking little of it, I later tried to find evidence of this rille in maps and images. What a struggle! The only proof of its existance I could initially find was an indication of a rille running along the crater floor, as recorded by Percy Wilkins, who was using the 33" Maudon refractor. I'd had it said to me by another, very well respected lunar observer, that I'd likely seen a line of sight effect caused by terrain artefacts. Later observations continued to show the feature as a definite rille, and not only that, other rilles began to reveal themselves at different points in the lunation. To date I have observed five within the Werner!    Then, a wonderful chap in the BAA sent me an email, in which he'd attached some images he'd taken of Werner, and guess what - there were the rilles. Though barely visible in the images, they were there and exactly in the same positions my small scope had seen them. 

Like many others, I'd started off using a 60mm refractor that gave me my first real view of the moon & planets, and it was a nice scope. I also bought a second hand pair of 12x60 binoculars, with which I found every Messier object above my horizon. From my experience,  small apertures can be very revealing and capable scopes, and having a larger aperture scope is no guarantee of a better view (unless of course we're considering the advantages of light grasp)! Reflector or refractor has always been a hot topic for as long as I remember, and again from experience,  I've seen instances were both have outperformed the other, dependant on target, seeing, transparency and optical quality etc. As regards the 3" refractor or 6" reflector advice of yesteryear,  I'd say ignor it completely and just use whatever scope you've got to the full. Only then will you realize its true potential!

[Alfian]

Thats a very interesting and edifying read Mike. Well done. Even with my limited experience and modest equipment I've occasionally observed a "doubtful"  luna feature which then having scoured my books to identify, found was real and not just a trick of the light. 

[Ags]

I tried stepping up from a 102mm scope to 150mm on the assumption that bigger is better and 150mm is the "minimum requirement", but I found that it was more of a disadvantage than an advantage. Even at F5, 150mm aperture had too much focal length for convenient object location with 1.25" eyepieces, also the scope weight was more than my mount could take at high mags, and with city sky background washed out  at 2 mm exit pupils and up, the aperture meant I needed magnifications in the range of 100x for pleasing views, which was too much for many targets (and too much for the mount). By changing to a Newtonian light bucket I thought I would get vastly improved star clusters etc, but all I got was clumps of hairy stars on a grey background. 

Retreating to a small refractor and Mak, I get dark sky backgrounds, proper-looking stars, wide fields* for easy object location and no issues with undermounting. I doubt I can do much "serious" astronomy, but I think if I wanted to get serious (e.g. measure light curves of variable stars, or do spectroscopy) I would still use a relatively small telescope, good mount, and lots of electronics.

* Of course a Mak does not give wide fields, but it is small enough to mount alongside the little frac, so I get the wide fields of the ST80 and the superb views of the Mak in one package - after some simple trickery with the tube rings to get the two scopes aligned.

Edited March 1, 2019 by Ags

[paulastro]

I've enjoyed reading this topic, lot's of interesting points put forward, not one of them without some merit.

Clearly the answer for any individual will be based on all sorts of things, their experience, what they want to see, how detailed a view they want, where they want to observe from, does it need to be portable or transportable  etc etc.  I think for any individual, the answer will be  a combination of the answers to two questions:  'which telescope will I use the most often' and 'which telescope would I keep if I was only allowed one telescope'. 

There is no minimum aperture which would suit all of us, as we all have an assortment of different requirements.

The answer for me is my smallest scope, my SW 72ED.    ?  If I never had access to any bigger scopes, it would always bring me great observing experiences and I'll remain a happy bunny.

 

 

[Peter Drew]

The post title begs the question "useful for what?". A 60mm refractor is ideal for solar sunspots and suitable double star observation but would be less useful for comet seeking. The urban myth that 3" refractors and 6" reflectors are somewhat equal has nowadays been debunked now that good quality examples of each are readily available. What is useful for me, these days, is a larger aperture to come close to seeing what younger keen sighted observers can see with smaller ones so my minimum useful aperture is gradually increasing. The bottom line, probably, is that the higher the quality the smaller the aperture that can be considered useful.  ?

[Carbon Brush]

For my twopennorth.

As a child I heard (or read) several times the view that 4" refractors and 6" reflectors were a minimum.

This opinion was supported by my older brother being given one Christmas (mid 60s) a 1" refractor with inbuilt eyepiece on a table top tripod. Views were as expected!
This was further strengthened a few years later when my father bought (1968/69?) a 60mm Prinz refractor with an awful zoom eyepiece and a table top tripod.

So I stayed away from astronomy telescopes for a very long time. Books being much better.

A very different sky view nowadays.

If I could get even my finderscopes into a tardis.........

David.

 

[paulastro]

22 hours ago, Peter Drew said:

The post title begs the question "useful for what?". A 60mm refractor is ideal for solar sunspots and suitable double star observation but would be less useful for comet seeking. The urban myth that 3" refractors and 6" reflectors are somewhat equal has nowadays been debunked now that good quality examples of each are readily available. What is useful for me, these days, is a larger aperture to come close to seeing what younger keen sighted observers can see with smaller ones so my minimum useful aperture is gradually increasing. The bottom line, probably, is that the higher the quality the smaller the aperture that can be considered useful.  ?

That's what I was trying to say Peter, perhaps not well enough!    (.....what they want to see, and how detailed a view they want....)  As I said, it depends on the indiviual's requirements.  Perhaps I should have put eyesight quality on my list of potential considerations . 

I seem to remember that one of the great comets of the 19th century was 'discovered' by a whole shift of miners one dawn as they left the diamond mine where they were working. Only naked eye needed for this discovery.  Also, the great George Alcock discovered one of his comets using a pair of 10x50 binoculars from the landing on his stairs.  Or was this one of his novae?   

Edited March 2, 2019 by paulastro