Minimum Useful Scope Aperture: Outdated concept ?

[John]

There are a number of terms used in some older books that deserve to be questioned, for example  "useful planetary work".

Perhaps today you need a launch vehicle, a robotic spacecraft, a team of engineers and scientists and a few million $'s to do useful planetary work ?   

 

 

[scarp15]

Today has an almost bewildering range of choices relatively financially accessible to many, through a strong retail section and thriving used market. Back then, regarding The Observers Book of Astronomy, there was equally a strong appetite, probably particularly among school kids for learning about astronomy, as this was the peak period for the Apollo space missions and Patrick Moor's Sky at Night was standard family house hold viewing. Then as an 11year old in 1973 I got my edition of the guide, which I currently have next to me, it still has its price tag; 75p. The front cover is an illustration of the Aurora Borealis as seen in southern England in November 1961 to the centre of the picture is the Plough. Along with A Concise Guide in Colour Constellations, many more books related to astronomy were obtainable through the library. Perhaps then it would have been more to do with learning, perhaps creative understanding to, I recall attempting to compile my own book form of gathered knowledge into school type jotters with many illustrations (still somewhere in the loft). Using or owning a decent telescope was almost inconceivable, there were only two in existence that I knew of and both were displayed in Dixons window. Lucky enough to get the smaller of the two for my 11th birthday, mostly for pointing at the moon it was pretty rubbish, but gave me a lasting astonishing moment when I pointed it at Venus.  

For anyone who would have been into visual astronomy in the 60's or 70's maybe if you lived on the edge of a Town or reasonably away from a Town or City, the light pollution would have been much less glaring, a 3" refractor, 6" reflector would have been quite effective if optically sufficient. Therefore my own thoughts or memories, were that then it was just the outstanding science through the Apollo launches and exposure on the TV, through the library, in the class room or obtained in the local book shop that was so captivating. Owning a telescope or even binoculars for night time use didn't seem to figure, just the knowledge of human endeavour, understandings of the Universe, with some classic science fiction on TV such as Star Trek counted, knocking a football around badly with two or three mates filled in the rest.  

[Alfian]

2 hours ago, John said:

There are a number of terms used in some older books that deserve to be questioned, for example  "useful planetary work".

Perhaps today you need a launch vehicle, a robotic spacecraft, a team of engineers and scientists and a few million $'s to do useful planetary work ?    

 

 

This idea of "usefulness" is thought provoking. Useful for what (to quote Peter) and "useful planetary work"-  but to whom with respect to above makes me reflect on the whys and wherefores of why I enjoy astronomy. Whilst the amateur astronomical community can make a valid contribution to the astronomical knowledge/research base,  for me its usefulness is purely personal - although I guess there is a very very outside chance I might spot a comet or something before any one else but I doubt it!  Keeping the grey cells active, enquiring, learning and staying engaged is useful! At the same time its enjoyable and fulfilling, most of the time. In this context the minimum aperture is one sufficient to float my personal boat which as someone has mentioned can be the Mk1 Eyeball or a small binocular. That's the minimum - the problems start when I wonder what the "ideal" (for me) would be and I'm still working on that one.

Edited March 1, 2019 by Alfian

[R26 oldtimer]

It seems that the trend these days is that scopes get smaller, and finderscopes bigger.... Sometimes bigger than the scope itself!

[Ruud]

I'm buying a 70mm with a 2+1+1 design. Good for photography, excellent for birding I think.

Robtics_robtics_70_mm_f5_4_element_flatfield_astrograph_quadruplet_nl.pdf

[Alien 13]

I still chuckle when people have a look at the moon through my 90mm Mak, their look of shock is priceless...

Alan

[Alfian]

44 minutes ago, Alien 13 said:

I still chuckle when people have a look at the moon through my 90mm Mak, their look of shock is priceless...

Alan

I can believe that - the 90 Mak is a proper little marvel.

[Astro Imp]

Although I lived through the Apollo missions and vividly recall waiting for Apollo 8 to make contact after it's first disappearance behind the moon at Christmas 1968 I was never in a position to own any equipment 'til relatively recently. 
Reading the thoughts of the experienced and knowledgeable members I have found interesting and enlightening.
Thank you to all who have contributed.

[F15Rules]

Fascinating stuff?.

Like many, I grew up on The Observers Book of Astronomy and absorbed much of what SPM said in his inimitable style.

To be fair to him, I think he really wanted to steer newbies to astronomy away from some of the truly awful "department store" scopes that sold in their hundreds (and still do!). 

Just look at eBay telescopes. You have to trawl through 100s of cheap and nasty "scopes" before you see a proper one. Now, how many potential amateur observers have had their interest crushed, having bought one of these things and been SO disappointed that they were turned off the hobby for life?

I still do agree that a half decent pair of binoculars is a much better way to start off in the hobby, unless you can afford a decent 70mm or so refractor.

My first scope was a Prinz 100 60mm F15 frac, made in Japan, and it had an inbuilt zoom eyepiece giving 15-60x magnification. It was "ok", not great. But my next Prinz was a 550, again 60mm F15, but this time on a nice equatorial mount, with a good objective, finder and 5 individual eyepieces. It.cost £39.50, which was a weeks' wages for my dad in the early 1970s..My parents paid £20 towards it for Christmas 1972, and I paid the balance over about 9 months from my Saturday job pumping petrol at a Mobil garage (30p an hour!)?.

With this scope I observed most of the objects SPM described in The Observers Book, including my first view of Saturn's rings, M42 and the Trapezium, Mizar etc - from then on I was hooked for life. So, for me, following SPMs advice to "get the best telescope you can afford" did really pay off.

Other factors that have changed beyond recognition, and for the better, since then include:

- eyepieces then were all 0.965" with very narrow fov and mediocre quality for the most part. Nowadays we take high quality 1.25" and 2" oculars for granted, with very well corrected, often very wide fields to boot.

- most starter scopes were small, 60mm F15 refractors, again with a narrow field of view and on rickety mounts and tripods. Chromatic Aberration was the norm, not too bad at F15, but Vega was described as a "bluish-white" star, in the OBA, largely because that is what it looked like in most small scopes (and beautiful it looked too!). Today, 70-80mm apertures are the usual starting point with very good lenses, often with ED glass (unheard of back then), and an almost pure white Vega!

- finally, "useful" aperture. As John says, "whatever that means"... but the context of the time is important to consider as well: no "information superhighway",  no amateur imaging to speak of, few, if any, Damian Peach standard planetary imagers (although quite a few gifted planetary sketchers - our own MikeDnight would have shone in their company)- and still a lot of UK amateurs (including Sir Patrick himself) making real contributions to earth based Lunar studies for example (before the advent of hi resolution space imaging).

Today, despite our high end equipment , few visual observers could claim to be making real, groundbreaking discoveries (possibly excepting Comet hunters), and even the fabulous photo images being taken through relatively small scopes are not likely to lead to many new discoveries not already found by huge ground based observatories and in - orbit automated scopes.

But we have so much to celebrate. In the 60s few of us would ever have dreamed of owning telescopes of the quality that we actually do own, and we have so much great equipment and accessories available to us, with so much useful information at our online fingertips...so we can just indulge and enjoy ourselves..but perhaps none of that would have become a reality in our lifetime without the Sir Patrick's and GED Alcocks of this world blazing the trail for us with their sheer dedication and persistence with the equipment they had available at the time?☺

Dave

 

Edited March 1, 2019 by F15Rules

[RobertI]

I think all the points made are very valid. Clearly telescopes used to be much more expensive and most people would make an investment in one scope to last them a long time, so that one scope had to be able to do a lot of things, and smaller aperture scopes have their limitations (although fewer limitations nowadays with the excellent quality optics available). 

Edited March 2, 2019 by RobertI

[niallk]

I have a ridiculous amount of fun with my 50mm Lunt 

Small, but expensive!  Big enough to give serious wow vies, especially when double stacked. 

[Dave Craft]

By far and beyond the best read I've had in a very long time.

Dave 

[ollypenrice]

I think John put his finger on it at the start when when he said, 'there are recommendations in there regarding the minimum aperture telescopes that are considered "useful" (whatever that means) for astronomy.' A lot of these books were rooted in amateur astronomers contributing to professional astronomy, very much a Patrick Moore thing. However, SPM was also a great advocate of the binocular. So if the 50mm binocular is worth having, why would the 50mm telescope not be? In a nutshell, if you are not doing science, but just enjoying the night, you can enjoy whatever you like.

The only instrument I own, and in which I can see the whole Veil Nebula in one go, is a slightly less than 3 inch doublet. This TV Pronto cost a couple of hundred quid second hand. It can also show me the Rosette Nebula in full. I find that 'useful!' 

Olly

[andrew s]

@ollypenrice is spot on "useful" meant getting if published in the BAA journal or equivalent. For a hobby there are not limits but a scope or binoculars with an aperture less than the size of your iris might be of limited use. But wait,  pin hole cameras have there uses! 

Regards Andrew 

[Adam J]

On 01/03/2019 at 00:45, John said:

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.

 

Well I am not one for observing but as my house was in the way of NEOWISE from my garden observatory I took the 50mm finder off my scope and observed NEOWISE from the front garden with the familiy by attaching it to a camera tripod. Worked perfectly, massively improved the view over nakey eye and had exactly the correct FOV for the target. So that was a 50mm aperture and it was indeed useful. Hence I dont think there is a mimimum useful aperture unless you want to be much more specific about what you are planning to do with it.

Adam

[PeterW]

Nice views of the Milky Way with a 16mm focal length lens... different things need different optics to observe....

 

Peter

[Alien 13]

Thankfully scopes dont behave as one expects, the naked eye will always gather the most light so even if you could look through a 10M scope you would have less photons hitting your retina due to losses in the optics combined with the pupil size limit. This means any scope of any size is useful, they just show different things.

Alan

Edited November 7, 2020 by Alien 13

[Paz]

As well as being able to deliver nice views I would add being accessible to what makes a scope useful, i.e. easy to use. I would guess many people these days have less spare time and more alternative past times competing for attention so for any scope to be useful it has that to contend with.

[DaveL59]

I've found when using the TAL-M 80mm reflector that I have on occasion forgotten to switch out the optical finder which is a 25mm objective that sits in the light path using the same eyepiece as the scope. A few times I've had a 15mm+3x Barlow and one time the extension tube as well which would get me to x139 via the mirror and was quite surprised at how good the magnified image thru the finder lens was. In fact one time I'd even stacked the 2 TAL 32mm barlows (x3 and x2 or 3) I have with the 15mm while looking at Venus and that still showed well.

Edited November 8, 2020 by DaveL59

[badhex]

This is such a brilliant post - huge credit to all the great replies so far. The only thing I will add, and which has been said before - is that "usefulness" really is in the eye of the observer, as it were. 

For a variety of reasons but mainly convenient setup/cool down times, I have found the recent holiday purchase of a cheap (but surprisingly good, optically) Celestron travelscope 70 to be much more "useful" than my SW 80ED, and certainly if I had any of my other, larger scopes currently available to me, I bet they would get much less frequent use. 

[Xilman]

On 01/03/2019 at 00:45, John said:

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

 

 

 

 

 

An image taken with a 2.2mm f/1.8 refractor appears at https://britastro.org/node/20154

[Xilman]

4 hours ago, badhex said:

This is such a brilliant post - huge credit to all the great replies so far. The only thing I will add, and which has been said before - is that "usefulness" really is in the eye of the observer, as it were. 

For a variety of reasons but mainly convenient setup/cool down times, I have found the recent holiday purchase of a cheap (but surprisingly good, optically) Celestron travelscope 70 to be much more "useful" than my SW 80ED, and certainly if I had any of my other, larger scopes currently available to me, I bet they would get much less frequent use. 

As has been observed many times before in many contexts, it is not the size that is important, it is how you use it.

Hence my slightly tongue-in-cheek (only very, very slightly) post about a 2.2mm aperture refractor.

On Twitter I have seen images of the likes of Ceres taken with nothing but a smart phone and a solid place on which to rest it.  One can do photometry of genuine scientific importance with a sub-5mm aperture refractor. Many variable stars reach magnitude brighter than 7.5 and a wide enough field of view is unobtainable with more conventional telescopes where comparison stars can be hard to find.

[Xilman]

On 12/11/2020 at 17:28, Xilman said:

As has been observed many times before in many contexts, it is not the size that is important, it is how you use it.

Hence my slightly tongue-in-cheek (only very, very slightly) post about a 2.2mm aperture refractor.

On Twitter I have seen images of the likes of Ceres taken with nothing but a smart phone and a solid place on which to rest it.  One can do photometry of genuine scientific importance with a sub-5mm aperture refractor. Many variable stars reach magnitude brighter than 7.5 and a wide enough field of view is unobtainable with more conventional telescopes where comparison stars can be hard to find.

This is a wonderful example tweeted today: https://twitter.com/PeterLewis55/status/1329375163218087937/photo/1

It contains all of Orion and Lepus, Taurus as far as the Pleiades, Canis Major as far as Sirius, and chunks of Eridanus and Auriga. It is possible to measure stars down to at least sixth magnitude. In particular the brightness of Betelgeuse can be estimated by comparison with Aldebaran.

It clearly needs some flats taking, but that is a detail.

 

Image taken from light polluted London.

[John]

My original post was really concerned with observing. That's all I have any experience of. Aperture and it's relationship with imaging is likely to throw up a different sort of discussion I would have thought ?

 

 

[Xilman]

45 minutes ago, John said:

My original post was really concerned with observing. That's all I have any experience of. Aperture and it's relationship with imaging is likely to throw up a different sort of discussion I would have thought ?

 

 

Ah.  My responses were also concerned with observing.

Many (most?) astronomers think of the term "observing" as "collecting new information on astronomical phenomena", which can be broken down into two categories, one of which is "visual observing" and the other is "instrumental observing".  The latter is further broken down into imaging, spectroscopy, astrometry, photometry, and so on.

Radio astronomers make observations but they certainly don't use their eyeballs as the detectors of incoming radio photons!

You clearly meant to write "visual observing".