Building 'scopes more portable than Dobs? Any ideas?

[madbadgalaxyman (Robert)]

Dear Colleagues,
I had to have a good laugh at an old thread in which lots and lots of older blokes like me complained about how increasingly hard we are finding it to manage telescopes that are bigger than about 14 inches in aperture!

The litany of complaints about the lack of usability and practicality of Really Big Telescopes (of 16 inches and over), for those of us in late middle-age or old age, is all too familiar to me!!.......
creaky backs that have to move massive objects around, increasing difficulties climbing ladders to reach the focus, long setup times, etc.

In fact, sadly, it is an all too common story for us seniors to have to downsize to a smaller and (at least for us) a more practical telescope.....
but this Extremely(!) Sad Story of giving away a Really Big Dob can hardly be regarded as progress!

So why do we just meekly accept that we have to abandon the use of a giant telescope? Why not say "we're mad as hell and we're not going to take this any more?" Why not look for a better solution than being forced to move to a smaller telescope?

So let's see if there is some kind of solution!?!?

Let's see if there is any way to go beyond the Dobsonian Revolution, and to practically and economically build and own a really BEEEG telescope which is more compact than a Dob of the same aperture and which does not weigh as much as a Dob of the same aperture.

In other words, is there a feasible and buildable telescope design which is manageable and usable enough for us "old b*ggers" to be able to use it at >=16 inches of aperture ??

So, ladies and gentlemen.....
......have you any ideas?


cheers, madbadgalaxyman

One well-known (but expensive) route to relatively greater manageability and usability for larger apertures is of course a compact "folded" optical system such as a Classical Cassegrain, a commercial Schmidt-Cassegrain , or a Ritchey-Chretien. For instance, the tube assembly of the Meade 16 inch LX200 weighs some 30 kilograms.....
still too heavy for some of us oldies, so one wonders what would be the minimum possible weight of a 16 inch cassegrain-type Tube Assembly?
(Hmmm...not sure if 30 kilos is right)

[Rainmaker (Matt)]

I am sure the usual suspects will tell you to look into EAA and Night Vision options rather than going down the large aperture path, however, until they create a NV device that shows all the beautiful star and nebula colours, I will be advocating for large apertures but in more compact structures.


The Rolls Royce of big scopes here in Australia is surely Pete Read's superb SDM scopes many of which I have been lucky to view through, but if a more manageable weight is desired then there are options in aluminium and carbon/kevlar that can be considered.

I like the stability of the structures like SDM builds in preference to the lightweight Hubble and Obsession Ultralights so I went with a clone of the Spica Eyes designs of Tom Osypowski.

When planning my scope I also saw a scope made by Bratislav Curcic with help from Stefan Buda, this was an excellent, compact and light
scope built to exacting standards with lots of custom components.

My own scope managed to shed about 25Kg from what I had previously built, achieved with the use of aluminium, carbon, kevlar and titanium components but that did increase the cost a little ( $$$$$ )

[N1 (Mirko)]

There won't be a single best solution to this question that's about as old as the hobby itself. I'd agree with Matt re lightweight materials in proven designs. That will surely be your best bet. But i also think regular physical exercise goes a long way in letting you use your existing gear for longer. I realise there'll be many to whom neither of the above will be available, and that's where clubs and observing buddies come in.

[madbadgalaxyman (Robert)]

Originally Posted by Rainmaker

My own scope managed to shed about 25Kg from what I had previously built, achieved with the use of aluminium, carbon, kevlar and titanium components but that did increase the cost a little ( $$$$$ )

That's a first-rate solution you have there, to the "big scope usability problem". Full marks!

[madbadgalaxyman (Robert)]

Hey, you ATMs and Optics Gurus,
would folding the light path of a Newtonian work?

While doing this may not have that much impact on the weight of the optical tube assembly, it would put the focus of a Really Really Big Instrument at a much more convenient height, thus saving us from having to perch precariously on a ladder!

There have been various attempts to do this:

https://www.cloudynights.com/topic/4...of-telescopes/

https://www.cloudynights.com/topic/112172-folded-newt/

http://www.jimsmobile.com/buy_ntt40.htm

https://www.cloudynights.com/topic/3...ded-newtonian/

https://www.johanneswilm.org/mike/telescope/


While a lot of attempts to fold the light path of a telescope (including by the Cassegrain-type route) do result in an unacceptably large central obstruction (at least for the optics purists) , I put it to you that what we are looking for in the biggest apertures is essentially a Supergiant(!) Light Bucket for deep sky viewing......
where - in very practical terms (for the Actual Observer) - the central obstruction interferes much less with contrast and resolution than it does during planetary viewing.

But can anyone think of a better way to accomplish the folding of the light path than these designs?

cheers, madbadgalaxyman

[glend (Glen)]

Robert, if you can go the Observatory route then it really doesn't matter how big and heavy it is. I have had a 16" Dob and had to let it go due to my age and health issues. Building my observatory was the best possible outcome. You talk of 16" Cassegrains, but think about the field of view at that sort of focal length and what you intend to do with it. Define your goals first before deciding on an optical design.
Personally, I am not a fan of "artwork" dobsonians, because up to 16-20" the commercial products are a much more cost effective investment, and you will generally be able to re-sell it down the track. It depends on what your buying it for obviously.

[madbadgalaxyman (Robert)]

Originally Posted by glend

Define your goals first before deciding on an optical design.
Personally, I am not a fan of "artwork" dobsonians, because up to 16-20" the commercial products are a much more cost effective investment, and you will generally be able to re-sell it down the track. It depends on what your buying it for obviously.

Hi Glen,
My own Very Highly Specialized personal goals have long been extremely well defined after 5 decades in the Astro game, so this thread Rather has the goal of prompting some creative thinking about solving some common problems that people have with the usability of larger apertures.

As I see it, I started this thread to gather together information and insight and knowledge and creativity...... about the way people are trying to solve the problems of huge mass and size, focus much too high up, etc., etc., in apertures of 16 inches or more, as most of us would like to own gigantic telescopes; but these problems are obstacles in the way of our practical usage and ownership of truly huge instruments.

In other words, how to build or get a REALLY BIG scope without enduring this sort of problem.......


So this thread, at least in My Own View of it, is about optical design and fabrication, rather than about "what telescope I want and need Right Now". Neither did I intend this thread to be about finding alternatives to carting about huge telescopes ......such as building an observatory.

cheers, Robert

[sunslayr (David)]

If you weren't going down the observatory route you could go with something like a collapsible binewt setup. That way you could go with two primary's with a much shorter focal length than one large mirror. I have always wanted to experiment with different materials for the primary mirror. The original newtonians used a metal mirror I can only imagine that a 20" aluminium mirror would be much lighter than a glass one. If the mirror was large enough it could compensate for any reduction in reflectivity from a poorer surface finish, not to mention aluminium is much faster to grind and shape than glass. The last idea I had was that stuff they sell at bunnings called Rust-Oleum Mirror Effect spray. It basically turns any smooth surface into a mirror, could be fun to experiment with that.

[madbadgalaxyman (Robert)]

Originally Posted by sunslayr

The original newtonians used a metal mirror I can only imagine that a 20" aluminium mirror would be much lighter than a glass one. If the mirror was large enough it could compensate for any reduction in reflectivity from a poorer surface finish, not to mention aluminium is much faster to grind and shape than glass. The last idea I had was that stuff they sell at bunnings called Rust-Oleum Mirror Effect spray.

Hi there, sunslayr,
some interesting ideas there!

I am not sure if an Aluminium primary mirror would yield much of a weight advantage over a pyrex mirror, but I think the general principle that you have outlined of trying to find a lower density material from which to make the primary mirror, is worthy of following up and investigating. Every little bit of weight saving is a good thing, in telescope design.

Here's an interesting thread on alternative mirror materials:'
https://www.cloudynights.com/topic/408925-metal-aluminium-primary-mirror-making/'

Here is a table of the comparative density of various solids:::
https://www.engineeringtoolbox.com/d...ds-d_1265.html
( I hope it is more correct than a lot of that suspect info on the web!)

[sunslayr (David)]

This site shows a process of creating a large glass mirror that has hexagonal cutouts in the back making it lighter. Not sure how practical it is considering you need access to a glass kiln big enough to make your glass blank. I guess you could make one large and flat using thermal bricks and kiln heating wire. Not sure of the effect of thermal expansion though with an uneven back.

[madbadgalaxyman (Robert)]

Telescope with an F1 (!) primary, built by our professional cousins.....


Not exactly relevant, but it illustrates the truly Massive ongoing efforts by professional astronomers to make extremely large telescopes as compact as possible. The point has been reached where gigantic professional telescopes are virtually "no longer than they are wide", to put this trend in homely layman's language.

I feel that the amateur community should try to adopt the same aim of "compactisation" for our much more modest instruments.

( Its the Large Synoptic Survey Telescope, though recently having undergone some name changes.)

[gary]

Hi Robert,

The trend in recent years has been toward the "Ultra Compact" Dobsonian designs.

As a disclaimer, Obsession Telescopes is a customer of ours.

In 2007, Dave Kriege saw the trend in the US to people downsizing their
motor vehicles on the back of increasing fuel prices.

In response to that he introduced the Obsession 18" Ultra Compact (UC)
at the 2007 Texas Star Party and assembled the scope on stage-whilst
talking in what I recollect was under 7 minutes.

As Dave points out, he sells more big Dobs than all his competitors
put together and fast forward to today, sales of UC scopes outstripped
sales of the heavier and bulkier "Classics" by such a large ratio
that he announced he would now only manufacture the UC's.

As you rightly point out, another factor is an ageing demographic.

I was custodian of a 18" UC here for a while and I could manhandle
it on my own. It fitted in the back of a SUV without having to resort to
folding down the back seats.

By comparison, an 18" Classic I transport takes up most of the available
space in the SUV, including having to fold the back seats down and
loading components on top of them. A pair of aluminium ramps then
needs to be loaded on the roof racks (I can get them inside the vehicle
in a squeeze) plus I need to transport a pair of "wheel barrow handles"
with wheels if I am to have any chance of getting the scope out of
and back into the vehicle.

In January 2020 I spent a few weeks with Dave Kriege on the Big Island
of Hawaii where he keeps a 22" UC in storage there.

Whereas with the 18" UC mirror I would lift on my own (32kg),
with the 22" the virtual mirror box assembly weighs just under 41kg,
so we did go through the precaution of two of us lifting to avoid back
injury. Compared to manhandling a 18" Classic though, it was trivial
and we would be up and going at 9000' within minutes.

As you are probably aware, there are also compact designs from scope
makers that use lightweight "sandwich" mirrors.

Anyway, in large aperture Dobs, the ultra compact designs are currently
in the sweet spot of what is selling. When we would drive down off
Manua Kea in the morning, Dave would be answering voicemail
messages on his cell phones and in the space of a 45 minute trip would
typically sell a few more.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[jamespierce (James)]

With thinner mirrors and sensible use of materials a 14 or 16 inch dob doesn't have to be too heavy. Once you go much past that though unless you use lots of exotic materials and are prepared for a very stripped back experience it's extremely hard to not end up with a heavy scope, just the mirror box alone will be heavy and hard to handle. Some of the creative approaches (string scopes, lots of carbon fiber etc) also don't really scale big. The mirror ends up being the most significant weight anyway.


This book is worth the read if you are seriously interested in this area - https://www.amazon.com/Engineering-C.../dp/0943396956 - Of course it just got harder to buy with William-Bell out of business.



I've had the privilege to look through many very big scopes and while you do get a little more on a really perfect night I'm now pretty much convinced it's rapidly diminishing returns after about 16 or maybe 18 inches. The extra effort involved in moving, setting up and observing up a ladder doesn't justify the improvement. That ignores the inevitable thermal equilibrium issues and flexure inherent in larger scopes.


On the earlier suggestion of an aluminum mirror it wouldn't work well, we need materials which are very thermally stable for mirrors - hence the shift to quartz vs pyrex at the high end.


If money were no object - a big CDK (or similar design) on an alt/az fork or L style mount in a dome would be the most comfortable experience - fairly consistent eyepiece height and position, feet firmly on the ground and a comfortable experience. Things to dream about.

[madbadgalaxyman (Robert)]

Originally Posted by jamespierce

(........) a 14 or 16 inch dob doesn't have to be too heavy. Once you go much past that though, unless you use lots of exotic materials and are prepared for a very stripped back experience it's extremely hard to not end up with a heavy scope,
just the mirror box alone will be heavy and hard to handle(......)
The mirror ends up being the most significant weight anyway.

If money were no object - a big CDK (or similar design) on an alt/az fork or L style mount in a dome would be the most comfortable experience

Thanks very much for your contribution, James.

You might (conceivably) be right that the ultimate limits of usability and practicality for apertures of 16 inches And Over have already been reached by telescope designers, due to factors such as the great weight of a big primary mirror. In other words, it is conceivable that we are already near to the optimum telescope design (for usability and practicality of Large Aperture telecopes) in terms of the parameters such as weight and tube length;
for instance with the short-tube R.-C./Cassegrain/CDK/Schmidt-Cassegrain designs, and with the Compact "evolved Dobson"/"Children of Dobson" designs such as the "Obsession Ultra Compact" and the uncommonly beautiful and very manageable 18 inch Newtonian of 'Rainmaker' which was shown in this thread.

But still......
"the person who said that they can't have a Huge Telescope must have said it with a broken heart", so I still think it worthwhile to open up this thread to see if there is some possibility of finding a New Way Forward towards our personal ownership of huge apertures!!

[jamespierce (James)]

Yes - My comments about ~16-18 being the sweet spot factors in those designs. One of the most beautiful light scopes I've used is owned by a german friend...

http://www.deepsky-visuell.de/ATM/20/Bau20.htm

Google translate will get you most of the way there, but you'll get the gist of it from the pictures. Very light design, no encoders, only a tiny heater on the secondary etc. Practical in their typical alpine observing locations, wouldn't work here in Victoria!

This scope makes an obsession UC look heavy! But it's still significant to move around. As soon as you move away from a dobsonian design the mount required becomes just as much a problem (assuming portable use)

I still dream of a 24" F3.3 to F4 scope too.

[madbadgalaxyman (Robert)]

Hi James,
Is that a 25mm thick primary in the very nice German scope?

I confess I'm having lots of trouble with that German language!

How much does this primary mirror weigh?

.....I know little that is detailed about supporting extremely thin Primary Mirrors, except for the "obvious" ( such as supporting it on lots of points!!).
But the reduction of the primary mirror weight, to an absolute minimum, seems a worthy goal for telescope designers and builders.

Maybe there is still room for innovation in this aspect of telescope design?
After all, with the E-ELT, our professional cousins have taken weight reduction to the extreme and divided its 39 meter Primary mirror into 798 segments!

[jamespierce (James)]

Try this link ...

https://translate.google.com/transla...20%2FBau20.htm

The primary is only 25mm thick - just under 10kg. And yes - the only way to end up with a truly light telescope is to start with a light primary. It does lead to another perhaps non-intuitive issue, with a very light mirror box your UTA has be extremely light also, balancing with heavier eyepieces and so-on becomes more of an issue also. Hence the single upper ring on Ronald's scope.

Of course it's possible to move the pivot point of the mirror box up, but we're back to needing more material etc to accomodate.

Also same designer - 27" for 50kg...

https://translate.google.com/transla....htm&sandbox=1

[madbadgalaxyman (Robert)]

To summarize this thread so far::

(1) we seem, so far, to have concluded that we may be approaching the limits in terms of "compactization" and weight reduction, in the very-short-tube Cassegrain/Ritchey-Chretien/CDK type designs, and in the short-tube (say F3.5) Newtonians of a "modified or improved Dobson" design, especially if the design also incorporates the use of advanced/expensive/light-weight materials.

(2) However, the idea of a Folded Newtonian seems worth developing
(as per a previous post in this thread), at least for the very biggest instruments, so as to bring the focus down to a manageable height.
(though the central obstruction is huge in some of these instruments, these are optimized "Light Buckets" for visual Deep Sky work, rather than planetary viewing, so maybe this can be tolerated)
http://www.siderealtechnology.com/28inch/

(3) Also, the ongoing Design Trend of reducing the weight of the primary mirror, seems worth continuing with. Let us hope for further innovation in this area.
(the 25mm thick 20 inch Primary Mirror in the previous post only weighs 10(!!) kilograms......impressive!)

As I've said, we're not talking about scopes for planetary viewing here, or about achieving the sort of optical perfection that the purists want!
We just want "a whole lot more light" for Deep Sky viewing, and to reduce the size and weight of our Gigantic Instrument to the absolute minimum.

[jamespierce (James)]

A couple more thoughts (Yes this is a topic I've spent a great deal of time thinking about and researching)

On folded designs, they are mechanically complex and fiddly to collimate - Unless they are very rigid (and therefore we are back to heavy) the design breaks down. It might be the ideal way to have a very large dob if it didn't have to move around though and keep feet on the ground.

On the final point about not needing ultimate quality, I actually think this is a misnomer. A small scope with a better quality image ultimately leads to better contrast and the ability to see very faint details. Big but crappy light buckets are superficially impressive, especially on objects which are already bright - but when you get to the limit they start to break down.

My 16 F4 for which it seems I got pretty lucky with a very good mirror often out performs 20s and even 25s which only have middle of the road optics when seeking threshold objects (~Mag 15 galaxies etc)