A new Dob - bigger or better?

[DRT]

We only live once...

That sums it up nicely

[astronymonkey]

Hi Derek,

For me an 18in f4 nichol mirror would hit the sweet spot for light gathering, weight and eyepiece height whilst using 82degree eyepieces. With ethos however I might push to a 20inch mirror but being a glasses wearer I prefer the eye relief of the t4 nags.

I have always tried to limit my exit pupils to 6mm and below and have based things on what would give this exit pupil whilst being able to deliver a 1 degree fov as this would show the orion neb nicely although a bit bigger fov would be better for extended targets such as m45.

As you have the ethos you have an excellent starting point so id get a good mirror from John and take it from there ..... you lucky man !

Cheers

[alan potts]

Thanks, Alan - I'm not worrying, just curious as to how all of this works.

I'm now even wondering whether or not it would be an option to pick up the mirror, drive onto a ferry from Hull, take it to the guy at Sumerian and see it being fitted to the scope before driving it all home.

I might ask if that is possible nearer the time

I don't know so much about that, I believe Michael tests them before shipment and I think on mine he made a slight shortening of the truss poles.

I also cannot really say too much about shoes for holidays as I have 40 pairs myself and about 7 of them have never seen my feet and now I don't need to dress up so I wonder if they ever will. I will have to wait for them to come back into fashion, one of the problems of platforms.

Alan

[alan potts]

While not quite optimum, the exit pupil provided by the 31mm Nagler will still be more effective than the 35 and 41mm Pans deliver in an F/4.3 scope.

Do you know when you point it out it is blindingly obvious, it is just that I have never really stood there at the scope and though about it. I tend to lay my cases down near the scope and just try what ever takes my fancy. I never stopped and thought about the 41mm Panoptic, which would have an Exit  Pupil of about 8mm with the Paracorr in place, it is still useable though, no secondary shadow, that I would not miss.

Even the 31mm Nagler will be about 6mm ExPu, but that I do find most useable for low power.

Alan

[niallk]

...one of the problems of platforms.

Alan

Hmmm ... platforms ... eyepiece height at zenith with an 18" scope ... Ah-ha!!!

What an ingenious solution

[cotterless45]

Just up on ebay, not too old , with the dual focuser. Good buy for someone wanting to jump into the deeper end,

http://www.ebay.co.uk/itm/Meade-16-Lightbridge-Truss-tube-Dopsonian-Telescope-/231643717736?hash=item35ef0aa068

Nick.

[ollypenrice]

I don't use a coma corrector in our 20 inch F4.1 Dob. I find that the TeleVues I use in it (13 Ethos and 26 Nagler) are remarkably clean off axis. I often expect the hard line visual buffs to complain about this but they never do. However, put in a Plossl with a far smaller FOV and it's a different story altogether. They can be quite queezy-feeling at the edge.

Regarding magnification and aperture, I really doubt that in big Dobs more aperture supports more magnification. I'd expect the reverse. That's why planetary Dobbers stop down their scopes. If you have massive aperture, though, you do have the possibility of observing the planets in daylight. I had a splendid view of a Callisto transit of Jupiter last year on a bright sunny afternoon. (0.8 metre F8 RC. Not mine, alas!)

Olly

[John]

Just up on ebay, not too old , with the dual focuser. Good buy for someone wanting to jump into the deeper end,

http://www.ebay.co.uk/itm/Meade-16-Lightbridge-Truss-tube-Dopsonian-Telescope-/231643717736?hash=item35ef0aa068

Nick.

Quite highly priced though I feel. You can get the GSO / Revelation 16" from Telescope House for £1,300 new and that has the same mirrors and mirror cells in it.

[swamp thing]

Get a "traditional" Dob structure - the extra weight is worth it for the extra rigidity versus an ultralight/ultra-portable design

^^^^ This.

Having used quite a few larger scopes I cannot agree more.

Nichol optics. would get my nod.

[DRT]

Thanks, Steve - unfortunately it will have to be a compromise between size and portability/storage as space at home and when traveling is limited.

Does a little 18-incher get me a ticket to Dobfest or do I need to be thinking of a proper big one?

[estwing]

Thanks, Steve - unfortunately it will have to be a compromise between size and portability/storage as space at home and when traveling is limited.

Does a little 18-incher get me a ticket to Dobfest or do I need to be thinking of a proper big one?

its not the size that gets you a ticket...its the amount of drink you bring...

[estwing]

treat yourself and come to SWSP in oct...usual suspects will be there..

[Uplooker]

treat yourself and come to SWSP in oct...usual suspects will be there..

Which in October SWSP????

Sent from my Fone

[estwing]

The main starparty near Brecon

[Uplooker]

The main starparty near Brecon

Ah, thanks. May try it at some point in the future. Too close to the Galloway SP

Sent from my Fone

[DRT]

its not the size that gets you a ticket...its the amount of drink you bring...

Sounds like advice for e teenage boy

[Dannae]

Hi Derek,

There has been a lot of good advice posted here already. I have experience of obsession 15'' 20" and 18 uc, a teeters 16"' a star master 18" and more than a few meade star masters over the years. I am currently having an artisan scope made (Pierre desvaux at the Dobson factory) with tracking platform from France with a 14" f4.5 mirrorsphere mirror as this is the size I am now comfortable to live my life with (13kg including mirror). I finished the ngc catalog a few years back and time is precious now for me so I have opted for high quality at all levels and components effectively created an heirloom for me. His blog is wonderful from his website and he posts pictures of every build. He has accommodated every wish for me. The euro is pretty good at the moment and I was tempted by a larger skyvision (also French) but loved the wood too much and the craftsmanship of the Dobson factory scopes to pass. I have never had the pleasure of seeing a Sumerian but apart from one bad experience here with an Altaid I have heard nothing but good about the scopes. I also have heard only great things about John Nichols mirrors.

I had a wonderful f20 mak from OO that I supercharged with a few obvious mechanical replacements but felt the few hours I spent with them had little to do with my aspirations and needs and I never felt comfortable dealing with them. The scope eventually exceeded my expectations and their mirrors are their strongest suit.

I wish you patience in choosing and patience in waiting for it to arrive and hope to catch up with you and all these light bucket nutters at some stage.

Regards

Dannae

[DRT]

Dannae,

Thanks for that, a very interesting post and another option for me to consider. I had never heard of the Dobson Factory or Pierre Desvaux before now so that is something else for me to research before making a decision, assuming of course that the funds become available to allow me to make one!

13kg for a 14" sounds very light so definitely worth considering from a portability point of view.

Thank you for posting.

Derek

[Dannae]

You are most welcome Derek. Pierre makes some wonderfully light 10 and 12" travel dobs too, though he makes whatever you want in whatever size. I normally binoview on high surface brightness objects like globs, clusters and solar system objects so I needed a more robust upper tube assembly otherwise I would have gone for one of these and saved another few kg.

There is roughly a 12 month lead time and I expect mine in early spring. Mirrorsphere the company that makes mirrors for French observatory telescopes have a very high reputation for the smoothness of their mirrors and I wanted the highest contrast I could get. They also are very close to his workshop so he picks them up and star tests them before despatch.

This is the highest quality dobsonian build I have seen of this type using wood on this side of the pond, websters and teeters are the two I would recommend from the U.S. If you want more bells and whistles but you will pay the import tax for the pleasure of owning one of these.

Regards

Dannae

[niallk]

Hi Dannae, what is your take on Obsession Telescopes? You have owned a great selection of scopes! Those Dobson Factory scopes look fantastic - best of luck.

Cheers,

-Niall

[Dannae]

Hi Niall,

I am obviously very biased but my obsession with obsession goes back a long way! The 15" was my first large dob, the largest scope I had at that stage was a 10" Meade reflector. I loved that scope and I bet I couldn't pry it out of your hands for any money I spent time in the U.S. and it was there that I discovered truly dark skies and what large aperture can do. I bought myself the 20" f5 around 7 years ago when I came back to the UK and that really was a beast though the skies didn't do it justice. Up until the recent trend in thinner, lighter, faster mirrors and the evolution out of the box that Dave Kriege incidentally popularised to the amateur telescope maker back in the mid eighties, they were pretty high up there amongst manufacturers. The obsession classic moves like a dream, the finish is good, and it just works beautifully. Fast forward many years and better finishes, components and options now exist from companies like teeters, waiters and websters who have taken the ball and run with it leaving obsession as it has always been. For example with every scope (untested - the mirror comes separately and never meets the mirror box until you put it in yourself) you have to centre dot your own primary, fit your secondary mirror and heater from a kit, drill and fit many parts of the servocat yourself, trim the truss poles to get to focus and basically do a few fittings that many others now offer as standard. I was happy and capable enough, if not a bit worried!, about doing this but considering the cost it would be something people might take for granted would already have done. Even the fan hardware blowing on the back of the mirror had no connector for a battery, just bare wires ( no big deal).

I helped a friend set up his 18" uc and fit the DSC and servocat and while it fitted its remit as an ultra portable scope, it definitely wasn't ultra light, Galaxy and Omi torus still use full thickness 2" mirrors to help with balance issues. The three strut truss I found not quite up to the job personally and I had to chase collimation all night when moving in alt. the 15 I had a go on was fine and the 22 I played with in the Isle of Wight I think at a star party there uses 4 struts connections which solves the problem. There was also a design fault with the flimsy plastic mirror cover if I recall when the connecting rod bracing the alt bearings was in place and the lid wouldn't fit over the mirror.

I'm being super super critical of things and admit I am a perfectionist. I was and am immensely proud and privileged to have owned an obsession scope and I highly recommend the classic range if you don't mind getting involved finishing things off yourself and have time to allow for cool down. A looked after second hand obsession classic in the uk is a no brainer:) I can't recommend the 18 uc for the two days I used it, and can recommend the 15 and 22 uc if portability and transport is the overriding concern.

My own personal opinion is that if you want everything done for you in the highest quality with the most modern advances (boundary layer fans, flocking, internal truss wiring, active counterweight system etc) and tested to the highest levels in a time scale that doesn't run into years then choose a teeters with a Zambuto/ light holder mirror. In the EU I would choose skyvision with their own mirror or wait a little longer and get a Dobson factory with nichol or mirrorsphere like mine. I would like to compare a Sumerian Canopus but have yet to see one in the flesh but it gives me the feeling of being everything everyone says it is.

Just my 2 mega parsecs

Regards

Dannae

[niallk]

Hi Dannae,

Thank you very much indeed for your detailed response - very sound of you!

My phone is about to die (in a rain lashed tent in the middle of nowhere) but appreciate your comments - will repost when I get charged up somewhere...

-Niall.

[stadaik]

Very detailed Dannae. I want more of these posts. Really useful.

[DRT]

Hi DRT,take a look at the Obsession website send for the dvd and drool,well worth a look:http://www.obsessiontelescopes.com/

HTH

Regards

Mike

Mike,

The pack arrived from Obsession yesterday and I read through it today on the train to London. I had to apologise to the person in the seat beside me for the amount of drool

Truly impressive looking kit and a definite contender. I love the look of the standard, heavier scopes and it has made me think twice about going for light weight.

Thanks for this tip.

Derek

[quaoar]

If you're bored it might be fun perusing the SDM website  http://www.sdmtelescopes.com.au/about-sdm-telescopes/ .

Peter Reads' scopes are very highly regarded. Crunch some numbers, I think our relative exchange rates are very favourable at the moment.  

JP Astrocraft in the US are held in similar high regard  http://www.jpastrocraft.com/