A new Dob - bigger or better?

[ollypenrice]

David Lukehurst does ultra portable Dobs and is highly respected in the community. He's also a very nice guy, which does matter. Another firm mentioned in the preceding posts is guilty of the most disgraceful indifference to their customers that I have ever encountered in astronomy.

Personally I like big Dobs and I like fast Dobs because many of those big scope targets are large and need the FOV. (Our 10 inch SCT has a longer FL than our 20 inch Dob.) Because I'm not going to use a big Dob all that much on the planets I'm not so fussy about 10th wave over 8th wave but a planetary Dob user might feel differently.

Olly

[DRT]

This is my take on it.

The faster mirrors will be dearer and you need to be more precise when collimating. Collimating the Sumerian is very easy but at f3.6 I'd be worried about a small shift when moving the scope, but I don't know how much might cause a problem or be noticed. You'll definitely want a coma corrector at f3.6 but probably will at f4.5 as well, but maybe not.

The pros of the fast scope are a shorter scope and wider fov.

I think I'd be happy at f4 or above

One more thing I've thought of, the exit pupil of a given eyepiece will be larger at f3.6.

Thanks, Richard - hadn't thought of the exit pupil thing.

That makes me wonder about how popular the 31mm T5 Nagler and 41mm Panoptic are with big fast dob fans - presumably lots of light gets wasted with the huge exit pupils those EPs would have in this range?

[Stu]

Thanks, Richard - hadn't thought of the exit pupil thing.

That makes me wonder about how popular the 31mm T5 Nagler and 41mm Panoptic are with big fast dob fans - presumably lots of light gets wasted with the huge exit pupils those EPs would have in this range?

I think that's one reason why the 21e is popular in large fast dobs Derek

[John]

Thanks, Richard - hadn't thought of the exit pupil thing.

That makes me wonder about how popular the 31mm T5 Nagler and 41mm Panoptic are with big fast dob fans - presumably lots of light gets wasted with the huge exit pupils those EPs would have in this range?

Thats why the 21mm Ethos is such a beauty in big fast dobs

Even in an F/4 you get an efficient 5.25mm exit pupil and still a good chunk of sky to browse.

Edit: Stu beat me to it !

(we are not on commission from TV, honestly !)

[DRT]

Sounds like my recent EP buying spree has been good preparation for this - I even snapped-up Alan's Paracorr this morning so I'm good to go

...just seem to be missing £5k to buy the Dob

[alan potts]

I'm about 5' 9", Alan. Might be 5' 10" when I get my new knees later this year

I've had a look at Telescope House but they don't seem to stock those TV building blocks - are they black and green and six times more expensive than the grey ones in B&Q? I'm sure they must make a big difference to the view!

Is it a Nichol mirror that you have?

It is almost a TeleVue brick , it has a Powered by TeleVue sticker on it.

More seriously the brick is 12cms thick so you can do the maths on F4 over F4.3.

Oddly on the 31mm Nagler, I favour this over the 35mm and 41mm Panoptic and I don't know why, I think it just works better somehow.

Alan

[John]

It is almost a TeleVue brick , it has a Powered by TeleVue sticker on it.

More seriously the brick is 12cms thick so you can do the maths on F4 over F4.3.

Oddly on the 31mm Nagler, I favour this over the 35mm and 41mm Panoptic and I don't know why, I think it just works better somehow.

Alan

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.

[Scooot]

I picked the 26mm Nagler because of the smaller exit pupil, the field stop is more or less the same as the 21 ethos.

[alan potts]

Thats a fantastic amount of money for a new system, and if you can afford it / write that amount off,  and get the maximum use  permitted from the scope against the UKs weather and seeing conditions, then go for it, you may not have another chance, but don't regret the expense afterwards?

I share your excitement with the windfall, I have two biggies  to collect this Year!  I know what I would like to get, but not sure how I will spend it?

Start a thread I am sure we will help you . I love spending others cash, oh you've noticed.

Alan

[estwing]

I think John will grind to order...so 3.7 or 4.2 are not impossible but I'd make it an 18"...it's the perfect size for light grasp and yet still remains portable..I lift mine into the back of a Volvo estate on my own...anything above is almost certainly a two man job..don't bother with mass production big dobs they're not scopes that are built to last...oh and one more thing don't shop around for mirrors just give Mr Nichol a ring you couldn't deal with a nicer chap and his work is outstanding

[Stu]

It is almost a TeleVue brick , it has a Powered by TeleVue sticker on it.

More seriously the brick is 12cms thick so you can do the maths on F4 over F4.3.

Oddly on the 31mm Nagler, I favour this over the 35mm and 41mm Panoptic and I don't know why, I think it just works better somehow.

Alan

In your 18" the exit pupil in a 41 Pan is over 10mm!, the 31 gives 8.15, still huge but not so bad. The 21e gives 5.5mm, much better.

I think the 31 gives around 1.4 degrees of sky, vs the 21e at 1.2 ish so you get nearly the same fov with higher mag and much darker sky background. Obviously exact figures depend upon paracorr/sips etc etc but the basic principle is there.

[DRT]

I think John will grind to order...so 3.7 or 4.2 are not impossible but I'd make it an 18"...it's the perfect size for light grasp and yet still remains portable..I lift mine into the back of a Volvo estate on my own...anything above is almost certainly a two man job..don't bother with mass production big dobs they're not scopes that are built to last...oh and one more thing don't shop around for mirrors just give Mr Nichol a ring you couldn't deal with a nicer chap and his work is outstanding

I think I'm there already - probably an f/4 18" Nichol.

I've been looking at the Sumerian scopes - they don't seem to have any variation in length to accommodate optics of a specific focal ratio. Are they made to order or are the adjustable so that you can place the mirrors the correct distance apart?

[Scooot]

Alan,

Talking of carrying them about, out of curiosity, do you know how much the mirror box of your 18" weighs. My 16" weighs 17.35kg with the alt bearing arms attached, which is about 1 or 3kg more that the OO 12" tube depending in whether you compare the VX12 or 12L.

[Scooot]

I think I'm there already - probably an f/4 18" Nichol.

I've been looking at the Sumerian scopes - they don't seem to have any variation in length to accommodate optics of a specific focal ratio. Are they made to order or are the adjustable so that you can place the mirrors the correct distance apart?

Made to order I think.

[jetstream]

I think I'm there already - probably an f/4 18" Nichol.

I've been looking at the Sumerian scopes - they don't seem to have any variation in length to accommodate optics of a specific focal ratio. Are they made to order or are the adjustable so that you can place the mirrors the correct distance apart?

Sumerian most likely waits until the mirror is completed to cut the truss poles. A mirror maker might end up with a perfect mirror close to the fl spec wanted and not want to risk proceeding further. I would trust the mirror makers judgement and have the poles cut accordingly by the structure maker IMHO.

[DRT]

So my mirror that will be made less than 100 miles away from home needs to be delivered to the Netherlands, fitted to the scope and then shipped back to the UK - is that correct?

[estwing]

Yes I think they make scopes to suit mirrors

[estwing]

No just tell them the focal length and go from there

[alan potts]

Alan,

Talking of carrying them about, out of curiosity, do you know how much the mirror box of your 18" weighs. My 16" weighs 17.35kg with the alt bearing arms attached, which is about 1 or 3kg more that the OO 12" tube depending in whether you compare the VX12 or 12L.

I don't have anything to weigh it on but it is a good deal lest than a 25kg bag of cement, which I do not find heavy.  I am guessing a bit but would say 20-21kg but it is easy to pick up as the design is a bit different to your's Richard.

Derek

Don't worry about the scope Michael at Sumerian knows how long to make things, I believe he may be working with John Nichol on some scopes because when I asked if he would do one for me with one of John's mirrors he personally found it quite funny that he was going over to using him and I was in there first.  I hope this has happened now as at the time it was under wraps so to speak and in the pipeline. For me you couldn't get a better mirror maker and nothing seemed too much trouble for him or Sumerian, I had all sorts of things sent to John's home for a free ride to Bulgaria, Cheshires from FLO for example and your Paracorr.

BTW Don't worry they know what they are doing, you should see the way they pack these things, I am still trying to get rid of the boxes and packing.

Alan.

[DRT]

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

[Charic]

Indeed - but it feels like an itch that I can't resist scratching

I've been looking at the Nichol's website and see that all of his "stock" mirrors come in variants of F3.6, F4 and F4.5.

What are the pros and cons of each of these for visual use?

To  me, the con of a faster ratio,  will be the additional  expenses incurred in buying  fresh new oculars for my scope?  I have already decided on  3-4 Tele Vue eyepieces for the 12" And yet I have never tried them?

TVs  are quality control tested,  to ensure the sharpest full field possible, down to f/4. That alone doesn't come cheap, and maybe,  possibly,   still need some form of coma correction ? ( It would be great if my EDs  worked  well in f/4 territory  with some  correction?  that will surely  help my situation )

For visual use only, I ignore the focal ratio, except as my guide to the shortest focal length EP I should work from?  I don't associate the focal ratio with anything photographic on my present system, 

But there really is a good advantage, and olly mentioned this in  another thread.......... On a Dob a fast F ratio is highly desirable because a scope with a shorter focal length can work at lower magnification and, critically, offer a wider field of view than one with a long FL. This means less 'nudging' as you track an object. [sic]

I've got a full range of EPs. but I always start with low Power! I only stop the increase when the image looks right.

Lastly, before I consider  the next scope, my only issue with the present  8"  is simple, the size of Jupiter?   As portrayed from the 8"  I would like to see Jupiter twice its  nominal size with  detail,  if the 12" cannot provide what I envisage, then its a no-no, my intention is not to buy anything bigger (    )

If the 12" provides,  and I can nudge a little less ( I don't actually nudge......I glide? )  and access the darker site more often, then I might upgrade this Year, sometime, maybe, possibly, who-knows!

I can go to darker skies, the 12" should still be just as portable as the 8" and I really  would like a standard tube Dobsonian. The Mrs wouldn't  notice the difference If I had the 12" Skyliner in the corner!

[Scooot]

I don't have anything to weigh it on but it is a good deal lest than a 25kg bag of cement, which I do not find heavy. I am guessing a bit but would say 20-21kg but it is easy to pick up as the design is a bit different to your's Richard

Alan.

I was a bit perplexed how to weigh mine, until Jane told me to just stand on the scales holding it, and then without it, and work out the difference. A bit of a light bulb moment

Thanks anyway, I was just curious as to how the extra 2" computed. I suppose the mirror being 26% bigger in area would make it about 22kg so that sounds about right.

[DRT]

I was a bit perplexed how to weigh mine, until Jane told me to just stand on the scales holding it, and then without it, and work out the difference. A bit of a light bulb moment

That's how I weigh my wife's suitcases before we go on holiday

It normally results in a discussion about exactly how many pairs of shoes you need for seven days

[DRT]

As portrayed from the 8"  I would like to see Jupiter twice its  nominal size with  detail

You need an 11" 2.8m focal length SCT in your life

More seriously, I also use Jupiter as a benchmark for my scopes and is something I miss since selling my 12" Flextube Dob.

Given how bright it was in that scope I think a Moon filter might be required in an 18" Dob

[niallk]

Best of luck with ordering the new scope - a 16" or 18" Canopus with premium optics will no doubt be a superb scope - an investment giving a lifetime of enjoyment! We only live once...