16",18" or 20" dob?

[jetstream]

What are the real world visual differences on extended objects between these apertures given equal (good) optics and optimum,dark sky conditions?In particular the brighter nebulas and more easily seen galaxies.

[nmoushon]

I can't give you actual examples as I have a mere little 114mm scope I use for visual in a very heavy LP area BUT I know the jump up 2" isn't going to make a HUGE difference but it will be noticeable. But the jump up in 4" will be pretty big. You'll definitely be able to pick out dust lanes in galaxies you couldn't with the 16". If you have the money and are willing to spend it the largest aperture is always better for visual. But have to remember the height of the EP will go up along size the increase in aperture(If the f/ratio stay the same between the scopes). So if you don't want to be on a latter the whole time your viewing then it doesn't matter how big the aperture is you'll hate viewing and just want to either downsize or just quit all together. Not sure how tall you are so you'll have to check that out for yourself. 

[scarp15]

Hi Gerry

Sounds like you have got the itch. Here is a comparison chart based on M13.

www.obsessiontelescopes.com/m13

[John]

I'd say the above is a decent indicator for globular clusters at least. Having viewed M13 through a 20" dob it really did smack you in the eye. M51 was pretty wonderful too

This particular 20" dob (a David Lukehurst) was fast so the eyepiece remained at an accessible height but it was still a pretty massive scope, or seemed so to me.

I agree about the 4" jump being optimum to make a real difference as well.

[estwing]

hmmmm....tough one I have to say 18" but then again!...I don't want to look at anything in a bigger scope...help mummy! and we all know its all about the skies anyway...and cost..and logistics..and setting up...on your own..and stepladders..and so on the best scope is the one most used. I'd love a 26" or 30" but for me my 18" is perfect

[jetstream]

hmmmm....tough one I have to say 18" but then again!...I don't want to look at anything in a bigger scope...help mummy! and we all know its all about the skies anyway...and cost..and logistics..and setting up...on your own..and stepladders..and so on the best scope is the one most used. I'd love a 26" or 30" but for me my 18" is perfect

Your telescope looks ideal estwing.I may try constructing one myself,just deciding on a few things...cost as well.Obsession isn't too far of a drive for me so I have to figure if building one is worth  the savings.The USA mirror makers seem to have a long waiting list,and OOUK has excellent mirrors so I was going to get one from them.Not sure if they do an 18" though.Was thinking either 16" f4 or 18"f4-the 16" 1625mm fl to preserve FOV or the 18" 1828mm fl for eyepiece height.I am 6'1" tall.How long to set up your telescope by yourself?

[Moonshane]

I have a 16" f4 and built Estwing's 18" f4. there's a pretty big difference in bulk between the two. the eyepiece height was really nice in the 18" for me at 6'3" and Calv was pretty ok too at 6'? it's also good that said on my 16" f4. bear in mind you lose some fov if you use a paracorr although other options don't do this I believe.

I think benefits between say a 16" and 18" would depend on your main observing site skies. e.g. I use my 12" f4 at home almost exclusively and rarely use my 16" f4 as there's a lot of light pollution (I mean a lot). at dark sites the effort of taking the 16" is worth it to me.

do consider:

1) cost

2) can you get the scope in your car

3) can you set up alone if needs be

4) can you pack up quickly if needs be (weather etc)

5) 2" extra aperture above 12" makes a surprising difference to the bulk of the scope/costs

6) if the above don't bother you then a 20" f4 would be astounding.

I am quite happy and content with my 16" f4 though.

[estwing]

10 mins tops to set up and I'm 5ft 10" or 6ft on shanes carpet !!

http://www.nicholoptical.co.uk/1109%2018%20inch%20F4.34.htm ..any good?

[jetstream]

Thanks for the information guys.I primarily travel to dark skies to observe and am finding better and better places,not too far away(thanks SQM),from home double stars,planets,star asterisms etc.....my best location so far has produced 21.5 mag skies,15 minutes from the house which I use a lot.I can get the 20" in the back of my truck with the seat folded up,but I think this may be a bit big....I heard that with a 18"+ scope from very dark skies that faint galaxies can be seen very plentiful in the EP....?

I am not familiar with Mr Nichol or his mirrors,for Calvin to use one & Moonshane to install one,they must be top notch- and the price looks great.I will check out this avenue for sure,Zambuto needs 8 months for a mirror and others have a longish wait too.You know maybe the FOV isn't as important of a consideration with this scope,as members have suggested in other threads,and a Paracorr will reduce it anyway.I'm going to try the Baader CC in the 10" with the 21Ethos to see how it will perform with no FOV reduction.Swampthings SIPS looks like an excellent choice,if going the Paracorr type route

[astronymonkey]

One thing that is often overlooked is the balance between fields of view, magnification, and exit pupil that you get with a large dob. Yes a big mirror gives great detailed views but there are few calculations you need to do first to make sure you are going to see what you hope to see. When I first got a scope 28 years ago I craved for more aperture and at the time a 12 inch scope was about the biggest you could realistically get, but with falling prices over the years bigger scopes are coming within reach of most people but these come with their limits where these calculations begin to pass the limits of the human body  ...... this may sound dramatic ..... but let me explain.

With a big mirror , In order to keep the eyepiece height reasonable you need to shorten the focal length and this increases the size of the exit pupil for any given eyepiece and coma.To counter this you can increase the focal length but then you end up with a larger magnification and an narrower field of view.

100 degree eyepieces have helped a lot but there are still limits which I feel you need to in ind.  For me this is an eyepiece height of 6ft when at the zenith (I cant grow any bigger and I dont want to use a step), a max exit pupil of 5.5mm (my pupils cant dilate any more) , min fov within these limits of 1 degree as this will allow the Orion Neb to fit within the Field of view (I cant make it any smaller)

So with this i mind an 18 inch 450mm obsession scope at f4.2:

• has an eyepiece height of 5ft 10in. 
• A 5.5mm exit pupil will happen with a 23.1mm eyepiece.
• A 23.1mm eyepiece would give magnification of 82x
• With this in mind a 21mm ethos would give a 1.1 degree field of view

Now I wear glasses so if I use a nagler type 4 22mm then I would end up with a field of view of 0.94 degrees which would have to be a minor compromise for me.

Moving up to the 20inch f4:

• Has an eyepiece height of 6ft 6inch
• A 5.5mm exit pupil would happen with a 22mm eyepiece
• A 22mm eyepiece would give a magnification of 92x
• With this in mind a 21mm ethos would give a 1 degree field but the nagler 22mm would give 0.89 degree field

stepping up to the 22inch f4.2:

• Has an eyepiece height  of 7ft 4 inch
• an exit pupil would happen with a 23.1mm eyepiece
• a 23.1mm eyepiece would give a magnification of 101x
• with this in mind a 21mm ethos would give a 0.8 degree FOV and a nagler 22mm would give a 0.76degree FOV

So Going bigger begins to squeeze the FOV and lifts the eyepiece height more and more.... so for me the compromises become too much at 20inches f4 as at this point I begin to need a step of some sort to use the scope.

Unfortunately these seem to be the laws of visual optics which you cant get away from so bear them in mind before you make your choice. Years ago when 12 inch scopes were the biggest you could get these limits were never met, but as you hit 18inches you need to get the calculator out Im  afraid to make sure you can realistically get the benefits that you hope too.

Cheers

[Joves]

I must say, the 18" Obsession Ultra Compact sure looks like a nice mix of aperture, EP height and portability!

[Mike73]

I went from 8" to 12" and got the wow factor, from 12" I moved up to my current 16"but I just don't think that 4" step up gave the same wow factor as my first two scopes, I'm pretty sure if I really needed that wow factor then the 18" is what was needed but it was a fairly large financial commitment just for that extra 2" of aperture.

Going from 12" to 16" has been a different observing experience though, before I had to work pretty hard and use a bit of 'averted imagination' to see spiral arms in Messier galaxies now I can see them with direct vision, globulars were resolved but nothing like they are with my 16" so it was well worth the upgrade.

I should imagine that moving from 10" to 16" would definitely give you that wow factor Gerry.

[jetstream]

I went from 8" to 12" and got the wow factor, from 12" I moved up to my current 16"but I just don't think that 4" step up gave the same wow factor as my first two scopes, I'm pretty sure if I really needed that wow factor then the 18" is what was needed but it was a fairly large financial commitment just for that extra 2" of aperture.

Going from 12" to 16" has been a different observing experience though, before I had to work pretty hard and use a bit of 'averted imagination' to see spiral arms in Messier galaxies now I can see them with direct vision, globulars were resolved but nothing like they are with my 16" so it was well worth the upgrade.

I should imagine that moving from 10" to 16" would definitely give you that wow factor Gerry.

My brain says 16",my heart says 20"...!Law of diminishing returns I guess,the Kriege book warns heavily of too big of telescope,I wish I could see one in person.The whole area's astro club has only 48 members around here-most 200 miles away and thats for about 26,000 square miles,so 26000/48=541 sq miles/member!!LOL!No wonder its dark here

[Bart]

I went from 6 to 10 and that was amazing. Then went to 16 and was also impressed. The globs are better and spotted dust lanes etc in the 16 that I didn't catch with the 10. The big difference was in hunting the galaxies which I really enjoy. Some if the old favourites were much clearer and as for hunting the fainties, it was wonderful. I love the 16 but find if I'm only shooting out for the little while I wheel out the 10 on my sack truck. I also find though that the 10 is easier on my back as the whole session can be spent in a seat

Saying that, if I know the night will be clear ( which hasn't been much in the last few months) I definitely get the big guy out.

Go for the biggest you can afford and maybe more importantly, can manage

If you're doing visual, you want to see as much as you can and as much detail as you can

Barry

[faulksy]

sorry to spam gerry, so when i move from 9" to 14" i should see a big difference then ?

[Mike73]

I also find though that the 10 is easier on my back as the whole session can be spent in a seat

I found that I just didn't like observing standing up and like to be seated while I do my sketches so I was tempted to get a f/4 mirror but glad I didn't because I bought the plans for a CatsPerch Pro  and I love it, worth a look Barry.

[John]

sorry to spam gerry, so when i move from 9" to 14" i should see a big difference then ?

I'm sure you will notice a difference on DSO's, but planetary / lunar might be less marked. My ED120 refractor gets surprisingly close to my 12" dob on the latter targets if the viewing conditions are less than very good.

[Mike73]

sorry to spam gerry, so when i move from 9" to 14" i should see a big difference then ?

Yep Mike don't worry, I reckon your socks will be well and truly blown off. 

[jetstream]

sorry to spam gerry, so when i move from 9" to 14" i should see a big difference then ?

spam?I'm not even sure what spam is.... this is a great question faulksy

[ollypenrice]

If you haven't seen a 20 inch, beware. Ours is F4.1 so a small one - but I wouldn't want to be carting it about. It stays firmly under its roll off. The guy who built it sold it because he was overwhelmed by the business of getting out to a dark site and setting it up.

Olly

[Beulah]

Now the maths has been very cleverly presented here's the practicalities;  If you are tall, strong, possess a good amount of patience, have lots of time and are relatively wealthy, get the largest aperture you can.

I would agree with Mike that the jump from 12 to 16 inches isn't as noticeable than from 8 to 12 inches.

However, in my situation, that would be difficult to quantify, as with each aperture increase the local LP has increased in my area! (should revert back to 6 inches  ) A very dark sky would show the dob's full potential.

Knowing your limits is very important. The cure for me with regards to aperture fever is knowing that the 22 inch can potentially have an EP height of 7ft 4 inches!

[estwing]

they say there's no cure for App fever,there is its called vertigo!

[swamp thing]

Reckon most seem to have this one covered.

My advice (for what it's worth )............Get the 18" classic Gerry

Ps the SIPS feathertouch rocks

[YKSE]

I went from 8" to 12" and got the wow factor, from 12" I moved up to my current 16"but I just don't think that 4" step up gave the same wow factor as my first two scopes, .

Some numbers may explain the difference:

From 8" to 12", the light gathering capacity increased by a factor (12/8)**2=2.25, i.e. 125% percent more, about 0.9 magnitude increase.

From 12" to 16", the light gathering capacity increased by a factor (16/12)**2=1.78, i.e. 78% percent more, about 0.6 magnitude increase.

  

faulksy

sorry to spam gerry, so when i move from 9" to 14" i should see a big difference then ?

 calculating in the same way, you'll get (14/9.25)**2=2.29, 129%, about the 0.9 mag increase, virtually the same Wow factor as from 8" to 12".

[astronymonkey]

I went from 6" to a 16" ...... as you can imagine I was gobsmacked by the difference.

the faint fuzzies simply jumped out and at the same time I went from plossls to naglers to keep the fov which made another huge difference.

With a big dob there is simply nothing that will come close for visual use under a dark sky.

As I've already said, an 18 inch f4 would be the limit for me due to the parameters I have set, but if theres a big dob somewhere on the observing field id be the first in he que to have a look, especially if its 20inch plus !

cheers