Lovely Third Light for my OO/Helmerichs 12"

[Captain Scarlet]

Almost a month after my wonderful first light for this new set-up, the forecast looked clear enough for long enough 2 nights ago (Friday) to risk setting up at the rear of my place here in Ireland. It’s a triangular grassy/scrubby hedge-bordered clearing with a good southerly view from NE to SSW. The forecast said high light cloud until 1am, and heavy cloud thereafter. In the event, I nipped out at 2135 to see how things were, only to find quite thick cloud everywhere! Cassiopeia was barely discernible, Saturn not at all. Oh well, I could look at Jupiter at least, bright enough at that point to penetrate the cloud cover.

When I went back outside just after 2200, it was all magically and majestically clear, and stayed that way all night. My SQM-L reading was consistently a shade under 21.6, pointing the unit directly at the Milky Way at zenith. That translates into a MW-clear 21.8-21.9. Seeing was not too bad. My views of Saturn and Jupiter demonstrated that, of which more below. During the night I saw perhaps 10 meteors, as I generally seem to during these sessions? Is that to be expected even outside a particular “xxx-eids” spell? And the extra effort to run a cable out etc for the hairdryer came in extremely useful, a lifesaver in fact when dew started to impinge after midnight. At one point, I noticed dew on the primary: a quick blast of the hairdryer down the tube cleared that up, and it stayed clear for the rest of the night. I recommend the following experiment: blow the hairdryer down the tube whilst pointing at, say, Jupiter, and nip back to the eyepiece to watch the image get utterly destroyed. Then keep looking, as it gradually settles and clears to reveal startling clarity.

In fact my night was 2 separate sessions, thanks to @John. I did my formal telescope session, the 300mm OO/Helmerichs on AZ-EQ6/Planet + Nexus DSC from 2200 until perhaps 0145 when I finished, dismantled and packed up all the gear. I came inside, ready for bed, quickly checked out this forum, and found John’s account of his Dartmore dark sky trip. His account, there and then, prompted me to go back out again, tired as I was at 2am, to try to see if, like him, I could make out M33 naked eye. And sure enough I persuaded myself that, with averted vision and after about 30 minutes, I could indeed see it, just-about and fleetingly. Perhaps I was imagining, knowing precisely where it was, but I am generally quite self-critical.

The Nexus DSC controlling the AZ-EQ6 was less glitchy than usual, quite well-behaved in fact. Only once did it randomly decide that what it was pointing at was suddenly 157 degrees away and below the horizon, necessitating a complete restart and re-align.

I’d made a rough short list to start off with, some Pegasus galaxies including Stephan’s Quintet plus a couple of PNs, and some globs. Anyway, on to what I saw.

Alignment – Polaris and Markab. I swapped my 18.2mm into my APM finder and its 12.5mm cross-hair into the main scope (at 146x). Using cross-hairs to centre the alignment stars, my GoTo was notably more accurate than usual, until the Nexus glitch.

Jupiter and Saturn 100-305x – My first target was Jupiter (obviously), initially with the 18.2mm for 100x. Nice enough, but no better than I’d seen before. During the night between other targets I kept returning to both these planets, at increasing levels of magnification. By the time I finished, I was on the Delos 6, yielding 305x, and it paid dividends. The GRS was plainly on display. I’ve never had a hint of it before, so I was (in the dark, on my own) jubilant. Just amazing. And the many bands were rough-edged with detail on view. So lovely, like some of the better images I’ve seen. I must learn what these details are called.

Saturn similarly was by far the best I’ve ever seen. The Cassini Division quite clear, most of the time. Stripes on the planet itself just about visible, I think? It was surrounded by lots of more or less faint dots, and I had to resort to SkySafari to see what they were. They were Titan, Rhea, Tethys and Dione. And one naughty star masquerading as a moon. And every now and then there was an extremely faint dot coming and going that may have been Mimas, it was in the right place.

Stu’s suggestions (Fuchsia) + M22 – I had texted my (formerly local) Astro group that the skies were looking good, so @Stu suggested I take in the Eagle, Trifid and Swan nebulae and the M22 glob. Unfortunately there is a close-by tall Fuchsia hedge just where those objects were at the time he suggested them, marking the Western limit of my vista from where I’d placed the scope. I was just about able to detect M22 through the shrubbery-gaps, but none of the others.

NGC 7331 + companions – NGC 7331 is the brightest of a cluster of 4-5 galaxies in Pegasus. So much brighter in fact that it was the only one I could see. NGC 7331 is said to be the MW’s twin, though bigger differences have been found lately apparently. I had a mental image of what I should be looking at, but I simply couldn’t see the much fainter fuzzies where I was expecting them to the left of NGC 7331. But there were some hazy indistinct averted-vision spots on the right. That’s when I remembered I was looking through a Newt, so I can declare that I saw NGCs 7335, 7337 and 7340.

NGC 7320 / Stephan’s Quintet – I’d been primed to not expect too much with only a 300mm scope, even in lovely dark skies. Stu said “Should be possible with your skies”. So my expectations were modest. In the event, I could just about with averted vision and concentration be aware there were a handful of fuzzies. Definitely not imagination, but certainly not very distinct. So, 12” scope and 21.8 skies are the threshold for me for this object, it seems. Quite pleased to get the tick though.

M2 glob – a very nice glob, one can never tire of them in dark skies with medium aperture.

M71 glob (surprisingly faint) – I thought I’d got it wrong to begin with, so I panned around. What it initially “GoTo’d” seemed to be an “on the dense side” Open Cluster, not a glob. I had to resort to SkySafari to check what the neighbourhood looked like, and it did seem I was in fact in the correct place. It was M71. Looking it up as I write, Wikipedia says “M71 was for many decades thought (until the 1970s) to be a densely packed open cluster and was classified as such by leading astronomers in the field of star cluster research due to its lacking a dense central compression”. Haha so I was in good company thinking it was an OC! I think that means I’m a leading astronomer too doesn’t it 😊?

NGC 6781 Ghost of Moon + Oiii – NGC 6781 is a faint circular PN. I’m not sure why it’s so called, as many PNs are circular. GoTo was slightly off, and I didn’t find it with an initial slewing around. I put my Oiii filter into the eyepiece, and found it pretty quickly whilst slewing around again. Very pretty, slightly smeared on one edge, as per the images I’ve seen.

M13 & Propeller – as soon as my mount had finished slewing, and on first presentation of the eyepiece to the eye, the propeller simply leapt out! It was odd, though. When I concentrated, it would come and go. There it was, plain as day, and suddenly, there it wasn’t! Rinse and repeat.

M27 Dumbell with and without Oiii – With the Oiii filter still in I went for M27, the Dumbell Nebula. With the filter, I couldn’t easily make out its “dumbbell-ness” at all. It was simply a large bright homogeneous oval. Without the filter, its eponymous shape returned.

M57 Ring Nebula with and without Oiii – I gave M57 the same treatment, with the Delos 6mm for 305x, and it was similarly impressive with and without the filter.

Pan 31: Ring, M33, M45, cruising - At this point I was deciding it was time to pack up, so I decided to finish off with the big Nagler 31mm for 59x / 1.6 degrees and cruise around. I was still pointing at M57, which now in a much larger field of stars was simply extraordinary. Such a tight extremely bright ring against a velvet studded background. Really beautiful. I also took in M33, which was a wispy smudge with hints of differences of shading. Even 1.6 degrees wasn’t enough of course for this object. I finished off with M45, The Pleiades: mainly Alcyone and most of “Ally’s Braid” (which I call the B2 Bomber), a shallow V-shaped asterism hanging just “below” Alcyone. Again, lovely pinpoints on a black background.

I brought everything inside, then as described above, came back out with my 15x56 binoculars and naked eyes for a second session.

All in all a highly memorable session, especially having two outings in a single night thanks to John. Bed at 3am.

I have, at the bottom and back of a cupboard, a Nichol 20” mirror awaiting the sale of my house to provide sufficient funds to get my dobsonian-build project under way. This session, especially Stephan’s Quintet, has made me all the more eager to get that dob built.

Thanks for reading, Magnus.

 

[John]

Great report Magnus

At the risk of leading you and your 12 inch further astray, NGC 404, the "Mirach's Ghost" is a small galaxy close to it's namesake star and well worth a look when in that part of the sky and also you should look out for NGC  604, a small patch of light near a star right next door to Messier 33 - it's significance is that it is an HII star forming nebula within the Triangulum Galaxy, probably the easiest DSO "in another galaxy" to see

Edited September 12, 2021 by John

[JeremyS]

2 hours ago, Captain Magenta said:

I have, at the bottom and back of a cupboard, a Nichol 20” mirror 

As one does…🤣

Seriously, though, Magnus: cracking report 👍🏻👍🏻👍🏻

[PeterW]

I’d be investing some pruning kit to keep the shrubbery at bay and improve you views! What f-ratio is the mirror in the cupboard? I’ve got one under a bed still waiting…. 
some good objects in there!
Peter

[Captain Scarlet]

4 minutes ago, PeterW said:

I’d be investing some pruning kit to keep the shrubbery at bay and improve you views! What f-ratio is the mirror in the cupboard? I’ve got one under a bed still waiting…. 
some good objects in there!
Peter

Mine's f/3.5. I didn't know you had a project-in-waiting ... a big one?

[Captain Scarlet]

2 hours ago, John said:

Great report Magnus

At the risk of leading you and your 12 inch further astray, NGC 404, the "Mirach's Ghost" is a small galaxy close to it's namesake star and well worth a look when in that part of the sky and also you should look out for NGC  604, a small patch of light near a star right next door to Messier 33 - it's significance is that it is an HII star forming nebula within the Triangulum Galaxy, probably the easiest DSO "in another galaxy" to see

Superb thanks John. The list for my next session, whenever that may be, starts here!

M

[PeterW]

13” f3 picked up a few years back and under a bed…. The focusser I bought for it has been used on my 8” as the stick one was UGH! No idea when it’ll get a scope build round it. 
f3.5…. Good speed, hope you’ll use the Explore corrector so you don’t lose any of the speed.

Peter

[Stu]

Wonderful stuff Magnus. Your skies are up there with Gerry’s and Olly’s I reckon, so a 20” will be amazing once complete.

M71 is a funny one isn’t it, seems to have a problem knowing what it should be!

M27 does look oval with larger aperture/darker skies as you can see the fainter outer regions. I recall a similar view with my 14” either in Pembrokeshire or Bignor.

NGC604 is a good one as John mentioned. I’ve seen it properly only on a few occasions, best being SGL10 with my 16”, that was a great view but skies were not as good as yours.

Hope you get more clear skies soon!

[Stu]

10 minutes ago, PeterW said:

Good speed, hope you’ll use the Explore corrector so you don’t lose any of the speed.

He’s not doing NV Peter so a little longer f/l won’t hurt too much 😉

[PeterW]

…. Not if you are a disciple of Bartels the IFN hunter… f3 to enable 7mm exit pupil with the 21E-bomb. (No electronics in sight).

Peter

[RobertI]

Lovely report Magnus, a fantastic haul of objects. What would you say is the biggest “wow” when moving from a smaller scope to a larger scope? I’ve never owned anything bigger than an 8” SCT,  but always felt that if I got a 12” or 14” dob, the biggest wow would be on globulars?

[Captain Scarlet]

1 hour ago, RobertI said:

Lovely report Magnus, a fantastic haul of objects. What would you say is the biggest “wow” when moving from a smaller scope to a larger scope? I’ve never owned anything bigger than an 8” SCT,  but always felt that if I got a 12” or 14” dob, the biggest wow would be on globulars?

Certainly in my experience globulars show the greatest improvement. My first scope was (is) a Mak 180 (i.e. 7") which I had delivered straight to Ireland (I was living then near London). Pretty much the first thing I looked at was M13. I remember thinking it was, truth be told, a little underwhelming, 21.8 skies notwithstanding. A brighter smudge than I could see in London certainly (through a friend's Mak 127), but not much starry resolution. Then @neil phillips SW 300p came up for sale, and on seeing M13 through that, the difference was nothing short of amazing. So globs in my experience respond both to aperture and dark skies.

Where I have been seriously and probably most impressed though has been in the "fuzzies". I recall cruising in wonder through Markarian's Chain, losing count of all the galaxies. But again, the 12" I had on tap was in 21.8 skies. My comparisons are tainted by the fact I changed two things simultaneously: bigger scope and much darker skies. In London where I was, 19.1 skies, I had an 8" newt, but that was probably as big as it made any sense to go at Bortle 8. The faint stuff was simply drowned by LP.

If you're near Colchester it seems your skies are a shade under 21.0? 12" aperture should be well worthwhile I think. Globs will be amazing of course and many galaxies should be able to make it through? @Piero I think observes from a similarly dark location to you, and I get the impression he gets satisfaction from a 16". It would be interesting to hear his thoughts.

Cheers, Magnus

[callisto]

Magnus, that was a fantastic report, I had all your sightings in my head while reading it 👍

 

Mark

[RobertI]

1 hour ago, Captain Magenta said:

If you're near Colchester it seems your skies are a shade under 21.0? 12" aperture should be well worthwhile I think.

Yes on a good night, and if the neighbours don’t stay up too late, the skies can be nice and dark. All too often though they seem to be ‘bright’ though, not really sure why. A 12” dob is definitely on my future list though - one can only try it out and see. 🙂

[Piero]

14 hours ago, Captain Magenta said:

Certainly in my experience globulars show the greatest improvement. My first scope was (is) a Mak 180 (i.e. 7") which I had delivered straight to Ireland (I was living then near London). Pretty much the first thing I looked at was M13. I remember thinking it was, truth be told, a little underwhelming, 21.8 skies notwithstanding. A brighter smudge than I could see in London certainly (through a friend's Mak 127), but not much starry resolution. Then @neil phillips SW 300p came up for sale, and on seeing M13 through that, the difference was nothing short of amazing. So globs in my experience respond both to aperture and dark skies.

Where I have been seriously and probably most impressed though has been in the "fuzzies". I recall cruising in wonder through Markarian's Chain, losing count of all the galaxies. But again, the 12" I had on tap was in 21.8 skies. My comparisons are tainted by the fact I changed two things simultaneously: bigger scope and much darker skies. In London where I was, 19.1 skies, I had an 8" newt, but that was probably as big as it made any sense to go at Bortle 8. The faint stuff was simply drowned by LP.

If you're near Colchester it seems your skies are a shade under 21.0? 12" aperture should be well worthwhile I think. Globs will be amazing of course and many galaxies should be able to make it through? @Piero I think observes from a similarly dark location to you, and I get the impression he gets satisfaction from a 16". It would be interesting to hear his thoughts.

Cheers, Magnus

 

Nice report, Magnus

 

@RobertI 

I just searched for `Colchester` on Google. It seems a really beautiful city with a lot of historical architecture. I will plan a visit, maybe next year.

Anyway, coming back to your question, an increase in aperture allows you to: collect more light and use higher magnifications keeping the same exit pupil. The latter benefit is critical. Small apertures a great for observing large targets, large apertures are great for observing small targets. .. and there are a lot of these small targets in the sky. In general, any object benefits from additional aperture. Objects are not all the same though. For instance, small galaxies and planetaries can simply be stellar (or invisible) at low/medium magnification (even under dark skies), whereas they pop up at medium/high magnification. The "companions" of NGC 7331 are an example. They are not really companions, but about ~300 mly away from us. I spotted 3 of them using at least 320x. Below that, I was unable to determine whether they were actual galaxies or not.

To use high magnifications, seeing becomes more and more critical, but I find that this threshold is higher than the 200-250x that most people repeatedly state. A larger aperture is more susceptible to a variety of things though. For instance:

• a large mirror is much heavier than the lenses installed on a refractor. Therefore, adequate bottom and edge support become more critical
• a large mirror is generally thicker. Therefore, cooling time is longer. This is not just the cooling time from indoors to outdoors. It is also the cooling time occurring during the night. The boundary layer of warm air 1 inch above the mirror surface (often looked at as atmospheric seeing) is related to this too.

There is no free cake here.. a thin mirror improves cooling time, but is picky on mirror support and viceversa.

• larger and therefore heavier mirrors require more solid supports and mechanics in general otherwise collimation will be affected (a large bucket for many issues here)
• faster optics require a coma corrector, otherwise the comatic blur can make the already faint object, invisible.

The lack of adequate mechanics / cooling can cause astigmatism, but also spherical aberration. Although annoying, astigmatism is easy to analyse and determine the source. Spherical aberration is trickier because it can be caused by a variety of things and some spherical aberration, although minimal, is also present in the mirror optical figure.

If you want to use high power with a large dobson, you need to take care of the points above, otherwise you limit yourself to use your dobsons within the range of magnifications typically used with refractors. By taking care of the issues above, I noticed that I can use my 16" with magnifications up to 400-450x quite frequently, and a few times even more. Dealing with the boundary layer alone allowed me to increase the mags for about 150x from the previous "seeing limit". I have tested this many many times now and the result is consistent. When the light shroud is fully down, the telescope performs like a solid tube and the boundary layer of air lays above the mirror. When I pull the light shroud 3" up from the mirror box, the incoming air flow is sufficient for dissipating the boundary layer of warm air above the mirror, without hitting the side of the mirror as this is still protected by the mirror box. The effect on stars and planets at 270x-320x is just striking. Bright stars shrink in size becoming dots. Planets like Jupiter and Saturn becomes covered with features. Their moons become clean disks and 1-2 Saturnian faint moons pop up.

It's a bit like with cars. You can drive a Ferrari in a little town and it does work, but it does not really shine. A utilitarian car does the job perfectly. Then of course, you might want the Ferrari for the pleasure of it, but that's another story. 

[Stardaze]

Enjoyed reading that Magnus, sounds like you have a great combo there. 

[Stu]

On 12/09/2021 at 15:08, Captain Magenta said:

I recommend the following experiment: blow the hairdryer down the tube whilst pointing at, say, Jupiter, and nip back to the eyepiece to watch the image get utterly destroyed. Then keep looking, as it gradually settles and clears to reveal startling clarity.

 

4 hours ago, Piero said:

Dealing with the boundary layer alone allowed me to increase the mags for about 150x from the previous "seeing limit". I have tested this many many times now and the result is consistent. When the light shroud is fully down, the telescope performs like a solid tube and the boundary layer of air lays above the mirror. When I pull the light shroud 3" up from the mirror box, the incoming air flow is sufficient for dissipating the boundary layer of warm air above the mirror, without hitting the side of the mirror as this is still protected by the mirror box.

My assumption that these two issues are related? Magnus’ hairdryer trick is disrupting the boundary layer so that when the tube currents subside, the image is much improved. Does that sound correct?

[Piero]

1 hour ago, Stu said:

My assumption that these two issues are related? Magnus’ hairdryer trick is disrupting the boundary layer so that when the tube currents subside, the image is much improved. Does that sound correct?

 

Yes the hair dryer would temporarily break the boundary layer of warmer air above the primary mirror. It should be used in "cold" mode, so it would act like a fan, which moves air but does not warm it. 

What Magnus described: "nip back to the eyepiece to watch the image get utterly destroyed. Then keep looking, as it gradually settles and clears to reveal startling clarity."  is correct. The image is looks very bad first, due to the created tube current, but few minutes later it sharpens considerably. I don't know how long this would last though. Maybe it lasts for 1/2h or 1h. 

 

Mine works because the mirror box is low enough that the air coming into the telescope from the gap mentioned previously does not allow this boundary layer to (re-)form. Note that this does not work in the same way with ultra-light dobsonians like Sumerians because they don't have a mirror box. Therefore, pulling up the light shroud, will increase stray light and potentially dew formation on the mirror surface as this would be completely exposed.

Another approach is the installation of lateral fans. See Teeters for truss dobsonians. 

For solid tube, the principle shown with this solution works too: https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/   

 

One thing I learnt from my 12" Lukehurst dobson is that this airflow for tackling the boundary layer must be directed to the mirror top surface, not the mirror edge. An unbalanced airflow directed to one side of the mirror edge causes astigmatism and spherical aberration due to temperature differentials. Here is an example. The opening (oblique cut) at the bottom/front of the mirror box in truss dobsonian was added by David Kriege (Obsession telescopes) to allow one to decrease the overall height of a dobsonian telescope for a few inches. This cut goes from the bottom edge of the mirror box to almost the back of the mirror. Lukehurst clearly interpreted this in a very different way (which is odd as Kriege is very clear about this feature in his book). In particular, he uses that gap as a way to slide off the mirror cell (with the mirror on top), discarding the principle of reducing the height of the telescope, completely. Whilst this seems attractive to him, and I believe some people, it has an important disadvantage. To allow this, the cut is made much deeper and it goes a bit beyond the top of the mirror, exposing the whole bottom edge of the mirror. Doing so, unless the telescope is pointed at the zenith, the outside cold air reaches that edge of the mirror and impairs the temperature of it. It does not have any effect on the boundary layer because the airflow does not reach the mirror surface effectively. Thankfully, this problem is fixable. One needs to close that opening (e.g. with a light plywood panel attached with some heavy-duty velcro). Doing so, the mirror box becomes similar to the design of John Dobson, except for the fan installed at the back of the MB. 

My point is that experimenting is a great thing because there are many things which can be optimised and some of these can give surprising results. Said this, one should at least try to understand existing solutions, rather than just applying them blindly. Anyway, in line with John Dobson's thought, in telescope making errors can be fixed (with some extra time and dedication).

 

Compare MB opening between these two:

Edited September 13, 2021 by Piero

[Stardaze]

Interesting discussion. I suffered a little astigmatism the one evening that was not normal toward the end of a session. Maybe the boundary layer was the culprit?

[Piero]

1 hour ago, Stardaze said:

Interesting discussion. I suffered a little astigmatism the one evening that was not normal toward the end of a session. Maybe the boundary layer was the culprit?

The effect of the boundary layer is very similar to poor seeing. Stars tend to be a bit larger, with variable plumes when defocused slightly. When you try to get a better focus, you get frustrated because the image doesn't get much sharp. The more you magnify, the more you see the effect. Essentially, it is due to the light path being disturbed by a layer of heat released by the mirror as this cools down and adjusts it's temperature.

Some astigmatism can be caused by the mirror changing temperature. A drop in 2C can show some astigmatism as well as spherical overcorrection. These tend to decrease as the mirror reaches the environment temperature. I suspect you experienced this.

Severe astigmatism is generally caused by optics supports. It's easy to distinguish whether it is due to the secondary or the primary support and which part. 

All the above assumes that good eyepieces are used (e.g. TeleVue - which don't have astigmatism) and that the optics are sound and of good quality.

If you are interested in these things, I would suggest you to get a copy of Suiter's star testing book. It's excellent.

Although old, Sidgwick's handbook for the amateur astronomer is a great resource too. In my opinion, Sidgwick is an outstanding writer. It's a real shame that his books are out of print. TBH, I find it awkward that Patrick Moore was made Sir despite of all his rough comments, and a gentleman like Sidgwick seems just ignored. Mad world.

Edited September 13, 2021 by Piero

[jetstream]

14 hours ago, Piero said:

When I pull the light shroud 3" up from the mirror box, the incoming air flow is sufficient for dissipating the boundary layer of warm air above the mirror,

Yes it works, been doing this for years myself 👍

Its an inherent advantage to truss dobs IMHO. I have no set distance to pull up, sometimes much more than 3" and just let the views be the guide. Doing this and a couple other things has allowed my 15" to go well over 700x on the moon and razor sharp.

[jetstream]

On 12/09/2021 at 09:08, Captain Magenta said:

NGC 7320 / Stephan’s Quintet – I’d been primed to not expect too much with only a 300mm scope, even in lovely dark skies. Stu said “Should be possible with your skies”. So my expectations were modest. In the event, I could just about with averted vision and concentration be aware there were a handful of fuzzies. Definitely not imagination, but certainly not very distinct. So, 12” scope and 21.8 skies are the threshold for me for this object, it seems. Quite pleased to get the tick though.

Excellent report Magnus!

Out of curiousity what eyepieces were you trying Stephans Quintet with? It actually matters IMHO. Using my VX10 I can get 3 of them, using high transmission eyepieces like orthos. Definitely worth a try as is upping the mag like @mdstuart told me years ago.. We observe under the same skies basically so it will be fun to compare- and yes its now raining here lol!

[jetstream]

On 12/09/2021 at 14:54, RobertI said:

 but always felt that if I got a 12” or 14” dob, the biggest wow would be on globulars?

The wow will be on everything IMHO. A 16" f4 -f4.5  truss dob would be perfect.

There is no comparison between an 8" scope and a 16" scope on the Veil for instance- in my own aperture upsizing I try to go at least one magnitude deeper on stars which results in amazing differences on all objects, skies considered IMHO.

The difference between the 10" and 15" here on the Veil is staggering...  and the Lobster Claw and...galaxies and...

[Wiu-Wiu]

On 12/09/2021 at 21:54, RobertI said:

What would you say is the biggest “wow” when moving from a smaller scope to a larger scope? I’ve never owned anything bigger than an 8” SCT,  but always felt that if I got a 12” or 14” dob, the biggest wow would be on globulars?

Moving from 8” SCT to a 12” dob the biggest ‘wow’ for me were the pinpoint stars. SCT’s always gave me a bit of ‘woolly’ stars, unless you had an edge or ACF, which were better, but not quite. 
the aperture also boosts contrast so the background tends to be a bit darker, and fainter stars come through.

the biggest ‘wow’ might indeed be globulars. They are much more resolved. 
 

I recently upped from a 12” to an 18” and the views are jaw-dropping. White ovals on Jupiter, a multitude of moons on Saturn, globulars resolved to the core, details in nebulae and galaxies,… . (but 16” with prime optics will do the very same thing.) The group of friends with whom I regularly go out to dark skies all own 16 to 20” Nichol and Lukehurst Sumerian dobsonians and the quality in all is superb. The bigger aperture ‘only’ brings out more faint detail. We are all awaiting one friend’s 30” lukehurst build, but that might still take a while. 
 

 

[John]

A few years ago I had some time at the eyepiece of a 20 inch dob under a reasonably dark sky and the view of the Messier 13 globular cluster and the Messier 51 galaxy (my choices) were jaw dropping 

At home (where I mostly observe) the practical maximum aperture I've found is 12 inches but that is with an Orion Optics based dob which weighs about the same as the Skywatcher / Meade / GSO 10 inch dobs.

I found 10 inches of aperture was where the brighter globular clusters really "came alive" for me

Having the scope well collimated and cooled is important as well to ensure that you get the best resolution of stars both at the periphery of the cluster and deep into it's core.

A nice observational target when viewing Messier 13 with a decent aperture is to pick up the nearby small galaxy NGC 6207 about 1 degree to the NE of the cluster. Lovely sight in one of those new fangled ~20mm / 100 degree eyepiece thingies