Now my question is: (And forget about the cost for a minute. I realize it can costs thousands on top of the scope for a big refractor) Purely visually. What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?
Regardless of which scope type you get, you will be making compromises. So up front wanted to make it clear that no single scope or design does everything well -- and by everything I mean everything and not simply the view under those hypothetical perfect condition (which rarely if ever happen in reality so they are just for the theoretical at heart).
1) First let's consider working magnifications for planets. Will only be addressing planets as that was the question. IME it is first and foremost about exit pupils. When the exit pupil gets too small, the lower light level makes to more subtle contrast features on a planet vanish from view. Examples would be some lighter and smaller festoons on Jupiter, small not so dark barges on Jupiter, the Equatorial Belt si sometimes ethereal so needs a brighter view, the edge details on Martian maria, etc. IME about as low as I like to go on the planets so I can retain brightness for those low contrast features is usually around 0.65mm. I will bracket that as 0.75mm to 0.5mm. Mars as example can usually hold up better at the dimmer 0.5mm whereas Jupiter does not and needs a bit of a brighter view.
So you need to be cognizant of this because the exit pupil will drive what magnification you can use. So @0.65mm exit pupil here are the magnifications you will get by aperture:
102mm - 157x
130mm - 200x
152mm - 234x
203mm - 312x
254mm - 391x
305mm - 469x
Now for planets, I personally can live with 135x and be quite happy as I have trained myself to be adept at the very small image scales. The details are there to be seen so nothing missed, just a more strenuous endeavor. 150-200x is certainly more comfortable and I rarely go above this even when the seeing allows it as 200x is a walk in the park once you are used to 135x to see details. But many folks have posted that they seem to prefer the 250x mark, or at least get excited when they can reach or exceed that. So you need to find out what you need in terms of magnification so you can choose an aperture wisely. Btw, the 0.75-0.5mm exit pupil rule I use does not work on the Moon as it is so bright and high contrast that the views look great even as low as a 0.25mm exit pupil.
Another consideration for the observer is how many floated they have in their eyes. You generally get more with age and they can be annoying when on-axis. So if you have a lot of floaters when working at small exit pupils, then shoot for the larger apertures so you can be at the image scale magnification you prefer at a larger exit pupil.
2) Second you need to consider ergonomics of the scope in the field. This is a BIG issue because it is a BIG topic as it covers things like size, weight, mounting needs, thermal acclimation, etc. So affects everything from if it will break your back getting it outside, if it will take hours to cool, if it needs assistance cooling so active devices like fans, if it also needs special boundary layer scrubbing which mirrored designs can need, if the mounting needs to be heavy and costly to keep the scope from shaking, if the entire setup will take multiple trips and significant time, if collimation is needed with each outing (truss Dobs many times), etc. So generally, the larger the aperture the larger all these p.i.t.a. items become more and more troublesome and time consuming. SCTs win on size but lose on thermal stability so you "can" be always futzing wit that. Dobs with their mirror mass need cooldown time and usually fans to help that and boundary fans also for really steady views. Refractors need the least care and feeding and at 102mm and smaller are fairly ready when you are and can be carried mounted. Get up to 130-152mm and now a bit of a heavier chore and needing a more substantial mount and taking 30-60 minutes and sometimes longer for optimum cool down. Some SCTs I've had never cooled down and the baffle tube plume was visible all evening. My 6" Dob would cool fairly quickly and well, but my 10" one would need fans on it for an hour to get it ready for planetary and then re-need them every so often if the temps were not stable outside. If you live in humid conditions then you might need optional dew heaters for secondary mirrors and have issues with the primary if it is just in a short rocker box. SCTs of course will need them as the corrector plate is not shielded. No refractor I've had has ever needed them as the standard dew shield has always been sufficient for 4-5 hours of observing even in the worst conditions here in Northeast. Collimation is usually a one time affair with a refractor and SCT, but a more regular thing with Dobs. And if a large truss Dob with a heavy secondary, may need tweaks during the observing session! Anyway, lots of issues to contend with here. As far as a p.i.t.a. factor, IME it is highest for Truss Dobs, less for solid tube Dobs, tiny bit less for SCTs, a lot less for refractors (even 6" ones). So important to know yourself here and how much futzing you enjoy as part of the process. Some like to tinker preparing for observing session and during the session, others hate it (I am the latter).
3) Next you should consider focal ratio and design. If working less than f/7 then you will need to be more careful with eyepiece selection if you want sharp to the edge or near edge. So your expense goes up with shorter focal ratios. At f/8 most everything works quite nicely and at f/9 really nicely. Now SCTs are f/10 usually but they are a special case as unless the Edge or ACF variety they have enough coma and field curvature that they behave IMO like a short focal ration Dob. Only problem is that little you can do to correct the views as no coma correctors and visual flatteners, although the standard 0.63x reducer helps a a lot. With Dobs they all have coma so upi really need a coma corrector, and these are pricey. But without them then you will not have much drift time as more than half the FOV is too aberrated for a clean planetary image. So I put them as a necessity when the focal ratio is f/6 or less. With refractors the off-axis will have field curvature especially if shorter than f/7, so the wider field longer focal length eyepieces will need to be better corrected varieties to get a completely clean off-axis. With Dobs the mirrors get dirty/dusty easy IME. So after a month or so of observing there is enough dust build up to visibly show more scatter - dust on a mirrored surface will show 4x the scatter as the same dust level on a refractive element (based on study of observatory optics to determine cleaning schedules), so why the mirrors need cleaning more often. So mirror cleaning is IMO a regular need, 2x a year minimum in my book. Refractors seem to accumulate less dirt and the front surface is an easy clean. SCTs usually stay pristine unless you use cooling fans for the interior without filters, and they they get dirty fast and much harder to clean. But I have had closed system SCTs for 20 years or more and the mirror is pristine looking and just the corrector plate needs cleaning as often as a refactor's objective, so not too much, sometimes as little as yearly. When my scopes are freshly cleaned and fully thermally stable, the views are scaled by aperture so one not really better than the other if all the same aperture, although the refractor will be a little brighter. However, the effort to get and keep them in that condition is way different, so why I just use refractors for planetary and do not want the hassle of the care and feeding the other designs need. My 4" goes outside and is generally ready in 15 minutes. My 152 is ready for planetary in 45 minutes or so. My SCT was never ready except twice in my life. My Dobs were ready half the time, other half could not get the mirrors fully tamed. So my compromise to refractors is mainly due to all the ergonomics of their field operation. Nothing to do but put them outside and let them sit for just a little and ready to go with no other attention. Dobs and SCTs need much more care and attention and not my cup of tea. Others though have no issues. Also why I shied away from Harley Davidson's and kept to Japanese motorcycles as the only tool kit I want to worry about on a long trip is whether I packed my ATM card for gas as nothing else will fall off or break on a 1000 mile trip. With the Harley though, even 2-3 hour runs with my friends would often need stop to fix or adjust this or that. As I said, not my cup of tea but others love futzing with their equipment.
4) Finally let's talk a little about seeing. There are probably other things we can talk about but these are the major ones IMO. Seeing is the last as it plays into the ergonomic issues of the scope choice. I do a LOT of planetary observing! And a pattern for me at several homes now in the Northeast, is that before 2am the seeing is an iffy thing for planetary. So if my viewing is only in the early evenings then I know I probably have about a 1-in-5 chance of the seeing being good enough to support what I consider productive planetary observing. By that I mean that the planet is detailed and stable for periods of time that are much longer than it might be unstable. So no "moments of best seeing" in my operating book because if the seeing is such that there are only moments, then the seeing is bad! So in my observing the planet is mostly rock steady most of the time and only the occasional few seconds of instability over a few minutes of observing. So for me about a 1-in-5 chance of getting that in early evening. From 2-5 am however, more like a 3/4-in-5 chance of getting that prime planetary seeing. So given all this, if you are getting a scope that needs some work to get it ready for an observing session, it can be a real bummer most times as more often you will set up and find the seeing is not there for planetary. So if you do that then get an 80-100mm Apo also so at least you can tell in 15 minutes if the seeing is good enough for wherever the planet is to put all the work into a setup of a rig that needs more care and feeding. Another reason why I go with refractors as I just put one outside and no hassle to bring it back in if the seeing is off. So they are just no hassle instruments and for that big pro in my book, I can live with the con of smaller aperture and less details. A larger aperture does get one nicer details, as example the eddies around the GRS and structure inside the GRS are easily seen and detailed in a 10" when it is magically freshly cleaned and collimated and actually fully thermally stable, and those things are still there but less detailed in my 4", but then I don't have to worry about the magic being there as the 4" needs to magic of all conditions needing to be just right. So again, does not mean one scope/aperture is better than another, just a matter of where you do and don't want your poison with whatever you choose as they all come with some level of poison.
Bottom line for me, a person that wants their equipment to get out of the way so all I worry about are the sky conditions and placement of the planet, a 100-130mm refractor is the best instrument out there as it is ready when I am for a 15 minute look or a 4 hour session. If I need to see more details, I can look at Hubble or Planetary Probe pictures as they will always show more details than any consumer scope. IMO a 152 Apo is not worthwhile better than a 130 for planets. Where the 152 shines is on DSO as it goes almost as deep and satisfying as an 8 SCT. So it is a nice mix instrument for both planetary and DSO, so a better general purpose refractor. For SCTs 6-8" are the least hassle IMO, 5" MAK and they are getting to behave more like refractors than SCTs. For Dobs 8-10" are the least issues, with the 8" being preferred as IMO they are much more thermally stable than 10" and be quite fine even without active cooling. So for least hassle planetary scope, least hassle first: 100-115 Apo, 5" Mak, 120-152 Apo, 6-8" Dob.
Edited by BillP, 28 September 2020 - 01:20 PM.