180Pro vs 10" Newt

[A McEwan]

Hi all,

Dedicated readers of my posts can probably guess where this is going, but what do you think of the difference in planetary performance between the EQ-mounted 180Pro Maksutov (f15) and a driven SW 10" f4.8 Newtonian? Visual only.

Ant

[CGolder]

How good are your eyepieces?

[A McEwan]

Pentax XL and XW, T6 Nagler, Vixen LVW, Meade S4k UWAs and Baader/Circle T Orthoscopics. Pretty good.

[blinky]

I would say (and I'm no expert!) but the Mak would be the best suited to planets - longer focal length = bigger planets

Maks are known as 'Planet Killers' for a reason!

[CGolder]

I think I have read before that the 180 takes an age to cool, depends where it is kept i guess. as you can tell by my sig i love Maks and SCT's so if i have the choice i would have the mak in a flash.. but i am not able to give a balanced opinion

[RikM]

I have just added a Skymax 127 to compliment my Explorer 150P, and the Mak is clearly better for planetary detail (at least on Jupiter). I know these are both smaller instruments than the 180 & 10" but the relative sizes and types are equivalent.

The previous owner of my Skymax has an obs mounted 180pro and is almost exclusively a lunar / planetary observer.

Rik

[sixela]

Hi all,

Dedicated readers of my posts can probably guess where this is going, but what do you think of the difference in planetary performance between the EQ-mounted 180Pro Maksutov (f15) and a driven SW 10" f4.8 Newtonian? Visual only.

Ant

Depends a lot on external factors. On both scopes, cooling is going to be pretty critical (ideally, you'd want something like a Lymax cooler on the Mak and at least a fan at the back on the Newt), and so is going to be collimation.

The Newtonian will need a much better focuser and observing with it is usually going to be less comfortable unless you have lots of floaters in your eye, as the Newt is going to allow you to observe at a given magnification with a larger exit pupil.

If the seeing really is very good (if you can observe at more than 300x) the Newt might beat the 180Pro if you have a good mirror, it's collimated well and its mirror is cooled.

But getting every bit of performance out of the scope sometimes depends on very subtle things: it's easier for your own heat plumes not to get in front of the aperture on the Mak, so ideally you'd want to stand downwind of the scope on the Newt.

Frankly, I'd be happy with either instrument, though I'd really want a dual speed focuser on the Newt.

[dweller25]

I am a planetary observer and had the 180 Pro mak - cooldown was a major issue as was contrast. At the same time I had a Skywatcher 8" Dob and it was excellent on the planets. I now have the 10" Skywatcher Dob which is even better.

The 10" Newtonian will beat the 180 pro easily on the planets.

[Astro_Baby]

Beg to differ, there's a reason planetary scopes have long focal ratios. It brings out the detail better. I have an 8 inch newt at f5 and my 4" fact at f 9 can blow the newt out of the water on planets.

At high powers f 5 scopes are going to be tough to find the sweet spot for focuing where a long focal ratio scope will have more of a snap to focus.

I also have a 180 but I can't give you an opinion because in the time I have owned it it's never been used.

The big downside with a monster sized mak is going to be cool down. If my 180 delivers the good it will be nodded to have it's own fans rather than messing about with a lymax.

[Mr Spock]

Beg to differ, there's a reason planetary scopes have long focal ratios. It brings out the detail better.

I'll beg to differ from that! My 250 f4.8 was as sharp as any other scope I've used, including f14 Mak, f15 refractor and my new 235mm f10.

All the longer focal length does is reduce abberations in less expensive eyepieces. So you can get good results in an f14 from a Plössl whereas it would struggle in a f4.8.

Detail is down purely to resolving power of the optics, which in turn is down to aperture and optical component quality.

So, I agree, a quality, properly collimated 10" Newt will show more than a 180 Mak.

[dweller25]

Beg to differ, there's a reason planetary scopes have long focal ratios. It brings out the detail better. I have an 8 inch newt at f5 and my 4" fact at f 9 can blow the newt out of the water on planets.

At high powers f 5 scopes are going to be tough to find the sweet spot for focuing where a long focal ratio scope will have more of a snap to focus.

I also have a 180 but I can't give you an opinion because in the time I have owned it it's never been used.

The big downside with a monster sized mak is going to be cool down. If my 180 delivers the good it will be nodded to have it's own fans rather than messing about with a lymax.

Mel, I beg to differ too ...

Planetary detail is a function of resolution (aperture) and seeing not focal ratio.

If your 4" refractor shows more detail than your 8" reflector then is there something wrong with the reflector ???

At high powers I have no problems focusing my 10" F4.8 Newtonian.

As I said earlier I have directly compared the 180 Pro with a Skywatcher 8" Newtonian and the Newt won - easily. Which it should as the Newt has an inch more aperture and a smaller central obstruction.

A 10" Newt will show an even bigger improvement over the 7" Mak - cheaper too

[sixela]

At high powers f 5 scopes are going to be tough to find the sweet spot for focusing

Actually, the sweet spot for focusing is easy to find on a short f/ratio scope, because it's narrow. It's also the reason that it's a pain to focus if you don't have a dual speed focuser. On a long f/ratio scope, it's actually harder to determine exactly where best focus is, because the depth of field is larger; for the same reason it's a bit easier to focus with a single speed focuser.

SCTs are, in this respect, "fast" scopes if you use the mirror mover to focus and "slow" if you have an extra Crayford focuser on the back. And fast scopes with a barlow and a helical focus on the eyepiece side of the barlow are "slow".

If a fast scope has no snap focus, that means it's either seeing or aberration limited (or you're magnifying so much that it's diffraction limited).

where a long focal ratio scope will have more of a snap to focus.

It has more depth of field, so it has less "snap focus".

---

Mind you, very fast scopes will generate spherical aberrations in simple eyepiece designs (Plössls and orthos) unless you use a barlow. But you're talking about f/4 scopes and below before you can see this on axis.

[A McEwan]

Ok, thanks for the input! Some specifics:

Cooling is not an issue with my 180 as I keep it at ambient temperature and only minimal cooling is required before images are perfectly stable. I've not had a session yet where the scope was not cooled sufficiently within setup time.

I like the planetary views through my 8" f6 Dob but it is not motorised and that's why I got the Mak- so I can have EQ-mounted planetary scope that is comfy to use.

I have not done a side-by-side between the 8" Dob and the 7.1" Mak but views at different times do not look too dissimilar, with (to my eyes) the edge going to the Mak because of larger image scale.

Now I'm thinking if a moptorised Newtonian is aavailable (in the form of the SW Auto Dobs) then I have a choice of larger aperture (at faster f-ratios) but also in a driven package.

So, there are options. One option is just to stick with the 180 Mak and enjoy it. Another option is to try a 10" tracking Dob and see how I like it in comparison. Another option is to get a tracking Dob and substitute the tube for a planetary Newt (f6.3, 1/10th wave, Hilux etc).

Interesting thoughts...

Ant

[Moonshane]

I have never used a closed reflector system so cannot compare directly but have a 12" f5.3 and a 6" f11 dobsonian.

I find that detail on Jupiter and general contrast is far greater with the 6" than the 12". I suspect that detail every now and again in fleeting seeing would be better in the 12" but generally the view is more detailed more of the time in the 6". That said, due to local seeing I have only been getting up to 140x or so recently. If I could get higher then I am sure the 12" would come into its own.

The 12" is better generally on the moon at higher powers and on decent nights I can get 400x+ with the 12" on the moon. I have dual speed focusers on both, they are excellent.

I am hoping to compare my 6" f11 directly with a 7" Intes MN78 (I think that's what it is) soon and will do a write up when I do this. I expect that the Intes will be far better than my 6" but at 10x the price so it should be.

[Astro_Baby]

Well gosh I'm sure then I have no idea why slow focal ratio scopes whether they be newtonians, SCT, Maks or otherwise are always considered as planet killers.

Obviously all the professionals of the late 19th and early 20th were totally wrong to concentrate on long focus refractors, perhaps that's why Percy Lowell got it so wrong on Mars

[A McEwan]

I suspect, AB, that long focus refractors were the most reliable telescopes that were available at that time with good optical qualities? Control of aberations is easier in long focus objectives. However, I'm not a historian so please don't quote me!

I think it's the associated small central obstructions in scopes (Maks, Newts etc) with high focal ratios that develops them as "planet killers"?

Ant

[Astro_Baby]

But ant you could equally well build an F5 scope with a smaller obstruction, yes you'd reduce filed illumination but that wouldn't matter for a planetary scope as you'd not be worried about the dye of the field anyway most likely.

Actually turn of the century scopes were always refractors because reflectors had become discredited after some notable disasters, mirror coatings were not long lasting and it was hard to figure large mirrors until the advent of high tech glass was available. Refekectors only got the edge when the max size of a fraction was reached at 40" because at that size gravity itself would distort the lens because the glass lens would be so heavy as you can imagine.

But......not conceding a thing here ......the long focal length was for fine detail and door larger mag because back then planetary was where it was at. Bearing in mind people hadn't found Pluto at the time the heat was on for that elusive planet plus peole wanted observations of surface detail.

You don't see anyone offering fast scopes as planet killers.....there's probably a reason for that.

This got said before that it was down to the poor quality of my newt but I don't believe that because it works perfectly well on deep sky and no worse or better than any other newt I have looked through of around the same size on planets it's never really a killer, it looks fine so long as you don't compare it directly with something slower and longer and on top of that it's super fussy on focus.

If I ever got the SkyMax 180 out I could give it a direct shoot out but unfortunately every time the SKyMax gets taken out the weather turns foul, or the dew hits mega bad or of late I get flu and now a serious eye infection in my viewing eye

[Moonshane]

hey A-B - check out your signature - surely things can only get better?

I'd be interested to hear about you 180 in due course.

[A McEwan]

Good points Astro-baby! Not going to dispute them - I'm in agreement!

The 180 pro is a great planetary scope. The few times I've used mine it has really impressed me. I hope you get a chance for yours to do the same to you too.

Good luck with the eye infection. And the dew. And the flu. And the weather....

Ant

[MartinB]

I'll throw my limited experience into the pot. I have done side by side viewings using a well collimated 10" LX200R at F10 and a 12" Skyliner dob at F5. Target- Saturn Seeing 8/10 The newtonian out performed the SCT in terms of contrast and resolution. Differences were easily observed - contrast a low powers and resolution particularly when moving above x200.

I have also compared both scopes with an ED120. In all conditions I felt the dob gave superior views. Comparing the ED120 with the LX200 was a bit more complicated. In poor seeing the extra contrast means that some features are more clearly seen with the frac but the SCT is a significantly better performer when the seeing starts to steady off.

Top of the heap for me though has always been the F5 12" dob. I don't think F5 is ideal but I have been convinced about the performance of newtonians as good planetary scopes

[sixela]

I like the planetary views through my 8" f6 Dob but it is not motorised

Well, for planetary observation, motorised beats non-motorised. It is a lot easier to concentrate and to use the moments of good seeing.

I have not done a side-by-side between the 8" Dob and the 7.1" Mak but views at different times do not look too dissimilar, with (to my eyes) the edge going to the Mak because of larger image scale.

Image scale is a function of the eyepiece selection. Obviously, if you're used to a Mak and your smallest eyepiece is a 10mm eyepiece, then you're never going to fully see what an f/5 scope can show you.

[sixela]

Well gosh I'm sure then I have no idea why slow focal ratio scopes whether they be newtonians, SCT, Maks or otherwise are always considered as planet killers.

Uhm -- prejudice? Historically accurate assessments that are no longer valid?

People genuinely believed the earth was flat once.

Obviously all the professionals of the late 19th and early 20th were totally wrong to concentrate on long focus refractors,

I have a 40cm scope and the Royal Observatory has a very good 40cm achromat (roughly f/60).

People tell me it's very good (it's seen service again now that someone found why they couldn't move the floor anymore --a small sensor on a cupboard was broken and the floor will not move if the cupboard is detected as open--), but somehow I'm not going to swap it for my Dob.

I would have trouble putting the building around it in my garden, to be honest, and I don't have a moveable floor to stand on while the eyepiece moves roughly 10 meters in height.

Ah, for the record, a few people specialise in asteroids and are still observing today, but that's with a 1 meter reflector, with a much smaller f/ratio (but a long tube; IIRC, it's a "lensless Schmidt" camera).

You don't see anyone offering fast scopes as planet killers.....there's probably a reason for that.

Ah? Is there? Try going to Florida (where they can observe at more than 1000x without image breakdown) and position your long focal length 8" instrument as a planet killer against an f/3.66 22" scope. Good luck.

If you make a planet killer, you make other compromises, and it makes sense to make it as long as practical. But once the scope becomes large, long is no longer practical. You can use a folded design but often that's worse than a short Newt in some respects (the central obstruction in most folded scopes is huge even compared to quite fast Newtonians).

This got said before that it was down to the poor quality of my newt

Poor quality or poor management. Many things can trouble a Newt. Collimation, cooldown, heat plumes from your body drifting in front of the aperture, pinched optics,...

If a scope doesn't perform well, I always look for the cause (one of the reasons is that it's usually not my scope, and waving my hands in the air uttering generalities to "explain" why the scope isn't doing well doesn't help the owner on bit). Before you have actually eliminated the usual suspects, you shouldn't jump to conclusions.

To quote Mike Lockwood, "Generally the people I hear complaining or pointing out the 'negatives' are those who have not used a quality fast instrument".

it's super fussy on focus.

Fast scopes are fussy on focus, almost by definition. But with a good dual speed focuser, that's not an issue.

[ollypenrice]

There are often several scopes on the go where I live and the curious thing is that which is performing best on what is not always all that consistent. Sometimes the 20inch f4.1 performs remarkably well on Jupiter but never on Saturn.

Sometimes the TEC 140 eats all planetary comers alive but it doesn't always do so.

The 10 inch SCT is consistently in the middle!

I have no explanation to offer, I'm afraid.

Olly

[Moonshane]

Someone I was speaking with once referred to the 'seeing coming into equilibrium with the 15" scope' he was using. I was not really sure what he meant and it was said in passing but I wonder if variance in the seeing conditions means that a certain aperture and focal ratio will be the best fit for conditions at a certain point and maybe this explains eg Olly's comment?

[sixela]

There are a large number of variables. The only constant is that the less good the seeing is, the less the advantages of large aperture are and the more the other drawbacks of these scopes (thick mirrors, observer in a position that can make heat plumes interfere, narrow focus range, exit pupil that is too large for the usable magnification etc.) start to make a smaller scope sometimes pull ahead.

It's not impossible to make a large scope perform like a smaller one --I frequently observe planets on my 400mm scope at outreach parties with so-so seeing and it's rarely bested by smaller scopes-- but it takes a lot more work to get it to perform optimally. Someone with a 120mm doublet refractor can simply put down the scope and observe, and I have to make sure I'm collimated, try to find a UTA rotation that will put people in the right spot, run the fans for at least half an hour, try the apodising mask to see if it helps, etc.