180Pro vs 10" Newt

[Rob]

I have to say I've owned 2 C8's, 127 mak, 125 ETX (one of the had picked white tube jobs), 102 f10 Vixen planet killer, 6" f5 newt, 8" f6 SPX Newt, and now 8" f5 DS Newt.

The 8" f6 or f5 newt 100% collimated under great skies will win hands down. It also matters how good your EP's are!

Rob

[John]

I...The 8" f6 or f5 newt 100% collimated under great skies will win hands down. It also matters how good your EP's are!....

I agree with this.

Trouble is that the reality is that much of our viewing is done under mediocre conditions with scopes that are somewhat less that 100% collimated - so refractors and other closed designs do often seem to "punch above their weight"

I'm always surprised just how close my 4" Vixen ED can get to the performance of my larger aperture scopes under all but the very best viewing conditions.

[dweller25]

Here is how I would rate the various scopes I have owned in terms of planetary performance - the figures are the telescopes THEORETICAL relative performance compared to an unobstructed scope ....

Skywatcher 127 Mak ...................(83mm)

Takahashi TSA102 Refractor ........(102mm)

Skymax 180 Pro Mak ..................(121mm)

Takahashi FS128 refractor ..........(128mm)

Celestron C9.25 SCT ..................(149mm)

Skywatcher 8" F/6 Newtonian ......(154mm)

Skywatcher 10" F/4.8 Newtonian ..(193mm)

But in the real world this is the order I would put them in ....

Skywatcher 127 Mak

Skymax 180 Pro Mak

Takahashi TSA102 Refractor

Celestron C9.25 SCT

Takahashi FS128 refractor

Skywatcher 8" F/6 Newtonian (Flocked and with cooling fan)

Skywatcher 10" F/4.8 Newtonian (Flocked and with cooling fan)

I do find Maks and cats under perform and I believe this is more with how I thermally manage them - ie badly.

YMMV - as they say across the big pond.

[lukebl]

Interesting thread, this.

I had been slightly regretting plumping for a 250mm SW Newt instead of my original choice of the sexy black Skymax 180. Not having them to compare side-by-side I thought I might be losing out on planetary viewing. Looks like I made a good choice, after all. And, of course, the shorter FL is better for DSOs anyway.

[ollypenrice]

The views are so good through so many different kinds of scope that just getting on with is not a bad idea!!

Olly

[sixela]

I agree with this.

Trouble is that the reality is that much of our viewing is done under mediocre conditions with scopes that are somewhat less that 100% collimated

As far as clusivity is concerned, I take that as an exclusive "our" when you address me. After all, I have a reputation to uphold...

I find cooling a much more serious issue. I'd gladly sacrifice a few proverbial goats to get that fixed faster than the laws of physics allow, and I'm always really jealous of all those people with fluorite or ED doublet refractors. Until my scope is cooled down and their objectives fill with dew, of course .

The worst thing is observing a planet with good seeing and the wind from the other side (I have the focuser set so that the dominant winds are in my face when observing South), with heat plumes from your body carried over the aperture. You can fix many things, but your own body temperature is something that's hard to change (at least without undesirable side effects like death). I've been known to observe Mars in very odd positions on my scope (standing as close as possible to the back, leaning over the rocker box and mirror box, instead of at the side of the scope).

[ollypenrice]

If cooldown is your thing, see my article in Nov Astronomy Now on Ralf Ottow's watercooled Newt. This has a waterjacket under the primary through which iced water is pumped for 10 to 15 minutes before being removed. Thermal sensors report to a display panel near the EP! Does it work? Oh yes, believe me it does. The telescope is astounding. The water tank holds less than a litre and the process very painless.

Olly

[John]

As far as clusivity is concerned, I take that as an excusive "our" when you address me. After all, I have a reputation to uphold....

Apologies there Alexis - I'll include an "exludes" clause next time I use that word, just for clarity

I really must spend much more time vetting my posts before I submit them - there are just SO many little traps to fall into these days

[sixela]

Apologies there Alexis - I'll include an "exludes" clause next time I use that word, just for clarity

<tongue firmly planted in cheek>

[MartinB]

Here is how I would rate the various scopes I have owned in terms of planetary performance - the figures are the telescopes THEORETICAL relative performance compared to an unobstructed scope ....

Skywatcher 127 Mak ...................(83mm)

Takahashi TSA102 Refractor ........(102mm)

Skymax 180 Pro Mak ..................(121mm)

Takahashi FS128 refractor ..........(128mm)

Celestron C9.25 SCT ..................(149mm)

Skywatcher 8" F/6 Newtonian ......(154mm)

Skywatcher 10" F/4.8 Newtonian ..(193mm)

The percentage obstruction relates to the diameter of the scope. The light catching capacity of a scope is determined by the area of the aperture, not the diameter. A scope with a 20% central obstruction has the area of the aperture reduced by around 4%.

[rawhead]

Have any of you planetary 'bods' done a comparison with a Tal 200K?

The open newt like design means cool down isn't such an issue. (or the dew for that matter)

[sixela]

The percentage obstruction relates to the diameter of the scope. The light catching capacity of a scope is determined by the area of the aperture, not the diameter. A scope with a 20% central obstruction has the area of the aperture reduced by around 4%.

But the effect he's modelling is the reduction of the contrast transfer function at certain spatial frequencies, and for that the "worst case" model is indeed dependent on the central obstruction ratio in percentage of the diameter.

It's not light loss that's the problem. It's also redistribution of the light from the aperture that is not obscured into parts of the Airy diffraction pattern away from the central disk.

A fairly simply worded web page that touches on the subject:

Thierry Legault - What are the effects of obstruction ?

It is, indeed, a worst case view (i.e. the "equivalent aperture" derived is extremely conservative), and you should take the "equivalent unobstructed aperture X" derived as meaning "this larger aperture Y should perform no worse than an unobstructed aperture X regardless of the feature you observe, and better on some features".

[MartinB]

But the effect he's modelling is the reduction of the contrast transfer function at certain spatial frequencies, and for that the "worst case" model is indeed dependent on the central obstruction ratio in percentage of the diameter.

It's not light loss that's the problem. It's also redistribution of the light from the aperture that is not obscured into parts of the Airy diffraction pattern away from the central disk.

A fairly simply worded web page that touches on the subject:

Thierry Legault - What are the effects of obstruction ?

It is, indeed, a worst case view (i.e. the "equivalent aperture" derived is extremely conservative), and you should take the "equivalent unobstructed aperture X" derived as meaning "this larger aperture Y should perform no worse than an unobstructed aperture X regardless of the feature you observe, and better on some features".

Fair enough! I think I'll stick with the mark 1 eyeball.

[sixela]

Have any of you planetary 'bods' done a comparison with a Tal 200K?

The open newt like design means cool down isn't such an issue. (or the dew for that matter)

They're pretty good, despite the rather large central obstruction (I can remember one night where my 16" was used at 720x on Mars with incredible detail in Achillis Fons, Idaeus Fons and Nilokeras, and the Tal 200K just two metres away was delivering fine images at 450x).

Well, I wouldn't say cool down isn't such an issue on a Tal. It's just not as bad as on a normal SCT or Mak , but you can control things more easily on a regular Newt (personally, I have a front baffle just above the mirror and a fan sucking air from the bottom, and the baffle make the air flow from the face of the mirror; it's something pioneered my Mauro da Lio and it works very well).

The overarching point I'm trying to make is this: if you have enough aperture, then details about exactly how to manage the scope to make it perform optimally make more of a difference than the actual optical design.

A good read (written by someone who can't be suspected of not liking refractors) is this by Roland Christen:

What is the best planetary telescope?

You will get dozens of different opinions on what is best, but I will tell you that for any given type of scope configuration, be it Newtonian, Cassegrain or Refractor, there will be excellent, just acceptable and rather poor examples for each type. No one type can lay claim to being the absolute best.

[rawhead]

They're pretty good, despite the rather large central obstruction (I can remember one night where my 16" was used at 720x on Mars with incredible detail in Achillis Fons, Idaeus Fons and Nilokeras, and the Tal 200K just two metres away was delivering fine images at 450x).

Well, I wouldn't say cool down isn't such an issue on a Tal. It's just not as bad as on a normal SCT or Mak ,

Thanks for that

Nice to get opinions from someone who has had the two side by side...

I struggled a bit with my F4.8 10'' newt. Collimation was my bugbear Don't have to worry about that any more thank goodness.

Cheers

[A McEwan]

Fascinating. * didn't realise I was opening such a far-reaching debate. It's all very useful and informative stuff, so thank you all for the input. I guess one point is that a good scope is a good scope - period, regardless of what the target is. Another is that all scopes need some 'input' from the observer to make sure it's performin at its best. This is something I understand completely, though I hadn't considered heat plumes from the observer's body before.

Ultimately I know I haven't plumbed anywhere near the depths that my 180 Pro is capable of, but it's nice to hear that with a little pro-active care a 10" f4.8 Newt (or 12" f5) could also just as readily (not "easily"!) be considered a planet-killer, and also have the good performance necessary for good views of DSOs too.

Ant

[Astro_Baby]

Sixela my 200 is perfectly collimated ( trust me I know these things ) it's also flocked, fanned and has a Moonlite, the optics aren't pinched and it's perfectly well set up. The fact it doesn't perform leads me to believe that either the mirror is rubbish (unlikely because I have had people view planets with it who own their own newts and no one has ever said .....good god that's a terrible scope)

What I can believe though is that a large scope under uk conditions is perhaps never able to punch it's true weight whereas a refractor with rather good optics can more readily exceed it's maximum theoretical limits with ease.

Here in the uk we could never imagine for the most part hitting x1000 mag and mostly will struggle to hit x250 and even then only on rare occasions. So In theory a large fast newt might well be able to outperform but if your stuck with viewing conditions that are limiting your performance then what scope might in theory perform better largely becomes moot.

I can also well ebleiev that different scopes will perform better on different nights but my own practical experience to date with the newt, assorted maksutivs and the newt is that the maksutivs and the new MAY perform but the refractor can be relied upon to do a good job no matter what the external conditions.

Scuse worse than average typing. I am working off an iPad in the back of a car.

[Rob]

Brings me back to collimation & conditions!..

I've run so many side by side tests with Russ over the years that nothing surprises me now. We had a ED100 wipe the floor with a C8, an 80mm Frac kill off a high graded OO Newt!... even an achro kill off a Apo

Its all down to collimation & conditions which include cooling, seeing yada yada yada!!.

Best thing to do is make sure your scope is top notch on collimation and grab the evening, as most of you have more or less found. Some nights are staggering!.. others are just pants!. Nothing has changed with the kit, you collimated it an inch from its life... the skies look clear and good, but there not!. Repeat the same the night after and bingo.. back in the zone.

I find there are plus's to many scope types. I've own 2 C8's and found them very soft on focus.. on one very crisp clear night my 150P and the C8 sat side by side. We went out for the evening for dinner and returned at 11.30pm, the scopes had been sat out for over 4 hours in icy conditions so cooling was out. The 150P f5 was fantastic, the C8 (Starbright Black tube) could not touch the newt on contrast!. Saturn was staggering that night.

On another occasion I've had my current 2 scopes out 150P & 200P DS.. the seeing was 5/5, and on a normal night the 200P DS I find has been my best ever planet scope, on this night the 150P stole the crown!.

So all in all.. if you have a mak, frac or newt. I sure you will manage fantastic images of planets over various nights..

Enjoy your scope!.. thats why you bought it.

Rob