Newton and high power star quality

[Hakann]

This is my very first thread at the forum and I will ask on a question that has my attention up.
High powers and Newton star quality.

If you don't like to read it all, read this lines and the last..



As not so many years as a visual observer ( 3-4 years ) I has of course not a great experience.
But I has take the time 'in my way' ( as I stepped in from exactly zero in astronomy ) to learn the sky and find Messier objects and try to learn on what scope design ( and size ) what I prefer and what I like to observe, and magnification, FoV, pupil size and so on.

In my astronomy I only go visual deep-space ( no planets no sun observing ) so a Newton is the way and the bigger - the better.
I did not go the traditional way to build a small scope first or what is common today to get a smaller China Newt or Dob.
I can’t stand the China telescope ( now is that said and it’s my way and it’s not critic’s to others who use them )
I got a 18” mirror here in Sweden but now I has stepped up to a CZ 18” ( f/4 ) but it’s still on order.
In this jouerny I did prefere observe whit others or go to SP or rent scops at darker sites.
Sweden has very limited nights in where the standard is the way I like it.
( Here I has enjoy bino observing and I has got me a good bino equipment whit good optic’s )
I know a Chinese scope in this sky we has might be all ok whit a decent EP, vs the diffraction vs price and size.

On my travel I has been on La Palma several times and to Oregon SP and Sahara desert and Bortel 1 in site in Australia and Europe alps for mention I has been serious to see great sky’s !
But even if I had some good sky it’s in my ’limited’ time and at a very small time frame ( vs a trip )
But I learned the hard way as ex 3 years in a row in La Palma under December it’s not real easy..
It can be dark but bad seeing or windy or ’calima’ or moister at other places is common. ( No new's for you guys, but I thought the stars was always up there ...'almost'.
And it can be not as dark one like or light polluted vs nearby cities or clouds.. or the Moon.
Here in Sweden it’s cold and snowy ( can’t stand that actually )

I has been in some very nice telescop’s.
Best I seen into so far is a Lomo mirror in a 25” in Germany.
But I has been in optic’s like Zambuto’s, Lockwood’s, Mel Bartels, Swayze, Kennedy’s, Galaxy etc, etc.

In La Palma last out my focus topics was Trapezium and the Horse head.
( I has been in 18” scoops here in Sweden but newer saw it, and I has been whit experience observes here )

I had a German made 20” rented Dobson in La Palma.
Sky was dark, and average over 21.7 and I was at 2.000 meters. ( of course ..medium seeing as it is ’always’ something )
I saw 6 stars in Trapezium and the horse head but I never saw the horses ’head’.
I had my Ethos EP collection there and the Nikon HW’s and Astronomiks filters.

What bothers me is I never really get great stars if it is jacked-up.
In Sweden one old observer told me we cannot get much over 200X here in a 16-18" Newton telescope. ( ex a decent sky and at 'what they say' ; 6 waves tolerance )
But the ’China users’ whit telescoops that price is less what a Ethos EP cost say 50X/” is a not problem here (?)
-If not they say the observer is lousy or to bad sky or bad mirror. ( they mean a GSO mirror is all we need vs they are on diffraction at 4 waves )
I personally has hard to believe a 12"-16" Skywatcher Dob has 4 waves.
I told them I had a hard time jack it up at ex Oz Sky ( Zambuto/DSM & Obsession telescopes ) or at 3 years in a row at La Palma sky etc.
Guys here mean you fall on the 3 examples - that's why you can't go over 50X/” !
At Oz Sky they said ( 2017 in April ) Ok now we has a limit, but on darkness we had near SQM-21.6-21.8.

What I do see if I jack it up is stars is big as coins, and I don't like that and wonder why.

But is this what one get in a Newton normally ?

Example in the 20” at La Palma I had ex Ethos 6 mm = 338X and that is 17X/” and stars was not what I like to see.
Blurred big time and I did collimate many times under nights whit my own Glatters tools and the 2-pupil AC.

In the Lomo 25” ( f4.5 ) we used Ethos 4.7 mm on ex M57 ( 600X ) and this was mediocre sky ( 1.5 hours by car from Frankfurt ) and around SQM 21.3, so bad but not that ’bad’.
I like it dark but realise if one is over SQM 21 one must accept that.
I also know I can’t demand refractor style stars all the time.
In the 25” Lomo we had 24X/” and I really enjoyed that mirror ( it was told to be a 8 wave ) but what numbers in RMS nm figure/surface was not mentioned.
Mirror was 70 mm thick and made of Sitall.
Owner told me on my question of fans - it is a zero expansions glass.
Now I know even at ZE glass, at a thickness of 70 mm on the edge, and that means a boundary layer above the surface as day was hot and night was not cold ( but of course cooler )
I guess he used fans as when I was there some hours around midnight 2 nights in
a row, the mirror did perform great.
He did not use a coma corrector but did prefer Ethos EP even of ’more’ glass ( and even if he was all into high powers ( builded the scope for that vs a small diagonal at 3.9” )

So what was the ’key’ here to the great performance ?
Was it the mirrors tolerance vs stiffness ( 9;1 ) or stiffness in scope, or the rather 'slow' focal or the small diagonal or if we had good seeing - I don’t know.
Owner told me you should use the scope in Austria alps at 2.500 meter at 1.000X on ex M57.
But towards my Sweden buddies, even if he goes 1.000X he is only at 40X/” up at the alp's whit Lomo optic’s in Sitall, and they is in low buck EP and China scopes in mediocre Sweden sky and +50X/” is a no problem (?)

I did try to explain to them how I see it and the sky I been at and in optic’s I look into and observes that has doing this for many years.
-But they say I’m not ’experianced’ yet.
( Ok so it can be if we was talking faint galaxys, but my eyes is ok so I know if a star is blurred or not )


If they can see pinpointed stars, why can’t I ?
( realise few or non invest here in good optic’s or bigger scoops as sky is so bad )

In my astronomy now I enjoy to learn to trace the object’s and ’eye candy’, but I know I will be chasing faint stuff down the road and I’m not afraid go to Chile or wildness Australia etc later on.

In scope build's I’m into cellulars design on the mirror and very stiff scope constructions.

From Aerospace I do know how hard ( if even possible at a 500 mm optic’s ) to test 10 nm RMS etc equal to ATM’s PV wf 8 wave.
I heard all kind if high numbers from ATM, but I shut for 15 nm RMS figure/surface @ 632 nm.
In ATM that is consider rather low ;-)
-No matter of glass material, thickness ratio or focal or dimensions.

I hope you might get my question.
Blurred stars in Newton's & sky quality.
Maybe one need to has blurred stars to see the faint stuff ?

[lamperti]

If stars are as "big as coins", perhaps you may not have enough out-focus distance. An extension tube or different type of eyepiece might solve that problem.
Al

[Hakann]

Al,

I’m a rouky but as I been obseving whit others, its not that issue ;-)
As I said I been observing allot whit experianced people, and we do see pratically the same.
As on the Oz Sky example.
In Sweden sky is a limit and I has been told this is what you get when jack it up. To see more but this is the price.

[Mark McCarthy]

Hi Hakann,

My guess is you're observing the effects of seeing -- probably mostly poor atmospheric seeing but maybe partly not-yet-ambient mirrors in the larger aperture scopes.

There are a lot of opinions about the effects of seeing and how to manage them. I found this (http://www.skyandtelescope.com/astronomy-equipment/beating-the-seeing/) article a nice general discussion.

Notice the difference between the large and small apertures in poor seeing as discussed in the article -- small apertures with small stars dancing around, while larger apertures have steady but bloated stars -- that sounds like the experience you described when comparing premium optics with GSO or similar commercial optics.

Best
Mark

[Hakann]

Mark,

Thank's for the reportage.
Allots of good info there on the subject.
Bigger optic's ( China made or premium ) I has the 'blurred' star issue.
Notice I been to dark places and high places. ( So not just Sweden observing.
But what get me the worry is that the Sweden observers in China Newt's can jack it up at no issue ( over 50X / inch ) and mean I has lack of experience.

This reportage cover up the issue.

As I been around ideas of a future fast 25" I had ask MLO ( even I'm no fan of plano mirrors personally ) and he mean he can go as thin as 1.55" if quartz is used but even whit fan's and that rather 'thin' edge it can take the whole night get cooled off. ( of course it depends where one are located )
He mean the mirror cell is the key here, but I'm more into a cellular design vs way better ratio ( thickness ) and a low weight to avoid the gravity effect that effects all optic's and especially planos.
But if the mirror is not at ebviniant 0.2C, how can one has a good wievs ? ( no matter of seeing.

Zero expenation material as Zerodur, ULE or Sitall has near zero ppm/c ( vs 9 on window glass or Pyrex at 2.5 but quartz is near 0.5. But even so, it is not the cure to be cloose to zero.
But the 70 mm thick Sitall mirror Lomo made I saw at 25", was impressing - so I know a Newt can deliver.

This reportage did not cover the deal using lightweight and stiff cellular made mirrors, that really get this air turbulance above the Surface way better.

I read a article on the old idea of a 6;1 ratio 6" vs stiffness to hold the parabola to the tolerance it was grinded, so NOT sag or gravity effect it negative.
If that is converted to a 25" it would be really thick, but that don't work in reality ( vs will newer cool-off )
Example a Dream cellular’s core could be lighter but they like to has a ribbed construction of the Delta series that are stiff, so weight is around 12 Kg.
Surface and ribs are 5 mm thick.

This don't cure fast seeing, or one can't be at ex LaPalma/Roque all the time and one can't bring around a 3 level high observatory and as a visual observer one is close to the light path and the optic's.
But I truly like the idea of a cellular design, as I has a issue of blurred stars.

Again, 3-4 years and living at 60 degree might eliminate all good nights that might be at other places, and I had has pretty good luck on my trip's but maybe not bad luck on seeing !

[Howard B]

Hi Hank,

You're not seeing sharp, pinpoint stars because the atmosphere hasn't been steady enough when you've observed to allow stars to appears as sharp as they can. It's a rare thing in most places to have seeing where an 18 inch scope can show resolution to its theoretical limit. Our turbulent atmosphere is to blame. Expensive mirror substrates that are perfectly figured will often just give you the sharpest image possible of blurry stars, so you don't need to spend a fortune to get a mirror that will always give sharp star images at high power. Sitall and Zerodur are wonderful mirror substrates but their real value is that their optical figures don't change as they cool. Much the same result is achieved with a meniscus plate glass mirror. However, a telescope that always gives perfectly sharp images doesn't exist, but you can take an average scope to a place like Mauna Kea, where sub-arcsecond seeing is common and you'll be amazed at how well it suddenly works!

A quick story to illustrate this point: a friend of mine from Washington state had a 25 inch f/4 Obsession that he was seriously considering selling because it never gave sharp images and very often they were terrible. He built an observatory for it in central Washington state under a nice, dark sky to get away from the lights, pollution and rain of the Seattle area but still he had only terrible images from his scope. In 2003 he brought his 25 inch to the Oregon Star Party for the first time. It was the year of the great perihelic Mars opposition, and he wanted to get some good views of Mars through other peoples scopes because his was no good.

As it turned out, we had several nights of very stable seeing and Mars looked fantastic and to my friends utter surprise - and delight - the scope that gave the best images was his 25 inch! Yep, a supposedly terrible mirror by Nova, made from two inch thick Pyrex and sitting at the bottom of a wood box, gave the best views of any scope on the OSP field. So his "terrible" telescope was actually a real gem - he had just never used it when the seeing was steady enough to show what it could do. Aside from having good optics, seeing conditions are everything when it come to image sharpness.

The difference in the images my 28 inch makes when I use it at home (very poor seeing) and the OSP (usually pretty good, sometimes spectacular) is amazing. But steady seeing is not an every night occurrence there, because the atmosphere is always changing.

My advice is to get a well-figured mirror but don't spend a fortune on it - the Zambuto mirror you ordered will work as well as any mirror costing way more. Use it consistently and get used to what it can show over time, and take it places where the seeing is often nice and steady. Over time you'll get a feel for what you want to have in your next scope, and then if you feel it's worth it, spend some serious money to get it. The experience you will have gained in the meantime will guide your decisions much better than anything I can tell you, and you may find that you don't want to spend big money after all. But if you do, you have to bring that scope to the OSP so I can look through it!

[Hakann]

Thank’s Howard for explain a great or perfect seeing nights is rare.
That’s good to hear from someone I know tell the truth and has a ton of experiance.

The S/T reportage said in the end ;
“Mostly, though, beating the atmospheric seeing is just a matter of patience. Just keep watching, and intermittent good moments may surprise you. One reason why experienced observers see more detail on the planets than beginners do is that they simply watch longer, ignoring all but the steadiest moments.”

I guess that is true as it can be said, but even if I said in introduction here at the forum I’m 50% in technology and 50% in observing, but that 50% in observing is not chasing Hershel list etc, as I’m more into real good optic’s and scope technology. So over time I has realise I’m not ( or will be ) a hard core observer in that matter of study.
But what I like to see, I like that in great optic’s.
But as experience of life I do know experience is always a key factor, so one must observe for learn.
The more – the better.
And I’m into what you recommend, as decent optic’s and take the time before invest seriously.
As you said, it’s rare to use diffraction tolerances anyway.

But a cast cellular mirror get one a great design at a decent price.
So if a mirror like that get way faster to abvient temperature and are stiffer vs gravity, the views will be better no matter of seeing is not perfect.
Correct, or ?
And if a scope is real stiff and hold collimation and has a correct collimation and one try to has a cover around the construction to avoid human body heat from the optic’s must also be a good factor to get the best out of situation.

I has to face it.
Sweden don’t hold my standards, as most clouds and bad seeing and cold and snowy in the winter and summer is off vs the sun and a very limited night ‘maybe’ at spring or fall, but moister is there.
It’s very rare to has a air conditions that a little bigger Newton can hold up over 200X.
That get me to trips, and a trip has a timeframe.

So for me it’s more important to has good optic’s and telescope design, and no matter of ‘aperture fever’ or what is reasonable, aperture do speaks.

[Howard B]

A cast cellular mirror has some great features Hank, and light weight is probably the most important for you if you plan to transport it. If it also has a perfect optical surface (and the rest of the scope is well made) it will give you the excellent views - but most of the time it will be the sharpest possible view of stars bloated and boiling because of atmospheric turbulence. Your Zambuto mirror will do just as well, although it will be a little heavier and take a little longer to cool. But a properly designed telescope structure can help it cool off faster, especially if the mirror is well-exposed to the night air.

As the S&T quote in your message says, time spent at the eyepiece is a big difference between beginning and experienced observers. Steady seeing comes and goes even on the best nights so it takes patience to get the sharpest views. But that matters most if you're observing the planets or tight double stars, and although important for galaxies and nebulae steady seeing is not quite as critical especially when you're just getting started. Also, and I think this is the most important point of all, your observing skill makes as much difference in what you can see as does a great telescope under a perfect sky. It takes time and experience for you to learn to how to see what your telescope can show, just like it takes time and experience to learn any skill.

It seems easy though - just point your telescope at something and look in the eyepiece, what could be easier? But learning to see everything possible in objects so faint that they require an 18 inch telescope to see at all - and with just one eye - takes time to learn. Which eye is more sensitive to faint light? Where exactly in that eye is the most sensitive area? How long can you hold your eye still enough to be able to recognise what's in the eyepiece? You need to find out these things and practice them until it's second nature and you do them without thinking. When you get to that point you'll have used you first telescope enough to know exactly what you want in your next scope, and why it's important for you to have it.

Scope technology is a lot of fun, which is one of the reasons I like making telescopes, so I understand that appeal. There are so many options and possibilities to combine into an instrument that's not only uniquely yours, but also works exactly the way you want it to. And this is just a hobby after all, so if you want to jump in at the expensive end of optics and telescopes and can afford to do so, then have at it. My advice is really centered on the idea that learning to drive really well first will let you enjoy the Ferrari a lot more later.

[Hakann]

Howard !

Yes it’s going to be some years now whit the 18", but later on if I go cellular design it's really not all of the light weight for transport as instead low weight vs cool-off and the stiffness factors ( for performance )

But it was good to hear your story of seeing and blurred stars effect.

As I love bino use, and you said the one eye in a Newton it take years to learn to be a good observer.
But that's also a fun part of a hobby - the road.

Now I know why I had a issue on the star quality.
-I simply had not yet that perfect sky, dark - yes, but seeing has affected me.