8 times better than a Ritchey-Chretien ?

[alpal]

Hi Guys,
Hypergraph's are excellent telescopes but
have a look at this link:

http://www.astrooptik.com/Komplettgeraete/hypergraph_frame_e.htm




Could this be business puffery or true?

[Shiraz]

it could be true but meaningless. The spot size (with diffraction) only has to be less than about half the atmospheric blur disk to get close to the maximum possible resolution - after that it becomes immaterial how much better it is. In Australian conditions, better than about 1 arc sec is probably good enough in most locations, so anything from a reasonably well made 150mm scope on upwards will be as good as it gets.

Looks like this is a hyperbolic system with a Ross or Rosin 2 element corrector. It will be pretty good, but the extravagant claims really are hyperbole.
http://www.telescope-optics.net/sub_aperture_corrector.htm is worth a read for the general idea.

[el_draco]

When a supplier wont list prices, there's usually a good reason; greed, guilt etc. Maybe I'm expecting to much but if I don't see a price I look else where.

[renormalised]

Those quoted spot sizes and such are usually from lab tests. Real life performance will vary and sometimes substantially. As Ray said, it might be true, but then again......

[alpal]

Thanks guys for your replies.

Have a look at the pics on this page:
many of them are taken with a Hypergraph 16” f8
from the Tivoli farm - high altitude dark site in Namibia Africa.

http://astrofotografia.com.pl/galeria.htm

They are incredible pictures.

However:

The only better pics I can find are from this site from Chile:

http://www.chart32.de/index.php/nebulae-n

32” Astro Optik Philipp Keller 80cm robotic classical Cassegrain with 3-Lens corrector.


Notice that it’s a much larger telescope so you’d expect better but it’s a Cassegrain

so how come it’s better?
According to the Hypergraph site it should only be a 10th as good.

[Bassnut]

It's funny, I've often wondered about these spot diagrams. Manufacturers of CDK, cassegrain,RC,and "corrected" anything all quote spot diagrams better than the others. I was swayed by "all serious OBS use RC", what do I know :P sounds good to me :shrug:. It's all marketing BS IMO, horses for courses.

[alpal]

I would have thought that the RCOS ion milled telescopes were the best
but as you can see - at the Chilean advanced Robotic Telescope
they have chosen a Cassegrain design - & wow - look at the results!

[Bassnut]

Well, how wrong could I be, I got done in right there. The Chilean advanced Robotic Telescope?.... what a name, cant beat that :eyepop:. Dunno about the images on the astrooptik gear site though, dont look that flash at all :shrug:.

RCOS is dead now anyway, must be crap.

[Geoff45]

Any spot size that is smaller than the diffraction disc is meaningless, even with perfect seeing, since it is using ray optics and ignoring the wave nature of light. I don't have the actual data, but reporting a spot size 20x smaller than some quality telescope is probably reporting a spot size smaller than the diffraction disc, so in real life the pic you get from it would be limited by the wave nature of light in perfect seeing (never going to happen) or, more realistically, by the seeing conditions.
Geoff

[alpal]

Still - the debate gets complicated when manufacturers make such claims.

Did you check out the Hypergraph 16” f8 pics?
It's hard to find better.

[RickS]

Spot diagrams don't tell you a lot. I want to see ray-fan plots!

[alpal]

We can only compare the results that manufacturers claim.

Still - I wonder if RC designs have been superseded?

No matter what system you buy it always needs a corrector
which is just extra glass that will introduce it's own aberrations -
especially halos on bright stars.

[Bassnut]

I sort of agree RC designs have been "superseded" in that similar performance can be obtained on "corrected" designs for way less money eg CDK.

But "No matter what system you buy it always needs a corrector" is not quite correct really. My RC set up has no glass at all, mirrors only. A flattener (glass) is recommended for larger sensors on my RC, but I get away without one. Assuming your correct that glass introduces aberrations, then thats one problem I dont have, so that must be an advantage on an RC not available on other designs I guess.

I dont know if not having glass makes any other difference. I focus every 2 weeks or so, does glass itself cause focus change with temperature?.

[alpal]

I wonder if a flattener would give you even sharper images
especially off axis?

Focus every 2 weeks?
I thought some people were using FocusMax & changing focus with
every filter & sometimes between frames?
Those electric focusers can have steps smaller than 5 microns.

I don't know about focus with a flattener & other glass versus temperatures.

[Bassnut]

Yes, a flattener would give better images but it's VERY expensive.

Focusing every 2 weeks is just lazy, but it doesn't make much difference, and with megadata, decon/sharpening makes small changes irrelavent anyway.

RCOS RCs are over engineered and expensive, that's why they went broke (I got mine S/H). Holding focus that well is uneccessary and not required for the amature market.

[gregbradley]

M CDK17 tends to hold focus for some time. It does change focus somewhat with significantly different ambient temps but otherwise in focus one night is likely to be in focus the next more often than not.

The joys of carbon fibre.

Greg.

[alpal]

I suppose too that at long f ratios focus is less of a problem.
I believe that bad seeing actually takes the target in & out of focus anyway.

Pity that RCOS went down - some telescope versions had ion milled mirrors.
I suppose you'd need sub arc second seeing for it to make any difference?

[alpal]

I use Maxim DL to check the last frame for FWHM while I'm waiting for the next frame.
I can then find out if the seeing & or focus is getting better or worse.
My limitation is the atrocious Melbourne weather -
it's been nearly 6 months without setting up my system.
I can't be bothered with even occasional clouds -
I want an all clear night forecast before setting up.


Still - this topic of RC telescopes being outdone by other designs is interesting.
Is it business puffery or not?

[gregbradley]

Hard to tell if true or not. I believe he would be achieving better spot sizes. Phillip Keller is very good at optics.

There is an astrophotographer up at Cairns who got one of his large scopes.

Greg.

[Bassnut]

I think RC is a bit of puffery these days, was the best in days gone by, but corrected designs are so good these days it's not worth the extra money IMO. But, There's always the placebo effect and "bugger the money I want the best" there's ALWAYS a market for that. Look at Marcus's Stella officiana RC, it's just amazing and utterly drool worthy, that can seriously make the ownership/imagining experience well worth the extra bucks. This is a hobby, how you feel about it alone can make ALL the difference.

[alpal]

Yes & with the usual ordinary seeing conditions I am happy
with just using a Newt. and a 3 element coma corrector.
RCs & other high tech designs can only take advantage of top seeing in my humble opinion.

[alpal]

Really?

Do you have a link to his images?

[Bassnut]

Of course, seeing is the killer, a rational decision to be sure.

Tell you what though, having a Ferrari parked in the garage, and cruising down to the shops for milk in it is pretty cool :P.

Then jus sometimes, you get to take it on a country trip and wet yourself.

Those rare still skies even urban bound, are a treat.

[Satchmo]

It is unlikely an RC with corrector gives you any tighter images than your corrected Newt.

[alpal]

Yes - seeing is everything.
When I first got my Newt. 4 or 5 years ago - I looked at Jupiter
many times & all I saw was a fuzzy ball with a few lines across it.
I thought something was wrong with it till I took
it to Mt Baw Baw at nearly 5,000 feet altitude & behold -
Jupiter snapped into view sharp & crystal clear at over 300 magnification.

[alpal]

Yes - then we are in agreement.
Look at the pics Mike gets with his AG12 Newt. - just amazing.

[Bassnut]

As I suspected, I was being flippant, makes total sense, even though I dont know much about this stuff really.

I'm hosted in very dark skies now, so I just live in a ego tripping world of hope and expectations with what I have :lol::lol::thumbsup:

[alpal]

Still nice to have a telescope as good as yours.

[gregbradley]

Its also unlikely you can do long exposures on a corrected Newt with a 16803 camera without severe flexure. Newts are great but having a focuser sticking out the side of the tube is an engineering nightmare. Perhaps one Isaac did not forsee!

Greg.

[alpal]

True -
many people have trouble mounting a tank like that on any telescope.

[Bassnut]

I must say, the RCOS is built like a tank and weighs like one too. You get of various length spacers with it that are engineered so close that they are next to impossible to unscrew when done up.But they are stupidly stable and anything hangs off the end with no flex. I've seen many neut setups with side mounted cams that just swung in the breeze. A notorious video of a 16803 mounted to an ASA neut showed many mm of flex just leaning on it a bit, what a disaster. 14 rolls of seletape around the whole shebang was required just to make it usable. That side mounting stuff must be a nightmare to manage.

[alpal]

Yes - you get what you pay for.
An RCOS can handle a big camera with ease.

A QHY9 is more than enough to hang on my Newt.

[Satchmo]

Orion UK seem to have the stability of the camera /focusser controlled on the Newts with the use of bulkheads either side of the focusser - see Mikes scope for example.
Avoid their CDK's though - there are examples that have serious issues.

[Peter Ward]

Well, yes and no...on axis it's a piece of cake.....but...the edges of his 16803 fields were proving troublesome.

[strongmanmike]

You are kind of right Greg. With the big heavy PL16803 + Atlas focuser and CFW-5-7, on the AG12 I had to limit subs to 5min and even then I could see diff flex more often than I would like but much of this problem was really due to the size of my guide scope (ED80) and how I had it mounted to the system (ie rather poorly :doh:) ...now that I have an OAG as part of the SX gear (all be it a much lighter system at 1/3 the weight) I can really see that when I eventually return to using the 16803 on the AG12 again I will be fitting a MMOAG in the image train..I think it, along with the camera and filterwheel, can just fit within the 79mm back focus.

Mike

[dvj]

I was able to use a separate guide scope on my ASA N12 for 30 minute subs - no problem. That was with the STL11000 hanging off the side. the larger problem was enabling good collimation.

[strongmanmike]

You may have had your OTA and guide scope attached more optimally than I John? For the very best results, I also think an image plane tilt unit is something that needs to be incorporated into these fast scopes and big chips.

Mike

[strongmanmike]

In the end there are a lot of people who search high and wide or wait yeeears and pay big dollars for that supposed perfect system (in their eyes) and along the way worry about every little nuance (perceived or real) but sadly even once they have found that seemingly holy grail get little or have nothing to show in the way of amazing images...in the end, is it really worth all the procrastination...an odd star shape at the edge of your field will not leap out and stab a dagger in your heart :lol:

Mike

[gregbradley]

Very true. All scopes except Astrophysics :rofl:ones have shortcomings of one type or another.

The fun is getting out and producing some images you are happy with.

The good news is the cost of quality scopes seems to have dropped a lot over the last 5 years. Perhaps this is why RCOS had trouble. High end high priced gear in a falling price market place with viable cheaper alternatives with too similar performance.

Greg.

[alpal]

Not only that - whatever you buy there will always be something bigger & better.

[Shiraz]

Mike, Greg, good posts. Agree, the fun really is in getting images you are happy with and modern low cost gear is able to do that for most people.

Agree also on the obsession with star shapes - it is cosmetically nice to have consistent stars across an image and something to be aimed at, but if the object of interest is a small galaxy in the centre of the field, it probably does not really matter a whole lot if the stars in the top left corner have 15% elongation. Stars should not be round anyway - they should be points. We are used to seeing round stars because that is what diffraction and seeing produces - but round stars are really only an artefact that shows how imperfect the "perfect" telescopes are - even the best of them cannot resolve stars as the points they really are. So the obsession with round star shapes is really a desire to produce stars that are equally wrong over the whole field.

[alpal]

Except that stars are a wave function so they show diffraction rings when out of focus.

[Shiraz]

Stars are not wavefunctions - they are just so far away that they are point sources with no effective angular extent.

Diffraction rings come about when the scope is in focus, but cannot turn a plane wavefront into a true point in the focal plane and you end up with an Airy pattern. that is the closest a scope can get to representing the point nature of far off stars and it is just an approximation to what is really there. We all seem to accept that it is natural for stars to be round blobs of varying sizes, even though that situation is actually a flawed representation determined by the shortcomings of our imaging systems and conditions.

[alpal]

I would say that star light is very much a wave function.
The star light is creating Fourier components.
This thread is all about spot sizes & a manufacturer claiming
that their spot size is 8 times smaller than an RC.
We can only ever represent a star as a spot of a certain size.
Given "perfect" seeing we would all like that.

[strongmanmike]

Weeelll a-c-t-u-a-l-l-y some have had to shim their AP focusers aaaand Planewave too, if I am not mistaken :whistle: in order to get good orthogonality sooooo if I could do that with the AG12 (which I haven't) I would have better field flatness across my 16803 field too ;)



Ah huh!! EXACTLLY and this is the key :thumbsup: besides, hows 2 finalist images in APOTY, two overall wins at SPSP and two features in IIS calendars sound for a not peeeerfectly flat field? :lol: :thumbsup: not that that is important at all but mega data and perfect flat fields at all costs can consume some when in the end it is completed and nicely processed images that count... regardless of spot sizes :)

[alpal]

Come on Mike - I know you want a Hypergraph.

[avandonk]

What is this obsession with tight stars? Like ray tracing they are the easiest things to record.

It is the Modulation Transfer Function of any optic that gives a complete assessment of the quality of the optic.

This takes into account all the variables and gives a mathematical measure of the performance of the optic.

In practice seeing, light pollution, weather, flexure and a multitude of other annoyances limit any system from its ideal.

Diffraction is the wall that all systems cannot pass through.

It would be quite informative if manufacturers actually published the MTF of the optic they are selling.

This is what Canon does.

http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_300mm_f_2_8l_is_ii_usm

Bert

[Shiraz]

Don't think it's going to happen Bert. Can you imagine for example the maker of a Riccardi Honders scope publishing data that shows that it's MTF performance is roughly equivalent to that of a similar sized unobstructed scope with about 1/2 a wave of SA? That would imply that the RH is no good (which it isn't), but how would you explain why it is OK - it would be a marketing nightmare..

Agree though that MTF would make a lot more sense than the geometrical spot diagrams that seem to be carelessly misused by some makers, as shown in the original post.

[avandonk]

Yep you are dead correct. In no way would an F3 optic compete with an F10 optic as far as MTF is concerned. The fact that the F10 optic will NEVER record what the F3 is capable of is a moot point.

Bert

[alpal]

Modulation Transfer Function or MTF is difficult to understand:

http://www.telescope-optics.net/mtf.htm

That may be why spot sizes & Strehl ratios are more often used.
Any telescope with an obstruction is going to worsen the MTF
due to the reduction in contrast.
The trouble is that if you want aperture i.e.
something bigger than a normal refractor -
then you end up with an obstruction.

Still - the pics are better once processed when using a large non -refractive telescope.

To me spot size does matter -
pics done with high end RC telescopes & the Hypergraph etc
mentioned at the star of this thread do look a lot better than
ones taken with SCTs & Newts.
The caveat is that the pictures I linked to were all taken at high altitude dark sites
& maybe that's where the real improvement was.
Seeing conditions are the greatest limitation.

[Satchmo]

Corrected Newts are capable of just as small spot sizes as any other design - the only reason high end RC look so good is that the owners usually have the money to buy an approprate size mount - and not be putting a 10" or 12" Newt on an HEQ6 mount for example.

If you look at the work of David fitz-Henry and Mike Sidoneo on this forum - both using well mounted tubes I would challenge you to find any RC shots showing better resolution .RC's tend to work at F9 or F8 which will exploit the best seeing if matched to the right sized pixels , but the spot size due to diffraction is actually larger than a typical Newt. There is always a trade off Newts will take in fields that an RC simply cannot , but to say that RC's are `sharper ' I'm afraid is nonsense.

[strongmanmike]

BINGO! :thumbsup:

[alpal]

Mark,


Do you have any evidence for that?

[alpal]

Have a look at this link:
http://www.nfilipovic.com/astrophotography/tal200k-ota-review

The graph shows 5 different optical systems.
The 5th row down of the optical results is for:



You can clearly see that the spot size for a Newt. is much greater than an RC.
(The SCT is shocking.)
I wish I could find a higher resolution image of this graph of optical results.

[RickS]

That's not a corrected Newt...

[Shiraz]

or a corrected SCT.... and the quoted data shows how good a couple of the scopes are if you ignore the curved focal plane!!. This is a fine example of why spot diagrams are such a dangerous indicator of performance - they are too easy to mis-read.

If you really want to see how good Newts and SCTs are Allan, have a look at what the planetary imagers do - Newts and SCTs are the main two types of scope used and both can get down to the diffraction limit in good seeing. For DSO imaging though, the atmosphere is so messy that it really doesn't matter how good the scope is (within reason) - the atmosphere will determine what you get, not the scope. Arguing over spot sizes completely misses the elephant in the room - the seeing.

[alpal]

Read it more closely.

[Satchmo]

The spot diagrams are still for a Newtonian without a coma corrector/ field flattener .

For example, Corrected DK's ( CDK's ) such as by Planewave are simple Dall- Kirkham Cass design with a corrector . Without the corrector they have the worst coma of any cassegrain design. With corrector they are beyond reproach .Comparing designs for imaging by looking at mirror systems without their correctors is just a waste of time. Even RC's require field flatteners which can be two lens involved.

The word 'astrogragh' implies a unified system designed to image the sky - lets stick to that definition.

[RickS]

Allan: if you want to look at the strengths and weaknesses of various astrograph designs then this is a worthwhile read: http://willbell.com/TM/TelescopesEyepiecesAstrographs.html

[gregbradley]

If you really want to see how good Newts and SCTs are Allan, have a look at what the planetary imagers do - Newts and SCTs are the main two types of scope used and both can get down to the diffraction limit in good seeing. For DSO imaging though, the atmosphere is so messy that it really doesn't matter how good the scope is (within reason) - the atmosphere will determine what you get, not the scope. Arguing over spot sizes completely misses the elephant in the room - the seeing.[/QUOTE]

I have wondered about this diffraction limited theory and limited by the atmosphere. It does not match my experience unless most scopes do not come close to being limited by the seeing.

Use an Astrophysics APO refractor or a top Tak or other tp brand APO and you will see much greater sharpness than say a lesser figured but still close to 1/4 wave ED80 or similar.

Longer focal lengths hit the wall of seeing much earlier than shorter focal length scopes down to FSQ type scopes not really seeing affected at all.

Rick from Planewave was telling me that they can get a mirror and measure it on a table and get 1/4 wave. But when the mirror is installed in a mirror cell that 1/4 wave can disappear and so a lot of attention was placed on the mirror cell. So these mirrors that are tested are no doubt tested on a bench before installation. If they were tested after installation they may reveal much lower ratings.

Nevertheless optics like Orion Optics UK mirrors which offer a 1/1th wave upgrade would on the surface seem a waste of money as 1/4 wave is considered diffraction limited. But as you can plainly see from Mike's and John's images the 1/10th wave mirror definitely adds to sharpness. Same with Rolf's Newt and David Fitz's mirror. They are really performing.

So perhaps getting the highest rated mirror/lens is the way to go to allow for the anomalies introduced by mounting the lens/mirror.

Marj Christensen posted once she has seen Roland rub a lens with his finger to get that last tiny bit of imperfection out. We must be talking some almost unmeasurable imperfection that would respond to that.
But you can definitely see it.

Perhaps its because APOs generally are short to modest focal length scopes and so aren't always hitting the limit of the seeing so much as the longer focal length scopes of about 1500mm plus.

This seeing limited argument does not bar getting the highest optics you can possibly get in my opinion.

Greg.

[Satchmo]

Its a classic example of reviewers/companies abusing peoples lack of knowledge about spot diagrams or optical designs to further their own causes.

If you would like to see what a large Newt can do with a 3" Phillip Keller Corrector on a Newt ( seeing and all other issues aside ) have a look at these spot diagrams of a 400mm F3.5 with 3" corrector to the edge of a 45mm diameter circle.

The square box is 20 micron in diameter . As these spots are just larger than the airy disc at that F ratio I think it is valid to look at spot diagrams, but seeing will of course bloat the discs larger .

[Shiraz]

Agree. Top of the head, I think that a 2 arc sec atmosphere blur circle will be about as big as the full 20 micron squares in the diagram - this system will be seeing limited across the field and as good as it is possible to get for seeing down to below 1 arcsec. That is, if I have interpreted the spot diagram correctly.

[Shiraz]

With your indulgence Allan

the angular size of the resolution spot from a scope will decrease with increasing aperture, but the seeing spot remains the same size. Small scopes have spot sizes roughly similar in size to the seeing spot, so the scope quality does have a major impact - the scope imperfections are not swamped by the seeing and a better scope is obvious. This also means that small short scopes are not affected so much by seeing because the spot size (or resolution) is mainly determined by the scope and not the seeing in many conditions. Sampling also comes into it when imaging, but that is another issue.

Above about 6-8 inches in Australian conditions, the seeing dominates and the scope quality becomes less important for DSO imaging - many of the best big scopes have huge secondary obstructions that really spread out their diffraction spots, but they still work perfectly because the resulting spot, while large compared to that of unobstructed optics, is still smaller than the seeing. Some people buy high spec mirrors and then put in an MPCC to fix the coma, which probably turns their 1/10 wave Newtonian into worse than a 1/2 wave system - others use focal reducers or field flatteners that do who-knows-what to the spot diagrams of their high end scopes - but none of this matters because the resulting scope is still better than the seeing. By all means get high quality optics - you will not go wrong that way. But for DSO imaging with apertures above about 6-8 inches, it probably does not matter much. What matters more is the mechanical quality of a good scope (eg the mirror cell/focuser/thermal) and that can have a profound effect on performance - nothing is worse than a scope that drifts out of focus and alignment with temperature and attitude - on a second rate mount.

The best illustration I know of showing how fundamental seeing can be is the before and after images of the 1987 SN taken by the AAT. Top quality big scopes do not work better than smaller scopes in poor to average seeing - they just work better in very good seeing.

http://www.astronnewsroom.com/2012/02/25th-anniversary-of-sn1987a/aat-50-the-field-of-supernova-1987a-before-and-after-march-1/

[Peter Ward]

I beg to differ.

Hubble pre and post servicing is a good example.

Throwing some light into the first or second diffraction rings I find to be rather moot.

"Oh dear my MTF is less than perfect APO refractor of the same aperture!"

All I can say is: lots of luck with your 12-14-16 inch refractor :rolleyes:

Fact of the matter is: a high quality optic will give you sublime focus, and with some exotic designs this will extend across an entire field (eg KAF 16803).

This is not the same is scattering incoming starlight all over the aperture and/or simply not being able to focus...which is what poor optics do.

[Shiraz]

Hubble is a bad example - no atmosphere.

I agree with you - as I said, central obstruction is not a problem in bigger scopes.

as long as the optics is good enough to outperform the atmosphere, the result will be as good as you can get. As Mark's post showed, a Newtonian with a corrector can do as well as anything.

[Peter Ward]

I guess you missed the point..... an atmosphere would only confuse the issue.

The RC design, polish, baffling secondary size etc. in the Hubble remained identical with pre and post servicing.

Problem was it had a 1/2 wave of spherical error....and images looked like crap. Even in a vacuum.

Co-star fixed the error. Images became world famous.

Now add seeing to the dud mirror....this is your (bad) baseline. Capish? :)

[Satchmo]

To be fair the error on Hubble mirror was 1/2 wave RMS - much greater than you will find on a lower end typical consumer scope .

There has been a thread on this before - but planetary imagers need focal lengths of 5 to 15 meters to literally oversample the airy pattern with pixels to exploit every bit of quality in their optics and detail possible in the image. Its all to do with pixel scale versus airy disc size vs. seeing . I think there would be few deep sky imaging rigs exploiting optical quality fully , but I do feel that the better the optics , the less havoc tilt and defocus of the wavefront due to seeing , will have on the FWHM. There is no doubt that the guys with known good optics that are well mounted and guided are coming up with the premiun images.

[Peter Ward]

Hummm.... I did have a gander through a dark blue tubed OTA...made in southern California at the time (they've moved further south since then)..... some years ago.... Couldn't see any bands on Jupiter.

"Jupiter has bands!!!???" protested the owner.

Reminded me of "Young Frankenstein" ...Marty Feldman saying " I have a hump!!???" :)

[Peter Ward]

Took this with an AP RHA with a 16803 sensor an average night...OK with an AO-X as well....

No decon. A single 20 minute H-alpha exposure. Flat/Dark calibrated only (hence the noise/gamma rays)

Here is the link (http://www.atscope.com.au/BRO/gallery279.html)

(sorry to keep it web friendly, 66% res)

Fish bums and star tightness come to mind at the edges... :)

[Shiraz]

That is undoubtedly true.

But ..when a system is finally working well enough that it outperforms the seeing, you have reached an absolute limit. It doesn't matter how much better the scope is than that minimum requirement - once it has enough resolution to be seeing limited, physics steps in and that's the best quality data you will get from it or from any scope in the conditions, regardless of pedigree.

So, getting back to the original post, having spot sizes that are 1/8 those of an RC is not likely to make the slightest difference to image quality, since a well made RC will most likely be big enough and of sufficient resolution to be seeing limited over a reasonable field - and that will be as good as it gets.

[alpal]

Stop making me so jealous. :)

[alpal]

Good post,
the best seeing I've had so far in Melbourne is a FWHM of 2.6 arc seconds.
I am using a Newt. + corrector & it doesn't seem worth upgrading
if that is the best seeing I can get.

[Peter Ward]

During nights of "fuzz-ball" seeing, non-deformable (read affordable ) AO optics really don't help much.....but during "slow seeing" tip-tilt AO systems can and do make significant improvements to FWHM's.

[Bassnut]

So what does that prove??, seeing doesnt matter that much??. You took this single non-deconed sub in Sydney skies right?. So, seeing can be beat with good gear/set up and AO. Thank you, so seeing isnt the limiting factor, obviously :thumbsup:

[Peter Ward]

Well...

I look at it this way....if an optical system is, to coin an old chestnut, is barely "diffraction limited" then *even the slightest* seeing distortion, by definition makes the images it produces, poorer than that limit.

If on the other hand, the optical system is better than "diffraction limited"
(yes... this is possible) it can suck up, for example, a 1/6th wave of atmospheric distortion before *total aberrations* take it into the 1/4 wave "diffraction limit"

Hope that makes sense :thumbsup:

[gregbradley]

I agree with Peter. He has expressed better than me but he certainly is pointing out the phenomena I have observed which was my point.

His RHA image says it all. You can't get those tiny stars with other gear. The other scope I have seen tiny stars like that is the infamous ASA 12. When it works its a tight machine.

There is apart from seeing and distortions the factor of scatter.

If your argument were true then there would be no point in getting those Ion Milled RCOS's that essentially had perfect figure. Yet have a look at those Rob Gendler images taken on a 14.5 inch RCOS with Ion Milled optics in WA on his astropic site.

They have very tight stars, show detail in commonly imaged southern objects you just don't see on images from this site except Martin's from Sierra Nevada.

So there is obviously another factor or factors that come into play than simply seeing limited, 1/4 wave limited etc. Scatter is one, sharp focus as Peter points out is another. Having had many scopes some definitely snap to focus better than others. Some have sharp focus all across the field and others super sharp in the centre and not so much in the corners etc. Addded to that would be deformations in the performance of the mirror/lens with different temperatures (Paul Haese and Bird can tell you about the importance of cooled optics) as well as deformations caused by stress from a lack of proper mirror support in the scope (as Rick Hedrick from Planewave was telling me, your 1/10th wave mirror once mounted may be way less than you think). So mechanical issues as above become very important factors in getting that sublime optical system which is quite a rare instrument. I guess that is why the high end stuff often seems overengineered like Tak scopes, AP scopes, etc.

Greg.

[Satchmo]

I've seen wavefront maps of these large RC surfaces before and after ion milling . The ion milling is more about honing down assymetries and non - rotationally symetrical defects in large surfaces to get them better than 1/4 wave and rotationally symmetric . Such localised polishing by hand would be very time consuming .A 32" F3 Hyperpola for an RC has a wavefront departure of over 120 waves on the wavfront from the base sphere. It is very hard to polish out this amount of glass without non rotationally symetric defects coming in .
A test of the combined wavefront with the highly aspheric secondary ( which also will have non- rotationally symmetric errors gives the operator of an ion milling machine a chance to smooth the wavfront better than 1/4 wave on these large surfaces , by milling down assymetries.

Robert Gendlers photos look good , not because the optics are 1/20 wave but becasue the combine wavefront error of the optical train could definately be better than 1/4 wave in terms of random - non symmetric errors.

[Satchmo]

Peter ,

I doubt that there is a single deep sky imager on this forum who is sampling the airy pattern with enough pixels or has near diffraction limited seeing with a mid size instrument to even put that theory to the test .

I don't think it is sufficient to point to experienced amateurs with high quality gear and say the differences are down to their having 1/10 wave rather than 1/4 wave optics , or that this extra quality is `money in the bank ' when it comes to all the different quality scopes trying to image a star that is say a real 1 arc second fuzzball in the sky .

Its worth noting that generally large observatory mirrors are specified by how much light they are expected to focus into X sub arc second spot, not by Strehl ratio.

The situation is made that more complex that smaller scale errors like 1/6 wave may be acroos 6 or 8" apertures but much greater over largere ones taking into account the tilt components between the cells .

My gut feeling is that fast optics will bloat more easily via the defocus component of the seeing error, but fast systems are the most likely to be well undersampled compared to the potential resolution of the telescope

The type of seeing is all important to . I'm sure there are many scientific papers around on the topic which are probably beyond our grasp .

There is some Hartmenn testing software around by SBig ( probably free if you ask nicely ) - it would be really interesting to see someone do some systematic measures at the beginning of their imaging session to see how often the real errors in their optical train are visible and compare to the FWHM they get with the actual images. I suspect most of the time you would be getting snapshots of the seeing .

Any takers? The question of `how good is good enough ' when it comes to amateur gear is one really worth looking into.

[alpal]

I thought that ion milling obtained 1/100th wave performance?

http://www.rcopticalsystems.com/telescopes/ion_milling.html

[Peter Ward]

Prior to getting the Honders, I had cursory look at other contenders like the ASA, Orion etc. that use Wynne correctors on their Newtonians. Yet it looked to me that the design still had residual field curvature, astigmatism, coma and chromatism.

The AP Riccardi-Honders, however has effectively perfect colour correction from 400 to 1000 nanometers, is fully corrected for spherical, coma, astigmatism, field curvature and distortion, as well as longitudinal and lateral chromatic aberration.

So I guess I'm one of those deluded nutters who waited years and parted with some serious bucks...:lol:

[Satchmo]

Thats nice advertising about what Ion milling is capable of in a research situation .

Have a look at the link on that page showing final ion milled results of an RC 24" - P-V is 1/10 wave - this would probably have been around 1/4 wave if only rotational symmetric polishing had been carried out ( easy enough to do with pitch polishing )

[Satchmo]

Nice one :)

Playing the devils advocate are there any comparison shots available with the competing gear that show spending all that extra money made a difference to the final quality of your image? Just asking :)

[alpal]

Actually here:
http://www.rcopticalsystems.com/telescopes/images/24RC_Interferogram_JN07.pdf

it says:
RMS = 0.024 ( = 1/42 wave )
Peak to valley = 0.1 wave

Remembering that the reflected wave is only half that value.

Still - I think it's a pity that RCOS went out of business.
Maybe ion milled optic telescopes will become collectors items?

[Satchmo]

Al - I suspected when you were throwing the figure of 1/20 and 1/100 wave around that you weren't talking RMS

I'm not sure what you meant about reflected beam being half that .
Those figures would be final focus wavefront error.

There is nothing particularly unusual about a 1/10 PV - 1/40 RMS surface for what I would consider a diffraction limited optic , but on a large RC with highly aspheric surfaces thats a very good result. I think Star Instruments surfaces were typically 1/4 PV wavfront .

[alpal]

It's not clear that they mean -

"final focus wave front error"

If you have a 1/20 wave mirror then it reflects at 1/10 wave accuracy.
If it then hits a 1/20th wave secondary then the final wave front is 1/5th accuracy.

[Satchmo]

An interferogram analysis of the quality of an imaging optical system is always supposed to be presented as wavefront error unless otherwise specified . The setting in the analysis software of the 'waves per fringe spacing' determines that the results are correctly presented and quantified .

In the case of this RC opticsal auto-collimation tested system the spacing would have been 0.5 waves per fringe. If you ordered a flat you would expect the results to be expressed as a surface error. It is possible for an optician to fiddle this aspect by setting a less sensitive wave per fringe factor than is technically correct for the testing set up , thus making the optics appear to be better than it really is . It happens.

[clive milne]

Peter,
it would be relatively easy to demonstrate the currency of your argument (that the AP RH) produces an intrinsically sharper image at focus compared to a Newtonian or SC, by providing a planetary image taken with it so we can do a real world comparison.
-c

[avandonk]

Completely different scenario. Planetary imagers are just recording the tiny peaks of the Airy Disc. They are recording quite bright images that can be temporally separated.

At f40 the Airy disc is about 50 micron. Please explain!

Bert

[Shiraz]

Hi Bert

at f25 the FWHM of the Airy pattern is 14microns - this sort of focal number is used with 5.6 micron pixels for planetary imaging

at f19 the FWHM is 10.7 microns, used with 3.75 micron pixels

basically, planetary imaging uses ~Nyquist sampling on the optics PSF, not the atmospheric PSF as used for DSO - that is the main difference.

Regards Ray

[Shiraz]

hi again Greg.

Tight stars do not of themselves indicate high resolution. Sure the optics must be good enough for seeing limited performance, but the star tightness is then mainly a function of the sampling. I think that Peter gets about 1.63 arcsecond pixels when he uses a 16803 with the short focal length RH, which is undersampling in most conditions. A star that would cover say 4 pixels in Peter's system (at 1.63 arcsec) will cover about 25 pixels in your CDK/16803 system (sampled at your system scale of about 0.64 arc sec). With the 694 on your CDK, the same star will cover about 100 pixels - so Peter's stars will look tiny compared to yours, as they should - not because of any optics magic, but because of sampling.

One can certainly get tighter looking stars (and a huge increase in sensitivity) by undersampling, but at the cost of not recording the full detail in extended objects. Undersampling is certainly a valid strategy though if you want to image large faint targets - eg as Bert does.

agree that mechanical quality appears to be a common characteristic of larger high end astrographs - and I would guess that it is very often the difference between an optical system being seeing limited or being a poor performer.

Regards Ray

[gregbradley]

I see. I can see the sampling being part of that. But if I imaged that same scene with my high quality APOs of around the same focal length I don't think I would get stars that small. So the accuracy of the optics and that optical system is having some effect there.

Greg.

[Peter Ward]

You conveniently ignore the physical size of the airy disk with this approach.

Airy disk size (assuming perfect optics) is purely a function of F-ratio.

At F8 the light is being spread over 10 microns. At F3.8 this contracts to 4.7 microns.

Not withstanding sampling, assuming the same aperture, you are concentrating the same flux into fewer pixels....

[clive milne]

I think it also worth adding to the discussion that field correction is a distinct issue separate from optical quality... if the component added to the image blur circle size by virtue of optical quality is below the sampling limit (on axis) then it (optical quality defects) will swell the image blur circle more or less the same at the edge of field.... the caveat to this is the usually unstated assumption that the location of the optical defect is coincident with the field stop. Unless you are talking about full aperture correctors located at the centre of curvature of the primary (lichtenecker, baker schmidt etc) then this is pretty much a given.

on somewhat of a tangent... but not totally unrelated... if you put the aperture stop at the centre of curvature of the primary mirror (ie, at twice the focal length) and leave the primary spherical, the system will have no coma or astigmatism...at all! The only issues with it would be spherical aberration which would limit the speed to about f8 for a 200mm aperture, you would have field curvature (1600mm radius) and the system would be physically enormous.
It is however a very useful hypothetical instrument to help you understand the true nature of coma and astigmatism in geometric optics....
c

[clive milne]

Yep... but you would have to increase the system focal ratio to around f24 (with a typical ccd ) before you are sampling the image at a level where diffraction becomes a sensible limit... this is why you can throw 50 percent of the collected energy into the diffraction rings without suffering any loss of detail at f4.... whether that is in the form of large central obstruction or a moderate amount of sa.... it really doesn't matter as long as the distribution curve is the same.

The design parameters of the hst are an interesting case in point.... when the goal is ultimate deep sky resolution, f24 (you would assume) is ideal coupled with a small secondary obstruction (15%) that doesn't throw a high percentage of its light out of the airy disk. The design parameters of the hst however, don't include an atmospheric component that would otherwise impose a different limit and therefore a different optical configuration optimised for those specific conditions.

[clive milne]

At the risk of sounding like pedant.. the airy disk size is proportional to f ratio and the wavelength of light concerned. It also decreases in size as you increase the size of the secondary obstruction.. if, however, the ca gets very large the amount of light concentrated in the airy disk reduces to the point where it is no longer meaningful to ignore the first diffraction ring as being a more sensible measure of the system performance..

[alpal]

Hi Clive,
what do you think about the manufacturers claim from "Hypergraph"
that it has a spot size 8 times smaller than a Ritchey Chretien?

[clive milne]

It is my opinion that the claim is most likely factually correct however, if it is implied that the hypergraph is 8 times better than an equivalent rc, 'ipso facto' by virtue of its spot diagrams I would suggest that the claim is specious at best....perhaps even disingenuous.

It would be equivalent to me putting a ridiculously high gear ratio into my honda accord and advertising it as being set up to go twice as fast as an Aston Martin at its rev limit.

Theoretical ray tracing is a valuable tool for the optical industry... but context is required before making assumptions about how relevant that information is to any given real world situation. It is important to view the (potential) performance of a telescope in terms of it being just one component of a larger system designed to perform a specific task... ie) convert the light flux from a distant object into a series of numbers proportional to its intensity and hopefully encoding spatial & wavelength data (colour and resolution) in the most cost effective means possible... ie) for a given investment you would hope to achieve the best signal with the least noise... if that isn't the merit function that is applied, then it probably reduces to an exercise in ego gratification... that is perfectly fine as an end in itself but it is better to be honest with oneself if that is actually the case imho.... the choices you make will be different.

if maximising s/n is your goal then the best bang for your buck is always going to be had from a permanently set up telescope at a dark site using a scripted routine on a robotic mount... A simple well made 14" newtonian on a paramount will likely produce more and better data under a dark sky and good seeing than you will ever hope to achieve with a hypergraph from central Melbourne. .. or even one that you have to drag out to the bush 5 nights a year.

best
-c

[clive milne]

On a philosophical note... in matters of opinion solicited in a public forum, you are pretty much guaranteed to come across mutually exclusive answers. A useful mental tool to apply to resolve conflicting conclusions is to rank them in terms of defeasibility.

[alpal]

I kind of came to the same decision right at the start.
I use a Newt. as the most cost effective way to do imaging.
I have not managed to take any images for over 6 months due to
every moonless night being cloudy.
I am hoping that I'll get a whole string of good nights one day soon
& I'm prepared to travel to get a dark site.