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  #41  
Old 17-11-2013, 10:41 AM
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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.

Last edited by Shiraz; 17-11-2013 at 10:57 AM.
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  #42  
Old 17-11-2013, 10:48 AM
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Originally Posted by Shiraz View Post
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, but even the professional astronomers do not regard that as a necessary feature - they need to know where a star is, what colour it is and how bright it is. Stars should not be round anyway - they should be points. We are used to seeing round stars because most telescopes have evenly illuminated circular apertures, 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.

Except that stars are a wave function so they show diffraction rings when out of focus.
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  #43  
Old 17-11-2013, 11:06 AM
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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.

Last edited by Shiraz; 17-11-2013 at 11:24 AM.
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  #44  
Old 17-11-2013, 11:34 AM
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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.

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.
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  #45  
Old 17-11-2013, 06:12 PM
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Very true. All scopes except Astrophysics ones have shortcomings of one type or another.
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 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

Quote:
The fun is getting out and producing some images you are happy with.
Ah huh!! EXACTLLY and this is the key 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? 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
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  #46  
Old 21-11-2013, 09:01 AM
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Come on Mike - I know you want a Hypergraph.
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  #47  
Old 21-11-2013, 10:19 AM
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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/consum...2_8l_is_ii_usm

Bert

Last edited by avandonk; 21-11-2013 at 10:47 AM.
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  #48  
Old 21-11-2013, 01:43 PM
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Originally Posted by avandonk View Post
[COLOR=black][FONT=Verdana]


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


Bert
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.

Last edited by Shiraz; 21-11-2013 at 05:11 PM.
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  #49  
Old 21-11-2013, 02:26 PM
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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
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  #50  
Old 21-11-2013, 10:15 PM
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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.
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  #51  
Old 22-11-2013, 11:58 AM
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Quote:
Originally Posted by alpal View Post

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.
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.
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  #52  
Old 22-11-2013, 01:06 PM
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Seeing conditions are the greatest limitation.
BINGO!
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  #53  
Old 22-11-2013, 01:31 PM
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Mark,
Quote:
spot size of an RC due to diffraction is actually larger than a typical Newt
Do you have any evidence for that?
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  #54  
Old 22-11-2013, 02:01 PM
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Have a look at this link:
http://www.nfilipovic.com/astrophoto...00k-ota-review

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

Quote:
5) Shape of the spot 10mm off-axis, in the plane shifted to compensate for field curvature.
Field curvature is defined according to the minimal RMS spot radius.
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.
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  #55  
Old 22-11-2013, 02:24 PM
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Quote:
Originally Posted by alpal View Post
Have a look at this link:
http://www.nfilipovic.com/astrophoto...00k-ota-review

You can clearly see that the spot size for a Newt. is much greater than an RC.
That's not a corrected Newt...
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  #56  
Old 22-11-2013, 02:53 PM
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That's not a corrected Newt...
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.

Last edited by Shiraz; 22-11-2013 at 03:22 PM.
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  #57  
Old 22-11-2013, 02:57 PM
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That's not a corrected Newt...
Read it more closely.

Quote:
Shape of the spot 10mm off-axis, in the plane shifted to compensate for field curvature.
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  #58  
Old 22-11-2013, 03:25 PM
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Quote:
Originally Posted by alpal View Post
Read it more closely.
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.
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  #59  
Old 22-11-2013, 03:33 PM
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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/TelescopesEye...trographs.html
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  #60  
Old 22-11-2013, 03:43 PM
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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.
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