SCT/Newt/central obstruction debate [split from 16" Dob]

[sejanus]

hey guys out of curiousity, does a dobsonian of equal size to a SCT absolutely kill it optically? is the difference in brightness really big?

i.e. is the SCT's only advantage compactness?

please note I don't want a 10" or 16" dob I am just curious :)


cheers

[janoskiss]

No. They will be similar in performance.


Yes, probably. And once you figure the mount and tripod into it the advantage might not be not that great (esp. if you opt for an EQ mount).

[norm]

Would a SCT be marginally dimmer compared to a dob due to the corrector plate ?:shrug:

[mick pinner]

Steve, what do you mean similar in performance? 3050mm fl compared to 1500mm l don't think so.

[Starkler]

That would depend on the quality of the optics as manufactured. Having said that, newtonian optics are easier to make to a high standard than sct optics which are more complex with more components and surfaces involved.

As for image brightness, I doubt you could pick a difference between a newt and an sct of equal aperture at the same magnification.

[janoskiss]

I mean for the same aperture the two types of scopes are capable of showing you similar things. One might be an f/6 or f/5, the other an f/10, but if they have the same aperture, you'll get close to the same brightness and amount of detail in DSOs, on planets and the Moon. Wrt FL the main difference there is that you might need a 20mm eyepiece with the SCT when with the Dob you need a 10-12mm for the same power. But what you see will be similar.


Marginally, but I doubt you'd notice. And the contrast might be a bit less because the central obstruction of the SCT is larger. OTOH on the Newt you have the spider. I wouldn't worry too much about it either way. But if you do, look through some scopes before buying one.

One other thing that I'm only guessing is that Newts would reach thermal equilibrium quicker because the tube is open - esp if a fan is used behind the primary mirror.

[Satchmo]

I wouldn't say `kill' but SCT's usually suffer from rough optical surfaces and the 40% obstruction at the secondary baffle lowers image contrast significantly over the smaller Newt obstruction. A 33% obstruction for example lowers the contrast curves equivelent to an unobstructed system with about 1/2 wave wavefront error. Throw in some surface roughness , and real optical errors and even good SCT's are struggling to get a Strehl ration in the low 0.70's. A well made Newtonian with small obstruction and single smooth primary surface can easily keep a Strehl above 0.95 ( Strehl ratio of a perfect system is 1.0).

Mark

[wavelandscott]

So out of curiosity, what kind of difference would this difference in Strehl ratio translate in terms of difference in observable magnitude? Another way of asking is, for a visual observer what difference might a "reasonably skilled" observer notice?

Thanks!

[janoskiss]

Yes, as I mentioned about that larger CO will marginally reduce contrast in an SCT. But don't you mean 6" Newt to 8" SCT?? But even so I don't think so. The effect of a change in CO from 25% to 30% CO is minimal compared with that of change of aperture from 8" to 6".


I've looked through a few SCTs, Maks and Newts, and aperture seems to be by far the most important factor when it comes to seeing detail in either DSOs or planets. e.g., a C9.25 SCT is certainly more capable than an 8" Newt, despite the larger CO of the SCT. Aperture rules. Everything else is icing on the cake, or lack thereof. ;)

[Satchmo]

One has to be careful of such comparisons, the seeing may have been limited to 8" cells on that night. However its generally accepted that a simple rule of thumb, assuming nice smooth optical surfaces is that you subtract the diameter of the central obstruction from the aperture to give you equivelent performing scopes in terms of equal contrast transfer function ( or MTF )

In other words , a C9.25 , 8inch Newt and 6" Apo refractor may all be capable of giving similar views , but I would say the 8" Newt would have the best potential for smooth wavefront with inly two surfaces in the optical train compared to 4 in the SCT.

Mark

[janoskiss]

Most Newts around have about 25% CO. The 8, 10 and 12" GSO Dobs (i.e. Newts) all have 25% CO. The 6" f/8 Dob looks like it has a bit less but I never measured it. 8" f/4 Newts have more, about 30% for mine, but that's a very fast scope. I did not realise most SCT's had such large COs.

[Satchmo]

Maybe in those small sizes. But 16" F5 ATM scopes typically uses 66mm flat ( 16%), 18" F4.5 uses 79mm ( 17% ) and 20" F5 using 79mm ( 15% ).

SCT's have bigger obstruction than you would think when you measure the actual diameter of the secondary stray light baffle.

Mark

[Starkler]

Its simply the primary mirror size divided by the minor axis of the secondary.

eg. 10" primary with a 2" secondary has a CO of 20%.



Good advice. In fact I wouldnt recommend a 16" of this type as an only scope unless you live in a good dark sky location and never want to shift it.

[Starkler]

Sorry I divided the wrong way. A two inch secondary is 1/5 the diameter of a 10 inch primary, hence 20%.

[janoskiss]

Goodness me, Rob! You're a worry...

The facts:

- Geoff obviously meant the reciprocal of what he said, i.e. obstruction diameter divided by primary mirror diameter, so 2"/10" = 50/250 = 0.2 = 20%.

- GSO Dobs have about 25% central obstruction, i.e., the diameter of the secondary obstruction is about 25% the diameter of the primary mirror.

- The difference between light gathering area of two equal-aperture scopes with 25 and 40% CO is about 10%, which would correspond to an effective aperture difference of only 5%. Not enough to make too much of a fuss about. The effect of the larger CO on the point transfer function of the instrument is a much more important factor as it impacts on contrast.

[janoskiss]

Well, I had not realised Geoff already answered your qn, and I could not have made my other points without clarifying that one first. No patronising intended by it.

I stick by my earlier statement that a Dob and an SCT of equal aperture will be similar in optical performance. Not identical but similar. That is certainly true within the context of someone like sejanus looking to buy their first scope off the shelf, so that we're talking about the likes of LX90 vs GSO Dob, not purpose-built hand-tuned Newts. But we probably managed to scare poor sejanus off by now anyway. :ashamed:

[jase]

Oh how I love debates on contrast and resolution based around central obstruction.:lol:

If you looking for some hard facts on obstruction, go no further than here:
http://legault.club.fr/obstruction.html

Infact, step back a notch and look at http://legault.club.fr. Check out the "About high resolution" section. Nice information of collimation, sampling etc.

Enjoy the educational experience.:thumbsup:

[Karls48]

You should also consider effect of focuser being fully racked in to reach focus with some eyepieces and cameras when comparing SCT with Newtonian. Defocus star image and you will see dark rectangle intruding to light circle. Surely, it must have impact on the image contrast

[bird]

It's very hard to properly compare scopes of different designs, especially if you're doing it at a star party where there are lots of different scopes set up and in use.

A lot of factors can effect the performance that you might not be able to measure or have any control over, e.g. whether or not the scope is properly cooled or collimated - these things can be radically different in scopes of different designs but the same aperture even when they are sitting side by side.

From what I've seen and read over the years, it seems to me that the humble newtonian can be the best performer, but it's also a bit cantankerous and needs TLC to get the most out of it. I think that's why the SCT has become so popular - sure the performance is down a bit compared to the equivalent newt, but it's a lot easier to transport and use.

If you want the best performing scope, and don't mind getting your hands dirty with some tinkering and maintenance then the newt is very hard to beat.

cheers, Bird

[74tuc]

Hi All,

Straight comparison of the two telescopes will show that t Newtonian can be made to perform better than an SCT. If you assume that the Strehl ratio is, initially, the figure of merit. The strehl of the 'scope is the product of the strehl ratios of the individual components in the light path.

For the Sct the Strehl ratio is the product of the strehl ratios of:

1. Corrector plate; 2; Primary mirror; 3; Secondary mirror;4.Secondary obstruction; 5; SR due to diffraction of central obstruction.

For a Newtonian the SR is the product of :

1. SR due to obstruction; 2. SR due to diffraction around central obstruction; 3. SR of main mirror; 4; SR of secondary mirror.

The Newtonian, because of its smaller CO, near perfect secondary (SR = 0.98) and with a hand figured main mirror, can be made diffraction limited for a small field of view.

The SCT, on the other hand, will have to have "neigh on perfect" components to approach the same performance of a Newtonian of similar diameter.

If you multiply the overall SR by the area of the main mirror you might consider this to be a figure of merit - The bigger the better.

Please note SR is meaningful for a field that displays no distortion, spherical aberration or coma a narrow field for most SCT's and Newtonians

Rgds,

Jerry.

[jase]

It is impossible to evaluate the contrast reduction caused by central obstruction by simply looking at a Strehl ratio. It is also impossible to evaluate the performance of any telescope with a refracting element (Schmidt corrector, Maksutov corrector, doublet or triplet) with the simple knowledge of the Stehl Ratio for one wavelength (usually green), which is sometimes given by telescope manufacturers.

Harold Suiter - http://www.willbell.com/tm/tm5.htm developed another ratio (EER), which takes into account central obstructions.

Encircled Energy Ratio - The EER is the ratio between the total energy inside a given angular radius from a mirror's diffraction pattern to the total energy inside the errorless circular aperture's diffraction pattern.

The point I'm making is that the SR alone can't deliver the final verdict.

[mickoking]

Give me a newt over a SCT any day of the week :thumbsup:

[Starkler]

I recall getting into a lot of trouble for saying similar things in the past :whistle:

[jase]

Perhaps to be politically correct, the perfect telescope has yet to be invented. All optical designs have strengths and weaknesses. The best telescope is one you use.
:nerd:

[74tuc]

Hi Jase,

1. Re:
"It is also impossible to evaluate the performance of any telescope with a
refracting element (Schmidt corrector, Maksutov corrector, doublet or triplet)
with the simple knowledge of the Stehl Ratio for one wavelength (usually green), "

Please tell us what you understand by "Strehl Ratio" and further why you believe
the quoted statement is true?


2. Re:
"Encircled Energy Ratio - The EER is the ratio between the
total energy inside a given angular radius from a mirror's
diffraction pattern to the total energy inside the errorless
circular aperture's diffraction pattern."

You have just given us a definition of Strehl Ratio called by a different name.

The above statement may be read as:

"Strehl Ratio - The SR is the ratio between the
total energy inside a given angular radius from a mirror's
diffraction pattern to the total energy inside the errorless
circular aperture's diffraction pattern."

where: "given angular radius" = Radius of the Airy Disc.

I leave it to you to figure out why Strehl Ratios in a system are multiplied.
When you find the answer you will discover that the first statement is not true.

Regards,

Jerry.:)

[74tuc]

Suiter's definition of EER (SR) is in the spatial domain in terms of energy in the Airy disc.

SR may also be described in the frequency domain as "The amplitude of the 0 frequency term in the MTF"

The use of SR is always accompanied with a caution on its use - the optical system must be Aplanatic.

BTW the use of corrector plates can introduce an insidious chromatic aberration due to oblique (off axis) rays and the inherent spherical aberration, this aberration goes under the grand name of "Oblique spherro-chromatic Aberration"

Cheers,

Jerry.:)

[jase]

The Strehl Ratio put bluntly, is the measurement between reality and perfection. More specifically, it is the ratio of the light intensity at the peak of the diffraction pattern of an aberrated image to that at the peak of an aberration free image. Correct or have I missed something:help:

A refracting element will refract wavelengths with varying performance. Some scopes are optimised for red, blue etc. Note: I have not mentioned anything about reflecting telescopes only refracting.
Potentially, it could be possible to have an achromat with the same Strehl ratio for green light wavelength as other designs (such as a high-end APO). This does not indicate the achromat is a better performer. Different wavelengths with different refractions. Some interferometers test at 550nm such as those used by Obsession telescopes (now I'm mentioning reflecting telescopes), so the red wavelength is used.

http://www.obsessiontelescopes.com/optics/index.html#Quality%20control%20with %20Interferometry

As a side note: Quoting from their website, there appears to also be descrepancies in the Strehl testing, so how can one be certain of the manufacturers specifications...
""Assumed" Strehl Ratio
There's a big difference in the Strehl ratios calculated only by zonal Foucault testing and interferometric testing. The Foucault test measures only a few points across one diameter of the mirror to generate a 2 dimensional cross section of the mirror's wavefront. There isn't enough data in this representation to accurately calculate an RMS or Strehl value. Some opticians take that cross section and assume that the mirror has perfect rotational symmetry. Because of this false assumption a Strehl based on Foucault test data is always over optimistic."

I recall reading something about EER and IIRC a long time ago in one of the ATM books. Let me try dig up some info. It could have potentially been the combination of both to obtain the central obstruction affects.:doh:


Please give me an example. I'm assuming you are specifically talking about RMS values? :confuse2:

I'm learning lots here...

[jase]

Hi Jerry,


I checked out the ol'telescope making book ("hand me down" with missing pages) for calculation of the SR associated with optical obstructions. Originally was thinking apodisation, but realised I got confused.

Central obstruction effect on intensity distribution of the diffraction pattern can be expressed in very simple terms. Obstruction of the relative size υ in units of the aperture diameter affects peak diffraction intensity I normalized to 1 as given by this simple relation:
I=(1-u2)2


The change in normalized peak intensity - given by dI = 1-I = 2(1-u2)u2 - represents the relative amount of energy transferred from the disc to the rings area. This attribute is consistent with the effect of wavefront aberrations, where the relative drop in peak intensity - for relatively small wavefront errors - practically equals relative amount of energy transferred to the rings area. The similarity extends to the consequential contrast drop-off for the range of spatial frequencies bellow ~0.5 which is the range of resolvable low-contrast details. In other words, for this range of spatial frequencies, the peak intensity I resulting from the central obstruction is comparable to the Strehl ratio for wavefront aberrations. They both indicate the relative amount of energy transferred to the rings area - the main factor determining contrast transfer at low- to mid-frequencies of the MTF.



Thus, with the RMS wavefront error w in terms of the Strehl ration "S" being ω=0.24(-logS)1/2, direct relation can be established between the relative linear size u of central obstruction and the similar in effect RMS wavefront error ώ (in units of the wavelength) for the low-contrast detail performance as:
ώ~0.24[-log(1-u2)2]1/2


For u=0.33, this gives ώ~0.076, or slightly worse than 1/4 wave P-V of spherical aberration level. Comparison with the effect of spherical aberration is most appropriate, due to both effects causing radially symmetric intensity distribution, with the predominant change being brightening of the first bright ring.

I'm still toying with these formulas to see the effects, but suspect I'm missing some values. If you have any great sites with more information on the above, I'd be interested in checking them out.

Cheers:thumbsup:

[Geoff45]

What about a refractor of equal size to the Dob or SCT?:whistle:

[jase]

Hmmm. Tough call.
Assuming both telescopes have the same focal length and focal ratio it would be a close call. I'd need to research this topic further. I have seen on other forums that it is has generated some debate amongst telescope enthusiasts.

I'm also assuming that you have sufficient funds for a refractor of equal aperture to a newt or dob.;) Cost differences per aperture inch is considerable.

Changing the topic...
I'd like to reference a comment by master optician Roland Christen (owner of astro-physics (http://www.astro-physics.com) the manufacturer of what many regard as the best refractors money can buy with a Strehl ratio of .99)

Reference: http://geogdata.csun.edu/~voltaire/roland/APO_testing.html

"Manufacturers use interferometers to compare the wavefront errors of a finished optic against some reference standard. In the case of the interferometer, it is a reference sphere of known high quality which is used to form interference fringes with the optic under test. When testing a mirror, it would not matter what wavelength was used, since mirrors are totally achromatic. In the case of lenses, it matters greatly what wavelength is used, since there is typically only one point in the wavelength range where the lens was nulled or figured by the optician. Testing at another wavelength almost always results in a lower wavelength rating."

My earlier statement of:
"It is also impossible to evaluate the performance of any telescope with a refracting element (Schmidt corrector, Maksutov corrector, doublet or triplet) with the simple knowledge of the Stehl Ratio for one wavelength (usually green), which is sometimes given by telescope manufacturers."
Is indeed true.

For those interested in more of Roland's optical wisdom and fantastic imagery please check the site out.
http://geogdata.csun.edu/~voltaire/roland/index.html

Regards

[74tuc]

Hi Jase,

I'd like to call time out and get back (for the time being) to the thread.
Sejanus must be thoroughly confused with all this
.
Question and Answer

The question posed was in the comparison of two types of telescope.
The answer to the question lies in the treatment of the telescope as an optical system - not just a primary mirror or lens.

In general:
To compare the telescopes optically we need to take into account the performance of each the optical element in the telescope then "combine" the performance of each of these elements such that we get a measure of the performance of the telescope as a whole. The overall performance of each 'scope is then compared.

The metric chosen here is the system Strehl ratio. As we have seen the Strehl ratio tells us the proportion of light reaching the Airy disk.

By definition:
SR = (Intensity at centre of PSF of the aberrated optic)
/(Intensity at centre of PSF of a diffraction limited optic)

PSF = Point spread function ie. the response of an optical system to a point image
cf. Impulse response in the time domain.

What happens next?

The psf of each element comprising the telescope must be known.
The PSF carries all the information about the optical element - to make life easy let us talk about one colour only.

I now try to describe in words the application of "Linear Systems Theory" to this telescope system.

(Why this theory may be applied to these systems is a long story that
starts with the fundamental question as to why we get diffraction in the
first place!!)

The light leaving the optical element is the light entering the
element modified by the psf of the element itself. This "modification" is
actually a mathematical operation called convolution (*). What may be said is that the function representing the incoming light (iin(x,y) is convolved with the psf(x,Y) to produce the output light function (io(x,y)) or

io(x,y) = iin(x,y)*psf(x,y)

For a system with a number of psf's in a line (eg. Telescope)
i(x,y) is the intensity profile. ii = input, io = output
io(x,y) = iin *(psf1*psf2*psf3)

Where psf1, psf2 etc are the individual psf's
and psft = psf1*psf2*psf3 ... psft = system psf

The equation: io(x,y) = iin(x,y)*psft(x,y) is very computationally intensive to implement - there must be an easier way (?) and there is!

We invoke an animal called the fourier transform - you may take the fourier transform of the above equation so:

if F{io(x,y)} = Io(wx,wy) ... notice change from i to I

note:x and y are spacial (eg mm) and wx and wy are frequency (cycles per mm)

In the time domain x and y would be seconds and wx and wy would be Hertz.
Io(wx,wy) = F{iin(x,y)*psft(x,y)}

or

Io(wx,wy) = Iin(x,y) . F{psft(x,y)} Where "." = multiply

note: We have changed the complex operation of convolution
to the simple operation of multiplication

Next, to simplify things by invoking the property of radial symmetry
we will remove one of the variables

So: Io(wx) = Iin(x) . F{psft(x)}

or write the above as:
Io = Iin . F{psft} ... Nice and simple!

Now remember that: psft = psf1*psf2*psf3

and

F{psf1*psf2*psf3} = F{psf1} . F{psf2} . F{psf3}

Now F{psf} is a complex function ie. It has amplitude and phase for
this exercise we will use the amplitude of F{psf} or |F{psf}|

In plain english:
|F{psf}| is the Modulus of the Fourier Transform
of the Optical Transfer Function and this is called the
Modulation Transfer Function (MTF)

Io = Iin . MTF1 . MTF2 . MTF3

or to extend the above to a 'scope with n components

Io = Iin.MTF1.MTF2.MTF3 ---- MTFn-1 . MTFn

MTF in optics is the same as frequency response in audio systems.

If we used the zero frequency term only then

I0o = Iin0o.S01 . S02 . S03 ----- S0 = Strehl ratio (SR)
I0o = Iin0o . SR1 . SR2 . Sr3 -----

So evaluating the component SR's in a telescope and multiplying them
gives us the overall SR which is a good measure on the performance of the scope.

The above sets the argument behind my original post. We have talked about the telescope as a system.
The next stage would be to discuss the points you raised but this is outside this thread.

It could be discussed as a separate thread but I am sure it would bore most people. The points would would address the tie between Strehl ratio and the phase front of the energy and how the optics affect this phase front. Once we understand this one could look at the atmosphere and the issue
of image de-convolution - notice many programmes have de-convolution algorithms and we have been talking about convolution - do you have a strange feeling that if we go down that path we will be going in the opposite
direction to the one we've been taking!!

Regards,

Jerry.

[74tuc]

Hi Jase

Re:
My earlier statement of:
"It is also impossible to evaluate the performance of any telescope with a refracting element (Schmidt corrector, Maksutov corrector, doublet or triplet) with the simple knowledge of the Stehl Ratio for one wavelength (usually green), which is sometimes given by telescope manufacturers."
Is indeed true"

Please Show us the proof that this is true yours or by someone else.

Thanks.

Jerry.:)

[jase]

Hi Jerry,
Thanks for the definition. This is now understood. Took me a while to get the flow. So the MTF can be defined as the magnitude of the Fourier transform result. It is the response of an imaging system to an infinitesimal point or line of light. Correct eerr perhaps I need to read your previous post again.

Agree, our conversations have become off topic. Apologies to all who are bored or scared of this thread. :lol:


Thanks again.:thumbsup: Perhaps we can brain meld another time ;)

[jase]

This is based on what I have read. Don't own a refractor, nor a interferometer to valid these results.

"Manufacturers use interferometers to compare the wavefront errors of a finished optic against some reference standard. In the case of the interferometer, it is a reference sphere of known high quality which is used to form interference fringes with the optic under test. When testing a mirror, it would not matter what wavelength was used, since mirrors are totally achromatic. In the case of lenses, it matters greatly what wavelength is used, since there is typically only one point in the wavelength range where the lens was nulled or figured by the optician. Testing at another wavelength almost always results in a lower wavelength rating."

http://geogdata.csun.edu/~voltaire/roland/APO_testing.html (http://geogdata.csun.edu/%7Evoltaire/roland/APO_testing.html)

If you can get your hands on a interferometer with reference lens, I'll get some refractors. I'm sure there will be plenty of IIS members who wouldn't mind their optics tested. :)

[Fox]

I'll add my 2 cents... A long time ago, after I bought my brand new 8 inch SCT, I took it to my friends place who had a home made (excellent) 8 inch Newt. In brief, the Newt just left the SCT dead in its tracks. Side by side, Centaurus A (Caldwell 77) and its dark lane were plainy visible in the Newt, but simply invisible in the SCT, not even with averted vision. OK, we had to use different eyepieces to achieve similar magnification, but in our minds there was no question the Newt just had so much more brilliance and contrast - it was such a disheartening experience since the SCT was brand new. Light gathering and resolution were OK, so planets, star clusters and globulars were pretty good in the SCT (IMHO). But when contrast became more critical, I reckon the SCT just seemed to fall apart.

Maybe at the time I just had a bad one, and maybe todays SCTs are generally better with better coatings etc. who knows, I have to say that modern SCTs look so inviting with Goto mounts. But I eventually replaced the SCT with my Genesis and never looked back. Fox.

[74tuc]

No Problems Jase. I find this stuff most interesting it is complex and the fine detail bores most people.

Yeah I agree about the Newtonian. I was at a viewing night when I looked through this chap's Newtonian (dob) - 8" F10 I think. The mirror was hand made in 1957 and it had a cardboard tube and hand painted; using orthoscopic eyepieces - this was one of the more impressive 'scopes I've seen through so much so I want to make one myself - I'm still looking for one of those amazing mirrors. This 'scope was excellent where it counted!

Jerry:)