Optical Performance of The Refractor
Commonly Encountered Wavefront Relationships
* Derived from 1/4 wave, P-V of spherical aberration, wavefront equaling an RMS of 1/14.05 or .071
As previously stated, the actual methodology of measurement, variation in type of interferometer and calibration may produce a degree of variability in results such that a variation in Strehl figures may be obtained by different laboratories or practitioners. This problem is not unique to optical testing and is often a bugbear of researchers - I know this personally as a former industrial chemist involved in refined analytical techniques.
Indeed, John Nicholl of Nicholl Optical (a respected UK mirror maker) rightly reminded me that the tollerance of any measurement is critical - in other words the statement that a mirror has a Strehl measurement of 0.99 is meaningless if there is a ±0.02 tollerance as the optic may fall into a range of 0.97 to 1.01, the latter figure obviously not possible.
However, as Royce points out, "The Airy disk of an unobstructed objective will contain a maximum of 83.8% of the energy from that star entering the objective. The first order ring will contain 7.2% of the light, the second order ring 2.8%, and so on, diminishing monotonically with each successive ring. Usually only the first ring is clearly visible on a steady night. Basically, what it's all about is getting as much of the light into the Airy disk as is possible. Arguments arise about whether additional light in the first order ring aids in the separating of double stars, but for all practical purposes the more light in the Airy disk, the better off you are."
In other words, the higher the Strehl ratio, the better the optical performance. Unfortunately very few manufactureres give specific, rathere than generic, test data and even fewer offer interferometer test reports as standard. And that's not where the story ends, because various other factors affect the optical performance.
So, now we have a system of comparative measurement it is necessarry to look at other factors which come into play such as:
- Telescope design : central obstruction - secondary mirror(s), spiders
- Collimation : how many variables in the system
- Eyepices : correction, number of elements
And of course, atmospheric conditions and the visual acuity of the observer also play a very significant part. Do you need excellent optics, given the UK's weather quality? Most certainly yes.The eye/brain system is superb for picking out subtle detail during fleeting, often sub-second, moments of optimum seeing. By comparison, digital imaging employs averaging/stacking of images to create a composite so clearly optical quality is important.
Basically, you cannot beat the laws of physics. There are websites that claim the central obstruction in an SCT (or other reflecting system with large secondary) have little or no effect on image quality. Various sources show computer simulated images with varying percentages of central obstruction, some of which show superior images with fairly large obstructions.
However, a definitive analysis appears on the Telescope-Optics.net site (www.telescope-optics.net/obstruction.htm) . This looks at the effect of central obstruction on the Modular Transfer Function (MTF), the most widely used scientific method of describing optical performance. It is, as the name suggests, a measure of the transfer of modulation (or contrast) from the subject to the image. In other words, it measures how faithfully the optic reproduces (or transfers) detail from the object to the image produced by the lens or miror. The following table is derived :
MAX. C.OBSTRUCTION (ο) FOR
mid-to-low MTF frequencies
entire range of MTF frequencies
This indicates that, for a 0.90 Strehl mirror, a 33% cental obstruction will reduce the system Strehl to 0.8 for the entire range of MTF frequencies.
It is also important to understand that the Strehl figure for each component needs to be taken into account, as the effects are cumulative. So, for example, a 0.9 Strehl primary mirror and 0.9 Strehl secondary results in a compound 0.81 Strehl system. Taking the cental obstruction into account, the system Strehl may be further reduced to, say, 0.71 - way below the Raleigh Limit and not diffraction limited.
This next table summarises the cumulative effect:
A comparison of the size of central obstruction found in different designs is revealing (secondary holders and spiders add to the problem and may introduce a smearing of the image):
Ritchey Cretian, Dall Kirkham, Schmidt Cassegrain, Schmidt Newtonian 30 - 45%
Newtonian 15 - 30%
Maksutov 14 - 30%
Refractors, Off -axis designs ( Herschellian, Schiefspiegeler, Yolo, etc) zero
This is why so many observers are disappointed when viewing high definition/fine detail objects such as planets, the Moon, double stars and small deep sky objects with fast Newtonians and SCTs and are amazed at the superb planetary images seen in refractors of modest aperture - many of the cheaper, mass produced Chinese achromats give Strehl ratios of 0.85 to 0.9 and and therefore comfortably outperform the off the shelf SCT system. The original Celestron 8, the Meade version and subsequent larger apertures and variants are all compromise systems which stray from the original Schmidt concept. Very good, portable and affordable telescopes they are but they are not optimised for high definition, critical observation whereas well made refractors are.
It is also obvious that slow Newtonians ( F8 to F10) and the Maksutov-Newtonians have very small central obstructions in the order of 14 to 20 % and provide very good image detail. Many reference articles state that a central obstruction of 15% or below is undiscernible and will give apochromat quality images. Indeed, it is argueable that a small obstruction may enhance the ability to separate close doubles and fine detail but small is the key here. JB Sidgewick recorded in his seminal treatise 'Amateur Astronomer's Handbook' that an aperture stop of one fifth to one sixth (20-16% approx.) may improve separation of close doubles, which is quite possible for point sources. Some websites use predictive programs to 'prove' the benefit of a central obstruction, but the size is the key - above 20%, the central obstruction wil always adversely affect resolution. An interesting and informative article appeared in 'Telescope Technology Today' volume 7 issue 6: Nov-Dec 2013, where it is demonstrated that 150-180mm apochromats gave comparable resolution to fast Newtonians of up to 750mm aperture when observing bar test targets. It should be noted that this article is not based on scientifically rigorous laboratory evaluation, also that resolution of extended objects (lines) is better than for point sources for all optics, but the comparison of telescope systems is basically valid.
(Note : reference to off-axis designs is for completeness - these are specialist designs with critical optical figuring and mechanical requirements.)
Just to refer back to measurement of Strehl for a moment, there are programmes that enable the user to measure optical performance by reference to star images taken through the telescope.An example is the Roddier Test and one site has compared a 9.25 inch SCT to Maksutovs and refractors for image quality, reporting the SCT as being "1/19th wave RMS and 0.90 Strehl". This effectively means that the mirror in this telescope has a Strehl of 1.0, improbable to say the least. As with all things, interpretation and rigorous technique is essential otherwise results are misleading.
This is, of course, a critical element in all optical systems and the ability of a particuler design to maintain optimum alignment is a function of the optcal quality, mechanical tollerances and materials used in construction. Nearly every Newtonian I have used has exhibited some movement of the optics at varying tube orientation, and on transporting or set-up. Compounds also exhibit collimation problems.
It is true that even the budget manufacturers are starting to produce well designed OTAs but in many cases the engineering tollerances are slack - it is an unfortunate fact that even some of the so-called "hand made" telescopes produced in the UK and abroad fall in to this category.
It is also surprising that observers will spend a vast amount of money on a superbly stable and accurate mount to facilitate imaging and yet fail to appreciate the need for engineering competence in the OTA.
Despite the relative simplicity of design, refractors will exhibit flaws induced by inadequate materials and engineering. A well designed refractor will have a tube of substantial wall thickness to minimise flexure, particularly when long focal lengths and/or heavy triplet objectives are involved. It is reasonable to expect no flexure, irrespective of tube orientation, and provided that the objective lens is precision made with zero wedge (edge thickness variation) then in reality it is not even necessary to have a collimating cell. For example, axial run-out of 0.003 inches on a 6 inch lens amounts to 0.03 degrees of tilt, insignificant for an aplanatic lens. What is required is a precision machined cell and a completely squared tube end.
This is a complex topic and I have yet to find two observers who totally agree on the merits of various types and brands. However, one criticism of refractors is often that the absorption due to the 2, 3 or more objective elements is severely detrimental to image brightness when compared to mirror systems - yet many of the premium oculars have 6 or more elements.
I think the choice of eyepice(s) is one of the more subjective areas of astronomy, but it is true that longer focal length achromats require less well corrected eyepieces and therefore fewer lens elements.
Another overlooked factor is that eyepieces will, of course, have a measurable Strehl ratio and further affect the tabulated figures above, but as far as the writer is aware no Strehl measurements for accessories are available.
Refractors - a summary
The optical performance of a well-made achromatic refractor may offer the best performance of any telescope type on many grounds. To summarise the above and related issues: