Changing Culture III: Aperture & Resolution.

On the left, a 90mm apochromatic refractor and on the right, a 203mm f/6 reflector enjoying a bout of late evening sunshine.

On the left, a 90mm apochromatic refractor and on the right, a 203mm f/6 Newtonian reflector enjoying a spell of late evening sunshine.












One of the ABCs of telescopic optics is that resolving power scales linearly with aperture and light gathering power with the square of aperture. These are fundamental facts that are demonstrably true and have been used productively over two centuries of scientific applications. And yet, all the while, there has been a consistent drive in the last few decades within a section of the amateur community that somewhat erroneously links performance to absolute monetary value. This largely corrupt movement is most ostensibly seen in the refractor market, where amateurs are apparently willing to shell out relatively large sums of money for telescopes that, in terms of performance, are severely limited by their small apertures. This is a worrying trend indeed, and has led many astray within the hobby.

In this capacity, I decided to highlight the anomaly by devising a simple test which exposes this ‘peashooter’ mentality for what it is; a gross misrepresentation of basic optical principles.

Materials & Methods:

Two telescopes were set up in my back garden; a 90mm apochromatic refractor retailing at £1017 (tube assembly only) and a 203mm f/6 Dobsonian, with a retail price of £289, but with some basic modifications (97% reflectivity coatings and a smaller secondary giving a linear obstruction of just 22 per cent) which increased its cost to  approximately half that of the smaller telescope. The Newtonian was carefully collimated before use.

The telescopes were left out in the open air during a dry and bright evening when the temperatures had stabilised and were fully acclimated. Both instruments were kept out of direct sunlight. The refractor had an extendable dew shield to cut down on ambient glare, while the Newtonian was fitted with a flexible dew shield to serve the same purpose. To remove the complicating effects of atmospheric seeing, the telescopes were targeted on the leaves of the topmost boughs of a horse chestnut tree, located about 100 yards away.

Both telescopes were charged with approximately the same magnifications, in this case, a very high power was deliberately chosen; 320x. Next, the images of the leaves were examined visually, being especially careful to achieve the best possible focus, and the results noted.


The 203mm Newtonian images of the leaves were crisp, bright and full of high contrast detail. In comparison, the image served up by the refractor was much dimmer and a great deal of fine detail seen in the larger instrument was either ill-discerned or completely invisible in the smaller instrument. Though less dramatic, the same results were obtained when a larger refractor (127mm f/12) was compared with the 203mm f/6 Newtonian under similar conditions, with the latter delivering brighter, crisper images with finer detail.


This simple experiment, requiring nothing more than a few minutes of one’s time and no complicated formulae or optical testing devices, clearly showed the considerable benefits of larger aperture. The images served up by the Newtonian were brighter and easier to see than those served up by the smaller instrument. Resolving power and light gathering power work hand in hand; you need decent light grasp to discern fine details and vice versa.These results were largely independent of the surrounding atmospheric conditions, as the targets were located at close proximity to the telescopes and thus had to travel through a short column of air.

These experiments were repeated with larger instruments; a 127mm f/12 refractor and the same 203mm Newtonian, with the same results, that is, the smaller instrument runs out of light faster than the larger and shows less fine detail in the images served up.

These results confirm that larger aperture is superior to smaller aperture. No amount of claptrap can change the result either. Complications may arise when the same tests are performed on celestial targets, especially during bouts of turbulent atmospheric seeing, when the larger instrument will be commensurately more sensitive. In such instances, it is the environment that introduces anomalies. But when conditions are good, the benefits of larger aperture will be seen, clearly and unambiguously. Absolute monetary value has little or nothing to do with the end result, in direct contradistinction to what is claimed by those who promote small aperture refractors in an unscientific way.

See here for further reading.


De Fideli

Changing Culture.

Octavius: instrument of change.

Octavius: instrument of change.















As  I have commented on in previous communications, an urban myth has been cultivated over the years regarding the unsuitability of Newtonian reflectors in the pursuit of double stars. In the last six months or so, there are encouraging signs that more people are bucking this trend using Newtonian optics of various f ratios and in the examination of pairs of various difficulty, including the sub-arc second realm;

Exhibit A

Exhibit B

Exhibit C

Exhibit D

Exhibit E

These are but a few examples, and I can only hope that the changes will continue so that more people can enjoy this wonderful pass-time.

De Fideli

Origins of Life: A Closer Look Part I

Some life scientists believe they can present a truly naturalistic scheme of events for the origin of life from simple chemical substrates, without any appeal to an intelligent agency.

Here is one such scenario, presented by Harvard professor, Jack Szostak.

I invite you to study the video at your leisure.

In this work, I wish to critically appraise each of the steps Dr. Szostak presents in light of the latest research findings that show that any such scheme of events is physio-chemically untenable from a purely naturalistic perspective.


Video Clock Time 00.00 -10.00 min

Here Dr. Szostak sets the scene for this thesis, exploring the varied landscapes and environments under which we find life on Earth. Dr. Szostak reasonably suggests that when life first appeared on Earth, it must have done so in an extreme environment with higher temperatures and in aqueous environments with extreme pH values and high salinity. What Dr. Szostak does not acknowledge is that life was already complex when the Hadean environment first cooled enough to permit life to gain a footing. For example, there is solid isotopic evidence that the complex biochemical process of nitrogen fixation was already in place at least 3.2 Gyr ago and possibly earlier still.


Eva E. Stüeken et al., “Isotopic Evidence for Biological Nitrogen Fixation by Molybdenum-Nitrogenase from 3.2 Gyr,” Nature, published online February 16, 2015,
“Ancient Rocks Show Life Could Have Flourished on Earth 3.2 Billion Years Ago,” ScienceDaily, published online February 16, 2015,

In a more recent study conducted by a team of scientists headed by Professor Von Karnkendonk, based at the University of South Wales, solid evidence for complex microbial ecosystems in the form of stromatolite colonies were established some 500 million years earlier at 3.7 Gyr ago.


M..J Van Krankendonk et al, Rapid Emergence of Life shown by the Discovery of 3,700 Million Year Old Microbial Structures, Nature Vol 537, pp 535 to 537, (2016).

Dr. Szostak claims the origin of life must have occurred via a Darwinian evolutionary mechanism, but the self-evident complexity of the first life forms strongly argues against this assertion, as there would not have been enough time to have done so. In other words, the window of time available for the emergence of the first forms of life on Earth is too narrow to entertain any viable Darwinian mechanism.

Dr Szostak continues by considering the vast real estate available for potential extraterrestrial life forms. Szostak presents the emerging picture; the principle of plenitude – that of a Universe teeming with planets. That is undoubtedly the case; there are likely countless trillions of terrestrial planets in the Universe.  However, new research on the frequency of gamma ray bursts (GRB) in galaxies suggests that such violent events would greatly hamper any hypothetical chemical evolutionary scenario. In December 2014, a paper in Physical Review Letters, a group of scientists estimated that only 10 per cent of galaxies could harbour life and that there would be a 95 per cent chance of a lethal GRB occurring within 4 kiloparsecs of the Galactic centre, and the likelihood would only drop below 50 per cent at 10 kiloparsecs from a typical spiral galaxy. What is more, since the frequency of GRBs increases rapidly as we look back into cosmic time, the same team estimated that all galaxies with redshifts >0.5 would very likely be sterilised. These data greatly reduce the probability that a planet could engage in prebiotic chemistry for long enough to produce anything viable.


In addition to GRB induced sterilization events, Dr Szostak completely ignores the remarkable fine tuning that is required to produce a planetary system that could sustain life for any length of time.


Dr. Szostak entertains the possibility that lifeforms with fundamentally different chemistry may evolve and that our type of life might be the exception rather than the rule. This reasoning is flawed however, as the latest research suggests that carbon-based chemistry in a water-based solvent is overwhelmingly more likely to sustain any biochemical system throughout the Universe. Ammonia has been suggested as an alternative solvent to water but there are some(possibly insurmountable) issues with it.


Summary: Dr Szostak’s introduction presents a gross oversimplification of the true likelihood of prebiotic chemistry becoming established on Earth and other planets. Szostak does concede that our planet could be unique but is unlikely to be. The emerging scientific data however supports the view that life will be rare or unique to the Earth.

Video Clock Time; 10:00 – 32:00 min
The RNA World
In this section, Dr. Szostak presents the central dogma of molecular biology: DNA begat RNA and RNA begat proteins. Origin of life researchers were completely in the dark about how this scheme of events came into being, but in the mid-1980s, Thomas Cech et al discovered that RNA molecules could act catalytically.
Zaug, A. J & Cech, T. The Intervening Sequence of RNA of Tetrahymena is an Enzyme, Science, 231, (1986).

This immediately suggested a way forward; perhaps RNA was the first genetic material and over the aeons, it gradually gave up these activities to its more stable cousin, DNA. Szostak gives some examples of how this ‘fossil RNA’ has been incorporated into structures like ribosomes, the molecular machines that carry out the synthesis of polypeptide chains. His interpretation of these examples as ‘fossils’ is entirely speculative, however.

Szostak then explores hypothetical loci where prebiotic synthesis of biomolecules could have taken place, including the atmosphere, at hydrothermal vents and on mineral surfaces. For the sake of clarity, let’s take a closer look at RNA nucleotides, and in particular, the pentose sugar, ribose. Dr. Szostak mentions the Urey-Miller experiments where supposed prebiotic molecules were produced when an electric discharge was passed through a reducing atmosphere including water vapour. Though widely cited in college textbooks, its validity has in fact, long been discounted by serious researchers in the field. Urey and Miller assumed the atmosphere to be reducing in nature, but it is now known that it was neutral, consisting of nitrogen, carbon dioxide, carbon monoxide and water vapour.

The Early Setting of Prebiotic Evolution, Shang,.S
From Early Life on Earth, Nobel Symposium No. 84, Bengtson, S. (ed.), pp 10-23, Columbia University Press (1994).


Even in the complete absence of molecular oxygen, this atmosphere could not have sustained the production of prebiotic molecules, including ribose. Only in the presence of significant quantities of molecular hydrogen has some synthesis been demonstrated.

Schlesinger, G, & Miller, S. Prebiotic synthesis in Atmospheres containing methane, carbon monoxide and carbon dioxide. Journal of Molecular Evolution, 19, 376-82 (1983).

The problem with this scenario though is that molecular hydrogen would rapidly escape from the Earth’s gravitational field and thus is entirely irrelevant to the question of prebiotic synthesis.

An Aside:

Video Clock Time: 20:00 min: The Narrow Time Window:  Reconciling Dr. Szostak’s timeline for prebiotic chemical evolution with impactor bombardment history.

At 20.00min on his video, Professor Szostak envisages the time during which prebiotic chemical evolution took place on the primitive Earth. He dates it to a period between 4.2 and 3.8Gyr ago (the supposed time of the beginning of the RNA world). Szostak presents a warm, aqueous environment during which all these reactions were taking place. But the planetary scientists modelling the impact history of the inner solar system have revealed a violent early history for the Earth. Extensive isotope analysis of terrestrial and lunar rocks, as well as cratering rate analysis indicate that the inner solar system was subjected to intense bombardment from the debris left over from the formation of the planets, which occurred between 4.5 and 3.9 Gyr ago. The cratering intensity declined exponentially throughout that era, except for a brief episode of increased bombardment between 4.1 and 3.8 Gyr ago. This is known as the Late Heavy Bombardment. One study has estimated that the total accumulation of extraterrestrial material on Earth’s surface during this epoch added a mean mass of 200 tons per square yard over all the surface of the Earth. Thus, Dr. Szostak’s relatively ‘gentle’ scenario is untenable. Realistically, the only oceans to speak of during this epoch are those of magma.


Anbar A.D. et al, Extraterrestrial Iridium, Sediment Accumulation and the Habitability of the Earth’s Surface, Journal of Geophysical Research 106 ( 2001) 3219-36.

Back to Ribose (a key component of RNA nucleotides discussed by Dr. Szostak). The only plausible mechanism for the synthesis of ribose is the so-called Butlerow reaction (also referred to as the formose reaction) which involves the coupling of the single carbon molecule, formaldehyde (methanal) in spark-ignited reactions, forming sugars of varying carbon numbers, including ribose. However, many side reactions dominate formose chemistry, with the result that the atom economy with respect to ribose is very low; up to 40 other chemical products being typically produced. This is the case in carefully controlled laboratory synthesis (read intelligently designed!), where the reaction is protected from contamination. Experimentally though, the presence of small amounts of ammonia and simple amines (which should be permissible in Szostak’s scheme) react with methanal to bring the formose reaction to a grinding halt.

Chyba, C. & Sagan,C., Endogenous Production, Exogenous delivery and Impact Shock Synthesis of Organic Molecules: An Inventory for the Origins of Life, Nature 355(1992): 125-32.

The concentrations of ribose would have been far too low to sanction any RNA world envisaged by Dr. Szostak. Compounding this is the added problem that ribose and other simple sugars are subject to oxidation under alkaline and acidic conditions, and since Szostak presents both hot and cold scenarios on the primitive Earth, it is noteworthy that ribose has a half life of only 73 minutes at 100C (near hydrothermal vents) and just 44 years at 0C.


Oro, J., Early Chemical Changes in Origin of Life, from Early Life on Earth, Nobel Symposium No. 84, Bengtson, S. (ed.), pp 49-50, Columbia University Press (1994).

But there are more serious reasons why Szostak’s scheme of events could ever have happened on the primitive Earth. This is encapsulated in the so-called Oxygen-Ultraviolet Paradox.
Szostak envisages prebiotic synthesis in warm aqueous environments, but on the primordial Earth, some 3-4 Gyr ago, the presence of much higher levels of radionuclides such as uranium, thorium and potassium-40 would have presented another proverbial spanner in the works. These would have been more or less evenly distributed over the primitive Earth and when the radiation they emit passes though water, it causes its breakdown into molecular oxygen, hydrogen peroxide and other reactive oxygen species. Oxygen and the associated reactive oxygen species easily and quickly destroy organic molecules; not just ribose and other sugars, but the other biomolecules mentioned by Dr. Szostak too, including fatty acids and purine & pyrimidine bases required for the production of micelles(see Part II) and nucleotides, respectively .

The other part of the paradox pertains to the produce of stratospheric ozone, which requires ultraviolet light. The ozone layer was not present during the epoch in which Szostak’s scheme of events would have occurred. The intense UV irradiance on the primitive Earth would have sundered any exposed prebiotics, further compounding the problem.


Draganic, I.G., Oxygen and Oxidizing Free Radicals in the Hydrosphere of the Earth, Book of Abstracts, ISSOL , 34 (1999) .

Draganic, I, Negron-Mendoza & Vujosevis, S.I, Reduction Chemistry of Water in Chemical Evolution Exploration, Book of Abstracts ISSOL, 139 (2002).

Dr. Szostak appears to be completely unaware of Draganic’s work (though citing Hazen and Deamer’s hydrothermal synthesis work @ 31 minutes) and indeed, in and of itself, would preclude any further discussions of his scheme of events. But we shall nonetheless persevere with this analysis.


This work will be continued in a new post (Part II) here.

The Life & Work of Dr. William Doberck (1852-1941)

The Makree refractor erected in Hong Kong.

The 13.3 inch Markree refractor erected in Hong Kong.

It gives me great pleasure to present an essay written by my compatriot, Dr. Kevin MacKeown (now at the Unversity of Hong Kong) on the life and work of the Danish astronomer, Dr. William Doberck (1852-1941), who, amongst other things, dedicated his professional career to double star astrometry.

Dr. William Doberck ( 1852-1951).

Dr. William Doberck ( 1852-1941).

Most significant for me was his spell at Markree Observatory, Co. Sligo, Ireland (my homeland), where he used a magnificent 13.3 inch refractor, with a lens fashioned by Robert A. Cauchoix of Paris, which was privately owned by a wealthy British gentleman, Edward J. Cooper. There the telescope was used to catalog 60,000 stars. After leaving the west of Ireland, Doberck moved the telescope to Hong Kong Observatory, where he served as its first director between 1883 and 1907.

You can view this essay here.


Neil English is working on a new book – Tales from the Golden Age – honouring the life and work of the greatest telescopic observers in history.

De Fideli.

Charm Offensive: A Spyglass Comes This Way!

A telescope that fits in the palm of your hand.

A telescope that fits in the palm of your hand.














Whichever way you slice it, the invention of the telescope revolutionised human civilization and extended our reach far into the wider Universe. And though the telescopes of the 17th century were very crude and unwieldy by today’s standards, the arrival of the achromatic doublet greatly increased their convenience and ease of use. The new technology, patented first by John Dollond in 1758, was quickly exploited to produce portable instruments for the military and the navy in the form of the nautical or ‘signalling’ telescope. And while Dollond & Co. made a fortune selling these telescopes to customers all over the British Empire and beyond, it was not long before an army of other opticians took their cut of the market, by fashioning their own renditions of the instrument.

An un-coated object glass of one inch aperture.

An un-coated object glass of one inch aperture.














As a result, although many authentic signalling telescopes built by the finest opticians in Europe and the New World can still be had for prices that reflect their provenance in the 21st century, many other antique models fashioned by more obscure artisans can be purchased relatively cheaply even in today’s market.

I’m no antique collector though, but I was pleasantly surprised to see that the classical spyglass, the signalling telescopes of old, are still being made today. I took delivery of one such instrument this very afternoon. Tipping the scales at about 380 grams, the telescope collapses to just half a foot (15 cm), making it fit snugly in your pocket, and extends to 15 inches – in three draws – when pressed into service.

An ornate eye lens.

An ornate eye lens.





It was only in the aftermath of World War II that anti-reflection technology in the form of a thin layer of magnesium fluoride was first applied to lenses to increase light throughput and decrease glare, but the ship telescopes that accompanied the soldiers and sailors of the 18th and 19th century were entirely un-coated.

In this capacity, though some of my contemporaries might baulk at the thought, I was actually delighted to see that the lenses in my ‘Ship Telescope’ were also un-coated and thus more consistent with those from antiquity. The instrument  has an objective of one inch (26mm) aperture and delivers a correctly orientated image with a power of 6 diameters. Objects can be focused from infinity to just a few feet away! Focusing is intuitive, achieved by slowly sliding the draw-tube in and out, as appropriate.Optically, the instrument isn’t great – compared to all the wonderful and inexpensive mass produced telescopes we enjoy now – but I believe it is more or less typical of what these instruments could deliver in their day. Specifically, it displays strong field curvature, some obvious lateral colour, minor internal reflections when turned on very bight objects, as well as moderate spherical aberration in a very narrow field of view. But to dismiss the instrument as optically shoddy is to completely miss the point! The centre of the field is good and sharp and perfectly adequate to the task intended for it. And what it lacks optically, it more than makes up for in other ways.

The three-draw spy glass extends to about 15 inches.

The three-draw spy glass extends to about 15 inches.

Although my model only set me back about £35 (inclusive of delivery), I was pleasantly surprised by the build quality of the instrument. The main tube has a stitched leather sheath and the three draw tube is entirely fashioned of polished brass. Each draw glides smoothly over the other with just the right amount of traction. Handling the instrument for any length of time imparts the strong aroma of cuprum et pellis in the palm of one’s hand that is so reminiscent of bona fide antique pieces. It also came with a nicely machined brass dust cap for the objective, though without a soft case or box to store it in.

I spent the odd idle moment enjoying the landscape around my home with the instrument and once my boys arrived back from school, they were eager to try out the new pirate ‘scope for themselves. All in all it was a big hit with the entire family and just a joy to use on a bright Spring day. A telescope like this would make a thoughtful and relatively inexpensive gift for someone who would like to see the world through the eyes of a child once again. It will bring you all the way back to the time of your youth, when the world seemed more innocent than it is today; an ornate and charming memento of a by gone age.

Oscar keeps a close eye on those roosting corbies.

Oscar keeps a close eye on those roosting corbies.














De Fideli

A New Filter for Planetary Observers

The Televue Bandmate Planetary Filter.

The Televue Bandmate Planetary Filter.

Planetary filters have been around for a long time. For many years simple, dyed in glass colour filters were used productively by keen observers of the bright planets Mars, Jupiter and Saturn. In general, the would-be observer would choose a filter with a colour opposite to that of the feature he/she wished to observe. And while these filters are still used productively by seasoned observers today, in recent years a new type of filter has come to the fore; the interference filter; designed to selectively block unwanted wavelengths and transmit selected wavelengths with high efficiency.

The interference filter is made out of successive layers of dielectric materials, with thicknesses ranging between one-quarter and one-half of the target wavelength. The coatings are deposited onto an optically flat glass surface in a vacuum using ion beam sputtering technology. The unwanted wavelengths are nulled by the reflections generated by the dielectric materials, which undergo destructive interference and thus are removed from the optical path. Because the designer can cut off and enhance any visible wavelength or waveband at will, they offer the potential to experiment in new ways with the human visual system.

A good planetary filter ought to accomplish a number of things.

1. Glare reduction, which almost invariably leads to an increase in perceived image quality.

2. Exaggerating differences in brightness between the various coloured features of a planetary image.

3. Overcoming to a greater or lesser degree, the image distorting effects of the atmosphere

3. Enabling observers to study different levels of a planetary atmosphere.

4. Enhancing the resolving power of a telescope.

In the last few years, Televue has brought to market a series of interference based filters designed to enhance details on the bright, extended objects like planets. Their latest product, the Bandmate Planetary (BPL) Filter, has different dielectric layers applied to each side of the optically flat BK7 substrate. At the present time, Televue only produce the filter in a 1.25 inch format, which is threaded to mate to most any eyepiece. Some have questioned why not a 2 inch version? I believe the company reasoned (correctly) that most planetary eyepieces are going to be of the smaller, 1.25 inch variety. When viewed in bright daylight, the filter gives a pinkish tinge to objects and seems to enhance brown, yellow and red tones.

Overall, the light transmission of the filter is good. Comparing it with a light yellow colour filter, a Baader Fringe Killer and the Baader Contrast Booster, the Televue BPL showed less light transmission than the all of them bar the Contrast Booster. Light transmission is important in determining the size of a telescope that would benefit from its use. If the telescope is too small and gathers only a limited amount of light, the filter will dim the image too much. For this reason, the benefits of filters are best revealed in larger aperture instruments.

I tested the filter on a number of telescopes, with apertures ranging from 80mm to 200mm My target was invariably the bright planet Jupiter, now well placed in the sky for observation. In my 80mm f/5 achromat, the filter imparted a reddish tone to the planet. Visible chromatic aberration was reduced but the image was quite dim, making any gains in contrast marginal at best.

When tested with a 5 inch refractor, the effects of the Televue BPL were more readily observed. The greater light gathering power of the instrument allowed the filter to transmit a decent amount of light to the eye. The modest amount of secondary spectrum seen in the unfiltered image was effectively removed with the filter and the elaborate banding of the giant plant was more easily seen. Close examination of the image under optimal magnifications revealed many shades of orange, yellow, fawn and dark brown. The Great Red Spot (GRS) was much easier to see with this filter than without it. The bright zones surrounding the belts came through in a plain white.

When I tested the filter on a 17cm f/16 Maksutov Cassegrain and a 8-inch f/6 Newtonian, its effects were most effectively manifested. Like all good filters, it reduced glare and greatly enhanced the dark Jovian belts. Under the best conditions, the filter brought out very subtle features, such as ovals and festoons in the planet’s southern hemisphere. The GRS was very obvious and with appropriate magnification, some substructure could be seen within it. Shadings in the polar hoods were also more easily seen with the filter.

When compared to other filters with a proven track record to enhance certain Jovian features, the Televue BPL edged ahead in terms of the amount of Jovian atmospheric details revealed. All in all, the images struck a nice balance between image brightness and the telescope’s contrast transfer.

The fact that the filter passes longer wavelengths better than shorter wavelengths may help stabilise the image. This is because of Rayleigh scattering which predicts that for a given sized particle, light is scattered in inverse proportion to the fourth power of wavelength. This means that the air is less turbulent when viewed at longer wavelengths. This effect was not vigorously tested however.

Cui bono?
What is clear from my tests is that the Televue BPL filter works very well indeed for larger aperture ‘scopes. Personally, I would not recommend it for ‘scopes less than about 5 inches in aperture but will work brilliantly with larger Dobsonians and catadioptric telescopes. It will also work extremely well in suppressing the chromatic aberration of inherent to larger achromatic telescopes and would thus be a useful tool used with old fashioned classical refractors housed in observatories up and down the country.

Planetary imagers will also likely benefit from using this filter, especially large aperture instruments. Because of its reduced light transmission, slightly longer exposures may be necessary in order to achieve optimal results.


In summary, the Televue BPL filter is an excellent filter to divine as much detail from Jupiter’s massive and ever changing atmosphere as is conceivably possible. If you’ve never seen the Great Red Spot, you will with this filter! Saturn and Mars should also benefit from its use. It is more expensive than other filters on the market but I believe the extra cost incurred is well worth it when you see what the filter can achieve. Albert Nagler has definitely done his homework with this exciting new product. Highly recommended!

ergo dixi vobis

Typical UK Retail Price: £113.

Typical US Retail Price: $140.

De Fideli

Neil English is author of Choosing and Using a Dobsonian Telescope.

Taking Back Visual Astronomy- Filters (An Update on Colour Blindness)

Are serious visual observers becoming colour blind?

Are serious visual observers becoming colour blind?

This work is dedicated to Rutilus

Do not let yourself be tainted with a barren skepticism.

Louis Pasteur (1822-95)

Filters work.

Sunglasses show up details in a bright, washed out image that are nearly invisible in its unfiltered counterpart. Narrow band nebula filters allow you to more easily see faint deep sky objects despite removing vast amounts of other information from the image. Atmospheric haze turns a good achromatic image into a great one. And colour filters, judiciously selected and attached to the eyepiece of a telescope, help you to see planetary details more clearly.

Filters are useful.

But  as I write these words, a new generation of amateur astronomer is running roughshod over tried and trusted traditions of visual observing. The condition is particularly perspicuous amongst those who delight in owning high-end apochromats and premium Newtonian mirrors. In love with aesthetic images, these uber emotive souls are shocked and horrified by the suggestion that adding a filter to the eyepiece might actually enable them to see more than their pretty unfiltered images render. Here are a couple of statements I’ve heard from two supposedly serious planetary observers

“I tried out various colour filters with my 12″ Zambuto mirror, but in the end, figured I was seeing everything without the filters than with them.”


” My TEC 160 image is so good, I wouldn’t dare cheapen it by placing a $5 filter between it and my eye.”

Or, how about this quote oft parroted on tinternet:

“Filters are unnecessary as it has more to do with the observer’s eye being ill-adapted to photopic mode observation.”

Well, try saying that to a bloke wearing polaroids on a bright summer day.

You see, these three statements amount to complete and utter nonsense!

Take a quick look at these three full moon images.



Green filter

Green filter

Red Filter

Red Filter






Are you going to sit there and tell me filters don’t do anything?

Simply put, filters can improve what we see by removing what we don’t want to see from the view. We can all understand that most contemporary observers desire the most aesthetically pleasing image possible from a ‘scope. Adding a colour filter won’t do anything to enhnace that world view but what it will most definitely do is exaggerate differences in brightness between the various features of a planetary or lunar image.

Colour filters have many uses, including;

1. Glare reduction, which almost invariably leads to an increase in perceived image quality.

2. Overcoming to a greater or lesser degree, the image distorting effects of the atmosphere

3. Enabling observers to study different levels of a planetary atmosphere.

4. Increasing contrast between areas of different colour.

5. While not eliminating optical defects, improving image definition even with bad or mediocre optics.

If you are in the slightest way sceptical about any of the claims above then you’ll be helping to topple the photographic industry. The most commonly used Wratten system, for example, was developed by Kodak in 1909 and has been the standard ever since. The Wratten number, usually found around the circumference of the colour filter gives precise information about the properties of that filter.

Colour Filter                   Wratten #                  Light Transmission (%)

Light Yellow                      8                                         83

Yellow-Green                   11                                       78

Yellow                               12                                       74

Deep Yellow                    15                                        67

Orange                             21                                        46

Light Red                         23A                                      25

Red                                   25A                                      14

Dark Blue                         38A                                      17

Violet                                 47                                         3

Light Green                       56                                        53

Green                                58                                        24

Blue                                   80A                                     30

Light Blue                          82A                                     73

As you can see from the table above, filters vary considerably in their ability to transmit visible light. But, get a load of this, they do it without sacrificing resolution (and may in fact increase it). A very important point I should think. In general, filters work better with larger instruments which have more light gathering power. That said, one of the most versatile filters – the light yellow Wratten # 8 – can be used productively with even the smallest apertures.

Most of the attributes of filters highlighted above are well known, with the possible exception of attribute 2. Meteorologists have known for quite some time about the scattering effects of particles in the atmosphere. Known as Rayleigh Scattering, it predicts that for a given sized particle, light is scattered in inverse proportion to the fourth power of wavelength. Thus, it can be shown that violet light (wavelength ~400nm) is scattered some 16 times more effectively than deep red light (800nm). That’s why the sky is blue and sunsets are red.

And, so the theory goes, employing a red filter during turbulent atmospheric episodes might mitigate to some degree the deleterious effects of bad seeing. Although I have not explored this as vigorously as I’d have liked to, I once tried to see Sirius B using a light red Wratten # 23A with my 4″ Televue 102 refractor some years back, and if memory serves me well, the results were encouraging. Weather permitting, I shall attempt resolving the Sirian Pup – always very low even at culmination from my northerly latitude – using a similar strategy early in the new year.

A violet (47A) filter is very useful for observing cloud features on Venus and although its light transmission is painfully low, it can be pressed into service with larger aperture ‘scopes.

Mars is a great planet to learn how good colour filters can be in extracting atmospheric and surface features. A simple light yellow (#8) reduces glare and increases contrast in smaller apertures (5-inches and less). An orange (#21) is great for pentrating haze and cloud in the Martian disk,as well as increasing contrast between the light and darker areas of the planet. A light green # 56 filter darkens both red and blue features, enabling the observer to prize the morphology of the polar cap more easily.

Jupiter and Saturn also benefit from coloured filtration. Blue and green ones are just dandy for bringing out the belts of the planets. A yellow filter can help reveal bluish features( festoons), while a red filter can help bring out the white ovals so cherished by planetary observers. The icy Saturnian ring system too can look majestic using a red filter.

I’d be willing to bet good money that a patient observer, sketching planetary details in red, green and blue light will see more than one observing a ‘luminance’  image. Every dedicated planetary observer should have a set. And while dyed glass filters are perfectly adequate ( and cheap as chips), one might gain some additional benefit from the newer interference colour filters manufactured by companies like Baader Planetarium., Germany.

The study of colour filters on the lunar surface is an unexplored frontier, as far as I’m aware, but think geologically (minerals and that)!

                                          Improving resolving power with colour filters

The resolving power of a telescope (in radians) is approximated by Lamda/D, where lamda is the wavelength and D is telescope aperture. The Dawes limit is closely matched to a wavelength of 562nm. Converting radians to angular degrees, we can easily compute that for a 4-inch instrument (0.1m), the Dawes limit is ~1.15 arc seconds. Yet, as I have shown elsewhere in my work, there are quite a few instances where this value has been exceeded. An overly sceptical person might doubt the veracity of these claims, but if the eye has a peak sensitivity at a lower wavelength, resolution can be improved.

Individuals who have a form of colour blindness called protanopia perceive red hues as essentially dark and have peak spectral sensitivities shifted to shorter wavelengths (typically 520nm) – quite similar to where a normal, trichromatic eye would shift when fully dark adapted (~507nm). Thus, even if these individuals were observing in photopic or mesopic mode, they would have no sensitivity to longer, red wavelengths but with no loss of acuity. A simple calculation shows that such an individual might derive a ‘new,’ lower Dawes limit of 1.0 arc second with the same 10cm ‘scope. What happens as the protanopic eye dark adapts – does the peak sensitivity fall further back as in the trichromatic eye? If that happens, even greater resolution feats are conceivable!

And what of deuteranopia (another fairly common form of dichromatism where the retina lacks green cones)? Under typical night time viewing conditions, wouldn’t their red light sensitivity decrease, inducing them to rely on their blue-sensitive (peak spectral response ~440nm) cones. Could these individuals resolve finer details still?

All this serves to illustrate is that we still know far too little about the human eye (in all its enormous variety) to devise over-restrictive rules that only serve to tell folk what they can and can’t acheive. I for one don’t want to be told what I can and cannot see. Away with the Universal, away with the ‘thought police’!

While all of this sounds like pie in the sky, it can be handidly demonstrated with colour filters. A violet filter working at 390nm will improve the resolution of a telescope by up to 30 percent. A blue filter; less so. Noted CCD imager Damian Peach, produced a cool illustration of this effect on his website. You will note that the binary system is unresolved at red wavelengths, elongated at green wavelengths, and cleanly resolved at blue (lower) wavelengths. See here. Neat huh?

Note addded in proof: While researching the life of the 19th century observer, G.V. Schiaparelli, I came across a curious account of colour blindness in William Sheehan and Steve O’ Meara’s book, Mars; The Lure of the Red Planet(pp 117):

That color blind individuals possess superior vision, at least for certain  types of observations, is attested  by at least one other case known to me. According to Donald Osterbrook, Lick Observatory, astronomer Nick Mayall was colour blind., ” and he believed that it made his eyes more sensitive to faint light so he could find and observe fainter stars, nebulae and galaxies than other astronomers  with normal eyesight. Certainly, when I visited him at the Crossley reflector one night around 1955, he was taking a spectrum of an object  that was too faint  for me to see, though he evidently could see it well and the spectrum  was a good one when he developed it the next day. Several other astronomers have told me that color blind observers can see fainter objects at night than those with normal eyes -WS.

Indeed, knowedge of this sort has helped resolve a few issues I have had with my own telescopes. I only recently discovered that my eyes are particularly red sensitive and I appear to have less sensitivity at shorter (bluer) wavelengths. While using my 5″ Russian achromatic refractor, I can see the faint companion to Eta Geminorum better using a fringe killer than without it ( it blocks off  deep red wavelengths very effectively, as you’ll see below). I find that while I can achieve 1 arc second splits quite easily, 0.9 arc second pairs remain beyond my abilities, possibly because I see too much of the red end of the diffraction pattern. By using a blue filter, I hope to finally smash that 1.0 arc second barrier, if anything, to prove to myself that it is my eyes that are found wanting and not my telescope.

Nosce te ipsum.

                                                            Polarizing Filters

The light that reaches us from the depths of space vibrates in every conceivable plane. Plane polarised light, on the other hand, vibrates in only one plane, greatly reducing scattered light in the eye (irradiance) and increasing contrast. You only need look at the effects of a polarising filter from a medium focal length lens to see how dramatic an improvement to a daylight landscape it can make.

Single polarising filters have been used successfully by double star observers , especially in cases where one component is significantly brighter than the other. The theory is that the glare from the brighter primary is reduced enough to render visible a faint secondary.

Many observers have employed polarising filters to observe deep sky objects during full Moon nights. There is apparently a big contrast gain when the telescope is pointed about 60 degrees away from Luna and amazing results when swung away to 90 degrees. Larger apertures and low powers naturally benefit from this more than smaller instruments.

Neutral density filters are often cross polarising in effect, where two polarisng layers are mounted in such a fashion that one can be rotated relative to the other, empowering the observer with ability to vary the brighness of an object. Moonwatching with a large Dobsonian  can be cool with one of these.

                                                         Minus Violet Filters

Minus Violet or anti-fringing filters have been round for over a decade now. By and large they have a tried and trusted reputation for improving the performance of achromatic refractors. They achieve this through selectively blocking (via destructive interference) selected wavelengths at both the violet (short wavelength end) and deep red (longer wavelength) end of the visible spectrum. Typically it is at these extreme ends of the spectrum that most of the unfocused light(secondary spectrum) arises in achromatic refractors. Because they block specific wavebands of visble light, they usually impart a yellowish tint to the image, which seems to bother some more than others. In addition to blocking off unwanted secondary spectrum they greatly help with focusing the instrument, particularly during turbulent bouts.

The most aggressive minus violet filters have a tendency to dim the image a little too much, especially if you’re in the habit of using smaller instruments, but there is one filter that I have studied at length that is particularly useful; the Baader Fringe Killer. Although more expensive than traditional yellow filters, the fringe killer works more effectively in my opinion. The Baader Contrast Booster is also excellent although it cuts off too much light for productive use in smaller instruments.

I have used the fringe killer quite extensively with my 5″ f/9 achromatic refractor, observing Jupiter at or near opposition as well as in the pursuit of double stars. It deals effectively with the unfocused blue halo round the planet but also greatly increases the contrast between the darker belts and surrounding bright areas of the atmosphere. Focusing the planet is child’s play too. If you’re a student of Jupiter using a moderate aperture achromatic refractor, then this filter is highly recommended.

These filters are also excellent for star testing refracting telescopes. They invariably clean up the spherochromatism that oft attends the intra- or extra-focal images of stars, allowing you to more accurately assess the quality of the optic.

But it’s not only achromatic images that can be improved by this filter, ED scopes seem to respond very well too. This is particularly the case with the new breed of econo-model ED scopes now on the market. One way these manufactuers get round the issue of producing a fairly short focal length ED doublet without introducing spurious blue fringing round bright objects is to over correct at short wavelengths at the expense of more lax correction at deep red wavebands. This reduces blue fringing alright but serves up slightly washed out images of planets like Jupiter. When I examined the image in one of these units, I discovered that it was quite dramatically improved by using the fringe killer. I attribute this to the cleaning up (by blocking off) of the loosely focused deep red colours. It was just easier to see the details in the planet’s belt with the filter in place. The filter transmits enough light that telescopes as small as 80mm can benefit from its effects.

Double stars too benefit from this filter. One of the most effective things the filter does is cut down on glare (irradiance) which can make seeing a faint, close companion more easy to pick off, especially when located right up next to a much more brilliant companion. While evaluating a 6-inch ED instrument, which produces very bright images of systems such as Delta Cygni, I found the fringe killer reduced the glare round the primary quite a bit making the companion easier to keep in view during a vigil.

And on my 5″ f/9 achromatic refractor, I discovered that the fringe killer was a fantastic tool to render tricky systems like the devilish companion to Eta Geminorum (Propus) much easier to see. On most nights I have trouble with this system, whether observing through an apochromat or achromat. Reducing the red glare of the red giant primary with this filter was a real eye-opener to me though.

Some might object that using such a filter reduces the aesthetic appearance of a pair, but that’s not really been my experience at the eyepiece. Famous colour-contrast pairs, such as Albireo and Gamma Andromedae (Almach) are just as beautiful with the filter as without it and the rich colours actually seem more enhanced to my eye compared to the unfiltered view. Only whiter pairs seem to give way to a yellowish cast.

If you’ve been round the block a few times, no doubt you’ve heard the show-stopping mantra of the apophiles whenever the subject of these filters comes up.

“It won’t turn your $300 achromat into a $3,000 apochromat.”

What does that mean exactly?

If they allow you to use optimal magnifications on the moon and planets with your modest achromat isn’t that enough?

These filters will often improve the images of both telescopic genres by teaching your visual system to concentrate on the most important wavebands – where the vast majority of the information from an image is imparted – over yellow green wavelengths.

Filters are tools; pure and simple. Find the time to use them skillfully.

References and Further Reading

Some additional background on filters here

Bakich, Michael E, 2003, The Cambridge Encylopedia of Amateur Astronomy, Cambridge University Press.

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