Investigation of telescope visual filters; (Part 1 Contrast).

Investigation of telescope visual filters; (Part 1 Contrast).

I remember my grandfather pointing out constellations such as the southern cross, the big dipper and the devil’s eye to me when I was a youngster, in our little town of Lowood in Queensland, Australia. I was hooked on what was “out there”, even then, those glittering jewels in the sky were mesmerising.

My first telescope was just a toy refractor given to me by my parents, in the 1980’s, at around the time of Halley’s comet. My elderly neighbour, Ma Kemp, regaled me of a tale of Halley when it had last visited the earth, when she was just a girl and how the sky had been full of something like fireworks. She laughed about how she and her sister had been terrified of the event and that they had thought the world was going to catch on fire and end.

I couldn’t wait to see it after hearing her tale of fright. Well, what a letdown when it did finally happen and wowo was I thankful for that toy refractor ..it did actually get me some Mag! Luckily, the newspapers were also full of amazing graphics of the Comet that you didn’t have to go cross eyed to get the **** thing in focus. My bedroom walls were plastered with them, photos of high-res ice tails and ultra-violet spectral filters etc.

My next scope was an old Japanese 3 1/4” reflector with 0.965 objective with wobbly legs from the second-hand store. It was great for the planets and landscape views but not so much for the DSO and faint fuzzies. A lot of Life happened since then and now and in the last 4 years I have become the proud owner of a second-hand, astronomical club made, 16” Newt Dob I call Delvira. Any night that is clear I try to get out in the dark with her. The things we have seen together!

I am basically self-taught in the ways of astronomy, a total amateur and am enjoying learning all the time.

My first set of filters came with the Celestron eyepiece kit I bought, shortly after getting Delvira. The kit came with the traditional #23 red, #80a blue, #56green and #58green (I suspect Celestron #58 could be #99 it just feels more neutral density in greenish, then a dark green, something lost in translation maybe 😉 ), #12 yellow #21 orange and neutral density filter #ND 0.9 that come standard with most kits these days. They didn’t overly excite me the first time I used them on Saturn and Jupiter all much too dark, although they all worked great on the moon for cutting that intense light back a notch. It was when I viewed Venus for the first time as that brightest of stars and thought to myself what if I try that #25 red. It was then that I really found filters useful, transforming stars into planets. I enjoyed watching Venus go through its shape change that first season with Delvira.

However, of course with the first cloudy night I immediately began searching for something to make the view of Venus better. I purchased a couple more filters over that first year including an adjustable polarizer, a second-hand original Tele Vue Nebustar to enhance nebula viewing and a #38A Blue for Venus views. That TV Nebustar is one of my favourite filters to date for all things nebular.

The Nebustar prompted me to purchase a TV OIII from Agenastro online, as I was going to have to pay postage from the states to Australia, I thought I may as well add a cheaper colour filter to the lot. I chose the Brandon Magenta filter as I had read a lot on Cloudynights about it while reading about what best to view Mars with and it had rave reviews. When the package finally arrived, I couldn’t wait to use that magenta only to realise it had non-standard thread ARRRRRRGGGHHH what will I do I aint paying $60 postage for a $15 attachment. I sacrificed the Celestron ND 0.9 and replaced the glass in it with the Brandon Magenta and yeehaw we were off.

Well, the Magenta really did improve my view of Mars especially once I stacked the Polarizer on it. But what was this I read about an #85 Salmon also improving the Mars view, hmmm, I better get that too. Eventually I ended up finding a cheap Tiffen 28mm set of salmon glass on ebay, not the darker salmon but #85 and #85C (interestingly, an #85B is just about the equivalent of 2x #85, while 85C is just under half as dark), I also had to set them in Celestron filter rings but it was worth the sacrifice as salmon (or amber as Kodak call it) really does some great things!

So, at the moment I have a suite of filters which can be seen in figure 1. the top half of the figure shows the filters in room lighting and are named by Kodak Wratten, apart from the Tele Vue oxygen three (TV-OIII), Tele Vue Nebustar (TVNeb1) and the Baader Contrast Booster (Baader CB). For a description of the wratten see Table 1 at the end of the document.

Meanwhile, the bottom half of the figure shows the filters held to the window with sunlight transmitting through them, a white sheet of paper is held behind them to keep the background view even. An interesting note on the figure while we are here is that both the Tele Vue filters when viewed by sunlight were much more a greenish-blue colour rather than the blue that appears in the Iphone capture while all the other colour filters in the picture appear true to what was observed by my eye.

Figure1. My filter suite

My latest purchase is the Baader contrast booster and this brings us to the topic I wanted to talk about. Coming to an understanding of what is happening to the light in these pieces of glass so that we behold what we behold!

When I really got started trying to understand colour filter use, I went back to my Art School days and revisited some colour theory, by drawing myself a colour wheel, see figure 2 for an example. Which I then held my filters over the top of, just for fun, to see what would happen.

Figure 2. The Colour Wheel

Basic colour theory Rule 1. Complimentary colours lay side by side on the colour wheel for example blue and green are compliments as would be green and yellow. Meanwhile, Rule 2, colours that lay opposite from each other on the colour wheel are contrasting colours, an example would be yellow is contrast to violet, as is orange contrast to blue and red to green.

The colour wheel and those two rules are our defining concept to explain a specific set window of energy wavelengths, known as the visible light spectrum and how they behave within the human eye. Which brings us to our next figure, Figure 3 the energy spectrum and the position of visible light within it, R E S P E C T to who it was that made the figure, beaut work mate and thanks to You for the free download!

Figure 3. Spectrum of Energy (click image to launch larger version)

Something to think about, the Baader contrast booster blocks light transmission between 500 and 530 nm and also between 570 and 600 nm. Baader, like Kodak, also has nominated colour filters, however rather than a wratten they are nominated by their wavelength in nm, as do several other modern producers of telescope filters. Dark blue is 435 nm, light blue 470 nm, yellow 495 nm, green 500 nm, orange 570 nm and red 610 nm. Yeah but when I look at his fig3 that yellow’s in the green spectrum, hmmm his figure is wrong,,, Hey hold yer horses! Here is something to think about. Did you know there are three, and only 3 true primary colours in light. They are blue, green and red. How is this so, what about yellow … maybe it’s like, but not really like, how birds can see iridescence and ultraviolet and dogs and cats see in the dark in far red while bats use radar. It is just how humans are designed, to ‘see’ some green frequency as yellow. Any way I digress, I want to talk about contrast and filtering light.

Basically, optical filters are coloured bits of glass, the colour denotes which wavelengths are transmitted through the glass, while the concentration of colour aka density of the colour contributes to how much of the selected and non-selected wave lengths are permitted through the glass. The Kodak Wratten chart takes all this into consideration and was concocted as a means of keeping its product batches of photographic light filters uniform and included calibration # glass in their chart. In fact, all the filters associated with a Wratten are still available from Kodak today in 2022.

However, what the chart doesn’t take into account is the modern telescope filters such as the OIII and Contrast Boosters, etc all have modern coatings which have various qualities ascribed to them such as reflective and anti-reflectivity and are usually patented secrets. These coatings help in giving the filter tighter selectivity for the desired wavelength of interest. These filters are specialists and are used for nebula and DSO’s mostly but that is not to say simple colour filters can’t achieve some desirable views also.

I have found coloured filters to be particularly useful for planetary views. Especially once I started to stack filters properly. One night I had been observing Saturn using the Brandon Magenta filter, I don’t know why but I enjoy the relaxing colour hue and it seemed to bring out some surface detail. I had recently purchased an old Meade #8 pale yellow, as I had always found the recommended #12 yellow for Saturn a hard colour for my eye to settle in to. Any way this night I decided to stack the yellows, first the #12 on the magenta, yuck it was kind of orange. But stacking the #8 pale yellow on the Magenta wow all of a sudden, the surface detail seemed to become noticeable with faint yellows and violet adding to the planetary view and the Cassini division still as clear as when viewed without the filter stack.

I quickly rushed Delvira over to Jupiter to see what the view would be like, hmmm not as satisfying but nice. Well let’s try something with those filters recommended for Jupiter, blues, #80a and #82a and they have been sitting there for a while even though I do like the look of Jupiter with the 82A. After various combinations of stacks with the blues I had decided to try the two Salmons #85 and #85C I had also recently added to my collection. Well, the variations on the two stacks were clearly evident as soon as I had tried it. What I had inadvertently done was put contrasting-coloured filters in a stack and equal density of the filters applied to Jupiter cut back the glow while boosting colour contrast, see figure 4 for a daylight example of the stack. Blue stacked on Salmon equalled contrast on Jupiter!

Figure 4. Filter stack. #80A and #85 showing contrast at the overlap, the background is looking out my study room window at trees on a clear bright sunny Australian afternoon.

The stack could also be modified, to feel cooler or warmer, by using darker salmon/amber on lighter blue or vice versa darker blue on the lighter salmon, in a stack see figure 5. I found the lighter blue stack on lighter salmon to be on par with the Baader CB, and to be truly honest the combo stack view is actually a little better as the colour was much more natural looking than the Baader CB. I found the darker blue and darker salmon did nicely too, especially while zooming in with the Tele Vue 11 and 7.4 mm plossl’s.  

                                              
Figure 5. Filters used for stacking experiment

These blue and amber selections in the Kodak series were specifically designed for photography to make tungsten lighting appear as daylight by cooling or to make daylight appear more like tungsten lighting by warming, however for astronomy the two combined in a stack can act as contrast. This is also the case for yellow and violet or green and red, hence why Saturn looked so pleasing in the afore mentioned stack, yellow - magenta, it is about using those combinations on the right views and off course what suits your eyeballs personal taste. So, what this ultimately gives us is options and the ability to vary the contrast with different combinations of these densities as can be seen in figure 4 the bright Aussie sunlight bleaches out the colour in the trees but where the two filters overlap the trees are not bleached out, not orange and not blue.

 Also worthy of note, the Brandon magenta stacked on the #12 made a darker salmon/amber colour, secondly, I found I could stack up to 4 filters, with #8 + magenta + #85 + #80a a current fav, before the amount of glass becomes a noticeable problem and 7 before it just got crazy, and there are a number of reasons for this which I won’t go into here. But that is not to say don’t do it! Just be sure your scope has the light capacity and don’t go cracking a mirror with your huge crazy filter tower while focusing or anything.

I have used various stacks on the moon. I found red-green contrast stacks were definitely useful at full moon, however, during dimmer waxing and waning, the Baader CB at high Mag (300x) proved to be just slightly sharper when doing those close up inspections of crater detail.

I definitely plan to try the red/green contrast combinations as a contrast booster for Mars and maybe Venus this planet season. However, Venus, I suspect may look better using a complimentary filter stack of violet and red or violet and blue rather than a contrast stack. I have found investigating your target and investigating your filters for the target are of great importance. I have recently come across a fantastic Astronomer called Charles Capen and his colleagues, Parker and Dobbin. Their investigations of colour filter studies on the solar system’s objects are extensive with complimentary stacks playing a large part. If you can find a copy of Observing and Photographing the Solar System, I suggest you grab it, even though the photography is in ol skool analogue SLR the science theory is solid, regardless. I would also suggest the Handbook of Kodak Photographic Filters as a good read too, go on Treat Yourself!

If I can impart just one piece of Capens wisdom, train your eye! Learn to look for the differences between views and filter changes.

In my playing with stacks, I have also used complimentary combo stacks to make green-blues. I did this in order to replicate something akin to #44 and #44a to view nebula. Why? Because I have noticed, most nebula filters are blue-green when held to bright light. With the reasoning they essentially must be a minus red filter and where the colour of the wavelength excitation emission is in the light spectrum (going back to my old research science days). These stack attempts have had varying success with brightness in the nebula image and were definitely not as sharp in the detail as the TV OIII and Neb, although if I could find a dedicated single glass filter unit #44 or #44a, which are very hard to find, to test my theory I suspect resolution may be somewhat better, however, I know it will never be as sharp in resolution as a coated nebula filter. I also suspect the magenta/violet filters, Figure 6, may be a cheaper alternative for Hydrogen Beta filters.. but have however to test these theories but hope to when dark and clear skies next permit.

Well now we have the concepts contrast and compliment, I am going to go do some gardening. See you in part 2 Modern Visual Filters and Visual Colour Filters a REALLY Full on Review

Clear skies and have fun with your stacks on those not so clear nights! 

Acknowledgements:

Anonymous for the light spectrum diagram

https://www.kodak.com for all the wratten spectral data free for view

...and Kodak - if you read this could you release a specific glass 1 ¼” and 2” telescope set of these mentioned filters please!! 😉

Table 1. list of some Kodak wratten (#) with my decription