Refractors, we of course love – and there are many advantages to enjoy over newtonian reflectors. Quick cooling downtimes (just leave it for 15 minutes before viewing usually does it), no coma (which is where stars appear elongated at the edges), increased contrast and the ability to use eyepieces that extend well beyond 32mm for truly wide views. But achromatic refractors do introduce an irritation – “color fringing“, or “color blurring” as it is also known (“colour” fringing for us Aussies) which appears when a bright object is observed.
What causes it?
First of all, there is nothing wrong with your telescope. It isn’t a defect, it’s a natural scope characteristic. When light is focused through two elements, they don’t all end up in precisely the same spot. Yellow is more on one side, and blue is more on the other end, and everything else sort of gets in the middle.
What can you do about it?
There are some great options to deal with colour fringing, here are some:
Option #1 – Ignore it
Some do not find it particularly objectionable. The fact is, you may not find the colouring particularly objectionable. If that is the case, don’t worry about it and ignore it. But what if you can’t let it go?
Option #2 – Enjoy lower magnification views, change what you look at.
The shorter the tube, the wider of a view it allows – and that is often the point of a particular telescope, for example the Kson Ke’sil is a 150mm 750mm long scope, a focal ratio of f/5 which at that aperture gives a lot of CA, but you would never use that telescope for the main purpose of looking at Jupiter (a very bright object) – you would enjoy the massively wide views it offers, and allows you to see things, particularly with ultra low powered eyepieces that are just unrivalled. With such low magnifications, CA isn’t even an issue.
Option #3 – Choose a longer focal length scope
This clearly isn’t an option if you already have a refractor (if you do, option 3 is similar) – but if you are making a purchase decision, go with a longer telescope. The longer the telescope tube it, as compared to the aperture, the ‘slower’ the scope becomes and the less apparent CA is. For example, an f/7 will have less colour fringing than a f/5. A f/9 will be better than a f/7 and a f/12 will be even better.
Option #4 – Aperture Cap / Mask
This option is like option 3 – except for those who have already have bought a telescope. You can ‘artifically’ make your telescope have a slower f/ratio by adding an aperture mask. Create an aperture mask. An aperture mask is simply a ‘cap’ that goes over the end of your telescope with a smaller hole! So now, your 100mm telescope becomes just 80mm, drastically cutting the CA. This is a wonderful solution, and Kson now provide professional aperture caps as part of some of their scopes. In the picture below, notice how the 127mm cap has a 2nd cap in it – reducing the aperture to 80mm. This allows the normally f/9.4 scope to work as a f/15 telescope, significantly improving any colour fringing or blurring. You can even try making one with black cardboard and clips to hold it in place.
Option #5 – Filters
Baader have a variety of speciality filters that can deal with CA. Just one problem – the heavier handed a filter you use, the more it will change the colour balance when you look through it.
- The most subtle is the fringe killer (click to see it). Takes the ‘objectionable edge’ off, while trying to retain most of the colour balance.
- The mid-range one is the semi-apo (click to see it). A bit of a more aggressive filter than the fringe killer, more effective, but does noticeably change the colour balance.
- A more heavy-handed filter is the contrast booster (click to see it). Very aggressively closes down the violet, but changes the balance significantly.
Read a great comparison of them here. (PDF Download)
One of the most interesting options is the inexpensive 495nm yellow filter (this is NOT the same as wratten yellow). These will add a yellow tint if you are looking at say, lunar or Jupiter etc, but for nebulae, and stars, it will not be very obvious. What will be obvious is the boost to contrast and clarity that comes from having offending areas of the light spectrum cut. M42 looked darker, more contrasty and the stars became pinprick clear. A real delight to use.
Option #6 – Try a different diagonal
Some diagonals just seem to add more CA than others – some suggest using a mirror diagonal in shorter focal length telescopes improves the image. To find out, why not use a friends, or take yours to an Astronomy Association for a nights view, and politely ask if someone would like to try getting a better image by using their diagonal? If you see a difference, it could be switching over to another type of diagonal, such as this mirror diagonal (click to see it) will help.
On the very sophisticated end of things, we have the CED, which is a special diagonal with coatings deployed straight on the glass that remove some of the violet – not bad! Of course, coming in at hefty USD$150 plus shipping, you would have to think about it.
Option #7 – Select simple eyepieces with a long focal length refractor
Complex, multi-element eyepieces can tend to add more CA – why not do the opposite, and go for simpler eyepieces? This is particularly effective when dealing with longer telescopes where you are trying to reduce CA, as these simpler eyepieces (especially orthosocopics – click to see them) perform very well at these ratios.
Option #8 – A combination of these approaches
Why not slap on an aperture mask, and apply a fringe killer? Or some other combination of the above? This can combine several highly effective methods!
Usually, these approaches or a combination of approaches works for most people. If it still doesn’t work for you, or you think it will really bother you, you might want to swap the set of problems with refractors for a different set of problems with a different telescope design, such as a newtonian reflector which does not have that problem.
Another way around it is to spend more on your telescope, and invest in a telescope with ‘ED’ (extra-low dispersion”) glass. This is rare, expensive glass and is specially formulated. Using rare-earth compounds, these glasses greatly reduce chromatic aberration (“CA”) (which is what happens when all the colours don’t arrive at the same focal plane). The price of ED has certainly come down, and we do have the SVBony ED scope, which is a marvel at the price, but there are other ways we deal with the fringing – which for you may be perfectly adequate and allow you to invest those funds in a better mount, extra eyepieces, or a decent Astronomy book that will add much more enjoyment than expected!
Up to you what you try, and that’s part of the fun! Don’t let uncertainty stop you from enjoying the unbelievable views that are now possible compared to even just a hundred years ago. Get out there and start observing!