It is undoubtedly true that by far the most prevalent reason why so many amateurs have dissed Newtonian reflectors in the past boils down to poorly collimated ‘scopes which lead to less than inspiring images. The amateur who pays close attention to accurate collimation will however discover the solid virtues of these marvellous telescopes and will soon forget the bad experiences of the past.
I’ve noticed a trend over the last few decades, where more and more amateurs have become lazy and impatient. They want instant gratification. This is one of the main reasons why many have turned to hassle-free instruments such as small refractors and Maksutov Cassegrains. It’s an entirely understandable trend, but in other ways it is lamentable. One of the downsides of this trend is that amateurs have become less concerned with learning practical optics, deferring instead to higher tech ways of obtaining optimal results in the field. One such technology is the laser collimator; a very useful device that has made accurate collimation far less of a chore than it was just a few decades ago. But while many have defaulted to using such tools as labour-saving devices, they have, at best, become less familiar, or at worst, all but forgotten the traditional tools used in the alignment of telescope optics; tools such as the collimation cap and the Cheshire eyepiece, and in so doing have less and less understanding of how their telescopes actually work.
The desire for super-accurate collimation has undoutedly been fuelled by the advent of faster optical systems; often supporting sub-f/5 primaries. Once, the traditional Newtonian was almost invariably made with higher f ratios:- F/7 to f/10 and beyond, and requiring very little in the way of maintenance. This is abundantly evidenced by the scant attention astronomy authors of the past gave to such pursuits. In contrast, modern Newtonians are usually f/6 or faster, necessitating much greater attention to accurate optical collimation if excellent results are to be consistently attained during field use.
In my chosen passtime of double star observing, I have acknowledged the need for accurate collimation. Such work often requires very high magnifications; up to and in excess of 50x per inch of aperture, to prize apart close double stars, some of which are below 1 arc second in angular separation. At such high powers, sub-standard collimation results in distorted images of stellar Airy disks, and that’s something that I’m not willing to put up with. In this capacity, I have tested a number of collimaton techniques using a few different laser collimating devices but have also spent quite a lot of time comparing such methods to more traditional techniques involviing the tried and trusted collimation cap and Cheshire eyepiece.
To begin with, it is important to stress that the methods covered in this blog can be achieved easily with a little practice, and I will gladly defer to recognised authorities in the art of Newtonian collimation, such as the late Nils Olif Carlin and Gary Seronik, who have done much to dispel the potentially stressful aspects of telescope collimation. Nothing I will reveal here goes beyond or challenges anything they have already said. My goal is to reassure amateurs that one can happily live without lasers, especially if your Netwonians are of the f/5 or f/6 variety.
Many of the entry-level laser collimators often manifest some issues; partcularly if they are not collimated prior to use. Thankfully, the inexpensive SkyWatcher Next Generation that I have used for a few years did come reasonably well collimated, but others have not been so fortunate. One easy way to see if your laser collimator needs collimating is to place it in the focuser of the telescope and rotate it, examining the behaviour of the beam on the primary. If the beam does not stay in place, but traces out a large annulus, you will have issues and will need to properly collimate the laser. This is not particularly difficult to do and many resources are available on line to help you grapple with this problem. See here and here, for examples.
Of course, you can pay extra for better made laser collimators that are precisely collimated at the factory. Units that have received very good feedback from customers include systems manufactured by Hotech, AstroSystems and Howie Glatter. Some of these are quite expensive in relative terms but many amateurs are willing to shell out for them because they deliver consistently good results. My own journey took me in a different direction though. Instead of investing in a top-class laser collimator, I re-discovered the virtues of traditional techniques involving the collimation cap and Cheshire eyepiece.
My personal motivation to return to traditional, low-tech tools was stoked more from a desire to understand Newtonian telescopes more than anything else. Any ole eejit can use a laser collimator but it deprives you of attaining a deep understanding of how Newtonians operate. In addition, I have frequently found myself dismantling whole ‘scopes in order to get at the mirrors to give them a good clean and this meant I had to learn how to put them back together from scratch. The simpe collimation cap has been found to be an indispensable tool in this regard, allowing one to rapidly centre the secondary mirror in the shadow of the primary.
Using just this tool, I’ve been able to set up all my Newtonians rapidly to achieve good results from the get go, at both low amd medium powers more or less routinely.
For the highest power applications more accuracy is required and I have personally found that a quality Cheshire eyepiece to be more than sufficient to accurately align the optics in just a few minutes. Not all Cheshires are created equal though; some are less accurate than others. For my own use, I have settled on a beautifully machined product marketed by First Light Optics here in the UK ( be sure to check out the reviews while you’re at it). For the modest cost of £37, I have acquired a precision tool to take the hassle out of fine adjustment. The unit features a long sight tube with precisely fitted cross hairs that are accurately aligned with the peep hole. It needs no batteries and comes with no instructions but with a little practice, it works brilliantly!
Because all of my Newtonians are of the closed-tube variety, they are robust enough to only require very slight tweaks to the collimation. I would estimate that 80 per cent of the time, it is only the primary mirror that requires adjusting in field use. I have found this overview by AstroBaby to be very useful in regard to using the Cheshire and would recommend it to others.
The Cheshire eyepiece is a joy to use when collimating my 130mm f/5. Because the tube is short, I can access both the primary and secondary Bob’s Knobs screws to whip the whole system into alignment faster than with my laser. With my longer instruments; partcularly my 8″ f/6 and 12″ f/5, collimation using the Cheshire is decidely more challenging as they both have longer tubes. That said, by familiarising one’s self with the directions of motion executed with the three knobs on the primary, one can very quickly achieve precise collimation. One useful tip is to number the knobs individually so that you can dispense with the guesswork of which knob to reach for to get the requisite adjustment. At dusk, with the telescopes sitting pretty in their lazy suzan cradles, and with the Chesire eyepiece in place in the focuser, I swing the instrument back and forth to alternately view the position of the primary in the eyepiece and the knob(s) I need to turn. Doing this, I get perfect results in just a few minutes; a little longer than can be achieved with a laser, admittedly, but not long enough to render the process exhausting or boring. It’s time well spent.
The proof the pudding, of course, is in the eating, and in this capacity, I have found the Cheshire to achieve very accurate results each time, every time. Indeed, it has made my laser collimator blush on more than a few occasions, where high power star tests and images of close double stars reveal that the laser was out a little, requiring a collimation tweak under the stars. Indeed, the Chesire is so accurate that it has become my reference method to assess the efficacy of all the laser collimators I’ve had the pleasure of testing.
While I fully acknowledge the utility of good laser collimators, I get much more of a kick out of seeing, with my own eyes, how all the optical components of the Newtonian fall into place using the Cheshire. Furthermore, the fact that it requires no batteries (and so no issues with the unit failing in the field for lack of power, as has happened to me on more than a few occasions), deeply appeals to my longing for low-tech simplicity in all things astronomical. The fact that the aforementioned amateurs also recommend the Cheshire as an accurate tool for collimating a Newtonian makes it all the more appealing.
Having said all this, the utility of a Cheshire eyepiece lessens as the f ratio of your telescope gets smaller, so much so that for f/4 ‘scopes ar faster, the laser technique will, almost certainly, yield more accurate results. But that’s OK. We are blessed in this day and age with many good tools that can make Newtonian optics shine!
Note added in proof: August 14 2018
If you do decide that you don’t like using a good Chesire eyepiece for precise collimation of your Newtonian reflectors, then I would highly recommend the Hotech SCA laser collimator. It’s an ingenious device (but costs significantly more than a regular laser collimator), but in this case you really do get what you pay for. I have tested the device on all three of my Newtonians and it gives accurate and reproducible results that agree perfectly with the Chesire. It yields perfect star tests at appropriately high powers (I’d recommend a magnification roughly equal to the diameter of your mirror in millimetres for such field tests) both in focus and defocused. I’d go for it if you can afford it. You will still need the collimation cap to centre the secondary before use however. See here and here for more details.
Neil English is author of Choosing and Using a Dobsonian Telescope.