Soon, I’ll be taking delivery of a 90mm (3.5″) achromatic refractor with a focal length of 900mm, which I picked up for the princely sum of £55, plus shipping. And though it has not yet arrived, I expect its performance to be very good. The instrument obeys (or very nearly so) the so-called Sidgwick Standard of colour correction, where its focal ratio (determined by dividing its focal length by its aperture) divided by its aperture in inches is approximately 3. For many decades, this relatively simple relation has been used to good effect to create many choice achromatic refractors.
Here, I want to demonstrate that its derivation is based on sound optical principles. Virtually all achromatic doublets are of the C-F variety, meaning that red and blue wavelengths are brought to the same focus with green wavelengths usually coming to focus slightly ahead of the coincident red (Fraunhofer C-line) and blue (Fraunhofer F-line).
In the absence of significant spherical aberration, the depth of focus of the telescope is approximated by +/- 4 lambda (f/D)^2 where f is the focal length, D is the aperture of the instrument and lambda is the wavelength. Typically, the longitudinal chromatic aberration for a standard crown/flint object glass is ~ 0.0005f i.e. about 1/2000th of its focal length.
Let us now see what happens when we constrain the colour aberration within the depth of focus of the telescope.
Equating both expressions, we have;
0.0005f = 8 lambda (f/D)^2
f = D^2/16000lambda.
Setting lambda = 550nm (green light) and then converting to inches yields 2.17 x10^-5.
Substituting the value of lambda in inches into the above formula yields f= 2.88D^2 or ~
f = 3D^2.
So, plugging some familiar apertures into the formula (outlined in bold above) gives a minimum focal length necessary to give wholly negligible secondary spectrum.
Here are some examples:
Aperture(“) Minimum Focal Length (“) Focal Ratio (f/D)
3.0 27 9
4.0 48 12
5.0 75 15
6.0 108 18
If we divide the focal ratio by the aperture we come up with 3; thus the origin of the Sidgwick Standard for achromatic doublets.
First hand experience with a wide variety of achromatic refractors will quickly convince you of the soundness of this result. The images served up by instruments with the above specifications give beautiful images. Indeed, the Sidgwick Standard can be further relaxed to 2 or even 1.5 without greatly affecting the information garnered by the telescopic image (the unfocused light being effectively attenuated or completely removed by modern minus violet filters).
The 3.5″ Skywatcher has a focal ratio of 9 and so is just under the Sidgwick Standard (~2.6) Thus, if properly executed, it ought to be quite a delightful telescope.
Well, it finally arrived this arvo (Tuesday, July 9); the Evostar D= 90mm f =910mm achromatic refractor.
The guy whom I bought it off packed it rather well and my first impressions were very good indeed. There were no signs of much mechanical wear and tear, the lens looked pristine and the focuser did what it ought to.
I scurried to affix a dovetail plate to the tube rings that came with the instrument and had just about had it mounted on the Vixen Porta II when I heard the thunderous sound of my wife boom from the kitchen.
“ What’s this? Another telescope? You said you were done buying and selling.”
“Yes dear, I am”, I replied, “it’s strictly for a review. It’ll be gone to a good home as soon as I’m done with it. Any way, folk need to know when there are bargains to be had,” I continued, “remember my poem about champions?”
“Of course!” she knew immediately what I was talking about.
Her menacing look turned to an enormous smile and I knew that all was well once again.
So what did my hard earned pecunia deliver?
It’s a lovely, light-weight tube; very portable. The dew shield is plastic but not a problem in itself.
The dust cap (also plastic) is neat looking with a 60mm aperture mask, enabling it to operate as a 60mm f/15. Cool!
Removing the dew shield reveals a pristine 90mm air spaced doublet objective with very nice and durable coatings applied to all surfaces. The instrument is not collimateable however, but neither is an ED80 or a Televue 85.
Checking collimation with a Cheshire eyepiece under a bright blue sky revealed good but not perfectly aligned optics but neither was it enough to worry about in the slightest. Unlike some early Meade 90mm f/11 OTAs I’ve examined, the paintwork on the inside of the tube was immaculately applied and the baffling was more than adequate, as evidenced by the shot I took, looking up the tube with the diagonal removed.
The focuser is the regular 1.25” Synta rack & pinion. Nothing to write home about but more than adequate for the task; it’s f/10.1 remember!. It handled heavy 1.25” eyepieces very well. Pity it didn’t come with a 2-inch focuser for use with modern wide angle eyepieces. But you have to remember this puppy cost me £55 plus shipping.
The ‘scope can be used with a regular 90 degree diagonal for astronomical use or a 60 degree, correct-orientation diagonal for terrestrial applications, like this one.
Day time assessment of the optics revealed nothing out of the ordinary. Indeed it fulfilled my expectations. Low power views of the corbies seeking refuge in the trees nearby were sumptuous; tack sharp and rich in detail, free of any secondary spectrum.
When the temperatures stabilised in the early evening of this scorching, July day, I inserted my 4.5mm Delos ocular delivering 202 diameters. Carefully focusing on the horse chestnut tree I was treated to an excellent image. The leaves were nice and sharp. Some small amount of secondary spectrum could be seen around their edges but nothing that could seriously detract from the excellent high power views this telescope was clearly delivering.
Did I mention that it only cost me £55 plus shipping?
Very happy with what I had thus far seen, I grabbed a bite to eat and awaited the semblance of darkness to fall on the landscape.
Wednesday, July 10
Unfortunately, while I was blogging last night, thick, low altitude clouds moved in from the east making any observations of the heavens a no no.
The clouds persisted right up until the late afternoon today, when the strong July Sun finally managed to burn them away. Eager to test the Evostar 90mm refractor out on ANY celestial object, I placed a white light solar filter on the instrument. It fitted snugly once the dew shield was removed.
Inserting a medium power eyepiece, I was delighted once more with the sharp, high resolution detail I could see on the solar photosphere. A string of sunspot groups were observed in a diagonal line across much of the Sol’s southern hemisphere.
Still awaiting a star test but I’m confident it will show well corrected optics.
From where I’m standing, even those ‘bargain’ ED ‘scopes in the same aperture bracket are beginning to look ridiculously over priced, especially if you’re a dedicated visual observer.
Under a Clear Sky
Shortly after midnight on Thursday July 11, I got an opportunity to star test the instrument under excellent seeing conditions. Contrary to what you’ve heard before, it only takes one good night to assess a telescope’s optics. Only on imperfect nights does one need to pass it through multiple star tests.
Centring the bright star Vega in the field of view of an eyepiece generating 150x, the Evostar produced a clean, white, sharply focused airy disk with a nicely formed first diffraction ring. The star was surrounded by a beautiful halo of purple; absolutely normal for this type of telescope and perfectly acceptable for visual use. Defocusing ever so slightly showed no signs of astigmatism. Examining the Fraunhofer diffraction pattern both intra- and extra-focally using a green filter revealed smooth, well corrected optics with only slight under correction. All in all – a very good result for a telescope that cost so little. Having had the privilege of testing out a number of small refractors of similar focal length, I am more confident than ever in claiming that such instruments are very easy to manufacture to very high standards.
Epsilon 1 and 2 Lyrae were perfectly resolved into fours stars. The greenish companion to Epsilon Bootis – now sinking lower into the western sky- was nicely separated from its orange primary at 150x. The same was true of Delta Cygni. Pi Aquilae, while certainly not resolved, was plainly shown to be strongly elongated. All in all, this budget refractor delivered up everything an unobstructed 90mm aperture ought to.
Its deep sky prowess was decent too. During the darkest period of twilight, I saw the beautiful incandescent annulus that is the Ring Nebula, M57, sharply defined at 101x. Moving into Hercules, I tracked down the globular cluster M13 and was rewarded with a bright, highly condensed image. With a concentrated gaze I could make out quite a few individual stars on its periphery. Switching to a quality low power ocular, I swung the telescope over to Cassiopeia and to the open cluster M52, now fairly high in the northeast. The telescope did not disappoint, revealing several dozen stars in a kidney-shaped arrangement, the eye being drawn to a striking 8th magnitude orange sun on its flank. The lack of field curvature of this ‘slow’ object glass makes examining these bright clusters a particular joy.
In summary, the SkyWatcher Evostar 90mm achromat is arguably one of the best buys in the small refractor market. Usually touted as a beginner’s telescope, it is much more than that. The Evostar 90mm would embarrass instruments costing ten times its modest cost new. It is super light, super portable and delivers fine views at low and high power. If I were in the market for a no-nonsense 90mm telescope, this would be the one I would unreservedly recommend. Because of its low cost and excellent durability, it would be especially useful to astronomy clubs undergoing public outreach. It would also serve as an excellent telescope for the instruction of undergraduates in astronomy/astrophysics.
Available in the UK for as little as £139 UK complete with AZ 3 mount, or as part of a slightly more expensive package complete with a lightweight equatorial mount. See here for more details.
Also available in the US under the Orion rebranding. See here for details.
January 13, 2014 update: Over the last few weeks I have thought about acquiring a modest additional family telescope for use while I’m on vacation in southern Ireland. In this capacity, I had an opportunity to re-consider the Evostar 90 achromat as a suitable instrument. After discussing the issue with my wife, I decided to pick one up on the used market for £85. The previous owner had the focuser re-lubricated, and flocked the interior of the tube and dew shield. I had a chance to evaluate its optics and, as I had come to expect, it more than delivered the readies. I will have the instrument shipped over to my sister’s home, together with a Vixen Porta II alt-az mount, in the coming week.
There! You see!
Truly a laird wi’ my telescopes three.
PostScript: I’ve since negotiated a better way forward with my CEO. I can buy items so long as I raise the money from selling some of my existing kit. Fair’s fair, I suppose.
Dr. Neil English is author of Choosing and Using a Refracting Telescope.
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