So what the heck is a Chromacor???

Transcription

1 So what the heck is a Chromacor??? Despite the fact that it s now been out of production for at least a couple of years, for those who might be interested I thought I'd share a few details of this unusual accessory now that I've had it a couple of months and had a chance to experiment with it a bit. The Aries Chromacor is a type of optical corrector that was developed by the company Aries Instruments who are based in the Ukraine of all places. The company is led by Valery Deryuzhin who was responsible for the design and development of the Chromacor which first appeared on the market in As far as I know it's no longer in production although a Chromacor II was sold for a while before that too seemed to vanish from the market. This possibly coincided with the availability of more competitively Chinese ED and apochromatic refractors although 6 inch specimens of these are still rather thin on the ground. Interestingly, around the same time as the Chromacor was being developed, Roland Christen of Astro Physics in the USA was experimenting with a similar device although he only ever made 2 of them one of which was advertised for sale recently in the USA at something over $3,000!. I can recall reading about the Chromacor when it first came out, probably in Sky & Telescope magazine which I used to get from time to time back then (pre-sgl days of course!). The claims made for the Chromacor were impressive - basically it was specifically designed to correct the majority of chromatic aberration (false colour - CA) in the Chinese 120mm F/8.3 and 150mm F/8 achromatic refractors that had recently appeared under brand names such as Konus, Bresser, Helios, Celestron and later of course, SkyWatcher. As well as dealing with CA the Chromacor can correct various levels of under / over correction that found in mass produced objective lenses. A range of correction errors were covered though a number of Chromacor versions each of which introduced a specified amount of counter-correction or, for those who were lucky enough to have scopes with objectives finished to within 1/6th wave accuracy, you could get a "null" correction Chromacor. So a "magic bullet" perhaps eliminating CA and correction errors at a stroke and transforming your humble Chinese achromat into a Tak-beating apochromat?. Well, perhaps unsurprisingly, it was not as simple as that. Yes, the Chromacors could make a big impact on CA, reducing it by as much as 80-90% and, yes, they can get objective lenses to closer to ideal correction levels. To achieve this nirvana there were some hurdles to cross however: - Cost: When available in the UK the Chromacors retailed for each - more than the cost of even the 150mm F/8 optical tube assembly.

2 - Optical Matching: For the maximum effect the Chromacor should be matched to the optical characteristics of the scope it is to be used in. Prospective owners needed to be able to correctly interpret star tests to determine which model to buy and that "risk" was theirs unless they could fork out for a matched OTA / Chromacor pair that some suppliers offered at a price of course!. - Collimation: Chromacors need the scope they are used in to be pretty accurately collimated for them to do their stuff well. - Fine Tuning: Chromacors are pretty fussy where they are positioned in the optical system in that they need to be as near as dammit right on the optical axis and a specified distance from the eyepiece focal plane. Positioning accuracy depends on the mechanical precision of the focuser and the standard Chinese R&P, while functional, is not known for it's precision. So the Chromacor option was not for the faint hearted or those on a tight budget. To put the cost into perspective though, the Vixen 150mm AT-Lux F/9 ED refractor (which I have coveted ever since I first saw the pictures in Astronomy Now around a decade ago) cost something over 4000 as an OTA as did Meade's ED152 refractor. From the research into the Chromacor that I'd done it looked at least possible that levels of performance approaching the above scopes could possibly be attainable for a much more modest investment in money but with some time and care. In 2001 however I had neither a suitable Chinese refractor (I actually had a rather nice TAL100R refractor at the time), or 500 to spare for an obscure accessory so I consigned the Chromacor to the "interesting but out of my league" file. Fast forward to a few months ago. I had recently purchased an excellent 127mm F/9.4 Bresser Messier achromatic refractor and had got the "big refractor" bug. A 150mm F/8 achromat came up for sale at a bargain price - an older scope, it was branded Konus and bright orange / yellow in colour!. It's the same optical tube that was marketed under the Celestron, Helios and SkyWatcher though so I made an offer and even got the OTA delivered as it's owner was coming my way one weekend. I tidied the big refractor up a bit although, optically, it was unmarked. A check with my cheshire collimator showed the collimation to be off a bit but the views through it seemed nice albeit with the inevitable CA that an F/8 achromat produces around bright objects.

3 The Konus Konusky 150mm F/8 A few weeks later and I was very suprised to see an Aries Chromacor pop up on E.Bay. As far as I recall the first I'd seen for sale for years. The one on offer was the O-1 model which is designed introduce 1/6 wave over-correction into optical tubes where the native optics are under-corrected. A crash course on refractor star testing and the diagnosis of the results was followed by a couple of clearish nights and the realisation that my 150mm was indeed a little under-corrected as far as I could make out - the co-incidence was too much to resist so I made a bid for the Chromacor and managed to win the device at what I considered to be a very reasonable price, given that there was some uncertainty over the impact it would actually have. I'd seen photos of the devices so I knew they were nothing much to look at. The reality was as per expectation and the Chromacor is a rather unprepossing object when you consider the original purchase price. I've attached some photos below which show the device in all it's plainness - a 7cm long tube of some sort of dark cast alloy with a wider base which is threaded to fit the standard 2 inch astro filter threads and a slightly narrower end which contains the mysterious cemented 3-element corrector lens which is reputed to be made of some very exotic and expensive glass types.

4 The Aries Chromacor O-1 The Chromacor is designed to be used with 2" diagonals - it screws on as a filter would and this is how it's held in the light path of the optical system. The notes that come with it are very specific about this positioning, in particular i) the shoulder of the Chromacor (where the 2" thread is cut) must be 161mm (+/- 2mm) from the focal plane of the eyepiece and ii) the central axis of the Chromacor must co-incide with, and be parallel to, the optical axis of the scope. During it's development, the Chromacor prototypes were tested using Intes 2" mirror diagonals and standard plossl eyepieces so the installation criteria are based on the length of the light path for those items. The device can be used with other brands of diagonal and eyepiece types provided that the required spacing between the Chromacor and the focal plane of the eyepiece can be achieved. For this purpose Aries recommend using 48mm filters with the glass removed as

5 spacers. My Chromacor came with 4 of these which allowed a fair amount of flexibility for fine tuning to suit different eyepiece / diagonal combinations. The Chromacor was delivered only a week or so before the SGL5 star party so I was keen to get it installed and operational for that event as I'd planned to take my 150mm F/8 there. There was one modification ot the scope that I needed to do before I could even use the device - that involved shortening one of the small screws that holds the focuser rack to the drawtube as it protrudes into the tube and would prevent insertion of the Chromacor. An easy "mod" although I had to ensure that I left enough of the screw in place to hold the rack securely. I had a couple of clear nights in the run up to the star party so I was able to play around with the Chromacor positioning and try and find something that put the correct spacing between my most used higher power eyepieces (Tele Vue Nagler Type 6 s) and the Chromacor, taking account of the light path through my old but good Meade 2 mirror diagonal. Three 5mm spacers seemed to be roughly right. The Chromacor Installed Control of CA seemed to improve when the Chromacor was rotated to a specfic position which shows I think that the alignment of the device on the optical axis was not quite spot on but despite this and some collimation issues, the scope / Chromacor combination performed reasonably well at the Star Party although I now realise that it was not giving it s best. Since the early testing I have managed to obtain much better collimation in the scopes optics through adjustments to the objective lens cell and the focuser. I have also replaced the three spacers with a single 14mm Baader Hyperion fine tuning ring which gives a more precise mounting for the Chromacor I feel while maintaining an optimum distance from the eyepiece focal plane. Now that the scope is properly collimated performance on lunar limb is a thin purple line inside of focus, a thin lime-green one outside of focus and little or no

6 discernable CA along the limb at focus this is a significant improvement over the levels of CA that the native optics of the scope show. Another noticeable benefit is that the scope now allows higher powers to be used. There used to be a noticeable cut-off in performance between 250x-300x (closer to the latter) but with the Chromacor my shortest FL eyepiece (3.5mm = 343x) is getting loads of use, providing crisp and contrasty views of Saturn, Mars as well as fine detail on the lunar surface and tight double stars. The ability to take power exceeding 50x per inch of aperture under good observing conditions is a sign of well corrected optics and is often remarked on by reviewers of good apochromatic refractors. On a bright star such as Deneb, at focus you get a well defined airey disk with a number of concentric diffraction rings and only a slight haze of purple chromatic aberration. The diffraction rings expand as you rack the focus slightly in and out and image is similar both inside and outside of the point of focus. Without the Chromacor diffraction ring definition was poor inside focus and the splash of purple much more prominent. Although I feel there are still further gains to be made in performance, the scope is now delivering some very satisfying views, for example defined crater pits for the Plato craterlets under the right lighting conditions, details on Mars when it was far past opposition and a sub 6 arc second disk, cleanly splitting double stars down (so far) to 1.2 arc seconds. There is still some evidence that the Chromacor is not precisely centered in the optical train as there is a slight increase in visible CA as a bright object moves towards the edges of the field of view so my next move is going to be to replace the standard rack and pinion focuser with a more precisely machined crayford unit sporting compression ring fittings which will hopefully hold the diagonal / Chromacor more precisely on the optical axis. I m not claiming that the combination of this scope and the Chromacor has delivered optical performance to rival Takahashi or Astro Physics but I believe the scope now approaches the control of chromatic aberration of a decent 6 inch ED doublet or a good achromat of considerably slower focal ratio (I d estimate an F/12 or slower). As I believe there is a bit better still to come and I don t happen to have 4000 to spend on a 6 inch apochromat, I m very happy to keep tweaking this clever Ukrainian optical gizmo to get the very best out of it. I do though still have a dream featuring a large refractor only now it s an 8 inch F/10 D&G achromat with the Chromacor fitted in it s drawtube!. John Huntley July 2010