This review look at another approach, the Aries Chromacor (manufactured in the Ukraine), which screws into your 2” diagonal. The Chromacor is a very different solution: dramatically more expensive, less flexible, but with nearly perfect color correction, and much better image detail. In spite of costing almost ten times as much, I chose to keep the Chromacor, and sell the Fringe Killer. But don't stop reading: the reasons why I made my decision may not apply to your achromatic refractor--or to your wallet.
As I discussed in the Fringe Killer review, my testing methodology was based on what I could see--not some theoretical measure of optical quality, and is necessarily subjective, reflecting the deficiencies of my retina, and how my brain processes the signals that reach it. I performed all of the tests in this article on one evening. To make sure that I was comparing equivalent viewing conditions, I removed and reinstalled the Chromacor repeatedly. Having picked a celestial object, I would observe it through an eyepiece without the Chromacor, then reinstall the Chromacor into the diagonal, and repeat the sequence with the same eyepiece. I used the same diagonal, even though it would have been easier to use two separate diagonals--but it would have added one more variable to the test.
I observed from my suburban Boise backyard on the night of March 23, 2004. The sky was clear, with very little moisture, but some turbulence. While my location was not perfectly dark or perfectly calm, it approximates what most refractor users will experience. My targets were an 8% illuminated Moon, Venus, Saturn, and Jupiter. (The Moon was on the horizon, casting little light, by the time I observed Saturn and Jupiter.)
I should add that I have taken this telescope to darker, more hospitable skies before, without the Chromacor. It performed considerably better in the uncorrected form than the results described below. Consider the following results indicative of the relative benefits of the Chromacor, not the best possible results for this refractor.
What The Chromacor Does
The Chromacor works much like the third lens that many apochromats use to bring all the colors of the rainbow back together. So why doesn't a Chromacor make your cheap achromat cost as much as an apochromat? Partly, because the Chromacor is at the eyepiece end of the optical path, and it is therefore smaller than a lens the size of your objective. Anyone who has priced refractors knows that the price rises with at least the third or fourth power of the increase in objective diameter, so solving the color problem in a smaller piece of glass, as the Chromacor does, is a cheaper solution.
In addition to correcting chromatic aberration, the Chromacor also compensates for some correction defects in the refractor’s objective. If your refractor is undercorrected, you can buy a Chromacor that adds some overcorrection, and vice versa. Chromacor comes in several “flavors”: undercorrection (1/6th wave, 1/3 wave, and 2/5th to ½ wave), overcorrection (1/6th wave), and null (for those lucky enough to get an achromat that is less than 1/6th wave from perfection).
Ordering the right Chromacor means that you need to star test your scope first. This was the most annoying part of the process. I’ve never bothered with star testing telescopes before; there wasn’t anything that I could do (I thought) about a refractor, and I certainly wasn’t going to refigure a working telescope mirror. Another reason for star testing is to see if your refractor has a consistent and symmetrical figure. The Chromacor will not correct for certain types of optical defects, such as severe astigmatism or pronounced zones.
I purchased my Photon Instruments 5” refractor used from someone who had owned it less than a month. (He had discovered that it was too heavy for his mount.) My first few experiences were quite frustrating; inside focus I had crisp, clearly defined diffraction rings. Outside focus, I had nothing but a fuzzy blur, even when using a yellow filter. With helpful advice from a number of other amateurs (and one professional, the CEO of a very well-known American maker of apochromatic refractors), I found that by loosening the front retaining ring of the objective, I was able to get diffraction rings consistent with 1/3 undercorrection, or perhaps a bit worse. This isn't great, but adequate, especially considering that I paid less than $400 for a 5" achromatic telescope.
I then ordered a Chromacor O-1 from Astrobuffet (http://www.astrobuffet.com/), a U.S. importer of the Chromacor. “O-1” indicates that this particular Chromacor provides about 1/6th wave overcorrection. (Unfortunately, there is no Chromacor O-2 for 1/3rd wave overcorrection, which might have improved my refractor even more.) Unlike the Fringe Killer, which simply screws into the filter threads of your eyepiece, Chromacor is a bit more complex to install. (John Hopper at Astrobuffet was extremely helpful, talking me through the process.)
The Chromacor is quite sensitive to the distance to the field lens of your eyepiece. It wants to be 161mm +- 2mm. Using a friend’s very compact diagonal, which did not give enough distance, the Chromacor was frankly a disappointment. There was only a limited reduction in violet haloes, and no noticeable improvement in resolution. Using a more commonly sized diagonal, the Chromacor met my very high expectations for color correction and resolution.
My first test subject was the Moon--a subject bright enough that chromatic aberration is readily apparent. I started with the 35mm Ultrascopic. Low power eyepieces are generally least affected by chromatic aberration, and this showed in the relative performance. There was a slight, not objectionable color fringe visible in the naked scope. The Chromacor showed a barely visible color fringe on the limb of the Moon--just a bit better.
Unsurprisingly, because of the low magnification, and the advantages of the orthoscopic design, the 18mm Omcon orthoscopic showed even less color fringing at the limb than the 35mm. The fringing was barely visible without the Chromacor, and even less visible with the Chromacor. Image detail was indistinguishable--but then again, at 18mm, the image was very crisp with or without Chromacor.
Raising the power revealed the advantage of the Chromacor not just on color, but also in image detail. At 9mm, the Chromacor allowed me to pick out detail on the floor of a small crater near the terminator--a crater floor that was featureless with just the uncorrected refractor.
At 6mm, the uncorrected scope had reached its useful limits under these viewing conditions. The image had turned fuzzy, and the violet fringe on both the limb and edges of craters was strong enough to impair detail. Here is where the Chromacor really showed its advantage: I was able to go up to 4mm--and I had still not reached the fuzz limit. Shadows were black, not purple, and I could clearly resolve those craterlets that were invisible at 9mm to the uncorrected scope. The Chromacor’s image with the 4mm was still crisp, although I doubt that I could have used much more power.
Venus is among the most demanding planets for a telescope. It is most visible when the sky is not yet completely dark, and the echoes of the day’s heat add turbulence. It is very bright. Even when you have resolved it, the best that you can hope for is a featureless mass of clouds. Aggravating whatever problems the telescope might have, even very good eyepieces often stumble because of their own chromatic aberration and internal reflections.
Venus was between 40° and 35° above the horizon. The 35mm Ultrascopic eyepiece with the standard telescope showed a violet halo around Venus, perhaps half the diameter of the planet. This problem became markedly worse with the 18mm, making it difficult to get a crisp focus with any of the higher-powered eyepieces. There was simply no advantage to raising the magnification; all I got what a larger, fuzzy, brightly colored blob.
Venus with the Chromacor was astonishing. I could not detect any violet halo or fringe until I reached 6mm, at which point the halo was substantially less than I saw with the 18mm orthoscopic and the uncorrected telescope. Even at 4mm, the Chromacor controlled the violet halo well. The halo extended perhaps ¼ of a Venus diameter or less, and it was neither oppressive, not did it prevent me from getting a crisp focus.
Saturn was about 10° past the meridian as I started my testing. Saturn, not being very bright, is not one of the more difficult targets with respect to chromatic aberration. Its rings, however, do present some opportunities for testing resolution of fine detail. Uncorrected, I could see slight color fringes on Saturn with a 12.5mm orthoscopic, and they were certainly visible (although not terribly offensive) at 9mm. The Chromacor eliminated all color from Saturn with every eyepiece that I tried--even the 4mm orthoscopic.
The Chromacor also provided significant detail improvement as well. Without the Chromacor, Cassini’s Division was barely visible at the ansae (the portion of the rings to either side of the planet) with the 9mm, and I could see a temperate zone cloud band on the planet. Moving up to 6mm was just above the fuzz limit, with no more planetary detail than at 9mm under these conditions. (Cassini’s Division seemed a little more visible than at 9mm.) The 4mm eyepiece was well above the fuzz limit, with Cassini’s Division now completely lost.
The Chromacor allowed me to see detail at lower magnifications, and to use more power. Cassini’s Division and a cloud band were visible at 12.5mm. Saturn has very subtle features (especially on the planet), and the advantages of planetary detail, although subtle, seemed to continue as I raised the magnification. At 4mm I was rewarded when the atmosphere calmed for fractions of a second, with Cassini’s Division visible all the way around the planet.
Unlike the uncorrected scope, which had clearly reached the fuzz limit by 6mm, the Chromacor at 4mm was still pretty sharp. If I had a 3.8mm eyepiece to use with the Chromacor, I might have been able to use it to get more detail, but I do not believe that there would have been an advantage to a 3mm eyepiece.
Jupiter was most of the way to the zenith by the time I turned the scope towards it. Without the Chromacor, the 18mm showed very apparent color fringing, with the violet halo becoming increasingly distracting as I increased the power. The Chromacor, however, completely wiped out halos and fringing until I reached the 4mm orthoscopic-at which point, there was just a faint hint of a violet fringe--enough that I was sure that I was seeing it, but little enough that I spent quite a bit of time convincing myself that it was present.
The Chromacor’s advantage in detail became most apparent at 6mm. At lower magnifications, the difference was hard to quantify; I had the impression that I was seeing more detail, but because Jupiter is so rich in detail--and how much is visible depends so much on seeing conditions--I spent quite a bit of time removing the Chromacor and reinstalling it. At 6mm, however, there was no question that the Chromacor provided much more detail than the uncorrected scope.
The move up to 4mm exceeded the fuzz limit for the uncorrected scope alone, and the color fringing made it impossible to get a decent focus. The Chromacor, however, demonstrated its ability to take more power. At 4mm, I could see three dark bands across the planet. Where the uncorrected scope had run past the fuzz limit, somewhere between 6mm and 4mm, the Chromacor was only limited by turbulence. There were fractions of a second where I could see detail within those three dark bands in such profusion that I could not hope to draw them. Under a better sky, I suspect that I could use a 3mm eyepiece, and get some benefit from it.
As should be apparent from my descriptions, the Chromacor compared to the Fringe Killer is the clear winner with respect to color and detail. You pay for it, however, in price, effort, and reuse. I paid $581 for the Chromacor. Some achromat owners are going to be a bit reluctant to fork out almost as much money for a corrector as they spent on the telescope and mount. For others, this isn’t really a choice; they simply can’t spend that much money on their hobby. For those users, let me comfort them by saying that, in my estimation, the Fringe Killer, gives about 20-30% of the color correction of the Chromacor for about 10% of the cost.
With respect to resolving detail, the comparison is a bit more complex. At the same power, the Chromacor definitely shows more detail than the Fringe Killer, and also allows me to use higher magnification without image breakdown. At least part of this is because, in my case, the Chromacor was not just correcting chromatic aberration, but also bringing an undercorrected refractor closer to neutral. A neutral Chromacor on a perfectly corrected telescope might have less of a detail advantage over the Fringe Killer. For the majority of achromat buyers, however, a perfectly corrected telescope isn’t a likely problem!
The Chromacor is a bit of work to get operational. The 2” diagonal that came with my refractor was not threaded for filters, so I had to buy one. If you find the prospect of spending a few hours fiddling with spacer rings and focuser collimation a little disheartening, the Chromacor might not be the right choice. In addition, because the Chromacor adds a lot of weight to your existing diagonal, you need to think about what happens if the screw holding your diagonal in place loosens up-and the diagonal, your eyepiece, and your $581 Chromacor, decide to make a quick exit onto the ground. (Recommendation: tie a lanyard around your diagonal, and to some sturdy part of the focuser--just in case that focuser screw gives way. I have had several occasions to be glad that I did this.)
The Chromacor ties you to the use of your threaded diagonal--so if you need to remove the diagonal to get your camera to focus with your current scope, you will either need to do astrophotography without the Chromacor, or you will need a threaded adapter that fits into your focuser without the diagonal. (I still haven’t found one.) Astrobuffet's John Hopper emphasizes that Chromacor is sold for visual use, not astrophotography, although I have be able to take pictures--at least at low power eyepiece projection. I can't get enough in-focus with my particular scope and Chromacor to do high power eyepiece projection.
The Chromacor, because it comes in multiple flavors of undercorrection or overcorrection, may not be suitable if you later buy another achromat. (You could, of course, buy the neutral form of the Chromacor, but then you would be giving up any correction benefits. Perhaps you should just plan to sell the Chromacor with the refractor!)
I have not tried a Chromacor with any of the semiapochromatic (ED glass) refractors. Since many of these refractors retain a very small amount of chromatic aberration, it is possible that adding a Chromacor would finish the process of making these into fully apochromatic refractors.
I should hasten to add that while the Chromacor gives most of the optical quality of a apochromat at a fraction of the cost, it does nothing for other issues. Adding a Chromacor will do nothing for your focuser, your finder, or the cell in which your refractor’s objective sits. If money were no objective, sure, I would buy the Astro-Physics refractor. But since I still work for a living, there’s a place for a Chromacor in my refractor.
In my case, I decided to sell the Fringe Killer, and keep the Chromacor. The Fringe Killer does a nice job for its price range, and for many achromat owners, it hits the perfect combination of price and functionality. The Chromacor, while much more expensive, lets me have most of the capabilities of a apochromat, for a quarter of the price.
After I completed the above test, I had a few nights of exceptionally good seeing from the backyard while using the Chromacor. The Moon, being a very high contrast target, tolerated what most amateurs would consider unreasonable magnification. At 382x (9mm orthoscopic plus Televue 3x Barlow), the Moon was still crisp and color-free. At 574x (6mm orthoscopic plus Televue 3x Barlow), the image was beginning to fuzz, and some of the shadows along the terminator were turning purple. Just for amusement, I put a 5mm orthoscopic into the Barlow for 688x. Unsurprisingly, there was less detail--I was well above the limits of the optics.
Clayton E. Cramer is a software engineer in Boise, Idaho. http://www.claytoncramer.com
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