Improving Achromatic Refractor Performance

While I'm awaiting the arrival of an Antares 6" F6.5 refractor purchased via the AM classifieds, I've checked reviews of filters designed to reduce chromatic aberration inherent with achromatic refractors.
I've found an interesting review published in SkyNews, a Canadian journal for amateur astronomers. The authors utilized an 120mm f 8.3 skywatcher and an 150mm f 8 skywatcher for their tests of six filters: Lumicon minus violet, Celestron-Baader contrast booster, Baader fringe killer, Orion v-block, William Optics VR1 and the Sirius Optics PC1.
To make a long story short, the Lumicon minus violet filter is the one you want. With the 120mm on Jupiter, for example, the review states, "The Lumicon filter presented the brightest and most detailed image of Jupiter." "Although chromatic aberration was still present, the Red Spot and other surface details were more easily observed." "The Orion and William Optics filters slightly enhanced the visibilty of the spot, but it could hardly be seen in the Sirius Optics and Baader Fringe Killer filters and was not visible at all in the Celestron-Baader."
In addition, with the exception of the Lumicon, all of the filters imparted a yellow hue to Jupiter.
Similar results were noted with the 150mm refractor: "subtle details in Jupiter's equatorial bands and polar regions, not visible without a filter, jumped out when using the Lumicon, Orion and William Optics filters." "The Lumicon filter revealed the most detail and kept Jupiter in a relatively natural hue..."
I called Anacortes to order the Lumicon filter but they don't stock it so I called a local scope city store and I plan on buying one in a day or two.
If anybody has any other tips on improving achromatic performance, please share them. I'm not considering a chromacorr at this point, due to the added cost, but if there are other alternatives to improving achromatic refractor performance let me know. The scope I'm getting is said to have excellent star tests and adding a filter that provides for significant reduction in chromatic aberration while preserving the natural hue of the object being viewed seems to be optimum in meeting my expectations.

>>a filter that provides for significant reduction in chromatic aberration while preserving the natural hue of the object being viewed seems to be optimum in meeting my expectations. >>

What you are doing in effect with any filter is removing the light from a particular wavelength. In an achromat, only the green and yellow come close to focus and the blue and red ends are basically out of focus, so the idea is to eliminate the red and blue light. There is really no way to preserve the natural hue of an object if red and blue is eliminated (unless somehow you put it back in and have it in focus). The object will be yellowish and your eye will eventually get used to it, but the blue detail is lost.

Rolando

Hi Rod,
The "secret planetary filter" that's been tossed around the Stellarvue Yahoo group site for years is the FL-D filter. Most of us get the 46mm size and then use a 48-46 step-down ring to attach it to the 2" diagonal or adapter. It will not eliminate the color fringing, but with my Nighthawk 80mm/f6 it does reduce it, and produces a more natural, but warm tone. Contrast is enhanced a bit with planets, especially Mars. I also use the warming filters 81B and 85A. The 81B (or 81A) is particularly usefull when viewing Jupiter in the dawn and dusk hours. The 85A has more "punch" to it and I use usually use it when the sky is dark. Although I use them most often on my 80mm and 102mm achros they are not really achro-specific and can be used with any type of telescope. If you decide to try these filters I recommend only the B+W brand, as they use Schott glass and have superior coatings to most other photographic brands of filters. (This is more important with telescopes than it is with cameras as the magnifications you use are much higher.) Everyone's mileage varies, but most SV members using these filters report an improved image on lunar/planetary targets.

David E

Rod:

As far as I can see there is no definitive choice. It really seems to depend not only on the scope but on the observer as well. One tester's opinion with two scopes may be valid for his own situation with those two scopes but I would not consider it a definitive result.

Pollux, who posts here occasionaly and more often on CN, seems to have tried many of the filters with many different achromats and has specific observations on which ones work for him with which scopes. I know he has owned the Antares like yours...

I have Williams optics VR-1 and have used it with several scopes including a 100mm F/6 and a 102mm F/9.8. While it does reduce the purple haze on Jupiter for example, it was still visable in both scopes and I really did not find I saw anything more, though to tell the truth, I am not looking that hard in 100mm achromat when there in an 6inch F/8 Newtonian I could have chosen were I really interested.

Jon

Rod Kaufman said:
If anybody has any other tips on improving achromatic performance, please share them.

You can try masking it. Masking that 6" f/6.5 to 4" would give a 4" f/9.75 which would certainly have far better apparent color, at the expense, of course, of a dimmer view and less resolution. Whether it was worth it would depend on your tastes and the object. It might be for some views of some objects at some powers.

Roland is absolutely right here. A filter is just that a filter. Like my other money pit hobby, high end audio, the presentation may actually be more pleasing to the ear (eye) if you can throw out an offending colorization. However, you are still throwing out information from the signal and overall resolution is reduced though some areas may experience higher contrast as the coloration is reduced.

The only corrector I know of is the Chromacor. And the darn thing works!

But there may be another. I've a really old AP 2" barlow. Using it ahead of the diagonal on my newly acquired AP 5" F6 (1986 vintage)actually seems to improve the color correction a bit.

Roland is that true or is some other factor at work here?

Jeff (an early adopter of your stuff).

8)