D&G 6-inch f/15 Refractor

I was lusting after the D&G 6 inch Refractor that is listed on the classifieds, and I was wondering, doesn't that looooog focal length practically eliminate any false color? Having never looked through one, I can only guesstimate due to the lack of false color on my old 60mm f/15. Is this correct? Chris Brown

Hi Chris,

I've never seen a DG in person so I can't comment on it from direct experience. I did however own a 6" f8 Synta that had a decent lens. I stopped it down to 4" which gave almost f12 and it still showed more color than my Vixen ED102 f6.5. Everything I've read would indicate that a 6" achro would have to be f20 or greater to eliminate false color. Have you given thought to a 6" Synta plus the Chromacor III that is coming out shortly? Or, better yet IMHO, an MN66 or c9.25.

Clear Skies,

Steve

Unfortunately, a 6" f/15 achromat is not chromatism free. It doesn't compare to 60mm f/15; the larger aperture, the higher f# needed to keep chromatism at the same level. So, 60mm f/15 compares to 152mm f/38, and 152mm f/15 to 60mm f/6, or to 100mm f/10.

This amount of chromatism compares to somewhat worse than 1/4 wave s.a. level, which is not bad at all. Due to lower "sensitivity" of an achromat to other error sources, a 6" f/15 refractor is likely to outperform high-quality 6" Mak-Cass with ~35% c.obstruction.

The 6 inch F15 does have some false color --
a sort of faint purple halo on bright
stars. Its many times less than the
Synta/Celestron/Skywatcher 6 inch F8 scopes.
But still much more than the 60mm F15 you
mention.

They are all-round superb scopes, and will
give near-perfect images for what they are.
Other aberrations like spherical, astigmatism,
etc., are undetectable.

Chris
I have owned a 6 inch f-12 D&G and it was an impressive scope but a bit long and hefty for my mount so I sold it.

D&G's lenses are excellent, they are polished well and have excellent figures. I do not think it is reasonable to speak of Synta 6 f-8 inch lenses and compare to D&G. D&G lenses star test really well, the Synta types I have used have not.
On deeper sky my lens produced better contrast on eta Carina nebula than a C-8. More structure was seen although the image was slightly brighter in the C8.

D&G are another class(compared to the mass produced Chinese scopes) in terms of quality and construction of the lens cell which can be easily collimated. Hence their higher cost.

My f-12 did not reveal significant chromatic aberration to the eye when viewing Jupiter at about 180 X. A faint purple halo was barely discernible and perhaps at say 250 X was visible. Contrast was very impressive, better than in a well made 6 inch f 8 Dobsonian a friend owned.

Saturn did not show false colour in focus at 200 X. Neither did the moon's surface at even higher powers although the image was slightly yellower than in my Zeiss 100 mm APQ. On the Moon's limb at 200 X false colour was not too bad. Better than in a Vixen f 9.8 4 inch at a similar magnification. To see false colour well with the D&G I had to push the telescope to ridiculous powers.

For visual planetary use the views were impressive, Mars did show a little false colour at 200 X and higher but still contrast was very good.

Vlad speaks of 1/4 wave out of focus colour. This is true but not seen by the eye as our eye when looking at bright planets picks up well focused green light.

Bright first magnitude stars at high magnification did show a little flase colour.

There was a review of a D&G 6 inch f-15 in "Southern Stars" a no longer published Australian magazine back in 1995. The reviewer compared the deep sky performance to a well made 8 to 10 inch reflector and the planetary contrast as good as any other 6 inch telescope bar an apochromat.

I tend to think this assertion is accurate after owning and using the f-12 version for over a year. On double stars I recall no other telescope around 6 inch that could blast tight doubles with relative ease in good seeing. That is except my APQ 130 which is able to do similar tricks.

Best wishes

Kevin