102 f9.8 CA ??

[Regulus]

Can anyone give me an indication of the extent of CA in the Black Diamond 102/1000 refractor (non-ED)??
And while on the subject, is there some simple math to calculate an ideal tube length for a give lens aperture using basic Crown and Flint. I understand that multi-coating might effect this equation.

Trevor

[johnt]

Hi Trevor,

Try this chart, (I think it gives what you are asking).

http://www.cityastronomy.com/CA-ratio-chart-achro.jpg

The "CA ratio" is "Focal length" divided by "Lens Diameter squared", all in INCHES. Or, just "F ratio" divided by "Lens Diameter in Inches". As they say, an Achromat with a CA ratio greater than 3 is getting quite good.

The 102 1000 Achromat gives CA ratio of 2.45, so although its not 3, its still quite good. (ie. F ratio=9.8, D=4", so 9.8/4 = 2.45)

Also, some further explanation is given in:

http://www.cityastronomy.com/color-correction-refractor.htm

So, that's why for Achromats to maintain a good CA ratio with increasing lens diameter, they have to increase the "F ratio" in proportion with the increased lens diameter. As the table indicates, a 4" F10 is about equivalent in CA to a 5" F12, and a 6" F15 etc...


John

[dannat]

While I don't have one now, the one I used to look thru was actually pretty good, CA wasn't a problem until you went above 200x power.
Also depends on the user, some are sensitive to CA, & see it everywhere, my eyes tend not to see that much

[brian nordstrom]

:)Hi Trevor , I had one of these a few years ago and was more than happy with it visually , it performed brilliantly on the moon and planets especially mars when at its closest to us and as Daniel says keep to max 200x and these work very well .

CA was there but it was very subdued and I ( personally ) had to look quite hard to see it so all in all my old one was a great all round scope .
On a side note its CA correction was about the same as my Istar 127mm f8 that I use now .

Be aware that CA is not only one thing that will degrade an image , I think SA ( spherical apperation ) is more damaging to seeing fine detail than CA is and all mass produced refractors and reflectors suffer from this .
I think that's why my Istar is such a good performer as its lense has been hand figured to reduce SA to very low levels and with the mass produced scopes out there its a bit of a lucky dip , if you get a good one , that's great but if you get a stinker .......
Brian.

[Regulus]

Thanks guys, that was pretty much what I needed to know.
Better prepared is always good when you a forced to catalogue shop (Tassie, doncha know?).

Hadn't really considered the Spher. Aberration problem in these 'cheaper' scopes, but had i given it a bit more thought then... Makes sense since they use almost the full objective edge to edge.
Camera lens makers avoid this somewhat by reducing the effective aperture on their cheaper lines so they can use a lesser quality objective etc. and concentrate the imaging on the centre of the glass, thus avoiding the poorer edge quality. You most often see this in primes where the edge glass is hidden behind a thick bezel.
I am puzzled given the knowledge of optics we have obtained, and the manufacturing and grinding standards we have achieved, that we still have problems in 'branded' products. I wonder what extra it would cost to lift this 102 for example to a consistent standard minus the spherical aberration etc.
Anyway, enough musings. Thanks for the valuable information.

Hope you all have a nice and happy festive season.
Trevor