View Full Version here: : achromat free of CA

14-09-2017, 12:09 PM
Does anyone know what f ratio is required for the best possible achromatic lens to become for all intents and purposes CA free? f/20-25-30?

14-09-2017, 12:14 PM
Depends on the aperture. As aperture increases the CA increases and so a longer ratio is required.

If you Google Sidgwick or Conrady standard you'll find the answer, or...

There are two well known criteria for small achromats of standard glass BK7+F2 combination. Conrady states that the focal ratio has to be at least 5 times larger than aperture in inches. Sidgwick criterion is less stringent - according to this rule it is enough if the focal ratio is 3 times larger than aperture in inches.

The attached chart is useful (also here: http://www.cityastronomy.com/CA-ratio-chart-achro.jpg)

14-09-2017, 12:31 PM
Thanks Morton.

14-09-2017, 12:34 PM
Just edited my first post to add more info.

14-09-2017, 12:44 PM
Thanks again.

14-09-2017, 01:16 PM
Conrady's rule is a good start but ... I've used two excellent refractors - 4.25" f/16 and 9" f/18 that were made long before Conrady which suggest that rule is overly conservative with good design.

Wiith modern glass types a doublet could achieve a focal ratio 2x the aperture in inches. BK7 + F2 is a very lazy solution.

14-09-2017, 01:44 PM
For some years iStar has produced achromat objectives with significant reductions in CA due to the glass used. IStar refer to these as: Anastigmatic objectives yielding reductions in CA of between 30 and 50%.


I built a 127mm f12 refractor using iStar's R30 127mm objective, which yields CA performance equivalent to a f15 scope. As you can see by the chart in the previous post, once you get into the green area CA is no longer an issue.

14-09-2017, 08:07 PM
Thanks folks.

15-09-2017, 01:48 AM
Indeed achros require a longer f/ratio for a larger aperture, so the minimum focal length is proportional with the square of the aperture.

However, my short 80mm f/6.5 renders decent images even at 130x. Bright objects do show color errors, mostly blue fringes, but it is not too bothersome. Saturn is rendered with a nice and crisp image. But for deep sky use, the color errors are unnoticeable.

Of course, my 110 ED renders better images, not only to the lower CA, but also the extra aperture which makes images (11/8)2 = 1.9x brighter.