APO doublet vs triplet question

I recently picked up the new Takahashi FS-152 doublet Herb had on sale here on Astromart. About the same time a friend of mine ordered an AP-140 and also a Tak TOA-150 to tide him over while waiting on the 140. He was pleasantly surprised when the AP-140 showed up (he was on the list early through Company 7). With two APO scopes he selected the AP-140 over the Tak and made me a deal I couldn't refuse on the TOA. I've been wanting to compare a doublet vs triplet for some time now but never expected 6" scopes would be the test. The FS and TOA are mounted side by side on a Robin Casady Triad bar atop an AP-1200.

The weather gave me part of the night Wednesday to do a quick first light check out before closing down. Though low in the sky I gave Vega a look. As expected, the TOA 150 showed no discernable color on Vega even moving in and out of focus. The FS-152 showed a greenish cast on one side of focus and a slight purple cast on the other. However, as it approached focus the color appears to go away completly. The results are the same using a Nagler 7mm type 6 or Nagler 3.5mm type 6. The question is, as focus is approached with usable magnification is it possible a doublet can be designed and manufactured that will become in essence color free at or very close to exact focus?

A doublet relies on the fact that your eyes selectively filters out the deep red and deep blue-violet wavelengths. Even though these colors are unfocused, they appear only faintly to your eyes, or not at all. Some eyes will see these defects better then others. Older eyes filter out the violet better than young eyes.

In an integrated time exposure the color error will appear as a violet halo. That is mainly why triplets are favored as imaging scopes over doublets, because they bring more of the violet spectrum into critical focus with the result that stars appear tighter with minimal or no blue-violet halos.

Rolando

Given that the f/8 Tak FS-102 and the f/8 FS-128 that I've looked through all showed colour error I find it unlikely that the larger aperture same design, same focal ratio FS-152 would not show even greater colour error than it's smaller siblings.

Please do note the the colour is not large and these are all very fine scopes, fully worthy of the term APO. But there is no way the FS-152 has no colour given that it's smaller siblings do. Look a bit harder, especially on brighter stars and for a yellowing on planets (esp. Jupiter when it's visible).

Philip

http://www.ancientstarlight.com/images/M1_Hubble_Palette.jpg

The link is to a shot (not by me) of M1 using an FS152. It is using a "hubble palette" to bring out colors in M1. But I call your attention to the stars.

There is no huge issue here with blue or violet swollen stars.

So it would appear that the FS series can do some excellent photographic work. Undoubtedly there are additional considerations of which I am ignorant, but this guy seems to be "living with the handicap" pretty well.

If you click on the picture you'll get the full resolution version. The photographer is a regular participant on Yahoo UncensoredTakGroup.

regards
Greg N