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Old 22-09-2014, 08:18 AM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
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hi Peter.

figure 149, example 17 in the reference shows the spots diags for your scope design (multi-band is the top spot diagram) - it definitely has an extensive halo at the blue end (about 10x the (black) diffraction spot) and presumably would be even less well corrected in the far violet. The reference notes that the straight triplet is best for visual and the designer could have chosen to pull in the violet for imaging, by sacrificing performance elsewhere. However, the general characteristic of a significant violet halo will probably still be there. The Trius has much higher blue/violet sensitivity than most Kodak chips, so is better able to detect any such halos.

if it is a violet halo due to residual CA in the APO, it would be in the lum image as well (as Greg has found), so a violet-cut filter would need to be applied there as well - probably best to just put one front of the camera and use it for all channels.

Eric, I think you can discount 1. and possibly 3. - halos do not show up in images taken with reflectors using the same camera (and I think using the same filters).

Last edited by Shiraz; 22-09-2014 at 09:48 AM.
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