Re: ATM Loveday Secondary

[Anthony Stillman <atmer@flash.net>]

Frank, others,

I plugged Dave's prescription for the Loveday into OSLO LT and fiddled with
it a bit.  Principally, I was interested in the effects arising from
decenter and tilt of the secondary, but I also wanted to explore other
variations.

This design, as expected, is limited by coma and stig.  However, as noted,
the large effective f/ and small field keep these abberations in check.  In
fact, the Airy disk is so large, 0.03mm, that the real field limit is
determined by the secondary obstruction and perferation and the vignetting
tolerance.

The major abberations arising from decenter and tilt of the secondary is
coma.  But for small allignment errors this too is smaller than the Airy
disk.  Specifically, decentering the secondary by 0.5mm or tilting it by
0.5 degrees has only a small effect on the MTF.  Tilt, which has a more
significant effect than decenter, becomes an issue when the secondary is
off-axis by about 0.9 degrees.

Likewise, defocus isn't that big of an issue either.  Variations of up to
+- 0.3mm from ideal focus can be tolerated with out severly effecting image
quality.  As David pointed out thouogh, small changes (0.05mm) in the
primary-secondary seperation can degrade the image if no compensation is
made for re-focus.

Lastly, though given the large Airy disk this point is rather moot, the
field is not flat.  In fact it is quite curved.  My best guess of field
radius for this design is about 570mm, convex toward the secondary.  This
implies that a flat centered 10mm CCD would use up less than 1/10 of the
focus tolerance.  Hence, the field curvature can be ignored.

I am incline to say, no offence Dave, that this prescription is not ideal.
I believe that reducing the inter-element spacing and lengthing the ROC of
the secondary will slightly improve performance at the cost of some EFL and
a bit more vignetting.  It would be interesting to look at a complete
analysis.


Anthony