[ATM] More Fold Flat Flatness

[Anthony Stillman <atmer@flash.net>]

There is a lot of information here, a conclusion sums it up.

Following up on
<http://astro.umsystem.edu/atm/ARCHIVES/FEB02/msg00294.html> from two years
ago,  I present here some of the aforementioned fodder.

In these models of newtonians, the field angle for each of the ideal
systems was chosen to yield a full field peak to valley wavefront error of
about 0.5 waves and a full field RMS wavefront error of about 0.1 waves.
The aberrations are in 589.3nm waves.  All of the various arrangements of
plus and minus power and spherical aberration were examined.  Focus is set
to minimize on-axis RMS. The models were evaluated by OSLO LT 6.1

In this first table, truncated to a single digit, are the worst case Strehl
ratios for an entry level telescope.


8" f/8
Diagonal 200 mm from the focal plane   Flat minor axis 38.7mm
0.277 degree field angle  Worse case Strehl ratios

          spherical aberration
          0       1/4      1/2
sag
 0        1       0.9      0.7
1/4      0.9      0.8      0.7
1/2      0.8      0.7      0.6


I also looked at a very slightly less entry level instrument, an 8 inch f/5
As with the 8 inch f/8, the effect of power is minimal.  However, unlike
the f/8, the effect of spherical aberration isn't.

The best case / worst case values are presented where significant.  Notice
the change in scale of spherical aberration.


8" f/5
Diagonal 200 mm from the focal plane  Flat minor axis 40.8mm
0.112 degree field angle

         spherical aberration
         0           1/16             1/8
sag
 0      1.00         0.82            0.54
1/8     0.96     0.89 / 0.71     0.63 / 0.43
1/4     0.85     0.89 / 0.57     0.69 / 0.32


The aberrations can be balanced.  Adjusting power, I was able to balance
out 1/8 of a wave of positive spherical aberration with about 1/3 of a wave
of concave power for a Strehl of 0.70

A little bit of displacement only slightly degrades these values.

I also examined a far from entry level telescope, a 25 inch f/5 with a
field angle of 0.035 degrees.  The four and a quarter inch minor axis flat
was placed 520 mm from the focal plane.


25 inch f/5 newtonian
Diagonal 520 mm from focal plane   Flat minor axis 108mm
0.035 field angle

          spherical aberration
          0          1/16             1/8
sag
 0      1.00         0.75            0.40
1/8     0.99     0.84 / 0.62     0.50 / 0.29
1/4     0.95     0.86 / 0.48     0.58 / 0.19

Balancing 1/8 of a wave of positive spherical aberration against about a
1/3 of a wave of concave power yields a Strehl of 0.62

Shortening the distance between the secondary and focal plane to 400mm
improves the numbers slightly.


25 inch f/5
Diagonal 400 mm from focal plane  Flat minor axis 83.4mm
0.035 field angle

         spherical aberration
         0           1/16            1/8
sag
 0      1.00         0.77            0.43
1/8     0.95     0.85 / 0.64     0.54 / 0.32
1/4     0.83     0.87 / 0.50     0.61 / 0.22

Balancing 1/8 of a wave of positive spherical aberration against about a
1/3 of a wave of concave power yields a Strehl of 0.64

I hope this helps you Scott.


Conclusion

The poorer the quality of the flat, the f/longer the primary should be.
Keep the distance between the flat and focal plane to a minimum.

Next, flats not at 45.


Anthony

contraria contrariis curantur
kawn-TRAH-ree-ah   kawn-TRAH-ree-ees   koo-RAHN-tuur
opposites are cured by opposites


_______________________________________________
ATM mailing list http://www.atmlist.net/