Newtonian 300 F/4 with X2.7 ED APM coma corrector Barlow lens
All OSLO simulations are courtesy Gerd Duering and Markus Ludes
1  Main characteristics of the X2.67 ED APM coma corrector Barlow lens:
X2.67 nominal ratio
105 mm optimal backfocus from centre of the last lens.
 62.9 mmfocal lens.FK 61 lens (identical to Ohara FPL51).
Transmission of over 99% from 400 to 700 nm.
Clear aperture 22 mm.
Designed to correct coma on Newtonian F/4 telescope over an APNC field.
2  The limitations of a 300 F/4 Newtionan for high resolution with a CMOSIS 4000 sensor :
The analysis is focussed on high resolution imaging with the large size CMOSIS 4000 detector (2000 x 2000 5.5 microns pixels).
The diffraction limited flat field of a Newtonian 300 F/4 is (in radius) is:
arcsec 
arcmin 
degrees 


396 nm  88 
1.46 
0.024° 
540 nm  120 
2.04 
0.033° 
In comparison, the field of the CMOSIS at X2 and X3 sampling and 396 nm and 540 nm wavelenght is:
396 nm 



540 nm 


Accordingly, a large part of the CMOSIS is not diffaction limited at 396 or 540 nm wavelenght.
OLSO simulations with two stacked X2.7 ED APM coma corrector Barlow (6.25 X resulting factor and F/25 resulting ratio) :
The light from the 300 mm F/4 mirror comes from the right side and the focus plane is on the left, 84 mm away from the last lens of the second Barlow.
The first Barlow lens works at X2.7 nominal factor while the second lens is at X2.3 factor.
The first Barlow is 40 mm inside the focus of the telescope.
The F/25 ratio gives a X2.0 sampling at 396 nm and a X2.6 sampling at 540 nm.
At 396 nm, the diffraction limited flat field is now : 3.3 arcmin (0.055°), larger than the CMOSIS at X2.0 sampling. The Strehl ratio is 0.94 on axis and 0.82 at 0.054° from axis.
At 540 nm, the Strehl ratio is equal to 1 on axis and to 0.95 at 0.054° from axis.