--8--

of the primary lieing to right and to left of the meridional plane the operator directs the center line of the secondary's tube-end image till it is exactly coincidental with the meridional diameter of the secondary. After that the operator turns the meridional adjustment- screw of the primary till the image of the secondary's tube-end exactly disappears behind the edge of the secondary. Herewith the reflector is pre-adjusted and must give considerable definition on terrestrial objects. The minute adjustment is done on an artificial star (suns image on a high polished bearing ball or Christmas-tree ball in a distance of minimum 200 focal-lengths) or better on a real star of second or third magnitude, when the seeing is excellent. We operate exclusively with the adjustment-screws of the primary till the extra- and intrafocal star-disks are absolutely circular. The nearer the focus we observe, the better the result will be. The 'how' of turning the adjustment-screws is quickly learned by experience. The difficult- ties in adjusting doesn't lie in the adjustment itself, but in reco- gnising the actual residuals, that is to say, in the actual seeing.

2) The coma-free device ( symbols v. fig.2 ) The inclination φ1 of the incoming beam of rays is the very same as in the anastigmatic device, defined by the condition for freedom of silhouetting. Therefore we get φ1 and the first axis-distance Δ, ceteris paribus, equal to those of the anastigmatic device. For eli- mination of the coma the inclination φ2 of the secondary must then be 6)Equation 6

The second axis-distance Δ′, which depends on the inclination φ2 of the secondary, becomes again 4) Equation 4

The exterior of the coma-free device differs from that of the anastig- matic device solely by the longer second axis-distance Δ′, respecti- vely by the stronger inclination φ2 of the secondary.