Lens Designs for 80mm SLR astrophotography



1.   Introduction

The medium format roll-film cameras such as Hasselblad or Bronica are famous for exceptionally high image quality of prints. This follows from the large film size and the extreme care taken with the lens designs and the engineering. Focal ratios of the commercially available long focal length lenses tend to be no faster than about F/4 because of the rapid escalation in weight and cost if faster lenses are specified, so easing the difficulties in realising lenses.

2.  A design for an 800mm F/8 lens

The very large relative increase in the image size requires a considerable increase in complexity over the designs for a 35mm film format.  It was found however that excellent performance could be obtained even if one allowed only 5 curvature values + plano.


               
           
                         


                    

                   

                                                    




3.  A design for a 600mm F/6 lens

A constraint was placed on this design that as in the above there should be only 5 curves + plano in a total of 6 elements.


                                    





                 
         

It's disappointing that the MTF has fallen below the design aim  at the field edge, nevertheless the image quality from 20% MTF at 8000cycles/picture width should be very satisfying. Allowing freedom in all curves produces only a modest increase in MTF performance.



                        
                       


4.  A design for a 400mm F/5.6 lens

The previous layout can be reworked to create a 400mm version, with a correspondingly wider field in excess of  11deg diagonal.


                              
                           


                                      The MTF is perhaps just that bit improved, even at the slightly increased aperture
 

                             



                               





5.   A design for a 600mm F/4.8 lens
By splitting the front and rear doublets of the previous design, and changing the glasses, the aperture can be increased to 125mm while achieving a diffraction quality image over a 6.3degree diameter field.   Notice that the two rear elements have a common glass type, despite being + and - .

       
           


             




The performances available from this design concept were then extended to the designs for digital imaging cameras.




© Don Barron 2005