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Lens Designs for 35mm film and electronic astrophotography

1.   Introduction

Modern 35mm SLR camera lenses from the very best names such as Leica, Nikon, Canon and Olympus seem at first glance to offer the possibility of high quality astrophotographs, given a good quality tracking mount and fast colour film. But any perusal of the opinions of users who've tried such lenses shows them to be generally less than satisfied with the results that they achieve.  There are a number of reasons for this.

Firstly, it is not actually possible to design a lens to other than a trivial specification which can perform to the limits set by diffraction.
 
Secondly, all real lenses designed and produced by the camera industry are a trade-off between what is theoretically possible and what is economically attractive to the purchaser.  A considerable price premium must be paid for any extreme care taken with the manufacture of the lens elements  and the metalwork, particularly the mounting flanges, sealing faces, the focus mechanism and the remotely controlled diaphragm.

Thirdly, the camera lens designer can allow the performances away from the centre of the field to fall.  For astrophotography this relaxation is hardly tolerable.  Sky photographs from such lenses can be very unattractive.

In this note I offer a number of designs which I prepared while looking for an 800mm F/8 telescope objective lens.  Although one seems able to buy such a telescope, quantitative performance claims are rarely offered.  While doing this work, I thought it might be interesting to see what is possible from a 400mm F/6 and a 200mm F/4 when used with a camera.

There are a number of ways of specifying the performance of lenses.  'Resolving power' is very much an arbitrary and subjective measure, and a bit old-fashioned in these days of powerful design tools and measuring equipment.  In this paper we adopt the polychromatic modulation transfer function, PCMTF, as the criterion.  A value of PCMTF greater than 30% at a spatial frequency of 100cycles/mm at any point in the field is desirable.   This value can be compared with typical camera lenses where the typical PCMTF has fallen to 10% at a spatial frequency of 50cycles/mm.

Notice however that this type of lens is considerably more complex than a simple telescope objective lens.  This follows from the very much larger field.

It's possible these lenses already exist as real camera lenses - but I doubt it.  Anyway, there may be some  enterprising ATMs who might be able to make them, or even a specialist manufacturer who could  tackle it.  Judging by the FAQ on the WWW, there does seem to be a large number of  skywatchers who could make good use of such lenses if they were available and affordable.  

2. A design for an 800mm F/8 lens

This is an especially interesting design.  It is almost perfectly corrected for chromatism between 0.45µm and 1.1µm.        

       

                          

                   

             

          
    

                                                
                         

3.  A design for a 400mm F/6 lens

This lens has an image field diameter of 6.2degrees



         

 

                                            




               

     

               

 



4.  A design for a 200mm F/4 lens

This lens has a field of view greater than 12deg. diagonal

                    

                 

 

                



 


     Although the image quality is no longer diffraction quality, it still exceeds the design aim of >30% at              100cycles/mm. at any point in the field.

© Don Barron 2005