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I’ve always admired wide field images of celestial object particularly the black and white deep narrowband ones. The idea of seeing an object in context, sometimes appearing to just hang there in space amongst nebulosity and stars always inspires me. Having commissioned the new QSI583WSG camera on my scope my thoughts turned to maybe using some of the manual Nikkor lenses I have for wide field shots.

There was a problem though. The registration distance for Nikkor lenses is 46.5mm and the distance to the sensor from the front plate on the QSI583WSG is 53.4mm, leaving just under 7mm for an adapter. The Nikkor lenses could not be used in this configuration. Sure, I could buy a WS front for the camera but that would mean no Off Axis Guider. I wanted a self guiding unit that was reasonably portable.

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After some research I found several makes of medium format lenses with large registration distances in the 70mm to 90mm range, ample distance for an adapter as well.

Another big advantage of medium format lenses is the imaging circle. They are designed to illuminate a negative 6cm square so illuminating the the KAF-8300 sensor in the QSI should be a breeze and show little vignetting.

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I monitored eBay for suitable lenses and one night was lucky enough to win a Hasselblad 150mm Sonnar f/4 lens. The main issue with any adapter would be the Hasselblad bayonet so I quickly sourced a second hand Hasselblad Extension Tube, again off eBay.

I did some quick calculations and made drawings and a meeting was arranged with my local engineering company. Bob the foreman spent over an hour discussing my drawings, measuring the extension tube and seemed genuinely interested in what I was doing. About one week later I collected the adapter and as luck would have it that night was clear for a short while. I knew I’d erred on the side of caution with my calculations but I was still nervous whether the lens would actually focus. I needn’t have been as the lens came to focus nicely around the 30m mark on the focus ring.

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Some trial shots were taken the next week and it became apparent that focussing this setup manually needed a light touch and patience. I already had a Robofocus unit running on my main scope so I purchased a second motor with mandel and belt and set about fitting this to the wide field setup. One Saturday morning I sat down with some aluminium plate and later that day I had a prototype built for testing.

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One problem was that the teeth spacing on the belt did not line up with the grooves on the focus ring of the lens, in fact they were only slightly out. I solved this but putting a rubber band around the focus ring and the belt grips on this. To support the lens and stop any movement due to the Robofocus I used a William Optics guide scope ring.

New kit equals cloudy skies so it was four nights later when I ran the focus routine in Maxim for the first time and it worked a treat!

The front element of the lens is quite exposed to the elements and thus susceptible to dew and airborne contaminants. I managed to source a Hasselblad Pro Lens Shade which is a remarkable piece of engineering. The hood part is reminiscent of the old bellows type cameras and covers all lens form 50mm to 250mm in length. The great thing for me is that is quite deep and would provide great protection form stray light and dew. The hood is bayonet fit and sits snugly and securely on the outer edge of the lens leaving just enough room behind for a small dew heater strip.

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The setup has performed very well over the first few months and the lens holds focus very well – much better than my Tak! Plate solves give a field of view of 6.80° by 5.12°, a resolution of 7.36 arc seconds per pixel and a focal length of 151.2mm.

The current assembly allows only one orientation of the camera – landscape mode. Plate solving several images shows that it is less than half a degree from being perfectly aligned to the Pole

I plan to add a bracket in the near future certainly before Orion rises as I’ll need portrait mode orientation for Barnard’s loop for which I’ve acquired a 50mm Hasselblad lens.

The next page shows first image taken with the wide field setup . . .

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