Binoviewing: Reaching focus by tube chopping?

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The irony is that the Vixen original eyepiece holder was short enough at 20mm high to allow the binoviewers to reach focus with 26mm Meade 4000 Plossls in place, but with no GPC fitted. In fact there was about 5-6mm spare drawtube showing. Close, but good enough. While the binoviewers would not quite focus by roughly 4-5mm, with the 30mm high, Baader adapter fully lowered, unless I shorten the telescope's main tube.

After I fitted the GPC there was now plenty of drawtube to spare at focus but the image was magnified by about 2x. [At a very rough guess] The image was still surprisingly sharp for an overcast day. I was able to clearly see the buds on the trees at 450 yards and recognise [inverted] Fieldfares resting briefly in the same tree tops.  

Before I decide whether to chop the main tube and by how much,  I ought to seriously consider using a star diagonal. Which has more light path to take up if I should want to use the binoviewer for lower powers without the GPC. My 45° Baader Amici prism would provide an excellent erect image binoviewing experience if room can be found for it in the light path. Searching online has still not shown its optical path length. This ought to be standard information in the description.

The Vixen focuser is 12.3cm long from the end of the main tube when fitted with the Baader 1.25" adapter. I then removed the focuser and measured the focus point of the binoviewer from the end of the bare tube. When measured 'straight through' the focus was at 12cm. With one of my no-name 90° star diagonals the focus was 4cm from the end of the tube = 8.3 cm optical path length. With the Baader 45° diagonal it was at 1cm. = 12.3 - 1 = 11.3cm optical path.

This confirms the optical path length of each extra component. The binoviewer will not quite reach infinity focus with the Baader adapter in the focuser. So the extra 3mm inward movement required matches my early guesswork. To be able to use the binoviewer in the focuser with the 45° diagonal I will have to cut the Vixen's main tube about 12cm shorter to be on the safe side. A little extra margin of freedom is always valuable in case eyepieces are not all parfocal. From past experience it is only rarely I want to look straight through the Vixen.

The problem is that once it's main tube is "chopped" the telescope can no longer reach focus with a single eyepiece. At least, without using a diagonal. Would I ever want to view anything so low as be be comfortably seen inverted [upside down] with a single eyepiece? An inverted image is hardly useful for terrestrial use.

However, cutting 12cm from the Vixen main tube is very likely to cause serious vignetting by the very long drawtube when moved inwards. Causing the equivalent of a smaller aperture.

Unfortunately none of my 'scrap' aluminium tube is of suitable diameter. The Vixen main tube is thin wall and around 90mm. I have 80mm and 120mm tube but nothing in between. The Vixen objective cell is threaded onto the outside end of the tube. While I could [theoretically] cut a new thread in the lathe I can't manage such a long tube in my lathe. Though I could rig up a temporary tailstock to run in a plug. That would allow me to cut a thread.

If only I had some 100mm Ø tube I could turn some packing rings to join the two diameters to hold the objective and the focuser in place. Postage for half a meter of 100mm aluminium tube from eBay is three times the price of the tube itself! The tube is quite affordable. After endless searching I still haven't found a single, Danish aluminium stockist which will deal with private customers. Yet there are lots in the UK and Germany advertising on eBay.

After checking again I realised that only a male threaded, 90mm Ø tube, like the original, would be sensible. The small flange on the Vixen cell prevents a smaller tube, than 120mm Ø, from fitting over the cell.

Unless, of course, the cell is reversed to fit neatly into a 100mm tube. That would provide firm support and hopefully, proper collimation from the long, cast sleeve which normally supports the dewshield. The objective lens would be reversed, inside its cell, to point the correct way.

A packing ring on the outside of the 106mm section, on the cell, would support a 120mm dewshield. That would leave the far end of the 100mm tube available for a shorter focuser which would not cause vignetting. Possibly even a 2" focuser for extra stiffness. Though would mean considerably increased weight and bulk.

Whoopee! I just found 1m of 100mm Ø x 2mm [wall] aluminium tube online from a most unlikely online source. Order confirmed for home delivery and cheaper than buying from abroad. 😊

Click on any image for an enlargement.

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