Going H-alpha: Focal length consumption.

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One half of one of the runners fell off my saw sled as I lifted it off so I had to re-glue and re-load it with weights. I used a lot of wood screws on the runner which did stick. I shall do the same to the repaired side.

Meanwhile, the sled resting on the saw table provided a handy, optical test bench.  So I brought out the Vixen 90M f/11 to check the assorted add-on components for their effect on focal length. Or, to put it another way: Where they came to sharpest, infinity focus. Infinity in my case meant trees edging a wood at 450 yards.

I used pencil marks on the silver finish draw tube and then used a steel rule to measure the differences in millimeters. Meade 4000 26mm 26 & 32mm Plossls were used throughout. The Meade are supposed to be parfocal and the difference is quite small. Probably less than a mm when used "bare" in the Vixen's 1.25" drawtube.

The Baader 45° diagonal consumes -100mm of the original focal length.

The T-S [Teleskop-Service] 90° diagonal uses up -86mm.

The T-S binoviewer with T-S 2.6x GPC uses up -15mm of focus. So the GPC does not fully neutralize the binoviewer's focus shortening effect as claimed by T-S. [GPC = Glass Path Corrector]

In practice, this meant that I could not reach focus with the binoviewer and the 45° diagonal even when using the 2.6x GPC. I would have to chop something like 15-18mm off the Vixen's main tube.

But I could just reach focus with the binoviewer, 90° star diagonal and 2.6x GPC with a couple of millimeters to spare.

The image with the binoviewer and 2.6x GPC was rather soft, dim and colourful due to excessive magnification despite the bright sunshine. Nobody would seriously want to use this combination for terrestrial viewing. Though I know, from personal practice, that it works on the Sun in H-alpha in my 150mm [6"] f/8 [120mm [4.75"] @ f/10 effective].

Since I could not reach [inward] focus with the binoviewer I decided to cut off 15mm of the Vixen's main tube. Once I was able to reach focus the view through the binoviewer without the GPC was crisp and pleasant enough but inverted. Not ideal for terrestrial viewing but fine for astronomical use.
The difference between the bare binoviewer and the binoviewer + 2.6x GPC is 98mm. Which explains why I was unable to reach focus with the binoviewer unaided by a GPC. It also explains my inability to use the binoviewer with the Baader 45° correcting diagonal + 2.6x GPC.

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