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Modding my DIY 16x70 RACI finder to a 14x70 RACI Finder, with DIY crosshair


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Some years back I converted a 70mm F/5 achromat (very, very cheap) to a nifty 16x70 finder, using a Prostar 22mm 70 deg Erfle EP with illuminated crosshairs. After the initial build, the only thing I replaced was the 45deg Amici prism tha came with the scope, by a 90 deg Amici prism from TS. I have thoroughly enjoyed having this piece of kit, and it is in part responsible for letting my DSO count skyrocket. I did notice that the FOV was considerably smaller than the rated 4.4 degrees. Recently, I had a careful look at the EP, and noticed that the field stop was a mere 22mm (57.3 deg AFOV , 3.6 deg true FOV). Comparing the image circle with that of a 20 Plossl of the kids confirmed the actual AFOV was far smaller than the rated one (too late to complain). This made me look for a better solution. As luck would have it, an Antares 25mm 70 deg EP appeared on the classifieds, and I managed to snap it up (thanks again Macavity). I had read a review (kindly posted in the advert) that the field stop of the 25mm was measured as 26.8mm or a true FOV of 4.39 deg, which is a huge gain. Just looking through the EP shows a much bigger image circle. The true or geometric AFOV is 14x the TFOV which yields 63.6 degrees. The rest of the apparent FOV size (which does look like 70 deg) is probably taken up by pincushion distortion. No worries, this is the biggest true FOV you can get in this barrel size, and that is what counts.

The bottom barrel unscrews easily to reveal the field stop, which I found to be a touch larger still: 27.3, just about as large as can be had in a 1.25" barrel (4.47 deg true FOV). I found that the top of the bottom barrel, when screwed back was flush with the field stop. By gluing to hairs (of mine, I plundered a dwindling natural resource ;)) on the top of the bottom barrel, and screwing it back, I could get the crosshairs in the right place. Quite some swearing was involved to get the hairs straight enough and central enough to my liking :rolleyes:.

Below are some images of the finished result, mounted on the C8. I am looking forward to first light. One very simple thing I like is the fact that the rubber eye-cup of the Antares 25mm can be stored folded back with a cap on, unlike the Prostar one. The latter is similar to the cylindrical eye-cups of the Vixen LVs, which I also always found annoying, because the rubber is very hard to fold back, especially when the cold has stiffened the rubber.

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Just an update about first light of the new finder: It works brilliantly, as can be understood from my observing report from yesterday. I liked the old set-up, but the new EP is a great improvement, hugely improving the ease of star-hopping. Nearly a degree of extra FOV is very useful. Furthermore, the hairs are a bit thicker than those etched on the glass reticule of the old EP. This means they can be seen a lot more easily without need for illumination (which I do not like to use). Well pleased!

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  • 7 months later...

What telescope was it from the beginning?

And how much did it costs to build the finderscope?

And do you think it will be too heavy for my 10" dobson?

I'm thinking about making one.

Thanks in advance

I used a simple department-store 70mm F/5 achromat on sale for 50 euro or so (with horrible Huygens EPs and a passable 45 deg amici prism). I then got a finder scope bracket for finders up to 80mm (about 90 euro, but I have since seen an Antares one for 70$). I later added a 90 deg prism (40 euro or thereabouts) as it is way more comfortable. In all I paid about 180 euro (plus the EP mod) so it is not that much cheaper than buying, although if you can buy some of the things second hand things get better. My guess would be it would be a bit heavy on a dob, unless you place it lower down the tube (which might be uncomfortable).

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Hi thanks for the reply

I'll think i will get a normal raci.

I mainly went down this route because I had already got the 70mm for some wide field observations. It became obsolete with the arrival of the 80mm F/6 triplet with its MUCH bigger FOV, and the 15x70 bins, with their minimal set-up time.

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