Proper first light of the Orion 2" 90 deg Amici prism (plus the home-brew mount)

[michael.h.f.wilkinson]

I have got a second-hand Amici prism from ABS-UK recently, and had enjoyed the initial views in daylight. I found the image was sharp, and even the 31T5 Nagler did not show vignetting. It did show a reddish "ring of fire" around the edge of the FOV, but I think it is down to the EP, as I remember seeing that with the WO Dielectric Star Diagonal I have as well. In the 22T4 the daytime image was flawless, and details in plumage of birds was magnificent. Cobwebs at 25m were very well defined. Mechanically the Amici prism is very well made, with fit and finish very like the WO (same factory, I would guess). For birding, my new 22x80, 3.76deg TFOV combo is awesome. So how about astronomy?

I have since been able to use it on astronomical targets, and have compared it to the WO 2" star diagonal. The first test was Jupiter, through my 80mm APM. It is often said that Amici prisms should not be used beyond 100x, so I tried 137x and 120x just to see what happened. The result was curious. Out of focus, there is a huge difference between the two, with lines appearing in out of focus disk with the Amici prism. Close to focus, stars (or in this case the Galilean moons) get a worrying, astigmatic-like distorted appearance. Jupiter itself became disconcertingly distorted as well. Once in focus, there was very little between the images, with the WO having a slight edge.

I then set up the mount in the back garden and had a look at some DSOs to see how the scope/Amici-prism combo fared in wide field DSO hunting. The home-brew alt-az mount sets up very quickly, but was at first a bit confused when I tried to find M65 and M66 by star hopping from Chertan (theta Leonis). I am so used to an EQ mount that alt-az has become confusing . However, when I (un)twisted my mind through 37 deg (90-53 ), I got the hang of things quickly. The view of the two galaxies was noticeably brighter than in the 15x70 Helios Apollo bins. You do need to focus very carefully to avoid some peculiar diffraction spikes around bright stars, but in proper focus the image is very good. M44 confirmed that I could get sharp star images across (almost) the whole FOV. Again, the WO star diagonal is a touch sharper, and a bit more forgiving of small focusing errors, but the ease of use of a RACI (right angle, correct image) wide field scope is not to be sneezed at.

The new mount worked well, with smooth motions in both altitude and azimuth, and given the ease of set-up I might well be using the APM 80 mm a lot more than before. I do not think it will replace the 15x70 bins, but it does put me off buying bigger bins. The damping time for vibrations is a bit longer than I first reported, but it is a huge improvement over the EQ1 mod I used before.

[Stargazer_00]

All in all a good, unbiased comparison Michael. Cheers for the write up and taking the time to post your conclusion. Sounds like you got a good deal after all as it saved you buying bigger bins and gave you more flexibility going forward

[alan potts]

Michael,

Very nice report on something we don't see every day, what was the spider like at that distance, scary.

Alan

[michael.h.f.wilkinson]

Michael,

Very nice report on something we don't see every day, what was the spider like at that distance, scary.

Alan

Thanks, Alan. The spider itself was well hidden, and spiders here are never really scary.