79 replies to this topic

[a__l]

The main differences are mechanical: the OVNI-M has the photocathode effective focal plane roughly 12mm from the top, and that's more than 30mm closer to the top of the device than a PVS-14.

 

I'm curious about the 12mm. How did you take into account the thickness of the glass and by how many mm does this shift the focal plane?

Let's start with this to determine the possibility of using P2 with head + NV.

Edited by a__l, 14 May 2024 - 02:14 AM.

[sixela]

You can use the visual top with an OVNI-M but you’ll have to misplace the Paracorr by 7.5mm (so that’s how much focuser in-travel reserve you need.) that accounts for just some focuser travel on both sides.

That’s empirical.

The new focal plane thrown out my the P2 is 11mm above the visual top.

The reason you need 7.5mm and not 1-2 mm is because the on/off button interferes with the visual top, fortunately on the outer ridge (otherwise you’d need even more).

If you put a TMB barlow on the 1.25” nosepiece of an OVNI-M then you get it to focus correctly with a visual top and the P2 at the ideal location (the nosepiece can be split but that’s going in too much detail). You do get some aberrations from the TMB barlow at the roughly 1.35x barlow factor that yields, but nothing too severe (in fact my eye’s aberration still dominate since you’re always using your entire dilated pupil with an NVD).

Without visual top you can get an NVD onto a perfectly positioned P2 with an adapter to M48 on the P2 body, but that holds for both an OVNI-M and a PVS-14. It’s just a lot more fussy when you want to change powers by adding or moving a barlow.

The main point is that if you still have 8-10mm of in-travel left when using a P2 with eyepieces you can use it with the visual top and an OVNI-M, albeit with the P2 slightly misplaced (which would be pretty bad for moderate exit pupil 100° eyepieces, but is less so with a 40° AFoV NVD).

Note I don’t always use the visual top with the OVNI-M.

For galaxies I do because I go from prime to barlowed prime with several barlows, but for detail in nebulae I go from prime plus Nexus to prime plus P2 and then I have spacers on the OVNI-M to be able to change just the coma corrector (without top) to go from 0.75x to 1.15x of prime.

The PVS-14 I always use with the P2 without visual top (I leave the set screw of the visual top loose so I can always screw it on or off).

Just a word: I’m not going to be splitting hairs if you try to derail the thread to show I’m “wrong” by a couple of mm. It’s certainly close enough for all practical purposes.

Edited by sixela, 14 May 2024 - 02:39 AM.

[Joko]

I went to a star party last week and the OVNI-M / OVNI-B were used in several telescopes with Paracorr P2. It works like a charm !

Several customers have been using for years their OVNI + P2, and it has always worked perfectly.

Edited by Joko, 14 May 2024 - 04:34 AM.

[a__l]

Just a word: I’m not going to be splitting hairs if you try to derail the thread to show I’m “wrong” by a couple of mm. It’s certainly close enough for all practical purposes.

My NV has a distance of 18mm including glass. I use two coma correctors. For afocal and prime. Which is what I recommend to PVS-14 owners.

[a__l]

The price of the P2 is comparable to the cost of the filters, so it makes sense.