Specifications:

Objective diameter:  100 mm
Magnification:  25 x
Exit pupil: 4 mm
Eye relief (acc. to spec): 15 mm
IPD:  56 – 72 mm
RFOV:  3.0  degrees = 52 m
Focus type:  IF
Prism system:  Porro I
Waterproof: yes (?)
Weight (measured, with eyepiece cover and objective caps): 4‘150 g
Made in: China

Remarks:
see review mentioned below.

Ratings:

Review:
The following is a brief user review comparing three 25×100 models currently on the market:
– Celestron Skymaster 25×100
– Omegon Nightstar 25×100
– Omegon Argus MS 25×100

EdZ and others had written excellent performance reviews of several 25×100 models some time ago on ClodyNighgts, and there is no intention here to go as much in-depth as they have done. Instead, this is only a very brief description of what I experienced when trying out the three mentioned binoculars recently.
Regarding the selection of models: it is my assumption that the Omegon Nightstar 25×100 is a „BA8“ design and therefore essentially the same instrument as the TS Optics 25×100, Oberwerk 25×100 Deluxe, Orion Giant View 25×100 and possibly further 25×100 models.
On the other hand, the Omegon Argus 25×100 is the same instrument as the APM MS 25×100.
If this is true, the current brief review would in fact cover most of the currently available mainstream 25×100 binoculars.

Some data:

Celestron Skymaster 25×100
Eye relief: 15mm
IPD: 56-72mm
FOV: 3 degrees = 52m
Close focus: 24.5m
Length: 405mm
Weight (with eyepiece cover and objective caps): 4’150g

Omegon Nightstar 25×100
Eye relief: 18mm
IPD: 60-73mm **
FOV: 2.5 degrees = 44m
Close focus: 25m
Length: 450mm
Weight (with eyepiece cover and objective caps): 4’712g
** see remark in text below

Omegon Argus MS 25×100
Eye relief: 18mm
IPD: 60-74mm **
FOV: 2.6 degrees = 47m
Close focus: 25m
Length: 390mm
Weight (with eyepiece cover and objective caps): 4’067g
** see remark in text below

General / Appearance / Mechanics
At first glance, the difference in length between the Argus and the other two is striking. The Argus is not only the lightest of the three, but also clearly the shortest. All three appear well finished and mechanically robust, the individual focus mechanisms working smoothly. A slight criticism concerns the central hinge of the Argus, which was not stiff enough so that the IPD always slowly collapsed when the bino was mounted. This could easily be fixed, of course, by tightening the screw at the front end of the hinge, but the bino should not have been dispatched in that condition.
Another more serious criticism concerns the tripod adapter of the two Omegon models. Although the IPD in theory could be adjusted down to 56mm, the vertical bar which gets screwed onto the tripod actually blocks such adjustment at about 60-61mm as soon as one moves the adapter to the center of gravity of the binocular, because the vertical bar interferes with the front part of the tubes. This means that people with a narrow IPD will always have to mount the binocular with the tripod adapter „out of balance“ resp. not at the center of gravity. This is less than ideal. Maybe a slimmer adapter could be purchased and mounted to avoid such nuisance. I have not tried this since I could just about manage with my 60mm IPD.
Only the Celestron allows the proper balancing of the gravity center of the bino on its tripod adapter without limiting the IPD adjustment to below 60mm.

Optics
Trying out a bino in nature, the first things I look at is the „ease of view“ (in German: „Einblickverhalten“: how easy is it to get a comfortable view when positioning my eyes behind the eyepieces, and how well can I comfortably observe when looking through the bino, without „kidney beaning“ and the like?).
The Argus in my eyes proved better in this than the Nightstar and the Skymaster, but all three provided reasonably good ease of view. The two Omegon models provide 3mm more eye relief than the Celestron, but I did not feel that difference to be very relevant (I observe without glasses, so this may be relevant for eyeglass wearers).

In order to compare the three binoculars on the night sky, I mounted them side by side and selected three objects which I observed in turn with each bino.

Moon:
All three binos showed actually less CA than I had expected (none of them has ED glass). The typical color fringes appear at the edges of the moon’s surface in all three, but it is not very prominent and did in my view not disturb the observation much. Of the three, the Nightstar showed probably the highest level of CA, the Argus the lowest.
The best contrast / sharpness performance comes with the Argus, which provides good central sharpness and also quite good off-axis sharpness for about 2/3 of the FOV. Further outside, the view gets increasingly blurred. In the central region of the FOV, craters and other features appeared crisp and clear.
The Nightstar would be second best, showing quite good central sharpness, just slightly below the Argus, but clearly less sharpness at the periphery of the image than the Argus, and a bit more color fringes also.
The Celestron seemed overall just a tad less bright than the Nightstar, and sharpness was at or slightly below the level of the Nightstar, in particular at the periphery of the image, which is unfortunate because it would have the widest FOV of the three. Nevertheless, it still produced some quite nice views of the moon and allowed the spotting of many fine structures on the moon surface (that were not all visible in the next smaller size Celestron, the Skymaster 20×80, which I just used for a quick comparison).

Venus:
Again, the Argus provided the sharpest and clearest image of the crescent which from my home currently looks like a U shaped bowl. CA, however, was more prominent than on the moon, but still not too bad; in addition, there was a bit of „glares“ and reflections surrounding the planet body.
The Nightstar produced a nice image too, not as sharp as the Argus, but not too far behind, however there were now several „layers“ of reflections around the crescent shape, and more color fringes than in the Argus as well surrounding the planet.
The image in the Celestron Skymaster was somewhat darker than in the Argus Nightstar, but the crescent shape of the planet crsip and clear and very well defined, with only just a bit of CA, and much less reflections and „glares“ than in the Nightstar, which was somewhat a suprise. It showed the Venus crescent overall at least as well as the Argus.

M42:
Atmospheric conditions were less than ideal under a relatively humid suburban sky, but still the three 25x100s showed nice views of the Orion nebula and surrounding structures, and using averted vision, quite a bit of the bright/dark structure of the nebula could be observed in all three binoculars.
The Omegon Argus provided again the sharpest views, and it allowed a good view of the trapezium, with a clear separation of the four main stars.
In the Omegon Nightstar, the four stars in the trapezium could just be split, while in the Celestron, you could guess the four stars more than really seeing them beyond any doubt, it was more like „three stars for sure and maybe four“.

Conclusion
The Argus wins by a certain margin with its compact dimensions, light weight and superior optics. But it (and the identical APM) is of course the most expensive (€ 749.- in Germany), over double the price of the Nightstar (€ 349.-) and significantly more expensive than the Skymaster (€ 432.- to €449.-).
The Nightstar offers a solid overall performance and is a bit ahead of the Skymaster in off-axis sharpness, but loses a few points due to the reflections and glares on bright objects. Moroever, it is the largest and the heaviest of the three candidates. On the other hand, it is the least expensive.
The Skymaster is shorter than the Nightstar and almost as light as the Argus, but it’s optical performance, in particular in terms of brightness and peripheral sharpness, is slightly inferior to the Nightstar, except when it comes to reflections on bright objects; at the same time it is clearly more expensive than the Nightstar.
The Skymaster does have two clear advantages over both Omegons: on the one hand, the Celestron warranty, which is in many countries „limited lifetime“, whereas the Omegons come with the only 2 year warranty common (and mandatory) in the EU. And on the other hand, the possibility to adjust the tripod adapter to the center of gravity even when the IPD is narrower than 60 mm.
All three binoculars are nice tools, provide in my view a satisfying observation experience under the night sky.
If I had to choose now, I would buy the Argus if money is no issue.
Otherwise, I would probably choose the Nightstar, despite its weight and better warranty; it may be optically a bit ahead of the Skymaster and is quite a bit cheaper. However, people with an very narrow IPD (59mm and below) might possibly be better off with the Skymaster.

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