Celestron 120mm Refractor

As far back as I can remember I have had a powerful interest in Astronomy. For years I longed to probe the mysteries of the heavens but lacked the proper equipment to do so, that is until I laid eyes on the sleek blue Celestron Omni 120.

Before we start I’ll quickly explain what makes this telescope what it is. Celestron is the company who made the scope, they are a reputable company who produce mid range mid price telescopes. Omni XLT simply refers to the coating on the lenses which improves light transmission and basically means more light ends up in your eyeball. 120 is the size of the objective lens in millimeters. The bigger the lens the brighter and potentially larger you can make the image

-Refracting Telescopes In General-

The omni is a refracting telescope, this means that the telescope employs a single lens instead of a pair of mirrors to focus light down into the pupil.

Usually a piece of equipment known as a star diagonal is used in conjunction with the refractor, this is effectively an angled mirror placed between the telescope and the eyepiece which makes it much easier and more comfortable to view through the telescope as less stooping is required (my star diagonal and my spine have fast become the best of friends).

The price of using the diagonal is that the image becomes reversed with East being to the left and West being to the right


East              West 


For any given size the refractor will produce the sharpest image but will also come in more expensive than reflectors or Schmidt cassegrains, these are different telescopes that use different ways of delivering light to your eyes.

A major complaint for all refractors is that unfortunately the lens does create a degree of false colour (a purplish halo of light around an object.)

This occurs as the lense seperates light into its constituent colours much like a prism. The false colour can be a problem to some observers but I find you quickly get used to it and after a while you don’t notice it anymore.

In a nutshell refractors are the best quality telescope you can buy but produce a degree of false colour and get very expensive indeed, especially as size increases (the larger refractors out there can cost as much as a small car running into the thousands of pounds)!

-This Refractor Specifically-

The Omni is a fine example of a mid to large size refractor at a very reasonable price (£280 now but £360 when i bought it grrrr). Setup is easy and takes a mere 5-10 minutes.

-The Focal Ratio-

The focal ratio of a telescope is worked out by dividing the focal length (the distance between the objective lens or mirror and your eye), with the size of the mirror. The focal length of the Omni is 1000mm and the size is 120mm so the focal ratio is 8.3333333, we express this value as F8.

Focal ratio is a good guide for what the telescope will be best used for, higher F numbers are generally better for objects of the solar system while lower F numbers are better suited for deep sky work such as galaxy hunting!

-The Mount-

The mount is a CG4 German Equatorial, this refers to how the telescope will be able to track things through the sky. It takes a little more research than other mounts to set it up properly but is worth it once you do.

The main difference between an equatorial mount and a dobsonian mount is that with a dobsonian you just push the telescope exactly where you want to look. But an equatorial mount is set up in such a way that it needs to be polar aligned before it can be comfortably and effectively used (this will require a lengthy explanation.)

As im sure you’d rather hear more about what the telescope will enable you to see rather than its mechanics I shall simply say the dobsonian mount (for reflectors), is much more straight forward and easy to use while the equatorial (for refractors) takes a bit of getting used to but will eventually allow you to easily track (or hold in view) objects through the sky.

Phew! Now the boring techicalities and fundementals of telescope design has been gotten over with…..It’s onto first light : )

-First Light-

The first time I used this telescope (and indeed any telescope) was a cool, clear winters night. I had always been fascinated by space and was now going to get my first proper view of the heavens.

First off, I pointed the scope towards Saturn, after a while I saw it come dancing across the field of view (FOV) at magnification of around 60x it was a tiny bluish dot but the rings were clearly visible and tack sharp.

I increased the magnification which held all the way upto 250x, at this size Saturn had a yellow hue and even very subtle bands could be seen in its atmosphere.

The rings at this point were positioned very nicely for observation, the cassini division (a gap between the planets A and B rings) was clearly seen.

I then pointed the scope at the moon, all i can say is WOW! Tiny rills and craters leapt into the FOV, words won’t come close. It was purely beautiful.

Still to this day it  reminds me why I fell in love with Visual Astronomy in the first place.

After an extensive study of our closest neighbour I decided to turn in for the night, it had been a wonderful evening.

I was laying in bed that night thinking about Saturn and recalling that I’d seen some stars very close to the planet that had looked a little bit odd, then it hit me… I wonder if those stars were actually moons?

That was it, I lept out of bed and went out once more into the breach, after carefully applying some clothes so as not to scare my neighbours,  and set up once again.

…”Mavis, get away from the window, that strange kid next doors got his shiny blue instrument out again…… And hes got a telescope!”…

I trained the telescope back over to Leo which is the constellation that the planet happened to be in, and once again the jewel of the solar system popped into view.

My mouth dropped as I registered what I was seeing, Tethys, Rhea, Dione were there, even little Enceladus occasionally twinkled into view using averted vision. But the real showstopper was the mighty Titan, the largest moon of Saturn and the only moon in the entire solar system to have a substantial atmosphere.

I don’t mind admitting that a little tear rolled down my cheek at this point….I was so happy and in absolute awe of what I was seeing.

-Deep Sky Objects (DSOs)-

Unfortunately from my semi light polluted skies (an 8 on the controversial Bortle Scale) a 120mm refractor was never going to detect anything but the brightest of objects outside our solar system, but over the next few weeks I tracked down….

the enigmatic Andromeda Galaxy (with an estimated 500 billion solar masses it’s interesting to think that this galaxy is quite likely to be home to thousands of sentient species)

The emission nebula m42 (Where baby stars are born)

and two of the galaxy triplets, m65 and m66 (I couldn’t quite make out the edge on spiral of NGC 3628)

all these objects were magical but of course very faint.

Since those first beginnings I have owned a few Scopes, and have since racked up a list of hundreds of DSOs from beautiful galaxies of all shapes & sizes to dying stars known as planetary nebula whose intricate shells give off an eerie greeny blue colour.

I’ll always remember though, my first telescope. Like your first love, it’s always the most significant.

To succinctly conclude, the Omni XLT 120 is a fabulous telescope for the price with false colour not being too prevalent and providing sharp bright images. Although i am highly biased as she was my first ; )

*In this review I have omitted any frustrations I had using this scope as they were mainly down to my inexperience and haste rather than any shortcomings with the telescope itself*