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Celestron Omni XLT 120


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Colin this refractor was voted a great buy in a review in the Sky at Night magazine. I think generally it is considered to be excellent value for money.

I think the package comes with a star diagonal which means that the image is upright but back to front so therefore different than a reflector.

The magnification is more difficult to answer because it depends on factors such as sky visibility and transparency. I doubt if you could go much above 200x and this will be a rare occasion. General about 150x/180x maximum would be the norm. The scope would be ideal for lunar/planets and double stars although the brighter DSOs would also be ok.

With regard to Chromatic Aberration you might need an Achromat & Semi-Apo filters available from FLO.

Mark

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Colin this refractor was voted a great buy in a review in the Sky at Night magazine. I think generally it is considered to be excellent value for money.

I think the package comes with a star diagonal which means that the image is upright but back to front so therefore different than a reflector.

The magnification is more difficult to answer because it depends on factors such as sky visibility and transparency. I doubt if you could go much above 200x and this will be a rare occasion. General about 150x/180x maximum would be the norm. The scope would be ideal for lunar/planets and double stars although the brighter DSOs would also be ok.

With regard to Chromatic Aberration you might need an Achromat & Semi-Apo filters available from FLO.

Mark

Very helpfull, thank you Mark

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Max magnification for any scope is usually 2x for each millimeter of the primary lens or mirror. So a 120 refractor should top out at x240 (not sure why Celestron would state x283).

An alternative is x50 for each inch of aperture which for a 120 scope (4.72") would mean a max of x236.

Normal UK seeing condtions tend to limit the maximum to x250 and often much less. I have seldom been up beyond x200 with a larger scope and more normally I use a max of around x130

Edited by Astro_Baby
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The x283 Celestron state is x60 per inch of aperture - which is possible but only on very good nights.

Realistically as AB says x240 is the max. This gives 0.5mm of light getting to the eye which is just bright enough.

For the price it's a nice refactor but a similarly priced 8" Dobsonian would visually beat it on everything.

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If you do get one then check the collimation. Every unit that I've seen, needed tweaking to get the very best out of it. Out of the box, they all have given decent images, but it was clear that the image seen was not the best one possible. After tweaking the views are excellent.

I have the Evostar versions, and I really like them. The views are every bit as good as my old Japanese made Vixen 102M.

Recently it gave me one of the best views of Mars I've ever had.

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If you do get one then check the collimation. Every unit that I've seen, needed tweaking to get the very best out of it. Out of the box, they all have given decent images, but it was clear that the image seen was not the best one possible. After tweaking the views are excellent.

I have the Evostar versions, and I really like them. The views are every bit as good as my old Japanese made Vixen 102M.

Recently it gave me one of the best views of Mars I've ever had.

Sounds promising.

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I have one at present mounted on a Vixen GP mount,

Yes you may need a Achromat & Semi-Apo filter.

and Yes you may need to initially collimate it, but after that it is an excelent scope.

I found that views were much improved after collimating.

Edited by mattifor
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Thats something new to me I thought refractors did'nt need collimating

I've collimated a few to get them spot on. Once done they hold their collimation very well. Quite a few refractor designs don't come with a collimatable lens cell which means that you need to resort to tilting and shimming the focusser slightly.

I have to say the collimation on the better quality refractors I've owned (Vixen and William Optics) was spot on and needed no adjustment - just as well because they had no provision for it !.

Just to complicate things further, you can get collimatable mirror diagonals as well - it's been know for diagonals to be out of alignment as well !.

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Just to complicate things further, you can get collimatable mirror diagonals as well - it's been know for diagonals to be out of alignment as well !.

Yes, that is a very good point. I adjust all my refractors without the diagonal in place. Get the lens and focus mount in line first, then you will easily see if the diagonal is out. I have had to shim a couple of diagonals in the past.

I often wonder how many people have bought and used a refractor and never even checked the collimation?

Usually it is a one-off job, even after 20 years some of my scopes are still bang on.

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Both my refractors are spot on but is there a thread on SGL somewhere to explain how to collimate a frac?

I would image that after the comments on this thread there might be individuals, who have a skywatcher or celestron etc, that might appreciate this info.

Mark

This is the method I use (including the cheshire sight tube):

Refractor Collimation

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