Celestron Omni XLT 120 tips

[Photonmonkey]

Hello all, I have just joined the forums after purchasing a Celestron Omni XLT 120 refractor, any recomendations on what I can expect out of this scope. I have the standard 25mm eyepiece and also bought a Celestron X Cel 8mm as well, thinking of getting a Celestron X2 barlow, tried to have a look at Saturn the other night but skies were pretty bad. Again this evening the skies seemed very smoggy, could only make out major stars in the main constellations so did not bother taking scope out, any advice gladly received ;o)

[Moonshane]

hi there and welcome.

I don't have it now but used to have this scope.

it's a really nice achro, great optics and a mount which copes well for visual. the 25mm eyepiece was OK too and the 8mm should be. not sure about the barlow although with the 25mm it would be OK - with the 8mm might be a bit too much (250x) except on the moon sometimes.

I actually found the 8mm to be the eyepiece which gave me optimum magnification a lot of the time (125x) but it could be pushed well above this on the moon.

it excelled at doubles and wide field star clusters and the chromatic aberration was well controlled at f8.3. I bought a 'fringe killer' type filter but did not like the yellow cast and the CA was OK for me even on the moon.

all round a really nice scope. being 6'3" I found that the tripod was a little low and made a wooden one to replace it and it benefited from a better focuser.

I went for more aperture in the end and sold mine when I got a 6" f11 reflector to accompany my 12" dob.

[John]

Hi - welcome to SGL !.

As Shane says, you have a very nice scope there. This evening was a bit murky but Saturn was worth a look and your scope will also be good at binary stars as well which don't need particularly clear skies to be observed.

My advice would be to i) download the freeware Stellarium software to find whats in the sky at a given time and ii) get a good guide to the night sky so that you can start to identify some target objects.

[david o]

Hi there, Welcome to SGL

Nice 'scope to get going with !

[Photonmonkey]

Thanks for all of your comments, I am glad I seem to have purchased a reasonable scope, I am going to try and get some good views of Saturn if I can, I was wondering what kind of magnification I could push the scope to under good conditions for planetary viewing?

[david o]

As a general rule of thumb the max usable magnification that a 'scope will give is about 50x per inch of aperture (or 2x aperture in mm). Beyond this you get "empty" magnification, gaining nothing but a dimmer image. The exception being bright targets like the Moon and planets.

So for your Omni XLT 120 it would be between 235x and 240x.

However, you'll rarely be able to use the full potential mag of the 'scope. You'll be limited by atmospheric conditions, known as "seeing" (this link explains it better than I ever can - The Atmosphere and Observing). The seeing usually limits the usable mag to somewhere between 100x and 200x.

Occasionally, on nights of exceptional seeing, you can push the mag to the 'scope's limits and beyond - depending on the target. Even on normal nights you can get fleeting moments of good seeing.

HTH

[Moonshane]

As a general rule of thumb the max usable magnification that a 'scope will give is about 50x per inch of aperture (or 2x aperture in mm). Beyond this you get "empty" magnification, gaining nothing but a dimmer image. The exception being bright targets like the Moon and planets.

So for your Omni XLT 120 it would be between 235x and 240x.

However, you'll rarely be able to use the full potential mag of the 'scope. You'll be limited by atmospheric conditions, known as "seeing" (this link explains it better than I ever can - The Atmosphere and Observing). The seeing usually limits the usable mag to somewhere between 100x and 200x.

Occasionally, on nights of exceptional seeing, you can push the mag to the 'scope's limits and beyond - depending on the target. Even on normal nights you can get fleeting moments of good seeing.

HTH

I think this is correct for the finest optical quality possible. in reality individual scopes never achieve this and therefore the max usable mag even in those rare occasions when the seeing conditions allow will vary from scope to scope. for 'mortal scopes' (i.e. ignoring the high quality APOs/fluorites costing £1500 and up for this aperture) I tend to work on 1.5x aperture in mm which would make this 180x for the 120mm.

my experience with this scope even with premium eyepieces was that it was rarely usable above 150x except on the moon and double stars. it's still a great scope though! and with lovely pin sharp stars with good colour in the wide field views

[Photonmonkey]

Thanks, Hopefully if it is clear this evening I will try and take a look at Saturn with my 8mm X Cel eyepiece which should give me about 125x. Just eager to get out there and start viewing the heavens again, unfortunately living on the outskirst of Birmingham in the UK we suffer from a lot of light pollution, would love to get out to some really dark sites! I havn't had a telesope since I was about 12 (a rather cheap 2" Tasco refractor) so looking forward to it.

I am also loving the forum, good to know there are so many helpful people out there.

[Moonshane]

you'll love it.

[Photonmonkey]

Moonshane you were right!!!!

managed to get out last night and get a look at Saturn, using the 8mm X Cel I even got a gasp of amazement from my hard to impress wife when she looked at Saturns rings, this was at 125x so I think I could probably push it a bit more than this as image was quite bright.

The only issue I have noticed, is with the CG4 mount the DEC axis there seems to be a bit of slack in the gearing as I can very slightly rock the scope a couple of mm each way even with the lock on, not sure if there is anything I can do about this, other than that I am most impressed with the scope and cannot wait to get out again ;0)

[Moonshane]

great stuff. there may be a way to tighten up the axes a little but cannot recall to be honest.

enjoy your new toy

[ollypenrice]

There is a way to tighten the backlash but I can't remember it either! It should Google. In the mean time, try balancing it a tiny bit back end heavy to encourage it to sit on one side of the mesh.

I had one of these and took it to Spain and Majorca on holiday before emigrating to a dark site. It is a fine scope, very competent. I'd agree with Moonshane, go for a max around 180, maybe 200. Don't budget for more, it will hardly ever be used and quality of view beats quantity.

Olly

Edit: just remembered: there is a block held on by two allen bolts, I think, with a little allen grub screw between them. The grub screw sets the play in the mesh. Back off the two big ones, screw out the grub a tiny bit and relock the big ones. Don't make any radical changes though!

[Photonmonkey]

Thats great, Thanks for the tip will give it a try. There isnt a lot of play but the other axis is rock solid so wanted it to be as firm as possible.

Thanks also for the advice on magnification, I think I will opt for another wide angle and possibly a 5mm to give me the 200x to max it out, to be honest even the view of Saturn at 125x was very pleasing.

Thanks

[Photonmonkey]

Just tweaked my DEC axis and hey presto no slack, many thanks Olly!!!!