Celestron Omni XL120 refractor - finding out the hard way?

[marwood]

Greetings,

I recently got my hands on one of the above and have had a few sessions with it when conditions have allowed. So far it has delivered some cracking results (up til now my observing has always been with binoculars) on the likes of Jupiter and the moon. Andromeda was quite a catch too. Where I've felt a little deflated is the clarity of views of stars. Observing a star cluster last night, many of the stars appeared as if they were a little distorted and just lacked the crispness that I've usually expected.

The fact that the scope delivers great views of the planets would suggest that there isn't anything wrong with the optics. Is there some other aspect that I may be overlooking?

Another tricky aspect I've discovered is manouvering the scope. Any time I want to look at anything directly overhead, the contortions I have to pull myself into just to get a look into the finder scope leave something to be desired. That is with the tripod extended out to its full height. It's as well that it's dark otherwise I would make for a less than dignified sight! Is that just me encountering first hand a drawback of refractors or are there any tips and tricks that might reduce the chances of me displacing a couple of vertebrae?!

[John]

Hi and welcome to the forum

Your Omni 120 should show good star images, especially if it shows nice contrasty views of Jupiter. Refractors are good at showing point point stars, or should be.

What eyepieces are you using with it ?

Refractors on equatorial mounts can require some contortions. I have mine on alt-azimuth mounts with a pillar extension when required which lifts the scope / eyepiece / finder up to around 160cm or so. A right angled finder might help too.

[marwood]

Hello Joh,

Nice to be here!

Thanks for your reply. The eyepieces that I have are the 25mm that came with the Omni, likewise the star diagonal. I also purchased a Barlow lens and a 9mm eyepiece, both from Celestron's X-Cel LX range. A right-angled finder would certainly be a good step in the right direction, I'll see what's available.

[marwood]

Hello Joh,

That should of course have read "John". Where is the edit function when you need it!

[John]

No problem about the spelling - we all do it !

On the lack of sharpness in stars - how long do you leave the scope to cool before using it ?.

Even though refractors tend to cool more quickly than other scope designs, the larger ones do need some time to reach thermal equilibrium and before they perform their best at high powers. 30 minutes or so should do it.

You may well find the barlow lens used with the 9mm eyepiece gives more power than the scope / conditions can usefully use quite often. I find 180x is more usable with my 120mm refractor on Jupiter, most of the time.

[marwood]

If it looks like a promising conditions for the evening, I usually put the scope out in the shed mid-afternoon to let it acclimatise so it gets at least a couple of hours.

I need to do a more thorough check and see if there are any combinations that work better/worse than others. I do know that I have had good crisp images of stars a few weeks back when I first tried the scope out.

Just a thought, would the fact that it was quite a frosty night have had an influence on the night that I noticed the poor star image quality? There was a fair amount of rime on the grass.

[John]

Sometimes the seeing conditions look good but when you come to view anything through a scope you can see that they are not and you can't get nice crisp images of anything at higher powers.

Another possibility is that the collimation of the scope needs a little tweak. The Omni 120's do have collimatable objective lens cells. I'd leave that until you have tried viewing on other occasions though, if the poor results are repeated.

[E621Keith]

I just wondered did you observe the stars before of after you observed the Moon, planet and M31? If you observed after you view these 3 objects, would it be possible your objective lens may have dewed up?

[marwood]

Thanks for all the replies. Just to give a quick update, I've been out a quite a few times since and not had a repeat. I think I can put id down to the seeing. I did purchase a right-angled erecting finder scope and that has helped a lot especially with some of the overhead objects that turn out to be elusive on the night.