Lateral Chromatic Aberration - collimation issue?

[Retrograde (Pete)]

I always intended that my North Group ED102 triplet would be my 'learner' scope.

It has served me pretty well so far and I continue to learn a lot about imaging but one thing I've noticed is that on bright objects like the Moon or planets there is quite noticeable lateral chromatic aberration that seems consistent no matter where in the field the object is - always red on the left of the object and blue on the right.

This is primarily noticeable photographically (see attached pic). Visually it seems pretty reasonable and star images look OK (if maybe not quite perfect outside focus).

My question is this:
would it be worthwhile getting it collimated by someone who knows what they are doing or is this about the best I can expect with a cheap scope?
I did have a couple of minor accidents early on when the altitude lock on my alt-az mount slipped whilst the scope was out of balance and the tube banged quite hard on the azimuth plate so it's possible the collimation has gone out because of this.
Whilst I do plan to buy something of much higher quality down the track I would still like to maximise what the scope is capable of in the mean-time.

Hope some experienced refractor people can offer some insight,

cheers,
Pete

[glend (Glen)]

Refractor collimation is easily checked with a star test. Just target a star and wind the focuser in and then out of focus. You should see a wonderfully round ring pattern off focus.
If the rings are offset in any direction you have a collimation issue. Most refractors can be collimated on a bench with a Cheshire tube. Just check online for links to the process. Don't pay someone to do something you can do yourself.

[alocky (Andrew lockwood)]

It's reasonably easy to collimate a refractor using a Cheshire eyepiece if the lens cell is adjustable. This looks like one or more of the elements is de-centred with respect to the other two, and is not a simple fix. Check that the shims between the elements are intact and in place (assuming it is an air-spaced objective) and is there any chance one or more of the elements were rotated with respect to the others? Alternately if the aberration doesn't change across the FOV then it may be a matter of a severely bent tube?
Good luck!
Andrew

[Paul Haese]

This looks more like atmospheric dispersion to me. A very common problem for planetary imagers.

It won't hurt to check collimation in any case. Star test at the zenith is your best bet.

[Kunama]

Originally Posted by Paul Haese

This looks more like atmospheric dispersion to me. A very common problem for planetary imagers.

..........

+1 I have seen this sometimes in the Takahashi as well. I would be surprised if it is collimation related

[wasyoungonce (Brendan)]

Originally Posted by Paul Haese

This looks more like atmospheric dispersion to me. A very common problem for planetary imagers.

It won't hurt to check collimation in any case. Star test at the zenith is your best bet.

Yep...typically you see exactly that red on one side blue on other. but as Paul said Cheshire or star test...try star test at zenith.

[Retrograde (Pete)]

Thanks for all the replies and suggestions.

Originally Posted by alocky

This looks like one or more of the elements is de-centred with respect to the other two, and is not a simple fix.

This is the kind of thing I was more worried about Andrew, rather than basic collimation. I will recheck collimation when time allows (& weather clears) but it looked OK last time I checked.
I can't see anything untoward with the objective cell but I'll have another close look. I certainly don't think its anything too drastic.

Originally Posted by Paul Haese

This looks more like atmospheric dispersion to me. A very common problem for planetary imagers.

I hadn't really considered this as I assumed the main atmospheric issues I'd encounter would be more random and Saturn & the Moon weren't that low - maybe 40-45 deg above the horizon.

I'm just starting to explore the limits of the scope in some areas and this was my first real go at imaging with the 4 x Powermate (at 2800mm fl) and the seeing certainly seemed pretty poor.
I've previously noticed some red/blue fringing without the Powermate which is why I assumed it was scope related but perhaps I'm worrying too much?
You can get an idea of the seeing at the time from this vid:

Click here to load the video

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[DavidU (Dave)]

Yep,atmospheric dispersion.