Colour aberration?

[Aenima]

Hello astro-peeps.

I have been using my 200p since I aquired it a month or two ago, and noticed that the images - stars especially - have a kind of rainbow effect where the white light seems slightly seperated into a shimmer of red-greenish-blueish spectrum at the edges and, when i use a bahtinov mask, it shows lines that are literally seperated into the basic three colours in the center of the star-X shape, only becoming white when the lines extend beyond the central cross area. Before using a bahtinov mask this colour aberration only presented in subtler ways but I'm sure now that something is not right.

If anyone knows what this might be or can shed light on the issue I would be extremely grateful.

Hopeful,

Regards

Jay.

ps. i have collimated as best i can using a cheshire but still wonder if this is part of the problem. Am looking for alternative solutions anyway.

[Tinker1947]

I'm sure somebody will have the answer, can you post what EP's you get this with....

[cantab]

Are these stars relatively low in the sky? If so, and stars high in the sky don't show the same effect, it could be the effects of atmospheric refraction.

[Aenima]

Wow, fast responses. Thanks.

This star was relatively high, Arcturus, and it happens with venus too. Just like a prism or similar does to white light, and the 200p is skywatcher with normal ep's that come with it - these same ep's were ok with my 130p but then again the effect isn't always as obvious. I use a barlow x2 ocasionally, and clearly the higher the x- Magnification the more it shows up?

I dont think it is purely atmospheric, but then I dont have much experience with most things optical.

Thanks again for th replies,

Jay

[perkil8r]

I have the same issues with mine. I don't have the same problems using higher quality EP's though and the barlow from SW is appauling. I have a TAL barlow, it is head and shoulders above it in quality. Sure collimation does help with the image you see, but EP's are a huge part. I put this colour seperation down to them at any rate.

[DirkSteele]

I cannot say I have come across this problem in a Newtonian telescope before. Mirrors are pure achromats (completely colour free) so I would suspect any spectrum you are seeing is likely due to problems with the lens elements of your set up, i.e. the eyepieces and barlows.

[umadog]

Yes, certainly it will be eyepiece related if the star is high in the sky. Probably the reason the eyepieces work well with your smaller scope is because the focal ratio is probably larger in that instrument. Eyepieces will show more chromatic aberration in a faster scope.

Does it get better if you barlow the eyepiece? That's not a great test, but you will effectively be doubling your scope's focal ratio so you should see the chromatic aberration disappear if it's due to the eyepiece. It may not, though, if the barlow is adding it.

How much of a problem is it, though? Do you only notice it on bright stars?

[Aenima]

Thanks guys,

Appreciate the input, only have stock ep's so dont have much to compare them to. I have a celestron omni 4mm and some kind of compact barlow but again these are not expensive so would be not much better.

Thanks perki8tr for the advice, I wonder if you can give me an idea how your getting along with the 200p/EQ5 set up, and what to expect from it in general terms. I have had a few issues but being a noob I think is part of the problem.

Appreciate the help, though, cheers.

Regards

Aenima

[Aenima]

Yes, certainly it will be eyepiece related if the star is high in the sky. Probably the reason the eyepieces work well with your smaller scope is because the focal ratio is probably larger in that instrument. Eyepieces will show more chromatic aberration in a faster scope.

Does it get better if you barlow the eyepiece? That's not a great test, but you will effectively be doubling your scope's focal ratio so you should see the chromatic aberration disappear if it's due to the eyepiece. It may not, though, if the barlow is adding it.

How much of a problem is it, though? Do you only notice it on bright stars?

No I think its a problem with all stars (and some planets) but with smaller objects the effect is smaller - my 130p is f5 too - and the barlows seem to add to it so they could be an issue. It is a major effect in all observing and most noticable when using a Bahtinov mask for focusing, and trying to focus on bright star or venus ~(venus sometimes has ghost images either side) - I'm a bit stuck really.

Appreciate all the input on this , thanx.

Jay

[perkil8r]

I'm still a relative noob myself. I found that the EQ5 is great when set up well, and a nightmare if not, so take your time setting up. I've not had that much time at the eyepiece since I got it really, but certainly eye pieces transform the scope to a completely new level. Have a go at Saturn with the 10mm EP and Barlow, you will get some colour seperation but the image of Saturn hanging there above you will stick with you for life! A truely awsome sight in our night sky, it doesn't look real because it looks just like you see on the films, which surely are sci fi not sci fact?! It will blow your mind, it did mine anyway. Go on youtube and watch some of the tutorials by Astronomyshed, they are great for the EQ5 set up and also for getting the bes out of your kit. I found them very useful indeed.

I had to adjust the clutches on my EQ5 as they were very sloppy, which meant that the scope would wander a bit. I found there was grease on the clutch surface, once cleaned off it made everything just that bit more positive. The EQ5 is starting to struggle with the 200p I think, certainly the weights have to be almost right at the end of the balance bar to get proper balance. I've since added a custom made weight which is a lot smaller. This along with the two stock weights has made balance a lot easier. Also I use the small one on it's own with the st80 which is much smaller and lighter.

To improve contrast and clarity I flocked the OTA on the 200p, it has made a big difference, and although a scary thought in terms of stripping it down to add it, it is well worth the effort IMO, and if you take your time it's not nearly as scary as it seems.

[perkil8r]

Your issues are almost certainly related to eye piece and Barlow, not the scope. Have you made sure they are clean and smear free? Be very careful how you clean them to avoid damaging them. There are tutorials for lens cleaning on youtube.

[rwilkey]

I have not found such problems with the 200P, my best guess is it's down to the stock ep's you are using, if you have a club nearby, you might be able to borrow better quality ep's to try out, Plossl's should certainly improve the image, try a 32mm if you can and then go from there.

[umadog]

No I think its a problem with all stars (and some planets) but with smaller objects the effect is smaller - my 130p is f5 too - and the barlows seem to add to it so they could be an issue. It is a major effect in all observing and most noticable when using a Bahtinov mask for focusing, and trying to focus on bright star or venus ~(venus sometimes has ghost images either side) - I'm a bit stuck really.

Yes, you're right, if chromatic aberration occurs at all then it will be present in all objects. The issue is whether it is occurs to annoying degree in any but the brightest stars. If you only noticed it in the brighter stars then you could perhaps cut you losses. What you're seeing, however, sounds pretty extreme. Ghost images shouldn't be happening. You'll end up buying more eyepieces anyway at some point so you might as well start now and see how that goes. You don't have to spend a bomb, just go for a Plossl or two from a big-name manufacturer such as Meade, Celestron, Televue, BST, etc. Look second-hand if you like.

[Aenima]

Nice1 ppl, cheers for the advice n guidance. Now I have my scope, mount, camera and software the only real place to go is the EP's - especially if they are partly the cause of the colour seperation, i was thinking the barlow from the SW stock set was pretty decent up until I used it with the 200p, now I wonder. Apart from my attempts at collimation the opticals had been - imo - not too shabby, but the bigger scope seemed to bring forward the limitations rather than show me more of the night sky... i want to fix the issue, and appreciate the advice, thank you.

Regards

Jay

[umadog]

I looked up the specs of your scopes and to my surprise both are f/5. I assumed that your 130 mm was slowish (f/7 or so) which would explain why you noticed no problems in it. So there goes the focal ratio hypothesis. If I remember correctly, chromatic aberration is worse at higher magnifications (at least for refractors). Your 200p is substantially longer focal length so yields higher power for a given eyepiece. Perhaps that explains the difference between the scopes?

Regardless, it is a fact of life that f/5 and faster scopes are rather picky about eyepieces. In fact, I've had bad views at f/5 with an eyepiece costing $120. So make sure whatever you buy is known to work well with faster scopes. If you're unsure and you're buying new, check in advance that you can send it back if it doesn't work for you.

[LukeSkywatcher]

Ive only suffered from CA with refractors or bins. I cant say its been an issue with my Newt or my SCT. So if your scope is not the problem, then it must be down to the EP's you are using.

Other than that i wonder if you are observing through double glazed window? or is the scope properly cooled down?

[Pete Presland]

you could always try looking around locally for fellow astronomers or a club/society and try some other eyepieces before you buy.i did find this place locally to you http://www.halien.com/TAS/

maybe if contact them you might be able to take your scope there.

[StarryBob]

Finger marks or smears on the secondary can cause this, I've seen it myself.

[Aenima]

This is welcome food 4 thought, and the ep thing seems recurrently central to colour problems, I have to admit my ep's are the basic ones from skywatcher and have only been added to with a 4mm omni celestron in an attempt to get more magnification to observe planets better. Also, as my knowledge and experience increases, the problem has probably become more of an issue, I now know that reflectors are mirrored and only barlows/ep's introduce colour aberation. Out of all the purchases the ep's were the most neglected as when i first started i was actually very impressed with them, only now have i caught up with the forgotton essentials like good colour and sharp focus. So, after buying a decent webcam (spc880/900nc) and a bigger scope/mount it will be a while untill i can get a new ep but thanks to many helpful posts/postee's I know now what to save up for, and to stop worrying over what the problem is - cheers folks.

PS. I observe outside as the window option would, as mentioned, be terrible - and im unsure of both accurate collimation (2ndary or primary - both scopes) and cleanliness of mirror surface. I used them from the box and have not dared to dismantle. But have done my best with a cheshire to collimate them.

Many thanks

Regards

Jay