SkyWatcher Skyliner 200P Coma

[orley]

Hi all,

At Xmas my wife bought me an unexpected but very welcome gift - a Skyliner 200 dob, purchased from FLO. Unfortunately through one thing and another first light with it wasn't until the end of last week. Out of the box the scope was out of collimation and I thought I'd collimated correctly, but in use I noticed what - to me - seems to be a large amount of coma. Stars in the centre of view were tight and sharp, but sharpness rapidly fell away towards the edges of the FOV. This was a disappointment, as I noticed a quite considerable increase in depth and detail. For example, the Double Cluster showed far more, clearly resolved stars than through my 100ED, but I couldn't get the view sharp across the field of view.

Is this normal for an 8" F6 newtonian? Or is the scope still not collimated properly? I did have a 6" F5 a long time ago and don't recall having the same issue. If it is normal, how much of the FOV of view should be sharp? I'm estimating the sharpness drops off significantly about half way towards the edge of the FOV. I should add that the effect was similar when using both a 2" SkyWatcher 28mm LET and an 18mm 1.25" BST.

Other than that I quite like the scope. The light grasp over the 100ED is quite noticeable.

Thanks in advance,

 

Tom.

 

[Tinker1947]

Yes its normal, a Coma Corrector will sort it out, i have the Baader and it works very well......

 

http://www.firstlightoptics.com/coma-correctors/baader-mpcc.html

[John]

At F/5.9 coma should not be a significant issue, visually (I don't see it with my F/5.3 12" dob). The 28mm LET eyepiece will show astigmatism in the outer parts of the field (it's just a 3 element design) but the BST 18mm should be better.

Could you describe in more detail the aberration you are seeing ?

Coma causes stars to grow little tails radiating away from the centre of the field of view wheras astigmatism distorts the stars into little "seagull" type shapes with the "wings" going around the field of view, as it were.

 

[DevonSkies]

What you describe sounds like "field curvature" to me. That's where the point of focus changes as you move from the centre to the edge of the field of view. Can you get the outer part of the field of view sharp by adjusting the focus? If so, then it's field curvature.

It tends to depend on the eyepiece used. Unsurprisingly, more expensive eyepieces tend to exhibit much less field curvature. I tried a BST 25mm eyepiece on my Skyliner 250PX (f/5) and it had terrible field curvature. Couldn't get more than the central 30% sharp. I replaced it with a Celestron X-Cel LX 25mm and the focus was then perfectly flat across the field. My other BST eyepieces show some field curvature, but I can live with it.

Hope that helps,

Ed

[John]

This article is very useful and worth a look. It describes and illustrates the principal aberrations that occur in scopes and gives some pointers towards causes:

http://umich.edu/~lowbrows/reflections/2007/dscobel.27.html

Of course sometimes we see more than one aberration type combined, which makes things a little confusing !

[orley]

Just now, DevonSkies said:

What you describe sounds like "field curvature" to me. That's where the point of focus changes as you move from the centre to the edge of the field of view. Can you get the outer part of the field of view sharp by adjusting the focus? If so, then it's field curvature.

It tends to depend on the eyepiece used. Unsurprisingly, more expensive eyepieces tend to exhibit much less field curvature. I tried a BST 25mm eyepiece on my Skyliner 250PX (f/5) and it had terrible field curvature. Couldn't get more than the central 30% sharp. I replaced it with a Celestron X-Cel LX 25mm and the focus was then perfectly flat across the field. My other BST eyepieces show some field curvature, but I can live with it.

Hope that helps,

Ed

Hi,

 

Yes, this is it! Apologies for the misleading headline in my post. Yes, in fact I could focus the outer part and have the inside unfocussed.

So it seems my collimation was okay then. I have an assortment of other eyepieces to try, so I'll see what the view is like with those.

Just now, John said:

This article is very useful and worth a look. It describes and illustrates the principal aberrations that occur in scopes and gives some pointers towards causes:

http://umich.edu/~lowbrows/reflections/2007/dscobel.27.html

Of course sometimes we see more than one aberration type combined, which makes things a little confusing !

Thanks for this! I'll have a good read.

 

Thanks everyone for your help

 

Tom.

[Axunator]

I have the exact same scope. Even with my longest focal length and widest field eyepiece, 26 mm Nagler Type 5 (82 degree apparent field of view, meaning true field of 1.7 degrees in this scope), I do not notice significant coma nor field curvature. I think your eyepieces are to blame, not the scope... 

What you get by paying the premium for premium eyepieces are wide fields that are extremely well corrected for all kinds of aberrations eyepieces may have. Now, Nagler 26T5 costs a lot more than the whole SW 200P scope, but there are cheaper alternatives with almost as well corrected fields.

It is a very nice scope by the way, I think. Especially the aperture vs. cost ratio is pretty incredible. Great wife you have! 

[orley]

Hi all,

Apologies for resurrecting this, but last night was the first chance I've had since to use the scope. This time I used a 20mm Meade 4000, a BST 12mm and a Pentax XF 8.5mm. The view was better - lovely on the moon - and the previous problem of the focusing either the inner or outer seemed to have stopped.

However, now I seem to have the centre and top half of the field of view in focus, but the bottom third (or so) out of focus. This was especially noticeable on the moon. Is this down to collimation? I had the same problem with all eyepieces I tried.

Thanks in advance!

Tom.