CA, diffraction spikes & coma

[Caldwell 14]

Why do we think CA causes so much distain from a large porportion of amatuer astronomers and yet diffraction spikes, coma and poorly collimated scopes seem to be meet with a meer shrug of the shoulders and be considered to be par?

Did I read somewhere that only 10% of Newtons are correctly collimated? Perhaps that was at a star party.

[ChrisThePuma]

Sorry - whats CA?

[Islander]

Chromatic aberration. It's colour fringing that occurs when an optical component doesn't bring all wavelengths of light to the same point of focus.

[Adz]

For me it was when I realised how much contrast and detail was lost from planetary viewing when the light isn't shining towards one point.

[John]

For me it was when I realised how much contrast and detail was lost from planetary viewing when the light isn't shining towards one point.

Me too. I found a way to remove most of the CA from a 6" F/8 refractor and the difference was remarkable. You don't know how much detail / contrast you are loosing to CA until you remove it.

Since then I've stuck with ED refractors with pretty low levels of CA.

I agree with the points on coma, diffraction spikes and poorly collimated scopes though. I'm not keen on those issues either

Just call me "Mr Fussy"

[ChrisThePuma]

Ok, next question - how can i improve (or irradicate) CA?

[Caldwell 14]

You don't have too, you have a Newton.

[ChrisThePuma]

Cool

[Caldwell 14]

I have a warning on semi apo filters BTW. They recently cost me a small fortune.

I sent back a Baader Zoom I had just purchased as it fell apart after 2 weeks (but thats another story) and as I don't have huge wads of money to chuck about (I am putting 2 daughters and my wife through University) I bought 4 BSTs. While I was waiting for my BSTs to arrive I, in an act of madness bought a cheap Semi Apo on ebay, put it in my diagonal the day before my BSTs arrived and forgot about it.

I tried out my new eps the next day and was suprised how very good they were for the money, I was suprised just how good the semi apo was at removing most of the CA (I still think CA in the TAL 100RS is fairly minimal) but my biggest suprise was that my scope now chucked light about all over the place and now the views resembled Aurora Borealis.

I posted on here looking for advise, twisting don the eye cups was suggested, I tried that and it appeared to work, but not for long, I blamed my BST's so I sold them on ebay (for very, very nearly what I bought them for) and bought a set Celestron X-Cel LX eps. This seemed to improve things, but again not very much and not for long.

By now I was starting to become obsessed with my little problem until my "Bingo" moment when I thought about my semi apo filter, "It couldn't be could it" Oh yes it could. The second I took it out all my problems disappeared. I tried just screwing it into the ep but my internal display of the Northern Lights returned.

The 25mm Celestron is a better ep that the BST but the rest are on a par and I certainly would not have changed them before my semi apo issue

Beware the Semi Apo, beware.