Chromatic aberration - depends on eye position?

[furrysocks2]

I received an Aero 35mm from an SGL member today and tried it out in the ST102 briefly this evening. The first 2" EP I've ever owned, pretty much maxing out the scope with a 7mm exit pupil and the reviews seemed pretty good down to f/5.

The moon was the first target. Very obviously visible were coloured fringes - "honey dew melon" on one side and "led blue" on the other. Despite the EP costing me only £5 less than the scope, I didn't cry because the moon was so big and bright and M42 looked fine as did three more of only six known targets in the sky - the Pleiades, the Double Cluster and M31.

My only other low-magnification EP is a 50 degree 30mm Vixen NPL - it's a lovely lens in the 3" lidl dob and the widest I've ever gone in the 8.5" newt (just under a degree). This new EP being 68 degrees giving 4.76 degrees FOV, I think, together with the size and feel of the rubber eye cup, it was like resting my face into a pillow if you know what I'm trying to say - a great first impression, I loved it. I look forward to that again but peering into darker skies - so much cloud and moon tonight so I didn't expect particularly good contrast. I don't know what it's like using a 90 or 100 degree EP - I read someone say something along the lines of 68 degrees being a sweet spot as far as matching the eye's capability. Regardless, I'm happy with this one and with any luck, unlikely to start a 2" collection.

 

Anyway, looking back at the moon I found that depending on where I positioned my eye, I could make the fringes disappear, giving a crisp edge to the moon. I could hunt for that reduction in colour and momentarily see both the edge of the disc and the terminator without fringes at all, but reliably able to remove colour from one portion or the other for extended periods.

 

So then, does the appearance of chromatic aberration depend on eye position in such a way as this? Does a non-dark adapted eye, smaller than the exit pupil, affect the same in any way? Or is my scope not collimated correctly, or not fully cooled?

Good to hear an explanation if anyone can give one...

[John]

An F/5 achromat is going to produce quite a bit of CA. Nothing can avoid that. When you are using wider eyepieces though it may well be that the extent and impact of the CA can be altered by moving ones head / eye position and maybe looking into the eyepiece at a slightly different angle.

I don't know for sure though because I've not owned an achromat for a while. Apart from my finder scopes.

I liked both the 35mm Aero ED and the Vixen NPL 30mm when I had them. Nice eyepieces for their price

4.76 degrees combined with an O-III filter will show the Veil Nebula in it's entirety when it's up, later in the year. A glorious sight and one that will not be affected by CA.

[furrysocks2]

Thanks, John.

It'll be interesting to see how much CA is apparent when viewing planetary targets, though my short focal length lenses are of very poor quality.

 

8 minutes ago, John said:

I liked both the 35mm Aero ED and the Vixen NPL 30mm when I had them. Nice eyepieces for their price

They both reviewed as good value. I'm happy enough with them as they didn't break the bank, but they're pretty much all I've ever looked through.

 

8 minutes ago, John said:

4.76 degrees combined with an O-III filter will show the Veil Nebula in it's entirety when it's up, later in the year. A glorious sight and one that will not be affected by CA.

Thanks for the tip - I have a 1.25" UHC, but nothing 2" presently.

 

[Moonshane]

I agree with you. Even ED scopes show some off axis CA on the moon when you look for it/move your head about.

[YKSE]

Yes, i think it's CA in the eyepiece. many wide angle eyepieces, even some 50­° plossls and othos (BCO e.g), show this CA in exit pupil..

Observing a full moon, I've tried the same balancing eye position(I've only had ED or mirror scopes), butcan't really get it totally disappear, when the eyepiece has this CA.

You can easily verify it in day time, pointing the scope at bright sky, if there's colour at the field stop, mostly yellow, green, blue or something inbetween, then you'll see this CA in exit pupil when looking at a full moon.

[alan potts]

I used to have 40mm version of this eyepiece and I found it very good for CA in all the scope I had at the time though it suffered from soft edeges more than anything else when the scope was as fast as F5.3, it was a jolly good buy for an F10 SC though. In my view most the CA is going to come from your scope though, you need to spend a sizeable amount of money to have a refractor that is free of it.

Alan

[rockystar]

I've got no scientific basis or knowledge for this though: but thinking about what CA is - the different wavelengths of light coming to focus at slightly different angles - could it be, that by moving to a different angle you can get some of the wavelengths to align?

[furrysocks2]

3 minutes ago, rockystar said:

I've got no scientific basis or knowledge for this though: but thinking about what CA is - the different wavelengths of light coming to focus at slightly different angles - could it be, that by moving to a different angle you can get some of the wavelengths to align?

I've got no scientific basis or knowledge for this either, but that's what I thought...?

[John]

There is an eyepiece aberration called Lateral Colour which causes the spectrum to separate slightly when a bright object is viewed off axis. Moving your eye away from the optical axis can increase or decrease the perception of this I think.

The eye itself may have a role to play in this as well. This link is mostly over my head but some bits make sense !:

http://www.telescope-optics.net/eye_chromatism.htm

[Moonshane]

I reckon this is what causes issues with me, the Moon and Ethos.

[Paul73]

On 13 January 2017 at 12:08, Moonshane said:

I reckon this is what causes issues with me, the Moon and Ethos.

I guess that 100° eyepieces have got to do a lot of light bending! Sometimes it feels like giving a bit on the magnification and using a narrower FOV eyepiece gives a more natural view. Of course, exit pupil issues then come into play.

In my limited experience, placement of eye will affect perceived abrasions/flaws. Try using an eyepieve with generous eye relief without the eyegaurd extended! 

Paul

[furrysocks2]

http://adsabs.harvard.edu/full/1879MmRAS..41...10.

Haven't read this in depth, but seems to support what's previously been said.