TAL 100 RT: Quantifying Chromatic Aberration

[John]

Back in another thread on the TAL 100 I mentioned that I was going to have a go at assessing how much false colour my example of this 100mm F/10 achromat showed.

I've been using the TAL 100 to observe the Moon this evening which is 54% illuminated so I've been attempting to put some quantative value on the CA that was visible around the lunar limb. I waited until the background sky was reasonably dark. I was using a 17mm plossl (59x) and 6.3mm plossl (159x) for this. At sharp focus though both eyepieces there was a violet rim around the lunar limb extending a short way against the background sky with it's outer edge gradually fading to match the background sky.

I decided to measure it's extent in terms of it's visible breadth from the edge of the lunar limb to where it faded out of visible sight. As far as I could see the rim was a consistent width all the way around the limb. Comparing it to lunar craters I found a few that seemed to be of a diameter that matched breadth of the CA rim. I picked craters reasonably close to the terminator so they were not viewed at an oblique angle. A bit crude perhaps but the best approach that I could come up with on the spur of the moment !

Checking with the data Virtual Moon Atlas, I reckon that the craters that I'd picked out had a diameter in the region of 30 miles. This equates to an angular diameter of around 36 arc seconds I believe. So, if the visual breadth of the CA rim that I was seeing was in the order of 36 arc seconds, thats 2% of the mean angular diameter of the Moons disk.

Interestingly, the breadth of the CA rim stayed relatively a similar size at both the magnifications mentioned above. I thought it's extent might have increased with higher magnification but, on this occasion at least, that did not happen.

My overall impression was that the rim of violet CA was not at all distracting at the level apparent tonight. The lunar disk and surface features were very sharply defined by the TAL 100. I could not see evidence of CA along the terminator or creeping into dark crater floors, as it sometimes can. A very satisfying view for a 10cm aperture telescope I felt. 

This result might vary under different observing conditions of course and if the Moon was higher or lower in the sky. I usually observe with ED doublet refractors that show no apparent CA rim around the lunar limb but the amount that the achromatic TAL 100 was showing was, I felt, pretty insubstantial and did not detract from my observing enjoyment.

"Your mileage may vary" though, as the saying goes   

[Piero]

Interesting analysis and great report (as always). 

These TALs really seem great telescopes beside having a nice aesthetic!

There is one thing I never understood about CA and that is why it is generally seen on the border of a bright object rather than on the centre. From your report, I understand that you analysed whether CA was present or not on the terminator (and you did not find it). I would expect the whole object to be distorted by CA, not just (or mostly) the border.. Any idea? 

[Peter Drew]

I too was looking at the Moon this evening. I was using my 220mm F12.5 achro refractor with a Hyperion Mk111 zoom. Up to about 200x CA was fairly negligible but higher power up to the maximum 339X showed an increasing amount including a little around bright crater peaks near the terminator. The rest of the lunar surface had a very faint violet caste if you had to put a colour to it. None of the CA I found to be distracting. The slightly longer FR than the Tal does not compensate for the more than doubling of the aperture so I would expect the Tal to show less CA on a side to side comparison.      

[John]

Interesting analysis and great report (as always). 

These TALs really seem great telescopes beside having a nice aesthetic!

There is one thing I never understood about CA and that is why it is generally seen on the border of a bright object rather than on the centre. From your report, I understand that you analysed whether CA was present or not on the terminator (and you did not find it). I would expect the whole object to be distorted by CA, not just (or mostly) the border.. Any idea? 

I think CA is present across the whole of the target object and I suspect it does affect the crispness of the view even if only slightly. I guess the much stronger tones of the target cut through the CA so it's not noticable until you reach the boundary between the object and the background sky.

Next time I think I'll put my Vixen ED102 on the same mount and try and do a sort of "blink comparison" between the two views of the lunar surface (at comparable magnifications) and see if I can detect any differences in the contrast and sharpness of the lunar features as well as whats happening at the limb.

I'm not trying to make a big issue of CA here. It's a fact of life with achromats and has been for 100's of years just as coma affects newtonians.

[Piero]

I think CA is present across the whole of the target object and I suspect it does affect the crispness of the view even if only slightly. I guess the much stronger tones of the target cut through the CA so it's not noticable until you reach the boundary between the object and the background sky.

Next time I think I'll put my Vixen ED102 on the same mount and try and do a sort of "blink comparison" between the two views of the lunar surface (at comparable magnifications) and see if I can detect any differences in the contrast and sharpness of the lunar features as well as whats happening at the limb.

I'm not trying to make a big issue of CA here. It's a fact of life with achromats and has been for 100's of years just as coma affects newtonians.

Sure, that amount of CA is very small and to me shows that that is a very good telescope, particularly if considering its cost. 

Thanks for the explanation and I look forward to reading about your next comparison then! 

[MattJenko]

All these recent achro threads has me hankering after one, but did I read this right:

I was using my 220mm F12.5 achro refractor ,,,,

I can't even begin to think what that would even look like!

[Peter Drew]

Matt. Go to the Astronomy Centre home page (astronomycentre.org.uk), click on members gallery then equipment. Scroll through the images and you will find the refractor.

It is self built as well as the mount and observatory.