strange colours on Moon

[jimmyh1555]

Just come in from looking at Mr Moonie through my XLT 120. Eyepiece was Delos 6mm, no filters. I saw some bright red markings on the walls of some craters - particularly Gassendi - and on that peak in the middle of Gassendi. It looked like it was a prism effect from the sun shining on the crater walls. Crystals?? Diamonds?? Or was it just a quirk of the XLT 120 glass or maybe refractions in atmosphere? Any ideas?:shrug:

[Wavytone]

It’s typical artefact from a fast achromatic refractor - lateral chromatic aberration seen when the subject has a sharp edge with extreme contrast (ie white vs full shadow) and is somewhat off axis. If you moved the scope to put the image on the other side of the eyepiece you’d see the shadows tinged with blue.

Try looking at stars at the same power and see what happens.

Achromatic refractors really should be f/15 - like Unitron used to do - to achieve excellent correction; the modern trend of making them around f/8 or even f/5 is a stupid compromise as opticalybsome false colour is inevitable. The downside of course is the size of the OTA and mount required.

Ways to eliminate false colour:

1. In a budget achromatic refractor, use a filter so you’re observing in 1 colour, this will also significantly improve the sharpness of the moon, visually.

2. Use an apochromatic refractor. Costs $$$ but the colour correction is vastly better.

3. Use a reflecting telescope - i see you have a lightbridge ...

[jimmyh1555]

Thanks for that, Wavytone! I thought that it must be something from the refractor. Next time, I shall use my Lightbridge - as you suggested - was a bit lazy last night as I have to get the sack trolley out with ropes, to move the Lightbridge:):)
Incidentially, when I used a 17mm EP I did see the blue fringing around the Moon's edge, which disappeared when I put on the lens cap and just removed the smaller central cap!