I've got a little Sky-Watcher Mercury 705 and recently I took a critical eye to the star test and was pretty dismayed. The diffraction ring is more like a diffraction triangle (even with all the screws on the lens cell loosened to the point the objective could rattle around). Additionally there was kind of a smear in the diffraction ring on one of the triangle sides, and it followed the objective lens if I rotated it.
Upon further inspection, it looks like the little black separator pads between the lenses have migrated inwards and are now affecting the views. I tried to move them but they are stuck on there really well. Addtioinally the lens has a lot of little scratches on it - I suspect at one point I put it away and the lens cover fell off and some other gear was rubbing against it.
I decided to buy a relatively inexpensive 70mm doublet from China ($28 USD / $40 CAD) from a store on AliExpress called Sophisticated Optical Tools Store. They had a variety of achromatic doublets available and I bought the D72F500 lens set. They came well packaged and the lenses are indexed so you can line them up properly. The spacer is a thin clear plastic ring instead of the older style black pads. Actually it took two attempts for them to ship me the lens - the first time I got a bluetooth camera remote shutter release! But the store was very helpful and they couriered me the lens which I got today.
I popped it into my Sky-Watcher over lunch and had a quick look at a squirell nest across the street - I was actually pretty surprised with how sharp the view was. I compared back to back with my original lens, and I'd say the new lens is sharper and easier to find focus. The new lens appears to be 1 or 2 mm smaller in diameter than the original, and the height is the same - but the new lens set has a wider crown lens and thinner flint than the old set.
I'll see if I can find time to break out my artificial star tonight and do some A/B shots on what the Airy disc looks like in both. I'm hoping the new lens at least has round stars! As for coatings, I noticed the old lens has purple and red reflections of my window, and the new lens has the blue reflections, but instead of red, the reflection was white. That tells me some of the surfaces likely aren't coated - and it's just a single coating, where as the original appears to be fully coated on all surfaces (I saw no white reflections on the old lens). Please, someone correct me if that assumption is incorrect.
Here is the new lens in the Mercury - did this during my lunch break...
For the price, however, even without the same coatings, I will be very happy if I can have proper Airy discs in my little Mercury - that would be fab!
Clear skies!
Rick