How does one get a matching field flattener? Is that something I can add on to my ES 127 APO?
Edit: My impression is field flatteners are for AP, but not fir visual...
If your Explore Scientific is of the FCD1/FPL-51 variety you do not, IMHO, have a true (photographic) apochromat. They just aren't quite that good and their focuser isn't desirable for imaging. I'd call it a visual apochromat because the 102mm I had was very good for visual work.
If you are looking for a visual instrument and you have any concerns about cost, then I'd advise you not to get a triplet at all.
For visual purposes there is (IMHO) insufficient benefit to going with a triplet. A good ED-Doublet will be close enough that many (maybe almost all) of us won't notice a difference in optical quality and our cool-down will be faster, our price will usually be lower, and we can either use a cheaper mount or put a bigger-aperture OTA on the same mount because it doesn't weigh as much.
For that matter, by most standards a far cheaper Dobsonian with a nice coma corrector is going to spank the view you'll get from a really good 127mm triplet apochromat.
Upshot is that I'd not consider buying a good triplet refractor in the 5" or larger aperture range unless I were planning to do AP with it - or just had plenty of money to spend for a beautiful functional work of art (which a superb refractor is).
So far as a matched field flattener? I'd look to folk like Stellarvue, WO, ES, Sky-Watcher, Astronomics, etc. and I'd check to ensure that they have a flattener designed for that particular telescope. Not just a flattener which has their brand name on it and is supposed to work with scopes in that range of focal lengths or focal ratios. The focal reducer is designed and sold for use with that particular OTA. That way if you find that you don't have great correction the seller/manufacturer of the flattener can't say that it is the fault of the OTA and the seller/manufacturer of the OTA can't say you have the wrong flattener.
Some years ago I had a very good triplet apochromat which was beautifully corrected and not overly costly. I wanted to use it with focal reduction and asked the maker/seller which reducer/flattener I should use. They advised me to get a particular one which is made by a great optics maker/seller. The correction wasn't bad, but you could still see fixable field curvature when you approached the corners of the images. The OTA was great and the reducer/flattener was great - but together they were good but not great. I sold both the reducer/flattener and the OTA. I don't have enough time to spend it trying to fix optical problems in processing.
Now I have an NP101is. It's a Petzval which (when paired with an APS-C sized sensor) does not have a significant field curvature. If I had a full-frame sensor I could have field curvature problems but I can buy a matched flattener to fix that. I was also able to get a reducer designed for that OTA which will give me a nice flat field at an even faster focal ratio and still fully illuminate my sensor. This makes for very nice imaging.
I've also a WO Gran Turismo. Good triplet and they sell a field flattener which is marketed for that particular OTA and does a very nice job which I find within my acceptable/beloved range.
Net effect is that from my perspective I'd not get an apochromat for almost anything but imaging. I'd not get an apochromatic OTA which I can't get with a field flattener sold by the OTA maker and specifically matched for that particular OTA. And then I'd test it to see if the field curvature is beautifully corrected - and if not I'd send both units back as being of unacceptable quality.
A further note? I've a Mak-Newt (Comet Hunter). Bigger aperture and has great optics for both visual and for imaging. I don't really like using Newts or Newt variants on a GEM for visual use, but if you don't mind that you can get a bigger aperture scope at a relatively low cost which I'd contend will come fairly close to a triplet apochromat (but not quite equal it IMHO) in quality and will exceed the triplet in light-gathering and do it with very fast optics and a nice focal length (from my perspective).
But what works for me is not necessarily what works for anyone else.