I am considering this scope:
https://www.teleskop-express.de/shop...FPL53-OTA.html
For dual purpose visual and for astrophotography with a KAF16200 sensor and perhaps a Proline 16803 if it can cover it.
I am not sure about a doublet being colour free. Its FPL53 and the 2nd element is Lanthanum. I have used a Tak FS152 before which was fluorite and crown glass element doublet. It was great visually but a bit false colour in images.
What do you think?
The other choice is either a high end 107mm F6 triplet or a medium end same size.
4 inch scopes are a bit lacking in aperture for anything other than widefield star scenes in my experience though.
There seems to be a lot of air spaced FPL53 triplet scopes on the market these days. They seem to have good colour correction and appear to be from Kunming in China up to 150mm in aperture.
Greg.