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One starting point is to take a look at the websites of APM and Teleskop Service in Germany - both have a huge range of APO's, doublets, triplets, quadruplets and more. Some are photographic only, to the extent they have a flat field and the OTA is set up for cameras and don't have the backfocus to accommodate eyepieces.
If you like what you see there the next question is whether you can find something better eg from Stellarvue, AP, TEC, ES... etc.
Need to decide what you want to photograph to understand the field of view required, with this and camera sensor size and pixel pitch you can calculate the focal length needed. Aperture and how much glass are determined by the depth of your pocket.
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