[JA]

Originally Posted by TareqPhoto

How should or could i answer you??!!!

Well, i have many DSLRS, some of them are full frame cameras, NONE of them came close to my cooled cameras, even my non cooled planetary cameras are doing better than any of my DSLRs or one mirrorless camera for DSO, so i will never waste time with a DSLR or mirrorless, and i will never send one for modify it for astro and i have no brave at all to do it by myself, so DSLR and mirrorless are out of questions.

Which Fullframe DSLRs and Fullframe mirrorless cameras do you have and have you tried in a back to back comparison against which Astrocams?

There are many incredible images taken with DSLRs, just take a look on Astrobin. Also the fact that you have a budget which precludes a Fullframe Astrocam or APS-c Astrocam + Reducer led me to suggest a modern Fullframe DSLR or Fullframe mirrorless to use with your 90mm f/6 since you wanted a large FOV. The same field of view your 90mm f/6 is capable of in fullframe equivalent terms.

Originally Posted by TareqPhoto

Now about the image circle of the scope, this is nothing to worry about or care about, because i can always use that for another targets, and one day i might buy another APS-C in mono, but for now i have smaller sensors cameras, people are using them with whatever scopes, so it is no point from you to tell me to forget about them, and not all astrophotographers using full frame astro cameras or even APS-C, so i have those small sensor cameras then i must use them, and for FOV i want using astronomy.tools site it showed me what focal length i should buy to use those cameras for certain FOV i want, so cameras are staying, i have to buy scopes to give me those FOV,

It's your money/choice, but whilst astronomy.tools may tell you the FOV with different focal lengths, does it also tell you how much light your telescope has collected and is now lost (not used) by the smaller sensor.

Originally Posted by TareqPhoto

... ASKAR FRA400 isn't only designed for full frame, also not all people who have this FRA400 using full frame or APS-C, so why they bought that, in fact even people who bought Takahashi FSQ-106 also some of them using small sensor cameras where the scope is even larger than full frame image circle.

The Askar 400 has a full-frame image circle and if you image with anything other than a sensor capable of using all the light collected by the telescope, be it a 1) Fullframe sensor or 2) APSc sensor with reducer, then you are wasting the light collected by the scope - LOTS of it, in your case:

, if you use a 4/3" sensor you will loose approx 2 stops of Light i.e: 75% light collected is lost or not used. It defeats the purpose of using multiscopes on the same image to save imaging time or go deeper in the same time.

Originally Posted by TareqPhoto

On the other issue of people buying smaller sensors for the TAK106 or whatever, that's a choice they make and they are presumably happy to give up lots light collection potential for a tight FOV. You want a large FOV hence my suggestion not to buy the Askar400, but rather work with your 90mm f/6 if it's a good instrument.

Long story short is making this more complicated, my 90mm triplet is nice quality and designed for full frame also, if i buy a reducer that will make it down for APS-C, and i think not FRA400 but another scope with a reducer will cover up to APS-C, so using smaller sensor won't be a big deal, are you telling me now to have all my scopes designed for full frame and i should have all my cameras as full frame or APS-C???

What I am saying is that unless you use the full image circle of a scope you are loosing valuable light collected. In your case up to ~75% of the light your proposed Askar 400 collects will be lost/un-used.

Originally Posted by TareqPhoto

I have to use multiple scopes, but i hate hate hate to use only one scope for whole imaging data per target, i remember i used my Canon 135mm lens at F2 wide open for Orion collecting HaLRGB, i spent 3 nights for that and the data was bad due to CA, so even F2 didn't help it to make it fast, i will never spend my life doing each target for 3 nights or more, and i will never spend more only to get RASA when a lens at F2 didn't help it as well, so i try to pick up several/some targets and try to collect data from multiple scopes, but i think using multiple scopes is like a bad idea from so many people, in this case no one will understand or help in this case, i saw few used multiple scopes for amazing successful results, each has own FOV or setup, i want to do mine for that wide field for now.

Using the 135 f/2 lens wide open may not have been the best path forward for astrophotography as it is primarily designed as a general use and portrait lens and has some softness at f/2, small CA which may appear worse in the corners with altered star shapes. If you like that focal length, stop down 1 or 2 stops or perhaps see if you can try a Samyang 135 f/2 or Sigma 135 f/1.8 for comparison.

One further comment:
The budget you mentioned of $1500 to 1800, you don't say if that's $US or $AUD or other, but I would have thought that could have got you an APS-c astrocam possibly new (depending on the make/model) or certainly used for that budget. Then when you have more funds, or if you get a good deal on the APSc camera, you can purchase an appropriate focal reducer.... and then you are good to go on your 90mm f/6.

Best
JA