Newbie question: ASI 294MC Pro + Tamron 150-600mm Di VC USD G2

Once I tried a similar set: An ASI294MM Pro +  ZWO filterwheel with LRGB filters + M48 to T adapter + Sigma 150-600mm Sport. There was no problem with back focus, but focussing was not easy. 
I guided with a ZWO 30mm Miniscope and an ASI120MM mini. 
The mount was an iOptron SkyGuider Pro on a good photo tripod. 

Unfortunately, the result was very disappointing. The stars in the corners looked like crosses, a sign for astigmatism. The lens is great for birding but apparently not good enough for stars. 

I hope you have more luck with your Tamron.

Olaf Fritsche:
Once I tried a similar set: An ASI294MM Pro +  ZWO filterwheel with LRGB filters + M48 to T adapter + Sigma 150-600mm Sport. There was no problem with back focus, but focussing was not easy. 
I guided with a ZWO 30mm Miniscope and an ASI120MM mini. 
The mount was an iOptron SkyGuider Pro on a good photo tripod. 

Unfortunately, the result was very disappointing. The stars in the corners looked like crosses, a sign for astigmatism. The lens is great for birding but apparently not good enough for stars. 

I hope you have more luck with your Tamron.

Why was it hard to focus? I currently capture DSO with my Tamron 150-600mm and Nikon D850 on the SkyGuider Pro and I don’t have any issues focusing it. Also my stars are fine corner to corner even with the full frame sensor.

I am considering the same move as the OP, upgrading to the same Astro cam as my next step while keeping my lens for now since it’s working good.

Thanks.

Hi Marcos,

as i mentioned above, i belong to the Canon-Family. Don´'t have any Nikon.
The  Sigma 150-600 contemporay is in use only for all everyday tasks, not for astrophotos.

CS

Günther

Günther Dick:
At least for the Canon EOS Bajonet there is an adapter available to M42 with internal M48 thread for filters.
It connects my ASI 2600 MC (Backfocus 17.5mm) to every EOS-compatible Lens and should mathch Your ASI 294 (Backfocus 6.5mm).

Olá Günther!
Thanks for the info, helped me a lot.
I also use a Canon so this adapter is exactly what I was looking for - I have already ordered it and it's on its way, thanks!

I will be thrilled when I get a beginner result as your M31, it's certainly a good start.   

Thank you.

If it is for a Canon then you set the desired aperture in-camera then, without turning the camera off, remove the lens. If it is for Nikon you need to hack the aperture tab.

Yes, sometimes it works for Nikon lens too but only occasionally (i.e., 200-500mm f/5.6 AF-S).