FOCUS

Is it maybe possible that the focus slides back during your session? is the fist image the same or better than the last one? 
I had the same, when the weight of my camera was to heavy for the focuser.

I agree with @Jens . The same happened to me more than once. But how are you focusing? Do you focus manually or use software or any other method? It looks to me as if you have the same problems as me when I started AP. I bought me a Bahtinov mask which helped a lot.

The optical things seems to be ok so far. The back focus distance is ok. You will see the difference only in the corners of the image. Yours look fine. Collimation seems also to be ok. The image itself seems to be out of focus everywhere, which assumes you might just have missed the focal point by a bit. The star spikes are quite faint, which proofs my opinion. They should be much brighter on the bigger stars.

If the guiding is problematic, the stars are usually elongated in some way. Yours are round. So I think, there is no major issue with your setup at all. Just tighten the right screw on the focuser so the camera can not slip and make sure, you absolutely nailed the right spot of sharpness.

CS
Christian

This is where the Bahtinov mask comes in. I built my first one out of cardboard myself. This worked as a first try. Focusing just by eye is usually not good enough. If you are trained, then it might work. So the mask will help you a lot.

You have to refocus after a while. Depending on where your Newton points to, the camera is always pulling on the focuser. I used a filter wheel and a heavy coma corrector as well, which is even worse. Depending on the temperature, your tube might change its length during the night. This changes the focus point as well. So it is good practice to refocus after a while.

But you are on the right way… it's just the learning curve

I forgot to mention I use batiknov mask

i will show just 2 different nebulas (Sadr area and bubble).its just 1 frame.its in focus…but its not at focus at the same time.there is a blur

On the Sadr image, you have a tracking issue. The small stars are elongated. This is bad tracking or your balance of the scope might be a bit off and it swings.

The second image looks better to me. But it's hard to tell if there is a bit of movement in the stars or if there are some abberations. How long is the exposure time? A small amount of loss in sharpness might be due to the seeing.

So then I am still thinking, you just need a bit more practice to focus. I always have some frames with tracking issues. Especially with a heavier scope like a Newton. Maybe you just picked one of those frames as example. Just give it another try. Now your senses are sharpened

I do think I suggested to check your focus and whatever other issues there might be by shooting the area around polaris, ideally the NP by switching of tracking and exposing for no more than a few (5) seconds and then maybe stacking a few frames although one might suffice.

YES,BUT I RELAXED THE SCREWS JUST A BIT

Did you modifiy the 150pds at all? If not, I don't think that should be an issue. If you put a bhatinov mask on, if you can get it in focus, you should be fine on that front. If not you can't get it into focus, then this could maybe be a solution.

Jens:
Did you modifiy the 150pds at all? If not, I don't think that should be an issue.

Kind of...

There may be an issue, but it is not related to the problems in your images. If the Primary is too far away from the secondary, the focuser needs to be closer to the tube (which seems to be the case). So the tube of the focuser or the coma corrector might get in the light path and cause additional diffractions. This is a common problem on cheaper Newts and a thing you want to avoid. So look at the tube and if the focuser gets into the light path, you might reposition your primary. That's not a big deal. Just screw in the adjustment screws half the distance you want to shift the focuser if possible (light passes the tube twice - in and out again - so half the distance).

However, @Jens  is right. This may not cause the problems that are discussed initially in this thread.

About that,yesterday I tried the telescope again (I have slept 3 hours…again.) and I screenshot a lot of things for you 
as you can see,I can see the tube on my donut

ooooh, just saw your image

that's not good; your focus tube is protruding

I still don't think that can cause sharp flaring like what you showed before; I think the flaring would be more diffuse

I still think it's a pinched mirror from tight clip(s) causing the sharp flare