Newt Primary's centering doughnut sticker showing up on images

[johneta]

Hi

Has any one had this before?

I imaged the Horsehead Nebula properly for the first time.

I only got 13 x 360 sec ISO1600 images on a DSLR. (Its Summer here which makes for very late nights when it gets dark late)

In the final stack- when I stretch it, I got a darker dim circle just to the right of the Horse head.

I ended up doing a HUGE amount of stretching on one image to see if it was on each frame. It was, and you can clearly see a doughnut shape.

The circle seems too big to be a dust spot on the sensor( plus I am using flats which should sort that in the stack).

My guess was, the intense light from the brightest star in the image is bouncing around the tube of the Newt and reflecting an image of the sticker onto the image.

Is that a thing?

Thanks for any help  John

[vlaiv]

No, light can't reach it as it is behind secondary mirror, so you can't see center mark sticker. Even if you could - it would be so much out of focus that it would not register.

What you are seeing here is dust shadow - a normal thing and is corrected with flats. Given the size of it and the fact you are using newtonian scope and hence very likely coma corrector - it is probably on the surface of coma corrector.

Take flats to correct it (in fact you should be taking flats anyways).

Here is screen shot of one of my flats from a RC scope:

It shows a lot of "center marks"

 

[kirkster501]

The mirror spot is too far away and too far out of focus to be visible.  What you are seeing is not the centre spot but a dust donut of something nearer the sensor, exactly like Vlaiv says.

[vlaiv]

Actually, now that I've inspected image further - it's not shadow - it is reflection.

At first I noticed just dark spot - but it is bright doughnut around this dark spot that is the issue - it is reflection of unfocused light:

Maybe this screen shot will help to see it easier:

That sort of thing happens when there is a bright star in the frame and it can't be corrected with flats - it is reflection - same as this here:

There is a faint halo around that star.

In your image it looks like culprit is Alnitak - it is same distance in opposite direction with respect to optical axis.

[johneta]

Thanks for the replies.

Vlaiv -Your second reply makes sense to me.

I did use flats in the stack I did of this image and it didn't fix it.

I was thinking about reflections as you suggested. I thought it might be a common issue with the Horsehead, as it has Alnitak in the frame.

Any work arounds people do?  Flocking the inside of tube help or other ideas?

Thanks John