Huge, weird spike in image

[BrendanC]

Hi all,

I need some help figuring this one out!

Imaging the Owl Nebula and Surfboard Galaxy last night, good conditions, no moon, seeing could have been better but everything was fine.

Then the subs started to come through and I noticed a weird spike cutting across them.

I went outside and looked around - no unusual lights anywhere. The only difference was that I'd cleaned the sensor beforehand.

I was also imaging the Sombrero Galaxy, so I waited for those subs to come through - no spikes.

The plan then went back to the Owl Nebula (I planned it so I caught the Sombrero as it appeared behind some trees). This time, it was after the meridian flip, so I figured that if it was the camera, the spike would still be in the same place but the image rotated 180 degrees. Blow me down wiv a fevver - the spike had rotated too! This implies to me it was a genuine artefact in the sky. 

I went out again, this time put the dew shield on and covered the base of the OTA with a hat, just in case any stray light was causing problems. Made no difference. It's a huge spike or trail or line, reminiscent of a satellite trail but really not. I notice that it's aligned with the diffraction spikes, but that's really all I can add. I took other subs, around the area, and some of stars, to see if I could replicate it, but it only seemed to happen in this fairly specific area of the sky.

Is it the 'zodiacal light' I've been reading about recently? Is it a VERY persistent contrail in the sky? Is it aliens?

Here's the stacked image, stretched in StarTools, just to show you it in extreme contrast:

It moves with the dithering, so it's not the camera or sensor.

So, I'm stumped. I'm going to try and lose the streak in pre-processing, but until anyone can give me a reason for this, I'm going to call it BrendanC's Streak.

Equipment was a Sky-Watcher 130PDS on an NEQ6 mount, astro-modded EOS1000D, guided 180s exposures, dithered by my usual value of 12 in APT.

Any ideas?

Thanks, Brendan

[tooth_dr]

It’s a reflection inside the optical train. 

[vlaiv]

It really looks like star spike - and for the moment I thought it is bright star just outside the frame.

Owl is in Big dipper and for a moment I thought it was Merak or some other bright star - but no, you are right - nothing major in that position in the sky.

Next idea was - aircraft flyby at low altitude and blinking lights - there are two spikes by the way - larger one and smaller one - so I thought main light / tail light or those lights on wing tips - but no, you have it in multiple subs.

However - it is much wider than other star spikes in the image - that can mean two things:

1. object is not stellar / pinpoint - it has dimensions

2. It did not move together with stars - it did stay in roughly the same place - but not exactly the same place.

Have you tried to see frame to frame difference in this phenomenon? We might get a clue if it moves or changes from frame to frame.

[BrendanC]

@tooth_dr Good idea, and it's what I was thinking too, but why wouldn't it appear with the other images I took?

@vlaiv I've looked to see if it moves and it doesn't seem to, but I can look VERY closely, almost pixel-peeking, and I'll let you know. BUT, I would still have expected it to have shifted by 180 degrees post meridian flip?

Edited April 6, 2021 by BrendanC

[vlaiv]

3 minutes ago, BrendanC said:

I've looked to see if it moves and it doesn't seem to, but I can look VERY closely, almost pixel-peeking, and I'll let you know.

I guess maybe easiest way to tell is to blink images that are further in time (not two adjacent subs - but say first and fifth that show the feature).

[tooth_dr]

8 minutes ago, BrendanC said:

Good idea, and it's what I was thinking too, but why wouldn't it appear with the other images I took?

Probably caused by something bright outside of the FOV such as a star 

[BrendanC]

So, I've been through them and as far as I can tell the spikes (yes, you're right, there are two) track the stars exactly. I can even see where they move around a hot pixel, but stay aligned with the stars.

I'm wondering whether Merak could have been the culprit? I think I tested the image by moving away from that side, past the Owl Nebula, which would have been in the wrong direction. I guess I should have done a series of subs around the image to do a better test.

Tonight is another clear night (amazing), so I'm going to have another go. I've actually rotated the camera 180 degrees this time, so I can see any effects from that, while retaining the framing I want. If the spikes appear again, I might try and get some subs from nearer the surfboard side and see what happens.

In the meantime I'm going to process the image by stretching the sky a lot less, and superimpose the objects. It's a bit of a cheat I guess, but DSS won't reject the spikes because they're an intrinsic part of the image.

Good to know it's not a result of the sensor cleaning (which I didn't think it was, but these things always seem to happen whenever you change anything!)

Thanks for the suggestions, really appreciate the help. Let's see what happens tonight...

 

[BrendanC]

So I was able to mitigate the spike(s) fortunately. Still, I need to sort this!

[BrendanC]

Forgot to mention, I also have a coma corrector in the optical train. I happen to know that it can cause internal reflections sometimes, so if I get the spikes again, I might try it without.

I sort of don't mind if it's very unique to this object/area of sky, but if it can be fixed, I'd like to fix it.