Puzzling "artifact?" in my test image of SH 308

[BruceG]

HI all, my first post over on this side as I wanted to ask about something strange in my image from last night. A couple of weeks ago I shot 4 hrs of OIII data on SH2-308 and had billions of stars but no nebula. I figured I may have loaded my new filter wheel with OIII and SII switched so last night I went out for a few test subs using the "supposed" SII filter.
As a test image to diagnose the fact that I had the 2 filters switched with each other in the software, it is 16 frames of 5 minutes (ASI1600mmm Pro) at unity gain with 5 darks. OK I have a nebula so problem solved.

Anyway, after processing the image and looking over it, I see what appears to be a little bubble like object just to the lower left of the nebula. I have checked various other images of this object including some done as late as January and February this year so I pass it over to you guys to possibly explain it. I know the larger dimmer spot could be a dust mote as I didn't shoot any flats, but what do you think the smaller, brighter blob is that makes the whole thing look like a bubble?
Higher res here (https://www.astrobin.com/397461/?nc=user)

[Placidus]

First up, welcome, and it is a very good image.

We confirm that there is nothing at that spot in for example the APOD (https://apod.nasa.gov/apod/image/1506/sh308_simon_2088.jpg) image.

We agree that it doesn't look anything like a dust bunny as it is light rather than dark. If you had done flats, using a different filter, which did have a dust spot, and now took the shot with a cleaner filter, you can end up with a lighter ring such as you have, but it doesn't look like that.

Don't think it's a true transient astrophysical object such as comet or a Klingon, but you never know. We've seen other people accidentally photograph a distant comet.

Our first guess was that it is a "residual" image of a bright star from a focusing run, but arguing against that you've got a CMOS camera and we think it's only CCD's that show residual or ghosting images. A characteristic of residual images is that they'd be fainter in each progressive sub.

Another guess is that it is a reflection of a bright star off the back of a filter or the chip cover.

One way of getting around reflection images is to move the scope by an amount large compared with the diameter of the artifact between subs. The idea is that the reflection moves in the opposite direction to the field. You then use statistics to find what's common across all the subs. The easiest is a median combine. More sophisticated and powerful is a Winsorized combine, which is what we use.

Have another go, next new moon, but frame it differently, and see if the blob is gone, or if it moves relative to the image.

Once again, a really good image, apart from the Space Invader that you've captured. Astonishing what folk such as you are achieving with these cooled CMOS cameras.


Best,
Mike and Trish

[BruceG]

Thanks for the detailed reply Mike n Trish,
AS stated in my first paragraph, 2 weeks ago I shot 4 hrs on the same object mistakenly using the SII filter and got just the star field. The same artifact is there on the SII but of course no nebula. So I guess it isn't dust on the filter but perhaps elsewhere in the optical chain or a reflection somewhere (probably the cover glass on the sensor) as others have complained about. Those that have complained are talking about square shaped flares around very bright stars, not necessarily a reflection projecting elsewhere on the sensor.
I guess I need to learn how to take decent flats and go from there. Thanks again for your input and advice.
Bruce.

[multiweb]

That's a really nice shot still. :thumbsup:

[Atmos]

It’s a symmetrical reflection caused by the bright star at 2:30 within the bubble.
I have seen this on a few occasions with random bright stars when using my D810 and my refractor.