All, I have had this ST8XME camera since 2006. Over the last year I have noticed this curved bright smudge like artifact that you can see in the lower left corner in this image. The weird thing is I just finished an entire project before starting this one and I did not see it at all. Now I am seeing it again. I have cleaned the chip glass. It is not filter dependent. I thought perhaps it was frost since it has recently gotten much colder so I have been cooling the chip down more, but after warming it up and changing the dessicant it is still there. It does not Flat out. So I don't know. Any suggestions? Thanks Dave
Perhaps a reflection from that extremely bright star? Try a couple of fields with and without bright stars and see if the "smear" is affected by the presence/absence and position of the star.
Looks like it might very well be some type of reflection as I took several images of star fields with no bright stars anywhere nearby, same exposure time (5 min) and no evidence of artifact! Interesting, as I do not recall problems with reflections before. I did recently change from using newtonian optics to RC. Wonder if that has anything to do with it? That's the most significant change in my system in the last year. Any thoughts on the cause of the reflection and what might be a solution, other than changing the target? Thanks Dave
I'm going with frost, the arched pattern in the lower left for sure. Likely the same for the streaks in the neck of the Horse Head. My guess is those are from the cleaning effort and become visible as the ccd ices up.
Why would it be only visible in certain regions of the sky and refractory to dessicant change and camera warming?
Frost usually occurs unevenly, and it can sometimes cause any slight contamination on the window to migrate and accumulate in a certain area, resulting in a streak after the frost disappears. As for the variation with position on sky, I would assume the contamination is so thin that you don't really see it unless there's a bright star in the field. Try the star / no star test I suggested earlier.
I have seen the same kind of artifact on images of the Horsehead and like you, they aren't eliminated with flats. I haven't seen these artifacts anywhere else in the sky. I also use a ST8XME. The orientation of my images were 90° from your image, but my image scale was in the same ballpark. The bright star in the left center of your image was out of the field of view. The curvature of the artifacts I see correlate with the bright star. The image of the bright star was formed between the Main CCD FOV and the guide CCD FOV. I took images at 3 and 5 minutes and the artifact was more noticeable in the longer exposure. I think the artifact is a reflection of the bright star from the edge of the guide CCD mirror. I blackened the edge of the mirror. But, I have not had the opportunity to go back to the Horsehead to test if the change I made effected the result. You can test the theory that the artifact is a result of a reflection internal to the ST8XME by rotating the camera, say 180°. Or, simply shift the image slightly to see the effect on the artifact. Rod Cook
Reflections from Alnitak (mag 1.74) are very common when shooting the HorseHead. If Alnitak is in the FOV, then the reflections are usually off the surfaces of the optics/filter surfaces and are a bright ring around the star. However if you use black anodized components like adapters or spacers and the star is to the side or near the FOV, then reflections can be large arcs. Black anodized surfaces reflect a lot of light, even though they "look black", especially infrared. The solution to these are to coat the black with black flocking paper. Don
Thanks guys. Yes I think I would lean toward some type of reflection artifact. Attached are 3 different test images. One is a "no star" image, one is an image with a bright star on the edge of the field and one is obviously a field with brighter stars in it. No artifact on any of these.
The thing about reflections is that they can be critically sensitive to exactly where the star is during the image. A star slightly outside the field can cause big reflections. Since this is a relatively rare phenomenon for you, I recommend you just keep your eye out for it. If you see it again, nudge the telescope to a slightly different position and shoot again. I'll bet the reflection changes or even goes away.