Reflected and scattered light in spectra (Edit)

[Thilo Bauer]

Dear spectroscopists,

Probably, it is a bit too late to publish this finding, one never knows.

A close investigation of my first light experiments with the ASI camera taken in the nights of 28./29. September 2017 yielded a mysterious broad emission on top of a faint continuum. in my spectra in a region of the nebula NGC 6888, where nothing should be in the line of sight of my Alpy spectrograph. The spectral feature shown here is the result of a 20 min exposure taken on 29. August 2017, 23:13:22 h UTC. This very faint spectral feature is also confirmed to be found on several 5 minute exposures taken in the night before, on 28. August 2017, 00:39:29 h UTC.

If anyone has a chance to observe with either imaging or spectroscopy, I'm very curious if one might be able to confirm my finding.

The image portion is marked in the DSS image of Aladin tool. A broad band color image taken with a modified Canon EOS 60D is provided for comparison. My own color image has been recorded on 21 August 2010, 23:15 h UTC.

I'm not sure what really has been recorded in my images. Maybe, some of you may contribute new results to this discussion or give an explanation of what it could be. Perhaps it is just an optical reflection.

Please don't hesitate to contact me for further discussion.

Thank you in advance!

Thilo Bauer

[Christian Buil]

I suspect presence of a ghost image of the main star spectrum in the top of the image (but not 100% sure). Try to take a second spectrum by the shifting the field along the spectral axis for blocking the star image (outside of the Alpy field).

Christian

[Thilo Bauer]

Hi Christian,

reflection is, what I would suspect, too.

I took several spectra of different regions of NGC 6888 and also Vega in these night, where ghosts are not visible. I found the ghost on different images of two nights, but only when taking spectra of the WR star. The slit was positioned at nearly the same, but not exactly the same location. Anyway, the ghost remained exactly with same distance to the WR star.

From a quick look onto the full frame taken with the ASI one can see, the light source is definitively visible within the slit (see zero order image of the grating). Probably this is the star close to the brighter reagion of the nebula itself. If it would be a reflex from the relatively bright 8 magnitude WR star, one would expect a WR like spectrum. However, this is not the case.

Below image shows a highly contrasted image of Vega taken in the same night, with no ghost visible. Exposure times are much different, however. It remains a mystery what will be recorded in these cases. It doesn't seem to have that much light towards the zero order image. Probably the "elongated" spectra outside the visible range in the blue end could be caused by variabilities of the grating itself (roughness), or scatter of light at larger scale period compared to the grating period.

Then I tried to rotate the image and paint a visible line into it. The last result of this operation shows, there is no obvious light source found in the slit where the bright emission and continuum is found in the spectrum.The brighter dots in the slit are caused by the stars accompanying the WR star in the field. Therefore, I would guess it should be a ghost of one of the emission lines of the WR star.

I will try to record overexposed spectra of other stars to see if and how reflection may occur having a star at exactly the same position.

I think you are right, Christian. This is a duplicate (mirror) of the WR stars spectrum due to the high dynamic range recorded in this case.

Neither was my Canon susceptible enough for these faint light sources, nor did it record such a wide field (due to the Barlow lens). Therefore, I'm wondering how to interpret these spectra now. No trivial task to calibrate spectra like this.

Thilo