spider image in Samuel Oschin Schmidt Telescope DSS imagery

Question

I was puzzled by image artifacts that appear in DSS-2 (blue) survey.

These images were taken by the Samuel Oschin Schmidt Telescope on photographic plates and later digitized. They are also available via the Stellarium DSS layer and can be found in the Stellarium web version too (e.g. search for TYC 1327-245-1 and zoom in).

In some images, one can even find a string of such artifacts, obviously resulting from the scanning process of the sky.

They look like images of the spider construction.

In example 1,4, a streak contains one such artifact; maybe a processing artifact?

In example 2, one will find a string of such artifacts (bottom right, outside the image; the image is limited to 60'x60', but there are several artifacts that run along a line, each about a degree apart, this is obviously related to the original sky scanning procedure). (Search for HD 44018 and you will find at least three of those on a line).

What is the optical effect here? Is this a form of stray light? Some form of internal reflection? It's a bit surprising seen the fact that these images were taken by professional users. The artifacts have a specific shape that probably reflects the original plate construction in the focus plane. Does anyone have a picture of that?

Some examples:

1. https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=6h19m24s&d=21h4m29s&e=J2000&h=60&w=60&f=gif&c=none&fov=NONE&v3=
2. https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=6h17m29s&d=20h48m26s&e=J2000&h=60&w=60&f=gif&c=none&fov=NONE&v3=
3. https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=6h18m10s&d=21h5m10s&e=J2000&h=8&w=8&f=gif&c=none&fov=NONE&v3=
4. (RED) https://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_red&r=13h18m39s&d=29h11m6s&e=J2000&h=60&w=60&f=gif&c=none&fov=NONE&v3=

for those interested, a few links to the web version of Stellarium where these artefacts appear (there is a small bug that prevents sharing URLs of stars that contain spaces)

• https://stellarium-web.org/skysource/V537Lyr?fov=0.82770&date=2021-05-05T20:55:44Z&lat=51.04&lng=3.71&elev=0
• https://stellarium-web.org/skysource/HD43509?fov=0.95220&date=2021-05-06T20:59:08Z&lat=51.04&lng=3.71&elev=0

Answer 1

Those are almost certainly "ghosts" caused by the internal reflection of light from very bright stars. (That is, the light is reflecting off the insides of the camera, filter, etc.) The pattern is an image of the "entrance pupil" of the telescope, which in practice means the primary mirror + the blockage caused by the secondary mirror and its support structure (hence the four thin dark "arms").

There's a possibly useful technical discussion in this paper, which among other things shows that the off-center nature of the pattern is due to the bright stars being off-center within the field. (If they're in the center, then the ghost has a symmetric, centered pattern.)

See this question and its answers for the general idea when your entire image is out of focus (i.e., all the stars will look like that). In the case of these DSS images, the ghost reflection images are out of focus because the multiple internal reflections change the total path length of the reflected light, while the rest of the image is focus for light following the expected internal path.

The nature of a wide-field or all-sky survey -- like the DSS -- it that you generally try to image all the accessible sky. This will inevitably include some regions with very bright stars in them, even if you might avoid them for more focused observations (e.g., if you're studying type X objects, you might leave out those which happen to lie very close to a bright star).