I was creating a comparison star chart to measure the brightness of variable stars in the M16 nebula. I found that one of the comparison stars, which is indicated on an AAVSO chart as 163, or magnitude 16.3, is missing from my image. I’m attaching the AAVSO chart and two images: one taken from the Kitt Peak Observatory in 1973, and the other taken last night from Madrid with a Seestar 50 and an IR cut filter (the star is to the right of the star indicated by the letter B). I’m also attaching an image where I highlight the star in an image from Hoag’s 1961 work. I’m leaving a GIF so you can see other stars that appear in the image and others that disappear, including some with large proper motions. The fact is that this star will be removed from the AAVSO comparison star list. Its ID is: 000-BLT-651
The problem I have with these comparisons is that if they are not made with standard filters it is difficult to confirm variability.
The fact that there are many stars that disappear is suspicious.
The star in question is UCAC4 381-104840 and it seems to have rather consistent results derived from the different databases. V= 16.44 from Gaia DR3, V= 16.44 from Pan-STARRS1, V= 16.44 from VPHAS+DR3, V= 16.42 from ZTF, V= 16.44 in 2007A&A…462..245G. All these results are much better than the APASS 16.33 Vmag. because it is close to its limiting magnitude.
The image was obtained with a Seestar 50 with an IR cut filter, that is, what is equivalent to an RGB image from any DSLR, the image shows stars much weaker than the star mentioned.
Also SkyMapper DR4 shows mean errors 0.009 mag in Sloan g and 0.012 in Sloan r from 9 and 6 observations, resp. Plenty of variables in this field, but this is probably not one of them. As Sebastian notes, this is most likely a passband-mismatch issue.
An inverse of this problem is the fact some of the standard fields have variables in them that are not marked on the standard charts!
I discovered a very red star near the center of the SA-110 field. Very bright in Ic. Never noticed it before but early in July 2025 I “found” it!
The standard, standard field charts don’t mark all the stars you might get signal from and don’t mark variables.
What I did to ID the “new” star was to run a standard VSP chart for the SA-110 center coordinates and checked show VSX variables. The “new” star is NSVS 1842309+000703 and is an SR type. Magnitude and range given in VSX are
15.033 (0.355) r. VSX gives the spectral type as M7. In the remarks the period is given as 161.1772066-days! Quite bright in Ic as mentioned and not marked on the standard, standard chart!
This is what I would call IRAS 18399+0004 or HaroChavira 26. The GAIA3 spectrum confirms the type M7 from Chavira’s 1967 paper, where the star was first identified (the types in that paper are usually reliable), and it is certainly a giant (M7III). It is strongly reddened in SA 110 region, located in the Aquila Rift zone of the Milky Way.
There are many reasonably bright stars not measured by Landolt in the Selected Areas he calibrated. He was using a single-channel photometer, getting one star at a time, so couldn’t cover everything. Once software is arranged, I intend to produce lists of high-weight standards in fields such as this one; I have scores of nights of data. I think many observers have the data to do the same given the right reduction scheme.
Last night, the collaborator who took the screenshots made a sequence of m16 but did not take the precaution of placing the short IR filter, and instead placed an anti-pollution filter, but the star appears in this sequence.
