I have been pondering standard stars a lot lately. The common 'lore' about such is that they must be done with a single, fixed instrument set-up. But it is also the case, at least for UBV, that the transformation process must accommodate filter/detector response differences gracefully, otherwise the UBV system would never have survived. Out of curiosity, I copy below an inventory of the various instrumental set-ups that Arlo Landolt used during his career. The details were published in the papers linked, all of which are well worth reading in detail. As someone who used US national facilities exclusively, he was at the mercy of whatever hardware the mountain staff provided for his runs at the various telescopes. Thus he used numerous single-channel photomultiplier tubes and several sets of filters. I omit for now the several papers with various spectrophotometric standards. These were all done during the main series, and do include many useful results. It is also worth mentioning that the data in the 1983 paper are a subset of what is in the 1992 paper, minus a few brighter stars appearing only in the first paper.
Landolt always reduced data using simple linear transformations, but always made minor adjustments to force results to be closer to the standard stars he worked from (Johnson's or his own). He also did the determination of extinction each night separately from the standard-star transformation. He once worried aloud to me about the possibility of some drift in the mean system over the decades. Nowadays one avoids these issues by doing quadratic fits (possibly cubic), and including terms for airmass and as a function of UT, and doing the solutions directly.
\Brian
Landolt 1967 Taurus UBV sequences
https://ui.adsabs.harvard.edu/abs/1967AJ.....72.1012L/abstract
--- KPNO 0.9-m and 2.2-m; 1963 Dec - 1964 Jan, 1964 Dec
--- "different 1P21 tubes", but same UBV filter set at both telescopes
--- 18" diameter photometer diaphragm on both telescopes
--- Johnson & Harris 1954 primary UBV standards
--- linear fits with "night corrections" for zero-point
--- Mount Agung eruptions 1963 Mar, May
the field 1 group is useful with CCDs, more so than field 2, if restricted
to stars brighter than V ~12.0 with several nights of data
Landolt 1973 UBV equatorial standards
https://ui.adsabs.harvard.edu/abs/1973AJ.....78..959L/abstract
--- KPNO both 0.4-m and both 0.9-m telescopes; 1969 Mar - 1972 Jan
--- "different" 1P21 tubes operated at 800v (i.e. lower than normal),
"virtually all with the same UBV filter set"
--- photometer diaphragm aperture(s) not specified
--- Johnson 1963 = Johnson & Harris 1954 primary UBV standards
--- linear fits with mean extinction and "night corrections" for zero-point
if not present in the 1983/1992/2009 papers, most stars brighter than V mag
12 to 13 and with at least n=7 observations on at least m=4 nights can be used
as standards; also select for errors <= 0.020 in V and B-V, and <= 0.03 in U-B
Landolt 1983 UBVRI standards
https://ui.adsabs.harvard.edu/abs/1983AJ.....88..439L/abstract
https://ui.adsabs.harvard.edu/abs/1983AJ.....88..853L/abstract
--- CTIO 0.4-m and 0.9-m telescopes; 1977 Sep - 1981 Oct
--- various RCA 31034A tubes; two filter sets, mainly the Graham set
--- 27" diameter photometer diaphragm at 0.4-m; 16" diameter at 0.9-m
--- Cousins 1973, 1976 E-region standards
--- includes results from a separate series at 0.4-m telescope on brighter
stars using 1P21 tube(s?) tied to the 1973 equatorial standards
--- linear fits with post-facto night corrections and multi-linear adjustments
to get E-region based UBV to match the 1973 equatorial zero points and
color-indices
--- R,I left on the natural system nominally tied to Cousins E-region R,I
--- the overlooked "equipment stability" paper stars observed at 0.4-m and
0.9-m telescopes using 1P21 or 931B tubes, reduced the same way
(E-region and 1973 equatorial standards mixed)
--- latter data are often good UBV standards, but only some have data on
enough nights to be R,I standards (though often pretty good even so)
Landolt 1992 UBVRI standards
https://ui.adsabs.harvard.edu/abs/1992AJ....104..340L/abstract
--- CTIO 1.5-m telescope; 1977 Sep - 1991 Feb
--- three GaAs tubes, RCA 31034A or Hamamatsu R943-02; the two filter sets
described in Landolt 1983
--- 14" diameter photometer diaphragm, a few nights with 10" aperture
--- 1983 Landolt standards
--- 1982 El Chichon eruption in Mexico affected extinction values
--- linear fits with post-facto night corrections and multi-linear adjustments
Landolt 2009 UBVRI standards updates and additions
https://ui.adsabs.harvard.edu/abs/2009AJ....137.4186L/abstract
--- CTIO 1.5-m telescope; 1993 Jun - 2001 Dec
--- four different GaAs tubes, RCA 31034A or Hamamatsu R943-02; same filter
set throughout = Graham recipe specified in 1983 paper
--- 14" diameter photometer diaphragm
--- 1992 Landolt standards
--- linear fits with post-facto night corrections and multi-linear adjustments
Landolt 2007 UBVRI standards near -50deg Dec
https://ui.adsabs.harvard.edu/abs/2007AJ....133.2502L/abstract
--- CTIO 1.5-m telescope; 1998 - 2001
--- single RCA 31034A tube and same filter set throughout = Graham recipe
specified in 1983 paper
--- 14" diameter photometer diaphragm
--- 1992 Landolt standards
--- linear fits with post-facto night corrections and multi-linear adjustments
Landolt 2013 UBVRI standards near +50deg Dec
https://ui.adsabs.harvard.edu/abs/2013AJ....146..131L/abstract
--- KPNO 1.3-m + GaAs RCA 31034 tube, filter set J; 1991 Oct 1995 Apr;
17".7 diameter photometer diaphragm
--- KPNO 0.9-m + same GaAs RCA 31034 tube, filter set J; 1991 Oct - 1993 Jul;
13".7 diameter photometer diaphragm
--- KPNO 0.9-m + CCD, 1992 Mar - 1994 Sep; 0".77/pixel, "Harris filter set",
"diaphragm set via software"
--- Lowell Perkins 1.8-m + Kron photometer, new Burle C31034A tube + KPNO J
filter set; 2005 Apr - 2006 Nov, and 2007 Aug - 2008 Nov; 15".7 diameter
photometer diaphragm
--- 1983 and 1992 standards adjusted to 2009 system by linear fits with
post-facto night corrections and multi-linear adjustments
Greetings,
Wow, excellent roll up of the systems he used. If he was able to get consistency across all those observing systems then he did a good.job! I suspect it improved the robustness in the standard star magnitudes produced.
I have even more telescope and detector combinations that I have used over the last years in retirement! I have used 18 different optics/sensor combinations, almost all portable observing--many, many setups/take downs! For my transformations I have always used linear fits, as any higher order fitting wasn't justified in my observations. For my DSLR and CCD systems I have always used the same methods to determine the transform coefficents for both DSLR and CCD-based systems. I used the standard CCD method given in the CCD observing manual. Of course, the DSLR fits are significantly noiser, generally a byproduct of the Bayer matrix non-optimal sampling of the psf but in the early days also non-round psfs because I was using non-guiding mounts and tracking often wasn't perfect. There is little bias between transformed DSLR (BVRc) and CCD (BVRc) observations in a sample of about 50 cepheids I have observed over the last 4 or 5 years. Yes, B and Rc DSLR measurements are noisier than CCD V and Rc. The B and Rc magnitudes also have more significant ouliers--an optimal focus is critical in Bayer sensors. I believe, hard to prove, that the red-extension stars you gave some tiime ago for SA-110 brought better fits for both sensor types because the color ranges are so much larger by including them in the fits.
Jim DeYoung (DEY).
Jim DeYoung wrote: "...a sample of about 50 Cepheids I have observed over the last 4 or 5 years." This sounds great. I have been trying to get reliable new complete multi-color Cepheid lightcrves for a few years, hampered for various reasons just now.
"...the red-extension stars you gave some time ago for SA-110 brought better fits for both sensor types because the color ranges are so much larger by including them in the fits." Glad this helps. If there's interest I can post a few more of these in other parts of the sky.
\Brian
I have the list you put on this forum recently of other red stars for calibration. I haven't yet ID'd and added any new stars in the the Landolt fields. Neither have I yet gone back and reprocessed old transformations with them. On my do list!
I tried to find the posting with the "some very red standard stars" from 2022 Jul 11 but have failed. I saved and have the list here but others may want the l link again.
The weather continues to be horrible for observing of any kind here in central Virginia at least.
Jim DeYoung (DEY)
>> "...some very red standard stars" from 2022 Jul 11 but have failed."
It seems the preferred approach with an imager (versus ye olde single-channel photometry) is to have several stars of diverse properties in the same field, rather than isolated stars of extreme colors. I'll post separately a few more sequences I've adopted that will be useful in the coming months.
>> "The weather continues to be horrible for observing of any kind here in central Virginia at least."
For various reasons I think many of us are not getting much reliable weather for astronomy.
\Brian
Arlo told me that when he did multi-night observing runs, though he determined second-order extinction in B band nightly, he used the mean extinction of all the nights to reduce his data.
Tom
The second-order B-filter extinction is usually difficult to measure accurately, with scatter in several determinations about as large as the value itself. Rather than measure it, folks often have adopted some value like -0.03(B-V)X. Since it doesn't vary much with the extinction itself, I would think the usual procedure would be just the opposite of what you describe: k''(B-V)X from a full run, but nightly determination of the main extinction coefficients. Landolt says exactly this in the 2009 paper near the end of section 2 of the text: "Each night's data were reduced using the primary extinction coefficients derived from that night, whenever possible. Average secondary extinction coefficients for a given run were used. Examples of the range in extinction coefficients which an observer in fact encounters have been tabulated in Landolt (2007b) [a conference paper]. Such tabulations should remind any observer of the perils of using mean extinction coefficients." QED
Surely to some extent slight systematic errors in the nightly zero-point and the extinction determination get soaked up in each of those adjustments and probably the color terms, too.
Another thing that I think is changing is the general increase in 'crud' in the sky, surely even at ideal photometric sites, due to climate change, the whole planet burning etc. I have seen only a few 'beautimous' nights in Flagstaff in recent years, at least in terms of low-level aerosols, even in winter after snowstorms. So the old lore possibly no longer pertains. I am hoping to be able to report on this eventually, including nightly trends in zero-point caused by changes in extinction.
\Brian
Arlo used the average 2nd order extinction of the whole run. That's what I was referring to as "extinction" :)
No problem. The jargon can be confusing. It is perhaps worth noting that there is no second-order extinction in V because the Chappuis bands of ozone flatten out the Rayleigh atmospheric extinction curve across the bandpass. The curve is flat enough for R and I that there is no need for a second-order term. There _should_ be one for U-B, but Johnson himself defined it to be zero, causing no end of problems with transformations for others.
\Brian