Understanding field star mags from 2MASS

[Marko]

This is kind of a far out there question but it is puzzling me. How to find really faint star magnitudes where the magnitude more or less represents mag of our own eyes perception limits (a sort of human luma curve adjusted value).

I am often trying to note in the field the dimmest field star seen when I look for faint targets to be a sort of limiting mag through the eyepiece with my trusty 7mmNag. Often I can only find such star data in Aladin using the 2MASS data. Problem is the Jmag and Hmag numbers seem 4 mag 'brighter' than I suspect these stars are. Example: For Minkowski 2-9 I noted an obvious field star NW of central star by about 1'. MegaStar say Mag 16, simbad in Aladin says B mag is 15.2 and R mag is 14.5. Then the 2MASS mags are even more confusing with Jmag 12.47 and Hmag 11.94.

My dimmest seen star was due east of central stellar point by about 1' and only has 2MASS Jmag 14.53 and Hmag 13.87

So should I say that mag is [MegaStarMag of Brighter Star] + [12.47 - 14.53] or in this case 16 + 2.06 or 18? or maybe stick with Blue mag for 15.2 + 2.06 for 17.26 (more believable). I would be basically lowering mag by the difference in Jmag values.

If there were a direct mag number in some huge catalogs like 2MASS that gave a 'human visually perceptable mag' that would be nice.

[Jimi Lowrey]

Hi Mark,

When I am trying to find out a field stars V Mag I use VizieR http://vizier.u-strasbg.fr/viz-bin/VizieR

I put in the Ra and Dec of the star and get way more info than I need but if you keep looking someone will have its V Mag listed in one of the catalogs.

Hope this helps!

[Marko]

Sounds like the sort of reference I was looking for. Also you have reminded me that it is the V Mag that I was trying to describe I wanted.
Found Vmag for the limiting star in my 18" with 8mmTmb to be 17.3 which is believable for a pure stellar object and the just slightly more apparent one at Vmag 17.15 which was even less noticable in my 7mmNag but the limit for that eyepiece. The other star I discussed that was much brighter shows Vmag 14.8 (Also believable)
Thanks Jimi

[Jimi Lowrey]

Was that 17.3 V MAG with a 8MM TMB Supermono or other type TMB ?

[Marko]

It was the 8mm TMB SuperMono showing dimmer stars than the 7mmNagler but yes, not an exact mag match. I have not had a proper shootout with the 6mm SuperMono but the 8 (except for the darn FOV 'tunnel') I think works out well in the f/3.7. What will be quite interesting is I broke down and now for next dark sky I get I am 'armed' with 6 and 8mm Delos (took advantage of TeleVue sale and yes, the 8mm shipped from OPT) and also found a 13mm Ethos for very good AstroMart price that came last week and looks pristine condition. Only have had the 6mm Delos out for one night and that was my 12" at a not so dark site so when I get them all out to a very dark site, skies permitting, with my main 18" will be a treat I suspect.

[Marko]

Today I found that 'The Sky X' with it's extra data disk has magnitudes of stars sometimes down to insane levels like dimmer than mag 18 which is beyond my 18" limits (but not beyond Jimi's 48" I expect).

The Sky X agreed with the earlier VizieR info as far as V mag goes. The VizieR info seemed often more extensive should I ever need that level of detail.

The Sky X has UCAC3 that says it supports visual mag to about 16 and then it has NOMAD which seems to be UCAC2 and several other catalogs combined.

Somehow The Sky X is showing and identifying stars to dimmer than mag 18 V

[Sue French]

Use the Aladin Sky Atlas online or download it to your computer. Call up an image of the area of interest. Choose VizieR -> Photometry to see catalogs that cover the area. Overlay catalogs onto the image to get magnitudes. There are several deep catalogs that give V-band photometry, such as NOMAD1, GSC2.3, TASS IV patches, etc. There is also an Excel spreadsheet on the web that will allow you to covert USNO B1.0 magnitudes to visual.

Sue

[Carter Scholz]

There is good information about photometry here:
http://brucegary.net/dummies/x.htm
including a method for converting J- and K-band magnitudes into Vmag.

Some theory here:
http://en.wikipedia.org/wiki/Plancks_law

Most stars are quite similar to black body emitters. So if you have the temperature of the star, Planck's Law gives you its spectral emission in any band. Unfortunately, it's not so straightforward to get the temperature from the band magnitudes. (I'm still researching this, so if anyone has resources, please share.) But for our purposes, the curve-fit methods on the first site should be close enough.

The PPXML catalog combines USNO B1.0 and UCAC. Its purpose is accurate proper motions, but it contains all their magnitude data as well.

[Carter Scholz]

Correction. That should be: The PPMXL catalog combines USNO B1.0 and 2MASS.