Hi Glen & All,
Thanks Glen for the info on the two galaxies
-- much appreciated (I shouldn't have been lazy and looked further -- but I hate the SIMBAD interface !!!), but ...
The surface brightness magnitudes you have provided from LEDA appear to be in square arc-seconds as opposed to the square arc-mins ones I provided (it came from Megastar) and therefore it _appears_ that there is a disparity in the data. I don't think it is a real (or at least significant) disparity -- basically just different units.
Professional astronomers tend to use the sq seconds measure for surface brightness and amateurs (for one reason or another) seem to use sq-mins (probably because it is used so much in amateur literature -- like Skiff & Lughinbul, NSOG, Megastar etc etc).
But, harking back to my earlier post, I essentially proposed that the brightest and best bits of IC 2118 appear to have _approximately_ the same SB as MCG -1-13-49 (PGC 16607). I also said that NGC 1752 seems to be about 3-4 times brighter than MCG -1-13-49. Therefore the SB of the best bits of the nebula are _about_ 3-4 times fainter than NGC 1752.
Now, disregarding the units for the mean time, it seems from the LEDA data that MCG -1-13-49 (PGC 16607) at S.B mag 23.9 /sq arc-sec is about 1.5 mags (or approx about 3.3 times) fainter in _surface brightness_ than its neighbour NGC 1752 (S.B mag 22.5 /sq arc-sec).
Therefore, proceeding on my proposal that the best bits of IC 2118 are on par with MCG -1-13-49, the nebula is therefore _about_ 3 to 4 times fainter in surface brightness than NGC 1752 -- this is what I guessed earlier and posted -- true?
Please let me know if I've got this wrong!!
A sq arc-sec is 1/3,600th the area of a sq arc-min. A rough rule of thumb I use to convert SB mags in sq arc-sec to sq arc-mins (without mucking around with log tables -- the magnitude scale is a log scale) is to subtract 8.9 from the arc-second figure to get arc-minutes. As near as makes no difference, 2.51^8.9th power = 3600 (well, 3606.6 to be _exact_).
Is the maths right?
Assuming it is, that would (according to the LEDA data) make the SB magnitude of NGC 1752 13.6 mags/sq arc-min, MCG -1-13-49 15.0 mags/sq arc-min and therefore, the best bits of IC 2118 about 15.0 mags /sq arc-min -- right?
(I think I guesstimated 15.5 before)
Either way, IC 2118 is perhaps one of the most challenging visual targets in the entire sky!
Attempts to observe it are very much encouraged from observers with large and medium/large telecopes -- a negative is just as (scientifically) important as a positive -- but you might have to wait till summer again!
Best,
Les D
Contributing Editor
AS&T