Akn 120

>> Possibly Jeff or someone else could clarify and add
>> some real substance to my weak memory.

The observation was made with a homebuilt 4.5" Newtonian. I was probably using an Abbe ortho at rather high magnification - I can't access my logs just now to be sure. The telescope had a very long (overlength on the sky side) tube, behind-the-mirror and under-the-focuser baffling, and a very black and rough interior - built optimized for faint stuff, basically.

The observing was done from near Coronado Peak in southeastern Arizona (south of Sierra Vista), in the late summer of 1997. This would have been at around 6530 feet, and the naked-eye limit near the zenith was 6.8. Prior to the observation, I plunged my eyes into total darkness for around a half hour, if memory serves.

The best star I could get was V magnitude 14.68 (stars a half magnitude brighter were seen very easily). The star was part of a calibrated photometric sequence around an active galaxy called AKN 120. The sequence is available from:

Hamuy & Maza 1989, Astronomical Journal, 97, 720

A couple of comments:

1) This observation was done with a significant amount of preparation as regards observing technique and sky conditions. I bided time and waited for an extremely clear night with very good seeing, both of which are required for observing the faintest stars. I also made sure I had clean optics and I had an assistant on hand to do some of the telescope manipulation while I was hiding my eyes from all light. And my red flashlight, which was put to use only briefly while drawing the field, and even then only while the observing eye was shut, is MUCH DARKER than those that I see people using at star parties like TSP and elsewhere. Going to the limits requires as many of these variables as possible to be optimized.

2) One thing I did not do in advance was study a chart of the field, though I'm really not sure I'm that susceptible to illusions of preconception - very often, I fail to see what I know is there and fully expect to be visible. But for those who think that anything seen on a chart will instantly be seen in a telescope, that source of potential bias wasn't present in this case.

3) A 4" refractor, which should have less loss than a 4.5" reflector, a lack of obstruction, and (one hopes) good baffling, should easily get into the same neighbourhood and deeper is certainly not out of the question.

4) The first sentence of the first posting of this thread was: "As I understand it, the limiting magnitude for a 4" (102mm) refractor is 12.0." This figure probably came from one of the tables that are frequently published in various astronomy books. Such tables are unmitigated rubbish and are useless for determining the limits of an instrument.

5) The GSC is also rubbish, as far as its magnitudes go.

>Another guy who has a finger on accurate
>photometry is Brian Skiff.

Actually, Brian is the guy who got *my* finger on accurate photometry, and thus enabled the observation.