Going to the Limit

By John W.  Reed

On July 25th I was headed to one of the deep sky outings we have been scheduling monthly.  As I made the final turn to the property, I noticed that it would be clear, but somewhat hazy.  The sky had a washed out look, the sunset glow was not red, but rather pink and diffuse.  Not a good night, I thought.

There were several members already there.  Bruce McMath was setting up his equipment to do some variable star observing, Michal Berrington was also there, along with several others.

Evening continued, darkening toward night.  A myriad of insects buzzed and chirped.  Mosquitoes began to bite.  On some summer nights at the property they can be a real problem, on others they are strangely absent.  A few stars attempted to pierce the mid-summer haze.

Rather than trying to concentrate on deep sky I became interested in observing with the club's 6" refractor (This instrument has now been unmounted and replaced with an Astrophysics refractor on loan from Jim Hoskins).  The lens was returned from testing to determine the extent of its problems, which are, apparently, a slight turned down edge on one of its surfaces.  The problem with its element spacing has been corrected.

Michal had the refractor pointed toward Antares in Scorpius.  She said she was trying to see the companion.  Now I have tried to see this alleged companion for many years and have never succeeded.  Always the bright orange disk of Antares' light blots out any hope of seeing the faint green companion.  Nevertheless it seemed an interesting trial for the newly tested instrument.  At first Michal and I could see nothing so we upped the power to about 200x.  This time when I looked I noticed that Antares' Airy disk was tiny, with two small, distinct diffraction rings.  As I studied it I realized that the "seeing" (the stillness of the atmosphere) was incredibly good even though the transparency was poor due to haze.  At times the seeing settled down even more and superimposed on that second tiny, orange ring was a small green spec.  Antares' companion!

The companion is 3 arcseconds from Antares and is listed as magnitude 6.5.  This seems like it should be an easy target, however, because Atares is first magnitude and 150 tines brighter, it is somewhat like trying to see a firefly next to a floodlight!

After that I suggested we go after more elusive prey.  I suggested double star Lambda Cygni.  Burnham's lists this pair as being 0.7 areseconds apart, one is magnitude 5, the other 6.5.  Now the problem is not that of one being brighter, but rather that they are so close.  Splitting two stars this close is like trying to resolve two quarters just touching at a distance of 3 miles.  The refractor did it!  Visible were two tiny Airy disks of white light, just touching.  Their faint diffraction rings were actually overlapping the oppsite disk!

Finally, we moved onto Saturn, it now being high enough to observe.  We slipped in the Barlow and studied it at nearly 400x.  The image was rock steady!  This now huge world floated in our field of view, occasionally showing ever finer detail.  The Cassini division 'was an easy black arc in the ring's "ears".  The planet’s disk showed slightly mottled belts.  The elusive crepe ring was visible close to the disk as a wispy lace border.  However it was the outer ring edge that I was studying the most.  Here the outermost ring "A" dims slightly before disappearing entirely.  It took several minutes before I believed it, but when the seeing settled to near perfection I saw the Enke division make a fleeting appearance.  It was a fine black thread near the outer edge, nothing compared to the Cassini.  Apparently this object is not a true division at all but rather a ripple in the ring material.  No wonder it was hard to see!

When Mr.  Dawes derived the theoretical limit to detail a telescope can see he did this assuming perfect atmosphere or seeing.  It has been my experience that this perfect condition  occurs rarely.  Perhaps that is why the 6" refractor has been felt to be less than perfect.  It may not be perfect, but it's close enough if seeing permits.  Now that the new Astrophysics refractor is mounted, compare it to the old refractor and see which is better.