Of Image Scale and Surface Brightness
A Tale of Two Scopes - And Two Nights
Argo Ventures On Alone
"And In Conclusion"

Of Image Scale and Surface Brightness

With the Moon approaching the end of the first quarter (Day 5) opportunities for deepsky exploration are getting a bit "claustrophobic". Last night's goal was to reproduce a series of observations done with the Pup two evenings earlier (Wednesday, the 16th) under very similar sky conditions (5.3ZULM + 7/10 stability).

Argo's 1800mm focal length limit's me to 52x as the lowest power (35mm Ultrascopic). While the Pup at 400mm peaks out at 40x using the 10mm - without barlow. My personal preference for any scope used to "sweep the night sky" is 30x. With 15mm ultrascopic in place, the Pup comes close to this at 27x. Sadly, Argo's 52x is almost double the desired magnification and this doubling effectively makes each "square" arc-minute of sky four times dimmer than it might otherwise be.

But there are other factors involved...

Under 5.0 ULM seeing conditions, the lowest magnification needed to get a satisfactory "dark sky experience" is just shy of one half a given scope's aperture in mm's. (Based strictly on personal experiences.) So with Argo, 70x gives decent dark sky views while the Pup needs 40x. On the few occasions when I've peered through 6.0 ZULM skies, 1/3rd the aperture in millimeters accomplishes this same effect - or 50x through Argo and 27x through the Pup.

Generally though, the Pup gives aesthetically better views at 27x than Argo does at 52x. Consider, for instance, that if we compare proportions of magnification to aperture ratios (Pup = 27/80) and Argo (52/150) we get the sense that the Pup effectively pulls in as much light per "square" arc-minute as Argo (.3375/.347=0.97). This suggests that both scopes tend to show about the same image contrast - with Argo gaining on image scale (52x/27x=1.93x) and the Pup gaining on field size. All this says that given a few conditions, the Pup and Argo should more or less be able to "find" the same DSO's at respective magnifications. It also suggests that whatever I see through the Pup at 27x is a scaled down version of that seen through Argo at 52x...

And it is this "scaling down" that I find especially appealing in a "rich field sweeper" such as the Pup. For given wide field views, the neighborhood becomes as important as the neighbor...

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A Tale of Two Scopes - And Two Nights

Like the Pup's occasion two evenings ago (January 16, 2002), I turned Argo on the M31 galaxy family. Here's the description from the Pup's observing Log documented on AstroTalk:

Even at 16x, all three close members could be located without trouble - but again like last night an exit pupil of 5mm's is too large to show a decently dark background sky. Moving on to 27x (3mm) improved things markedly so that a good sense of aesthetics was possible. And at this magnification the first dark lane separating M31 and M32 became readily apparent while the second one -further out - was hinted at as well. 27x also gave a good sense of knot NGC206 - but it wasn't till after I boosted the magnification to 40x that I was able to pin down its location between two wide parallel rows of 10th magnitude stars oriented north toward M110. Dropping down to 27x again I was able to see the stars as well. At 40x, M110 started to come out and play. Nice sense of elongation and central brightening.

Frankly, the view through Argo at 52x was almost identical. The only problem was I couldn't squeeze all three components plus the main galaxy's spiral extension into a single field of view! Happily knot NGC206 was easily acquired and found by referencing the same landmark. Through Argo, I also took a look at the knot at 120x - but couldn't find it. (The thing dissolves on magnification.) I suspect that the 25mm 70x view would have been optimal...

From the Pup's log:

With stars to magnitude 5.3 visible, swung the Pup due north and tracked down NGC185. Couldn't make it out definitively until I dropped in the 27x ep. At that magnification, got a sharp point of central condensation surrounded by an elongated flash of scintillation. NGC147, as usual, was very tough and once again, despite generally knowing where to look I will make no claims to finding it.

In fact, last night Argo outperformed the Pup here. The view of NGC185 showed a sense of structure (central brightening to dim halo), while I was able to definitively locate the faint glow of NGC147's core region.

Swung the Pup over to M33. Strong sense of NS elongation with the northwestern spiral arm better developed. This low surface brightness study seemed to need the lowest power possible that would just darken the sky - so 27x gave the best view...

Last night - through Argo - saw a larger version of same. However 52x made it possible to just hold M33's bright knot - NGC604 at the tip of the northwestern spiral arm. With slight aversion, 604 even took on a definite sense of "definition".

Also decided to check M76 - the Double Planetary - at low power. Although I could find it at 16x, it didn't really begin to condense until 27x and required 40x to give any real sense of frontier - strictly averted vision of course.

Through Argo, and again at 52x, some aversion of the sight made it possible to get a definite sense of orientation (east-west). Meanwhile, clearly noticed more central condensation in the western component than that to the east...

Made another effort to locate edge on NGC891. Before doing so noticed that Gamma Andromeda could be recognized double at 27x but needed 40x to fully resolve. Had no trouble finding the large "box kite" asterism of 7th magnitude stars due west of Gamma. At 16x even noticed an angular flash just east of the northern half of the kite. Couldn't see it at higher magnifications whatsoever. This 13.9 ASB baby wants the lowest possible image scale in such a small scope...

Gamma at 52x is a very easy and pretty split through Argo. Colors appear warm yellow and gold to my eye at this magnification. (The secondary is actually blue and this is the color seen at higher magnifications through Argo.) The Pup of course, is incapable of even hinting at Gamma Prime's closer .5 arc-second companion. On a very stable night, a 540x Argo shows the primary as a slight ovoid.

I had no better luck locating edge on NGC891 through Argo than through the Pup the previous evening. (In fact I came closer through the Pup!) Sky conditions are paramount for this particular study. This is probably due to the fact that the galaxy's core is muted appreciably by the dark lane that bisects its lenticular face end to end. My experience has been that a good three and change inch scope on a 6.0 ULM night at 40x will definitively reveal this "scratch of light". On such a night, Argo will reveal the central lane on eye movement, and most of the galaxies 13 arc-minute expanse on moderate aversion.

On the previous Pup viewing occasion, I serrendipitously found a colorful double very near 5th magnitude star (SAO55793) used as a guide to Galaxy NGC1023 in Perseus. Through Argo, I noticed that the fainter member appeared more orange than the red suspected through the Pup on that occasion. Meanwhile, precisely in the same locale described below:

One degree SW of the double caught a hint of a large ill-defined elongate region oriented roughly NS at 27x. But nothing definitive about this being 9.5 mag 9x3 arcmin Galaxy NGC1023.

As anticipated, I did come across NGC1023 through Argo - in all its edge-on glory at that same locale. Orientation, however, was clearly east-west and not north-south. The galaxy is located just north of the cup of a large parabola of 10 and 11th magnitude stars. Like most of the brighter edge ons, the view was surprisingly satisfying. Core condensation, spiral extensions, and faint halo encompassing everything. NGC1023 however, is no match for NGC4565 in Coma Bernices which in turn can not hold the proverbial candle to M82.

So as a series you have the elusive and ghostly NGC891, followed by the quite satisfactory NG1023, then the well-defined NGC4565, concluded by the extraodinarily present and well-defined M82.

Not mentioned in the Pup's report is the fact that - while locating M31 earlier in the evening - I had dropped by Beta Andromeda to view galaxy NGC404. This one is a bit of a "contrast test". Second magnitude Beta can easily overpower this faint (magnitude 10.1) small (4 arc minute) face on galaxy. Notes from January 16, say that through the Pup, the galaxy was "barely detected reliably" at 40x.

In fact, last night (the 18th) through Argo - and at 52x, I was able to not only locate the galaxy but distinguish a starlike core from faint core region. The interesting thing is that this was accomplished by contemplating the flashing spurious image of nearby Beta direct (using "softeye" techniques) while really "looking at" the nearby galaxy. However when directly viewed the galaxy dissolved, and though present, showed no structure whatsoever...

Had no trouble turning M52 up. At 16x, a faint ball of luminosity with a dozen stars resolved to magnitude 10. Strangely, adding additional magnification did little to improve the star count. Definitely not an 80mm study - especially at low powers...

M52 was appreciably better resolved through Argo at 52x - as one might expect. In fact, at that magnification two dozen stars in the shape of "Alladdin's Lamp" could be held direct.

I also had views of Open Cluster NGC7789 through both Argo and the Pup. At 40x, the 80mm Pup showed a "cloud of luminosity" with a distinctively brighter concentration of components to the west. Very few members resolved. While at 52x, through 150mm Argo, several dozen faint members magnitude 11 to 12 could be held. Even so, I was not impressed with Argo's view at low power through this less than optimal sky. It is pretty clear to my thinking that 7789 is a study that requires 5.5 ULM conditions to leave one breathless through a six inch scope...

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Argo Ventures On Alone

To say the least the last few evenings of observation have been very full. The rainy, overcast weather of November, December, and early January has completely reversed itself. There has been a solid week or more of clear skies and C O L D temperatures. The first few days of this period were quite turbulent. Beginning on the 12th, stability began to improve while transparency remained solid - if not optimal. This all culminated last evening when I turned Argo on M42 and got the kind of rave views of the Great Nebula seen at the SCAC star party last Saturday evening (the 12th) in Bonny Dune. However, stability has improved slightly since that occasion. As a result it was possible to hold both 11th magnifutde "Sextazium" members (E&F) direct at 120x. Dropping down to 70x also showed E direct, but low power flashing of the bright Trap-C member made F suspect.

The sky was so steady last night that I made a 120x run on winter's only globular cluster of merit - M79. Could pick out a half dozen resolved members with the cluster some 25 degrees above the horizon to the south. (This part of my sky is less light pollution afflicted.)

Last Saturday at the SCAC star party, visited with M81 and M82. The view last night was very fine, but the Boulder Creek lightdome (to the north) took the edge off the contrast.

Also compared two disparate pairs through Argo - Theta Auriga and Zeta Orionis. Clearly Zeta is the tougher split. Requires 120x to show any kind of distinction between the two bright components. While Theta's dimmer companion was well-spaced and both stars showed nicely defined airy disks.

The moderately disparate but well separated Rigel pair were a real treat last night. No flaring, or overpowering luminosity on the part of the first magnitude primary. Bright one and dim one casually sharing space - just beautiful.

No luck with either Aldabaran or Upsilon Ursa Majoris. The 11th mag 109 arc-second companion is a given for Aldabaran - but this is hardly anything other than a faint come. It's the 32 arcsec companion of the 11th magnitude that busts my chops. Its supposed to be there, and someday I hope to know where...

Before knocking off, ran the magnification up to 540x and took a look at 1 arc second disparate Omicron Persi. The delta on this double is roughly 4 magnitudes - so I wasn't expecting anything. In fact, my expectations were rewarded - no sign of a secondary whatsoever. However, first diffraction rings were quite mobile which means the possibility to be pleasantly surprised still exists...

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"And In Conclusion"

You will recall that the original thesis was that the Pup should give views comparable to Argo on deepsky studies - but at half the image scale. It's easy to see that this does not apply to any study that requires magnification - NGC604 in M33 for instance, or those that involve stellar components - (M52, NGC7789). But what does surprise me is that even with intermediate size studies - (NGC147, 185, 404, 1023) - Argo did a better job. Where the Pup really excelled was on the big 'uns - M31 and 33. It also excelled at simplifying navigation and "setting the scene". There is a lot to be said for covering "great gulps of the sky". One's sense of "orientation" is far better supported when two or three degree fields are possible - anything less seems "myopic". But once you get there - and as long as the entire study fits the field of view - Argo give's the superior view...

Obviously the best of all observing worlds would be to mount the Pup along with Argo on a beefier mount. Add an SCT in the 12-14 inch range and the Night Sky would become a veritable smorgasboard of deepsky and lunar-planetary delights. Wrap an observatory around the whole thing - somewhere conveniently located under 6.0 plus ZULM skies - and these two feet would barely touch the ground...

But until then, it's just Argo, the Pup and me in Backyard Boulder Creek. And that, dear friend, may very well be enough excitement for one life.

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