Tales from Santa Luce, Tuscany, Episode V: Great Globular and Wide-Field Shoot-Outs

[michael.h.f.wilkinson]

The day after my extended globular cluster jaunt, the skies were clear again (this is Tuscany, after all), so the C8 was set up early, in part in the hope of catching Venus at a higher altitude. That hope was vain, and although I could sort of work out the phase, the contributions of our atmosphere were pretty horrendous. Saturn was a touch better, holding up to 169x in the 12T4. The XW10 was a bridge too far.

Onward to DSOs. I quickly picked up NGC 6517 and NGC 6539 again, on my way to IC 1276, which I had not been able to confirm last night. This time I could make out a faint, circular glow at the right location. I repeated the exercise with 22T4 and 17T4, and each time the glow seemed to appear in the same spot. This is a very hard globular, with very low surface brightness.

Just southwards, I picked up NGC 6604 as a tight little knot of stars around a mag 7.5 star. Not the most impressive open cluster, but tha may in part be due to the rich star field it lies in. As M16 was just around the corner I had a look at it, and M17 nearby. As I enjoyed the view of the latter, it seemed I spotted a hint of reddish colour in the nebula. Has anybody else spotted that? I suppose the surface brightness of the Omega Nebula might be high enough to trigger the cones in the retina, but I have never noted it before.

NGC 6426 was next. This is a low-surface-brightness globular near Celebrai. Not very remarkable, but another Herschel-400 object bagged.

I then walked to the front of the house, to look further south through a gap in the trees, using the Helios 15x70s. Tr (Trumpler) 24, NGC 6231 and NGC 6242 could be made out quite readily. These three open clusters are not that remarkable, but it felt like bridging a gap, as the only time I had seen these before was from Sydney, Australia. M6 and especially M7 were much more of a treat.

Back to the C8, I decided to make a comparison of a couple of the best globulars visible from up north (rather than down-under).

M5 was first. This was quite superb, with quite a few bright stars showing, plus a cloud of fainter ones, and quite an intense glow in the background. The brighter stars seem to give it a slightly "ragged" appearance.

M3 was next. It is about the same size as M5 visually, but a bit lower in surface brightness, with a more regular sprinkling of stars.

M13 was clearly bigger and brighter than M3. Surface brightness equals M5 roughly. Nice touch is the "nearby" galaxy NGC 6207.

M22 was the last of the quartet. A bit bigger even than M13, bit higher in surface brightness and with slightly brighter individual stars sprinkled across it. It always seems slightly flattened, like a mini elliptical galaxy.

Verdict? All great objects, of course, but I feel M22 wins in visual impact.

Finally, I decided to remove the C8 from the mount, and set up the APM 80mm F/6 triplet, to do a comparison of my two wide-field instruments: The APM 80 and the Helios 15x70 HD. To make the comparison more interesting I used the 31T5 to give the APM 80 a magnification of 15.5x to match that of the 15x70 as close as possible. I used the Orion Optics 2" Amici prism both to ensure image orientations were the same, and because the Denkmeier Filter-Switch Diagonal won't come to focus in the APM 80 . In terms of field of view, the 80mm+31T5 wins hands down, with a whopping 5.3 deg vs 4.4 for the 15x70.

First test: The Veil

The Western Veil was easily spotted in both instruments. It appeared to be a shade brighter in the 15x70 bins, probably because the brain combines the information from two eyes, yielding the theoretical light grasp of a single 100mm instrument. The image quality in the APM was WAY better however. Stars where pin sharp, and slightly more detail was visible. That is the difference between a 70mm F/4 achromat and an 80mm F/6 triplet apochromat for you (plus the difference between a 31T5 and Erfle EPs). The visual impact of the 80mm was clearly superior. The Eastern Veil showed up slightly in the 80mm, but just gave hints of its presence in the 15x70. I then added The UHC filter to the APM 80mm, and the scope just blew the 15x70 away. Both Eastern and Western Veil showed, and even Pickering's Wisp could be made out.

Second Test: The North America Nebula and Pelican.

Both objects were visible in both instruments, but the Pelican showed much more detail in the 80mm with UHC filter. The sheer image quality of the APM 80 was staggering. The Helios is no slouch, but the image through the APM had more impact, and definitely showed more detail.

Third test: Trawling the Milky Way.

For ease of motion, the 15x70 were obviously better at this, but I don't mind the quirks of an EQ mount as I simply released both clutches and moved freely along all the Messier summer goodies. In part it is the superior optical quality, in part the larger FOV, and for a large part the ability to insert a UHC filter that makes this combination brilliant. I ended up at M27 which shone like a little gem in the star field. Even at 15.5x you could make out the shape of the Dumbbell, and the large exit pupil meant the egg-like shape of dumbbell plus the fainter parts could be seen.

For portability, the 15x70 has the 80mm beaten by a mile, but for sheer image quality and visual impact, the 80mm is just a lot better. I will not part with either instrument any time soon.

Next episode: Of Pisa and Planetaries ...

[cotterless45]

Chewing the carpet here waiting for a clear sky. It is amazing to see the Cygnus nebulae with low power, the Eastern Veil has ruled the sky up here. Lovely report, keep posting !

Nick.

[Astro Imp]

Great report as usual Michael, I've been following your reports from Tuscany and have enjoyed them immensely. Looking forward to the next installment.

Thanks for posting.

[KevUU]

I'm really enjoying these Michael, really good to whet my appetite after a while away from things. I'm off to find the next one now Thanks for posting.

[michael.h.f.wilkinson]

I'm really enjoying these Michael, really good to whet my appetite after a while away from things. I'm off to find the next one now Thanks for posting.

Thanks all, I added a link to the next episode

[Special K]

Firstly: what an excellent set of reports, you must have been knackered in the mornings! But on holiday you must have had all day to get over it!

Naturally the difference in investment between the bins and the APO is significant. However looking at web details of that APM is making my mouth water! I'm catching up on my astronomy having taken the summer off and this is really getting me back in the mood! Now if i could just rustle up a thousand pounds to blow on new gear....

[alan potts]

A very nice report Michael, the only thing with the heat is it messes up the atmostphere. I tend to have the problem for about an hour after the sun goes then it seems to improve. For some odd reason it rarely ruins the high over head views, it always seems to be low views, like Venus at the moment. Even Antares was not too bad last night but its like a disco ball now.

Alan