Getting Lost in the LMC

[Glenn Dawes]

Hello all,

Having been a city based observer for all of my life but hooked on deep sky, I’ve been restricted to owning scopes that I can carry around in my car (mostly sedans). For this reason, in recent years I have used a 10” Dob, with my only experience with larger instruments under dark skies being confined to brief interludes spent with O.P.s (other peoples’) such as at the SPSP.

This all changed over the last few months with my purchase of a 16” Lightbridge. Even then I was restricted until I could modify it to fit entirely in the back seat of my Ford (I’d be happy to start a thread on this if anyone is interested in what I did).

Over Xmas I stayed with some friends who are fortunate to live under the dark skies of Bethanga (east of Albury). I’d previously thought the LMC was very impressive, but the impact through the 16” was amazing. My friend Robert described the Tarantula as an Octopus with its numerous, detailed bright nebulae/arms and associated star clusters. Before this trip I had never appreciated the complexity of the dark nebula winding across this satellite of our galaxy – in many cases the dark regions were as obvious as the bright nebulae. Another first, I found using a 9mm Nagler (200X) made it much easier to distinguish between open and globular clusters – the extra magnitude and resolution of the 16” over the 10” really made a difference. Previously the individual members, of many of these clusters was either borderline or beyond the 10”. I also found having an Argo Navis was great in confirming my identification of open vs. globular. However this was only effective in regions away from the central bar. When on the bar it was not unusual to have 4-6 clusters and nebulae in the same field which made it difficult even for the Argo to sort out.

If anyone wishes to get serious in the future sorting out all these individual objects I’d recommend coming armed with the Herald-Bobroff charts or ‘The Night Sky Observer’s Manual, Vol 3, The Southern Skies’. Scanning them, reversing them east/west (to get a Newtonian image), printing and laminating them I find useful at the eyepiece.

Anyway enough of my ramblings – I could go on for another 2 pages with the new views I had of other old favourites elsewhere in the sky.

Regards

Glenn

[Kevnool (Kev)]

The 16" aperture is truely stunning....and its the image scale that gets me excited.......with the Argo-Navis onbourd over 100+ objects in one observing night.....ahhhhhhhhh.........Cheers Kev.

[Paddy (Patrick)]

I absolutely share your excitement, Glenn - these objects through 16" take on a whole new life (having upgraded from 12"). I sometimes spend much of my nights just trawling through the Magellanic Clouds and I can get lost in the Tarantula for very long periods.

[ngcles]

Hi Glenn & All,

I can 100% relate to what you and the others have posted Glenn.

The LMC is wonderful in any aperture but the level of wonder really takes-off when you move into the 16" + range and have a truly dark sky. Most northern hemisphere observers have absolutely no idea about what they are missing out on. There is more detail to be seen in the Large Cloud with 10x50 binoculars than a 20" will show in Andromeda.

One of my favourite pass-times at this time of year with the 18", is to centre on a part of the LMC, insert the 20mm TII Nagler at x111 (44 arc-min true-field) with or without (depending on mood and what you want to see best) a Lumicon UHC filter. Then hit the circular search button on the Servo-Cat.

The 'scope slews slowly in ever-increasing outward circles, about 1/2 a field wider each time around the origin point out to a circle about 2 1/2 degrees wide.

Mouth-watering. It is interesting to watch and listen to others taking this "trip" -- and the oohs and ahhs. The extensive clusters around the Tarantula (NGC 2070) are best of all, though the western-end of the cloud is very good too.


Best,

Les D

[astroron (Ron)]

Great Report Glenn, You get an entirely different view of things Astronomical when you join the 16" and above class of Telescope
I hope you have many more nights of great observing

[Merlin66 (Ken)]

Invest in Mattie Morel's atlases of the LMC and SMC; they will add to you experience!

[mbaddah (Mo)]

Would be interested to know how you modfiied your 16" to fit in the back of the Ford ?

[astronut (John)]

Great report Glenn,
I know the feeling when you have a significant jump in aperture.
Three years ago I went from my 4" Unitron Refractor ( which gives killer views) to my 12" Lightbridge, WOW what a difference.
I've spent many a night "Lost in Space" in the Magellanic Clouds.
With the aid of a dark sky + a U.H.C. filter the detail was like a 3D photograph!!

[Glenn Dawes]

Originally Posted by mbaddah

Would be interested to know how you modfiied your 16" to fit in the back of the Ford ?

Hi mbaddah,

I've just put up a thread covering this - please let me know anything that needs further explaining.

Regards

Glenn

[Smirnoff]

Great report Glenn! The LMC complex really is something when seen thru lkarge apertures, but really it is arguably the richest area in the sky for just about any instrument

[RobF (Rob)]

I wonder if Mike could build a new filter into the forums?

"Hide posts relating to 12" scopes or above"

Too great a danger of CAF (Contagious Aperture Fever)

[Glenn Dawes]

Originally Posted by ngcles

One of my favourite pass-times at this time of year with the 18", is to centre on a part of the LMC, insert the 20mm TII Nagler at x111 (44 arc-min true-field) with or without (depending on mood and what you want to see best) a Lumicon UHC filter.

Hi Les,

Thanks for your response and for sharing your LMC observing experiences. I find your choice of eps (power) interesting. I found, even with the 16" I needed the 200x of the 9mm Nagler to get a clearer view of the small clusters (you know the dozens of them listed as being 1 arcmin in dia). However with the 9mm I might still have a wide field, but the true field was only 20 arcmin - no wonder I kept getting lost! For those turned off by the Naglers because of the cost, from my experience the 16mm Meade 5000 series (super wide angle ep) gives a great image at f4.5. Even with the 16 LB I get a 32 arcmin diameter field. Bintel currently sell them for $239.

Regards

Glenn

[Smirnoff]

Yup, these clusters are all some 170,000 light years away, unlease as much magnification as possible on them!

[ngcles]

Hi Glenn & All,

Originally Posted by Glenn Dawes

I find your choice of eps (power) interesting. I found, even with the 16" I needed the 200x of the 9mm Nagler to get a clearer view of the small clusters (you know the dozens of them listed as being 1 arcmin in dia). However with the 9mm I might still have a wide field, but the true field was only 20 arcmin - no wonder I kept getting lost!

Of course you are right -- when it comes to seeing maximum detail in small low-contrast objects, high-moderate magnification rules. The 2mm exit-pupil rule is important.

Accordingly, my two most used eyepieces are my 12mm T II and 9mm T I Nagers which give x185 and x247 with exit-pupils just over and just under 2mm. I find the 16mm T II frames The Tarantula (NGC 2070) -- by itself beautifully but to take in the context of the surroundings, the 20mm T II and 26mm T V are wonderful. And as for this motorised sweeping around, they are hard to beat on the issue of breadth of field. You really feel like you are flying over the LMC

Glad your'e happy with the series 5000, I've never looked through one myself so I can't comment on them. I have looked through many of the earlier series 4000 UWA's and they were certainly sharp pieces of glass. Eyepieces are an intensely personal thing. What "brand" or "design" works for one person won't necessarily find favour with all others. It's a pity you can't drive before you buy.

Haven't looked through any Ethos yet either. I'm too afraid ... for my wallet !



Best,

Les D

[Paddy (Patrick)]

Originally Posted by ngcles

The 2mm exit-pupil rule is important.

Please excuse my ignorance Les, but what is the 2mm exit pupil rule?

[ngcles]

Hi Paddy & All,

The 2mm rule isn't so much a hard and fast rule -- more a rule of thumb that has existed since Moses played fullback for Jerusalem.

Most experienced observers agree that for small low-contrast objects, and to achieve maximum resolution of faint stars in things like G.C's, LMC O.Cs, LMC star-clouds etc etc, a telescope produces its maximum visual contrast when the exit pupil produced by the Aperture/Magnification/Eyepiece combo is about 2mm.

The exit pupil is the diameter of the beam of light that comes out of the eyepiece. A telescope used at low power with a longer focal length eyepiece will produce a wider exit pupil than higher power.

Beside directly measuring the diameter of the beam, the exit pupil for a particular Aperture/Magnification/Eyepiece can be approximated by dividing the telescope aperture in mm by the magnification a particular eyepiece produces.

Example: 200mm aperture used at x100 produces a 2mm exit pupil.

250mm aperture used at x125 produces a 2mm exit pupil.

456mm aperture used at x185 produces a 2.46mm exit pupil.

456mm aperture used at x247 produces a 1.85mm exit pupil.

So, to achieve maximum visual contrast on small low-contrast objects, experienced observers almost universally agree a 2mm exit pupil is best. However, large low-contrast objects are better observed with a 3.5-4.5mm exit pupil.

As I said it is not my rule, it isn't a hard and fast rule but a rule of thumb.


Hope this helps.


Best,

Les D

[Paddy (Patrick)]

Many thanks, Les. Interesting and useful!

[Paddy (Patrick)]

One more question, Les. Would you consider that this is also a useful rule of thumb for galaxies?

[ngcles]

Hi Paddy,

Yep, very much so for small, low-contrast objects of any type -- including galaxies. When I say small, I'm talking say less than approximately 4 arc-mins in size. There are a heck of a lot of small, faint galaxies and I customarily use the 12mm or the 9mm (or both) with my 18" giving x185 or x247. Usually more power than that (ie an even smaller exit pupil) spreads the light of a small faint object over too much of an area in the eyepiece field making it harder to see. On the other hand, too low a power will make the field brighter reducing contrast and making it too small to see easily. About 2mm just seems to do it right.

For small high-contrast objects like PNe and your "average" globular you can use more magnification again to enlarge the object to make the detail that is there, easier to see. Your eyes will tell you whether you are using too much because you will say to yourself something like "that looked better one eyepiece down".

That's the conventional wisdom anyway.

Hope that helps.



Best,

Les D

[Paddy (Patrick)]

Thanks again Les, it is very helpful indeed!