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From the title of the thread its obvious Adrian is after a low power wide field eyepiece to view M31 and other extended objects within the field of view. That's fine.
However I want to add a little on what can be observed within M31 as a couple of posters have alluded to the fact that M31 is a fairly featureless target and best viewed in binoculars, or a smaller telescope to maximise the FOV and frame the target. That's fine if you don't want to see much detail in the target and frame it as an extended object within the FOV.
There is in fact a wealth of detail and features to be observed within the structure of M31 in telescopes of 10" aperture and larger. The critical thing is dark transparent skies and good seeing.
There are several dust lanes within M31 which are very easy to see in a 10" telescope. In bigger telescopes a few more dust lanes become apparent. I am also attaching a map of M31 which appears on page 16 of the Night Sky Observers Guide. This shows over 40 individual targets within M31, including globular clusters, open clusters and stellar associations. The quality of the photograph, by todays standards, is very poor as it was taken years ago on film, but as a map it is accurate.
Several of the targets on the map are visible in a 10" telescope under good conditions by a skilled observer, including NGC 206, G76, G233, G252, G280 and C107. I have observed 15 of the targets listed on the map in my 14" SDM and over 20 of them in my 18" Obsession. It takes patience, perseverance, very good conditions (ie. good seeing combined with good transparency combined with dark skies) a good telescope with good optics and a bit of skill.
Importantly, you need to change your approach to observing these types of targets as they are extra galactic. The same thing applies to a slightly lesser degree when observing targets within the Magellanic Clouds, but they are much closer than other extra galactic targets, being 175,000 and 200,000 light years away compared to M31 which is 2,500,000 light years away. Many of us are accustomed to viewing globular and open clusters within our own galaxy. This is pretty easy as most of them are bright < mag 10.0 and fairly large where their dimensions are measured in arc minutes. For instance 47 Tuc and Omega Centauri are 30' and 36' across and both naked eye targets. Extra galactic globular clusters and open clusters are measured in arc seconds. Most of the targets on the map are between 1" and 4" wide. Consequently you need good seeing (tighter than the size of the target) to be able to determine them as non stellar. Whilst they are not resolvable in amateur telescopes, you can determine most of them as non stellar. The other thing which is really important is to use medium to high power. This helps to resolve them as non stellar by expanding the apparent size of the target and it also helps to improve the contrast. To properly observe detail and structure within galaxies you need to crank the power up. In my 14" I will often push the power up to somewhere between 250x and 350x and in my 18" scope I will push it up to 350x to 500x.
Cheers,
John B
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