Eyepiece choice for view M31 Andromeda

[TheUKDave]

So ... you hear a lot of people saying things like, "you want the least magnification/widest field of view you can get for DSO/andromeda/etc", and I was basically wondering where you draw the line with that.  For example, one line you might draw is that since Andromeda has an an angular size of about 3 degrees, is there any point in an eyepiece that goes wider than that? Probably not, right?

Another line you might draw is with exit pupil, on certain scopes, certain focal longer lengths might have such a large exit pupil that you'll waste aperture.

For me - Skywatcher 100mm ED DS Pro refractor 900mm focal length (f9) - an exit pupil too large is not too much of a concern, since I'd need to go beyond 50mm to get wastage there, and being a refractor it's only about wasting aperture as opposed to seeing a central obstruction.  So I'm just wondering how I would realistically be able to just get the best view.

Without mentioning brands, and just combining focal length and apparent field of view, on my scope (100 ED DS Pro 900mm/f9) I could get something like a 45mm 60° giving the full 3 degrees with an exit pupil of 5mm ... or perhaps a 40mm at 68° (same total field with a smaller exit pupil) ... or potentially a certain 25mm 100° eyepiece which would give almost 3 degrees with a much smaller exit pupil.

It feels a bit like swings and roundabouts.  Do I pay more attention to eyepiece quality, or to it's native specs (focal length/apov)?  And if the latter, then should I be attempting to get the full view in a longer focal length eyepiece, or a shorter one with a wider field?  Which would look better considering how faint Andromeda is?

Cheers,

Dave

PS - mentioning brands now I'm considering a bunch of wide field pieces, but to name a few, there's the ES 100° 25mm, the ES 82° 30mm, ES 68° 40mm eyepieces that all give about the same total field, but with different designs and focal lengths.  I don't know which of these would be 'best' for M31.  Don't get me wrong, there's plenty of other brands, for example the Panaview 70° 38mm also offers the perfect 3 degrees - and at a much lower price point.   

But I was thinking more conceptually at first.  What 'properties' would you look you for to view M31? not so much what 'brands' would you look for?

[Charic]

I chose the 2" 32mm Panaview for M31, and could possibly go wider, but more than happy with the result.

Al Nagler says ....frame the subject.... a little space around your target gives you some spacial definition, but how much you need, and at what cost is a choice only you can decide.

I chose my high power EP based on focal ratio and multiplied the same focal ratio against my entry pupil (approximate ) to find a suitable low power eyepiece, anything inbetween just helps me to frame the subject. Im also very happy with a 60°afov.

Note, when using the Panaview from home, M31 is nothing but a small misty patch inside a lot if space, but from darker sites, M31 fills my view and is just stunning.

[jetstream]

I like an eyepiece that shows the dust lanes of M31 the best and my 18mm ES 82 is no slouch in the SW120ED scope which is similar to yours, same fl different ratio. ES make very good eyepieces. Are you going to join the dob mob at some point?  If so the 40mm eyepieces might have limited use with the faster ones, like the excellent SW 250 f4.7. It would work nicely with an OIII though.

M31 just gets bigger and bigger the darker your skies with M32 getting caught in the glow...point is trying to get it all in the FOV most likely won't happen with a telescope, eventhough a wide apparent field of view is great. The 20mm ES 100 would be great in your scope on M31 and other fainter galaxies like M81, M82.

[Stu]

Dave, much of this depends upon the quality of sky you are going to be observing from. Under LP you won't see much more than the core anyway, so a widefield would be wasted. Under very dark conditions, as charic notes, M31 is huge, but the outer regions are very subtle. 3 degrees is probably enough to frame the parts that are viable unless you are under a pristine sky.

In general it is better to have a wider field with smaller exit pupil as this controls the sky background better. It also dims the object so it's a balancing act really. Around 4 or 5mm is probably best, but it does depend on your sky conditions as the larger exit pupil will seem more washed out under light polluted skies.

Something like a 35mm Panoptic would be nice under dark skies. X25, 4mm exit pupil and 2.7 degree field.

As Gerry says though, a little more power would probably bring out some more dust lane detail without showing the whole galaxy.

[Moonshane]

It's often cheaper and more effective to buy a short fast scope for wide field views. I bought a 80mm triplet apo used for £200. To make the point this gives 3.3 degrees with even the humble 32mm plossl.

[John]

It's often cheaper and more effective to buy a short fast scope for wide field views. I bought a 80mm triplet apo used for £200. To make the point this gives 3.3 degrees with even the humble 32mm plossl.

I tend to agree with this. Your ED100 at F/9 is a fine scope but 2.8 true degrees is about the widest view it's going to show whatever eyepiece you put in it. You can get a 102 F/5 achromat on the used market for around 1/3rd of the price of some of the eyepieces you mention and that will show a 3.2 degree true field with a standard 1.25" 32mm plossl or a whopping 4.9 degrees with a 2" 30m 82 degree eyepiece in it.

[SpaceWalker]

But I was thinking more conceptually at first.  What 'properties' would you look you for to view M31? not so much what 'brands' would you look for?

As practically everybody has said above, for M31 (as well as for all galaxies) most important is not an eyepiece but a dark sky.  A month ago I travelled to rural Florida with my family and got an excess to ~ Bortle2 sky. I had just my cheap Chinese 90mm f/8.9 frac. The best views were in my 32mm TV Plossl (25x, exit pupil 3.6mm, 2*TFOV) and 20mm ES68 (40x, exit pupil 2.2mm, 1.7*TFOV). Boy, it was amazing! All three, M31, 32 and 110, in a single field! M31 stretched from edge to edge and M110 was quite large and was shining  like a street light  . I usually don't see M110 in my red zone near DC even in my 8" Dob, and the others two look like just small blurbs.  So, if you find a really dark observing spot I'm sure you'll be happy with what you have. Any eyepiece that max out your scope TFOV (2*52') will be appropriate, be it the 25mm ES100, 30mm ES82, 31T5 or 40mm ES68.  But it makes sense if you have access to a dark sky, IMO.