The M31 "Twist"

[Howard B]

At the Oregon Star Party a couple weeks ago I finally got my 8 inch f3.3 scope together enough to be functional, and had a blast checking out some well known objects that are too large to enjoy in their entirety with my larger scopes. The most startling and exciting observation with the 8 inch was of M31, which has a strikingly different visual appearance than I expected. The near side (facing M110) seemed quite straight and two dust lanes were seen, and the far side (facing M32) was slightly curved. The unexpected part was that both ends of M31 appeared wider and curved away from the body of M31 in opposite directions, like water coming out of a sprinkler!

I was using a Paracorr 2 with a 25mm ES 100 degree eyepiece (3.22 degree fov) and the Ethos 21mm eyepiece (2.7 degree fov) for the quick sketch below, which was so quick I didn't bother to add any stars. By the way, my pupils still open up more than 7mm so I'm getting the entire light cone using both eyepieces.

1349 1350

I've never suspected this appearance in other scopes, and until I started to describe what I'd seen to Mel Bartels - who immediately knew what I was talking about - no one in my vicinity had ever seen this either. As you may know from a recent Sky & Tel article, Mel has made a 6 inch f 2.8 and a 10.5 inch f 2.7 and he calls this the M31 Twist. As we know, M31's shape is a flattened disc from our perspective, but with enough aperture and a wide enough fov, the twist becomes its dominant visual feature - fabulous!

Has anyone else seen the twist in M31, and if so what scope and fov did you see it with?

[Ivan Maly]

Excellent instrument and observation. So this is what it takes to actually see the spiral structure in M31 (this is not a disk warp but ends of arms). The NE end is primarily the giant association (star cloud) A102. I have seen it separately in my "narrow-field" 16". Naturally it is much smaller then visually. Symmetrically opposite is a fainter arm segment marked by A185; I have not explored that end yet. Maybe I should get myself this kind of instrument after all.

[Ivan Maly]

Actually I re-checked Boyd's Andromeda Galaxy book, and there is a warp as well as an outer two-arm pattern at least in HI. In modern optical images, however, I see pretty much normal arms at the distances in question.

[Uwe Glahn]

Hi Howard,

do you mean this "Twist"

http://www.deepsky-visuell.de/Zeichnungen/M31.jpg

And I thought I was the only one who saw these bend structures. The Instrument was a 4" f/4 Newton. The magnification was between 17x and 26x and the field around 4,7° (26mm Nagler) and 3,8° (17mm Ethos).

I'm not very proud for the sketch because the "Twist" is much to bright in the sketch. But it was there and it changed my picture of the famous galaxy as well.

[Ivan Maly]

I should let Howard answer, but my reading of the two sketches is that Uwe's twist (I remember this sketch) is the inner arms, outlined in the NE by A49-54 and in the SW by A78-82. 49-54 are seen individually in 16", but the opposite arm (their relative brightness is the opposite of the outer arms that I believe Howard depicted) is so bright that the individual associations are plain even in 4" (probably at ~70x but I don't remember any more).

[Howard B]

That's what I saw Uwe and even though the twist may be shown too brightly for what you saw, it is rather accurate for the view through my 8 inch. Excellent sketch, hopefully I can improve mine in the next couple of months.

[Howard B]

I just came across this image of M31 - a mosaic taken by the Mayall 4 meter scope - that shows the twist. More subtly than the view through my 8 inch, but have a look: http://tinyurl.com/oljvo5e

[Ivan Maly]

These are the "inner", bright arms. The outermost, faint arms are only traced by a few HII regions in this photo. I really thought from the proportions in your sketch, Howard, that you saw the outer arms.

[Uwe Glahn]

Ivan,

for what I interpret Howard and my sketches these structures are the outermost arms.

I found an photo of Roger Clark which shows the arms in green color.

http://clarkvision.com/galleries/gallery.NEW/web/m31_300mm.17min.c09.26.2014.IMG_2327-44.f-bin4x4c1.html

These different outer arm colors could also be the answer why it is visible more easily than (deep) photographs show. the outer arms, or what Howard and I saw seems to be very blue from the young star clusters and this blue could be detectable "relative" more easily than the red older central parts or better the bridges from the very bright central region to the outer blue regions.

[MelBartels]

Howard Banich has happily dragged me into this fine discussion. Here's my sketch of the Andromeda Twist along with a Milky Way feature that I call the Andromeda Shelf. M110 floats beautifully above it.

1351

The 6 inch f2.8 has a 4+ degree field but with 6 inches aperture, so many if not most objects I view through it I see new features and structures. Wonderful in dark skies!

Howard's 8 inch produces views that fit nicely between my 6 inch and my 10.5 inch f2.7. It's a very nice sequence of scopes because each scope reveals something unique.

Mel Bartels

[Jimi Lowrey]

Howard would you post a picture of your richfield scope? I am sure lots of us would like to see it.

[Ivan Maly]

I second that, and:

Uwe: Thanks for the Clark picture and the clarification. I labeled what I meant in Aladin - see below. The twist of both labeled arms on the SW end well past the major axis, with a dark space centrally from their tips, is plain in a conventional instrument such as my 16". The NE end is, well, messier, and this is where the wide field helps to see a symmetrical twist, I guess. Depending on the aperture of the wide-field instrument, I imagine, the "inner" or the outermost arms can produce the "twist" appearance.

1352

http://3.bp.blogspot.com/-VRehELueZC8/VAioYWGRVQI/AAAAAAAABcA/9zpy3-2T-XQ/s1600/M31%2Barms.png

[Howard B]

Here you go Jimi: http://bbastrodesigns.com/osp14/osp14.html

Scroll down this page a bit and you'll see a couple of photos of the scope while I'm talking about it during Mel's Telescope Walkabout at the OSP a couple weeks ago. The small table it's sitting on was borrowed from my living room (with permission!) but will be replaced with a portable base I'll make in the near future. Also, the entire thing will be reworked to be airline portable so it can go to Hawaii. Or Texas!

[Ivan Maly]

It is not an economical alternative, but can one get the same views by sticking an eyepiece into a Takahashi Eclipse astrograph (180 mm f/2.8)?

P.S. Epsilon they are called, not "Eclipse", sorry.

[MelBartels]

It is not an economical alternative, but can one get the same views by sticking an eyepiece into a Takahashi Eclipse astrograph (180 mm f/2.8)?



But $5400 + shipping (US dollars) for the tube assembly alone? I built all three of my fast scopes for $600, not counting the expensive precision focusers and eyepieces/filters/coma corrector. Parabolization is 'just so' but not 'hard' compared to slower focal ratios. The best way to get one of these fast mirrors is to make it yourself.

The goal is an apparent field of view of 100 degrees at an exit pupil of 6-7mm. I prefer the 21mm Ethos over the 25mm ES: it's a tad sharper and a tad brighter. Taking the 1.15x of the TeleVue P2 coma corrector into account, that means the telescope's focal ratio is best set to f2.6 to f3.0.

The views are special. Though my sketching isn't, you can get some idea of what is visible at my webpage, http://bbastrodesigns.com/6inchF2.8/6%20Inch%20F2.8%20Telescope.html.

Mel Bartels

[Preston Pendergraft]

What a unique telescope and an awesome observation report.