Size of M31 in my 8" dob

[Vaizki]

Ok deep breath..

First light on my first scope (SW 200 dob) yesterday and I sought out M31 by finding Cassiopeia, then the Pegasus square and Andromeda was easy through there.

Used the 25mm eyepiece that comes with the scope, not a great performer I'm sure and listed at 52° aFoV. I used Stellarium and the ocular view to check what I might expect in terms of size and I saw the following data from it:

- Magnification 48x

- FoV: 1.0833° (but on the bottom of the screen it says 2.08° - which is right?)

From the M31 item data I saw:

- Size +2°58'00"

AND in the Stellarium ocular view I'm seeing M31 fill the field of view with the edges of the disc outside the view. Which was expected when comparing size / fov.

BUT when I look through the real world scope, I'm seeing a much smaller object, something comparable to the size of M110 in Stellarium's simulated view.

Where did I go wrong? Or is this just a case of bad seeing which is losing everything but the brightest center of M31. The weather was not great, it had rained an hour before I went out.

Oh and I did re-find the object 3 times trying different routes because I though it might not be M31 after all. But I still might have gotten it wrong - though I doubt it

[Spec-Chum]

Or is this just a case of bad seeing which is losing everything but the brightest center of M31.

That

Keep starting at it for a while, you should see more and more of it "appear" in your 200p M31 will appear to run from about 1 o'clock to about 7 o'clock and fills up the 25mm EP.

Oddly, as stated on another thread, I find I can see more of the darker arms when using my UHC filter. I found this out by accident after I left it on and forgot. I wondered why everything was purple lol

[miguel87]

You certainly found M31, but as you said, what you were seeing was the core of the galaxy and some minor detail of the spiral arms. I'm not sure exactly what kind of site you are viewing from but it would have to be pretty exceptional to match the images on stellarium, even if you set light pollution to 10/10 on that program, objects such as M31 are certainly over exaggerated. Did you allow 20mins for your eyes to adjust? (it's amazing the difference this can make).

[Stu]

Hi vaizki

The correct numbers are x48 magnification and 1.0833° fov based on 1200mm focal length scope, 25mm ep with 52° fov.

Mag = 1200/25 = 48

Fov = 52/48 = 1.0833°

Andromeda is a huge object, but unless you are in nice dark skies, most of the outer regions just get lost amongst the light pollution so you don't see them. It is most likely that you did see M31 but all you could see was the central core which is much smaller.

Hope that helps

Stu

[RikM]

It's not bad 'seeing' (which is unsteadiness of the atmosphere causing blurring at high magnification), but it is probably just the light pollution or the general sky brightness has washed out the rest of the object. It never looks much more than a faint oval smudge in even the biggest amateur scopes and the darker and clearer the sky the better. The earlier rain probably helped since it tends to wash out a lot of the dust in the atmosphere. You can often get the clearest views shortly after a spell of rain.

M31 is one object that I find looks better in a decent set of bioculars than a telescope.

[Matt Scunthorpe]

As the others have said, what Stellarium shows is not going to be what you see in reality. The book 'Turn Left at Orion' is good for this kind of thing if you dont already have it. Shows you how find many objects with a diagram of what you will actually see through the EP.

[Vaizki]

Well this was a simple first try out from my yard in the "city" but it's right on the city limits and pretty dark sky. No streetlights etc fortunately. So less than ideal and with the rain leaving moisture behind I'm sure it was about as bad as it can get then My real site will be at the summer cabin though where there is much less air pollution though I'm sure the sea will create some moisture issues.

Is there an easy way to visually evaluate "seeing" and how is it expressed so as to give people here an idea about the conditions? Naked eye + faintest star magnitude?

I could see the milky way but faintly, only close to the zenith. Pleiades was unfortunately behind trees so couldn't really check how I could see stars there. I'm sure I spotted M31 with the naked eye after my eyes adjusted. I probably didn't get to 20 minutes of continuous adjustment as I used my red flashlight to check the star atlas too often due to my doubts.

If I got my jargon wrong please feel free to correct me but no lashes please

Good to know my finding skills are not completely hopeless then.

[Vaizki]

Yes I wasn't expecting the kind of detail or any colour like Stellarium shows, just maybe the general size of the thing to be comparable but it was more like 1/10th of it or smaller. I don't think we have a lot of dust in the air here in Finland usually but the air was very moist.

I'll take a look at the book too, thanks for the tip!

[Matt Scunthorpe]

I gauge the setting conditions by how much stars are twinkling. Crude but affective lol.

[Stu]

I guess how much of Andromeda you see is down to how dark your site is, and how transparent the sky is. Clear nights vary greatly in terms of the number of stars visible depending on the conditions. I must say, we have had a run of quite a few good ones recently.

Stu

[Stargazing_Cliff]

I could see m31 just little bit using me small newt and 25mm ep

I even set occulation on stellarium no way does it show it like how occulation does.

probably look lot better if wasnt for road orange glow street lights.

[acey]

The quoted size of M31 is based on photographs, it never looks that big to the human eye, even from a very dark site. The figure usually quoted is around 3 degrees, visually it's more like one degree, but if only the core region is visible then it's a lot smaller.

[Aenima]

This '7-timer' weather site is excellent for checking conditions in your area like seeing and transparency - hope the link works.

http://7timer.y234.cn/V3/index.php?product=astro&lon=-3.594&lat=50.417〈=en&ac=0&unit=british&tzshift=0&site=

Regards

Aenima

[Capricorn]

To get all of M31 in view you are going to need a long focal length eyepiece or a wide one, probably both.

Equally with a 200mm scope you should get enough light in the image mainly because the image will be small.

M31 is about 3 degrees to get that in view the magnification will need to be low,

Even at 26x that would be only 2 degree view and means a 45mm plossl. So to see all will take some doing and wide eyepieces tend to have poorer edge performance simply because they are so wide.

With an 82 degree ES to get all M31 in mens a magnification of 26 so a focal length of 46mm.

M31 is big and most do not realise the problem of seeing all of it.

[Stu]

31t5 nag in my 106mm refractor gives 3.6° field. From a very dark site, you can detect the outer regions of M31 but it's pretty subtle.

Stu

[SeedyF]

Through my old 150 I could hardly see anything, through the 250 two weeks ago I was amazed how big it is! Obviously before I was only picking out the brighter core but the 250 and a high position in the sky meant I was able to see much more of it.

I've had a few moments since getting the 250 and that was definitely one of them

[ismangil]

I would suggest finding globular clusters, they certainly looks more impressive in telescope rather than his.

I use bins as well, so speaking from experience - M31 is more impressive in its entirety in the wide field of my 10x50, much like M45 as well.

On mobile (excuse the strange predictive words...)

[Vaizki]

Bins.. bins.. bins.. I knew when I got the dob that good bins would be in my future.. and you lot are not helping!

15x70.. or just 10x50.. and on a local forum a nice pair of 25x100s for sale. Argh!

Oh and Chris, do not tempt me with bigger newts, I just got this one

[Vaizki]

Oh wow.. Aenima, that 7-timer site is trying to get me to quit the hobby:

[RikM]

Bins.. bins.. bins.. I knew when I got the dob that good bins would be in my future.. and you lot are not helping!

15x70.. or just 10x50.. and on a local forum a nice pair of 25x100s for sale. Argh!

Oh and Chris, do not tempt me with bigger newts, I just got this one

Biggest lens size you can afford and stick with a low power 8 - 10x is fine. Too much magnification and you restrict the field of view and can't hold them steady enough.

[umadog]

I was lucky enough to attend a truly dark star party last week and when M31 was near the zenith, M110 looked like M31 one does from light polluted skies. M31 easily filled a 1.2 degree eyepiece field. I didn't make an estimate at the time, but I'm sure I was seeing M31 extend to about 2.5 degrees of angular extent. The outer regions are visible if you place the brighter regions out of the field and rock the scope back and forth. M33 was even more stunning: HII regions everywhere and the faint outer spiral arms were visible.

[jabberwocky]

At kelling I could see M 31, M32 and M110/NGC205 all in my 38mm 70* eyepiece, fantastic view, couldnt take my eyes off it, low power wide field Is def the way to go, much better than through my binos. Even though the eyepiece is a cheap panaview copy the view was great, I'm sure through some high end glass it would be even better