M13 - is this normal?

[Nick P]

Just managed to get an hours observing in, between the clouds rushing over, and got my first view of M13, but I was surprised how dim it seemed in an 8" reflector.

At an apparent magnitude of 5.7 I expected something much brighter - I couldn't resolve any stars in my TV 32mm plossl (28x) and it disappeared in the TV 8mm (112x) , with best views coming from my old meade 4000 15mm! (but still no stars resolved)

Collimation seemed OK, all stars resolve to points, and had given plenty of cool down time, I am wondering if its an effect of poor visibility - although stars were not twinkling which I thought meant conditions were quite good - or has my eyesight deteriorated drastically now I am over 40

How do you normally gauge whether the seeing is good/average/ or poor?

[toml42]

first of all, what's your light pollution levels like?

secondly, Averted vision is your friend! i have an 8" too, and the first time i looked at m13 i didn't see any stars either, the trick is to not look at it...

counterintuitive, i know, but if you look a few degrees away from what you actually want to look at, it becomes much brighter and more detailed. this is to do with the rather silly way our eyes are assembled for nightvision

the first time i tried this with m13, the stars seemed to burst out of the cloud! now i can see stars in m13 without averted vision, i think it's a matter of training your eyes, but av always gives an extra 'pop' to what you're looking at. Using av i can see stars in globs much fainter than m13

personally, i feel that in my f5 200p, m13 looks its best at 125x with my 8mm 60' eyepiece, though you may find otherwise

good luck!

[richbyers]

Had your eyes adapted? if you are using a star atlas get a red torch to keep it when searching for things, it sounds obvious but can make a real difference.

For instance once i was viewing the moon and after taking my eye away from the eyepiece it could harldy see any stars, but close that eye and the other was perfect.

rich

[Nick P]

Hi Tom, thanks for the response, light pollution is a problem, although M13 was at around 45 degrees so appeared to be well out of the 'glow'. Tried averted vision, still very little detail as it was so dim.

Hi Rich, thanks, eye adaptation may be a consideration as my viewing spot looks over ground level lighting in a harbour and shop lighting, which I cant avoid!

[Doc]

I have never owned an 8" dob but I'm surprised that 200mm of aperture cannot resolve a few of the stars in M13, I used to have a 10" and that resolved quite a few form a LP back garden. As others have said, maybe observe M13 for longer, train your eyes and wait for those moments of good seeing.

[Nick P]

This is what surprised me too Doc, the view was similar to my old 90mm refractor - admittedly that was used under dark skies - I would have thought more resolution from the hugely increased light grasp would be expected.

Is it me or is it the scope!

[Llamanaut]

i saw m13 in my 15X70 bins last night, dim but unmistakble

[iamjulian]

Was looking at M13 night before last. 6" scope. At low magnification is is just a blur, but once you get up past 100x the outer stars are very sharp and as your eye moves around to take in the view, averted vision makes different stars visible so the whole thing appears to twinkle. At least, that was my experience. You have more light grasp than me so it definitely isn't the scope. My EP was a basic plossl. My sky isn't very dark, limiting mag about 4.8

Perhaps it was a bad night for transparency or seeing or whatever it is called?

[Ags]

I'm surprised you could not resolve stars in a 200mm scope - I can definitely resolve the stars in my 100mm scope from my back garden with nearby street lights. Maybe there was a thin layer of high level cloud ruining your view?

The other thing that occurs to me is that more aperture also magnifies light pollution - maybe you need to go to a higher magnification to attenuate the LP?

[Nick P]

In the words of Sherlock ' eliminate the impossible and whatever remains no matter how improbable must be the solution'! The scope should definitely resolve stars in M13, seeing was OK (no twinkling that I could detect), collimation was OK (pinpoint stars in the EP), other observers have resolved stars in M13 with light pollution equal to mine (and applying additional magnification with the 8mm plossl to 112x just lost the image).

So I think that leaves three alternatives,

a problem maintaining night vision due to the ground level light pollution I have to look at when observing.

a thin high level cloud cover - which is quite possible as we have had an awful lot of cloud lately and the satellite picture on SAT24 showed I was looking through a hole in a huge bank of cloud.

OR

The mirrors knackered and isn't reflecting at its optimum level - are there any easy tests to check this?

Would anyone agree with my logic here, or have I missed something blindingly obvious?!

[ollypenrice]

Last night it was just visible to me naked eye, blinking in and out with averted vision. I am pushing sixty with lousy eyesight but I live in a dark place. On a good night direct vision is possible. An 8 inch at a dark site will resolve the outlying stars. A big Dob will resolve to the core. It is all down to LP, I guess.

Olly

[John]

I'm another who is suprised that you are not resolving the outer parts of M13 into stars with an 8" scope.

My 4" refractor will resolve some stars, my 6" quite a lot and my 10" newt really shows it well. The view through a 20" dob at the SGL star party in April was jaw dropping !.

Stuff like high level cloud can impact resolution though and you are not always aware that it's there. Light pollution does not help either.

[Nick P]

Spent the last hour or so observing again and much better results tonight!

The 32mm plossl (28x) shows a bright round blur (as last night)

The 15mm plossl (60x) shows a larger more irregular round blur but I could detect several bright points within the structure - oddly within the core too so this may have been an optical illusion.

The 8mm plossl (112x) - The Wow moment - everything 'popped' into view at focus - a much larger irregular shape made up of chains of bright stars linking with less resolved stars to form tendrils of light radiating from the brighter core - the overall shape reminded me of a crab!

Many thanks to all of you for your help and advice, perseverance paid off, and a) taking much more care of my night vision, and observing a bit earlier at a higher altitude to get rid of as much LP as possible - and of course, there was less cloud about...

Best

Nick

[toml42]

glad to hear you had more luck this time i think it looks a little like a starfish...

[Nick P]

Thanks Tom, I can appreciate what you said about using higher magnification, quite stunning and nearly filled the useful fov of my 8mm!

[ollypenrice]

Ah, this is a heartwarming ending! M13 is just out of this world at high power.

Olly