Planetary nebulae - what's the knack?

[Hungrymark]

Just been out with the scope and had what was to me (I've been observing with a scope (8" dob) for about a month) a great session. M31 and M42/43 as a warm-up, then M36-38,  all great. Then found M1, which I've heard can be tricky so I was pretty pleased,  as I was with M81 and 82. A few of the Orion dobsonian doubles from 'Turn Left At Orion' split, and I felt like a king. So, dander up, I thought I'd have a go at the Eskimo Nebula and I'm blowed if I can see anything. At all. As I was when I had a look for M57 a few weeks back. Is there a knack for planetaries? I don't have a filter but am considering a UHC.

[Merlin66]

The renowned Aussie observer - Hartung (" Astronomical Objects for Southern Telescopes, p38") used a small 60 deg prism held behind the eyepiece to assist him in finding and observing small planetaries.

The prism breaks the surrounding starlight into small spectra, whereas the planetary appears as a small star like object. ( due to the limited emission wavelengths)

Try it, you'll see it works very well......

 

[niallk]

Fascinating post Merlin66!!!

To the OP, it can be tricky! Especially for the smaller ones.  Makes it very rewarding though when you do find them

I find it is a balance between finding them (lower mag makes it easier to recognise where you are, and sweep around without getting lost), and having enough magnification to discern that something is non stellar and fuzzy. Planetary nebs often require piling on the power.  I tend to switch between direct and averted vision a lot to try to spot a halo of fuzziness around an initially stellar looking object.

A filter can be a great help - I lean towards an O-III if you were to purchase only one, but it might be good to eventually plan to get both as funds allow.

[Victor Boesen]

1+ for the Olll

I have just come inside after a session where I observed the little dumbbell with my 10 inch dob. Without my Olll filter the nebula was really faint, and difficult to find, but when I put in my filter, the nebula was way more dramatic, and hints of the outer arms and detail in the core could be decected. So if you ask me, I would say either a UHC or both a UHC and a Olll would be a must have, not only for planetaries but it also improves the view on M42 and other nebula.

Victor

[Joe Adlhoch]

Hungrymark, I agree with the comments above. An OIII filter is a must in my book for many planetaries. A UHC filter is a close second. Many planetaries are small and can be almost indistinguishable from stars at low power. High magnification helps!

[niallk]

Here's a great report on the performance of various filter types on a selection of nebulae by Dave Knisely:

http://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/

[Merlin66]

Guys,

for the occasional visual observer, a $10 prism will work as well as a $100 filter.

 

[Hungrymark]

Thanks all. I'd been reading about filters and was under the impression that a UHC would be better for general use, with an O-III more useful for certain targets? And that's an interesting prism suggestion, I'm going to borrow one from work.

[Hungrymark]

Such an idiot, I've just noticed the thread entitled 'Planetary nebulae - a little guide' literally next to this one, and which contains all the information I could need. As you were! 

[Stu]

12 minutes ago, Hungrymark said:

Such an idiot, I've just noticed the thread entitled 'Planetary nebulae - a little guide' literally next to this one, and which contains all the information I could need. As you were! 

Not at all. I've been around here a while and had never noticed that before!!  

Good luck with finding them. I will say that if you managed M1 then you should easily spot M57 and the Eskimo.

I think size can be the most confusing thing, they are often very small and as the other guys suggested, can take some mag to make them look on stellar. Three screenshots from SkySafari attached, at the same scale showing how much smaller  M57 and the Eskimo are than M1.

[Hungrymark]

Thanks for that, that's helpful info. I think it's the size that's confusing me. I'm aware that they tend to look like slightly fuzzy stars at low mag, so it's a case of 'well, which one?'. There are loads of slightly faint stars in my field of view and I'm not yet experienced enough to know my way around the fainter ones, so I'm not sure which one I should be going in on higher power. Oh well, onwards and upwards! 

[John]

That is the challenge with many planetary nebulae - they are rather small targets !

Many need high magnifications to differentiate them from a slightly fuzzy star. Even M57 (the Ring) can be overlooked if viewed through a smallish aperture scope at low power. M27 (the Dumbell) is rather exceptionally large. 

A good star atlas is a great help in picking "which star" but sometimes even then you need to go from suspect star to suspect star applying some magnification and possibly a filter to see if it's really something else

 

[Hungrymark]

Thanks John. I've got the Cambridge Star Atlas and the Sky and Telescope Pocket one, as well as Turn Left At Orion. 8" dob as well in pretty dark skies, so the only thing I have to blame is my own incompetence haha

[laowhoo]

5 hours ago, Merlin66 said:

Guys,

for the occasional visual observer, a $10 prism will work as well as a $100 filter.

 

Not only that, but I can't wait to see the look on the kids' faces. Teachers should be happy too. This has been a WHOAH!  moment.

(Just saw a BBC history of astronomy docu w/ Jim Al-Khalili, and they had Newton holding a magnifying glass while looking at the prismatic effect of light that he'd just experimented for. Jim yelled, "Put the magnifying glass up to the light!" but it took another hundred years to discover spectroscopy.)

[Nyctimene]

7 hours ago, Hungrymark said:

the only thing I have to blame is my own incompetence haha

Not incompetence - inexperience!  As a newbie long ago,I would have been very proud of having observed M 1, 31, 42/43 and 81/82 within the first four weeks, as you did. The Eskimo nebula is well within reach of your 8"; you will be able to observe it even without a prism or UHC filter (the same goes for M 57); try to spot a star image, that is not quite as sharp/focused as the neighbouring stars; then crank up the magnification. Of course, a decent UHC filter (don't scrimp on this - Astronomik filters get good reviews) will help with the details and the "Eskimo hood". You can also try  the "Cat's Eye Nebula" NGC 6543 in Draco, mag 8,1; good luck with the hunt!

Stephan

[Moonshane]

I agree that they often need to be observed with 100x or more to even distinguish then as anything other than stars. I bit like M32 I suppose.

From my perspective, the trick is often not to look at them. By this I mean that using something called averted vision often makes them 'pop'. Wither averted vision the nebula puffs up and the central star (if obvious) disappears. Look at the central star and the nebula disappears again. This is a good characteristic of a PN although not all do it. Sometimes it's also mist on your eyepiece 

 

[Hungrymark]

YEAH! BAGGED ME AN ESKIMO! 

Now there's a sentence i never thought I'd say ?

[Stu]

1 hour ago, Hungrymark said:

YEAH! BAGGED ME AN ESKIMO! 

Now there's a sentence i never thought I'd say ?

Nice one, well done!

[Joe Adlhoch]

Congratulations! I think the Eskimo is one of the better planetaries.

[laowhoo]

On 1/7/2018 at 03:28, Merlin66 said:

Guys,

for the occasional visual observer, a $10 prism will work as well as a $100 filter.

 

Merlin, we found a prism last night but wanna be clear before we buy cuz it's sorta lab-grade/BK7 and all that--how to use? Just take any old hypotenuse/triangular prism and what? Lay it on the EP? They come in all shape and sizes, so we wanna get the right one, and we're at least covering the field stop. Thanks again for the great idea.

[Merlin66]

It needs to be a 60 deg dispersion prism.

Focus the eyepiece as usual, and hold the prism with one face slightly tilted behind the eyepiece, look through the prism at a slight angle and you'll see the stellar spectra.

Two minutes practice, and you'll see what I mean.

I actually mounted a small 25mm face prism across an old plastic film cassette, at  the best angle, with a cut-out "viewing port" and this just fits over the end of the eyepiece.....

I'll upload a couple of images showing my arrangement later.

 

 

[laowhoo]

Thank you Merlin. Here it is

http://www.ealingcatalog.com/right-angle-prism-lab-q-bk7-35mm.html

Look forward to your pics, and thx for the detailed explanation. We'll have a few months practice b4 our dark sky trip.

[niallk]

Cool I'm going to try this too! 

[Merlin66]

No, that would be the wrong prism....you need a 60 deg equilateral prism.

http://www.ealingcatalog.com/30-mm-equilateral-prism-bk7-glass.html

The prices are pretty high... I'd recommend trying Surplus Shed; they always have loads of neat stuff cheap.

https://www.surplusshed.com/pages/item/L1851D.html

There are also some flint (F2) prisms which are even better.

The last image shows the prism sitting on a TV eyepiece with the "viewing port" towards you.

 

[laowhoo]

 

Ours was a clearance item so not sure we can do a straight return. (Man, that Surplus Shed is a bargain. Gonna have to scrounge around there. Thanks.) Those pics are great. We're waiting to hear back.

Well we couldn't do a return, but they agreed to exchange for the one you found. Thx Merlin. Even intercepted it early in transit so we'll save some postage.