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Damo636

M57 Central Star

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Quick question guys, is the mag 15 star at the centre of M57 doable in a 12" dob? I tried last night but the dew put me out of it before I could get a proper stab at it !

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Hmm! This is a very tough challenge. It has been suggested that it may be variable too.

Never seen it myself even from very dark skies with my old 16" perhaps others have?

I think it requires around 18" to be sure of seeing it.

Perhaps others know better?:)

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I once saw it in my mates 22" f5. even in that it was not obvious.

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Thanks guys, I had a feeling this one was a tough cookie! However, I'll have another go tonight even though its highly unlikely I'll spot it (clear two nights running over here :icon_cheers: )

God loves a trier :grin:

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I think one of the problems with observing it is contrast, the centre of the ring nebula isnt quite dark it glows slightly this makes picking out this star notoriously difficult.

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Isn't the maximum mag for a 12" aperture 15.1?....In theory I mean.

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Isn't the maximum mag for a 12" aperture 15.1?....In theory I mean.

it's probably one of those perfect optics, collimation, seeing, darkness and eye quality things.

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Thanks to this question I've found the following page which struck me as interesting:

http://www.cruxis.com/scope/limitingmagnitude.htm

What really surprises me is that according to their model, large increases in aperture don't make very large increases in limiting magnitude.

James

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What really surprises me is that according to their model, large increases in aperture don't make very large increases in limiting magnitude.

James

I noticed that when I went from an 8" 13 something mag to a 12" 15.1? Couldn't figure it out but don't think I will ever get all the points like Shane mentioned in one night anyway.

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To think i had myself convinced I would never need anything bigger than the 12", who was I kidding :rolleyes:

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To think i had myself convinced I would never need anything bigger than the 12", who was I kidding :rolleyes:

Completely agree, we may aswell face the fact that sometime soon we are going to be building our own monster dobs. :cheesy:

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According to that site, even my ST120 can manage 13.something. Given what I know I've seen in fairly poor conditions I don't think that's too implausible either.

James

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Completely agree, we may aswell face the fact that sometime soon we are going to be building our own monster dobs. :cheesy:

The problem is, if you're going to make your own monster dob mirror ideally you really need to get some experience with some smaller mirrors first. And if you already have a 12" scope, who wants to go back to a 6"? :)

James

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Well I just don't understand it? Hope someone can explain?

According to that site, even my ST120 can manage 13.something. Given what I know I've seen in fairly poor conditions I don't think that's too implausible either.

James

Edited by Mike73

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To think i had myself convinced I would never need anything bigger than the 12", who was I kidding :rolleyes:

Oh dear! Didn't take long for the aperture fever to return, did it? :)

Completely agree, we may aswell face the fact that sometime soon we are going to be building our own monster dobs. :cheesy:

Good call. Don't buy, build your own, you'll get a much better product than the budget scopes in the larger sizes.

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As a rule of thumb, doubling your aperture adds around 1.5 magnitudes to the faintest star shown, this is why you need increasingly large optics to make much apparent progress visually, magnitudes are logarithmic, not linear. As far as the M57 central star is concerned, I seem to recall that it's spectral class is another factor that makes it a difficult visual object .

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According to that site, even my ST120 can manage 13.something. Given what I know I've seen in fairly poor conditions I don't think that's too implausible either.

James

That sounds plausible. I managed to pick up SN 2012aw in M95 earlier this year with my ED120. It was around mag 13.2 at the time.

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Not only is it faint but a very blue star (class O or B) which our eyes are not as sensitive to as white, yellow and red stars. So detecting it is very difficult visually even in monster scopes.

Edited by Mr Q

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Hmm! This is a very tough challenge. It has been suggested that it may be variable too.

Never seen it myself even from very dark skies with my old 16" perhaps others have?

I think it requires around 18" to be sure of seeing it.

Perhaps others know better? :)

i could have swore blind i saw it a few weeks back from our dark sight but no one else could confirm it so i ended up doubting myself but the conditions were the best i have seen in a long time some at the local society have seen it through the 30" we have there :p

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Phil Harrington discusses this as Challenge 171 in his recent book "Cosmic Challenge." He says that with his 18" reflector, he's seen the central star from his suburban backyard in conditions of perfect atmospheric transparency and seeing. Seeing this star "requires transparent skies, but not necessarily dark skies."

From a really dark site, under perfect conditions, he says that telescopes as small as 11" in aperture have shown the central star.

This is a great book, by the way, and any deep sky observer with an interest in questions like this should have a copy.

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Thanks for the heads up on the book Will, sounds like an good read. Its encouraging to hear that the central star in M57, although extremely difficult, may still be possible under optimum conditions.

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With the central star of M57 you're trying to see it against the faint central haze of the nebula itself (and with the bright outer ring very close by), rather than against a perfectly dark sky, and that's what makes it harder than its magnitude alone might suggest.

Also, don't put too much faith in online limiting magnitude calculators. They're based on a mathematical model of a statistical average. In other words, the average person, under all the specified conditions, would be within a certain margin of the calculated number. The margin is non-zero and not everybody is average. Bottom line: go look for yourself and find out what your limit is, given your particular eyes, scope and site.

And with regard to magnitude, somebody said that going from 13 to 15 is not much. A two magnitude difference is a heck of a lot - the difference, for example, between a light polluted town (limiting mag 4) and a dark sky (mag 6) where you can clearly see the Milky Way. One magnitude difference is a factor of 2.51 fainter: a magnitude 13 star is about 6 times brighter than a magnitude 15 one. Increasing your aperture by 50% (e.g. 4 inch to 6 inch or 8 inch to 12 inch) will increase light grasp by 125%, and that should turn into about 0.9 magnitude difference. That's enough for a "wow" factor. Doubling aperture gives roughly a further 1.5 magnitude, and that's a super-wow. Increasing your aperture inch by inch means a proportionally smaller magnitude gain as the aperture gets bigger: going from 4 inches to 5 inches is a 56% increase of light grasp, and would show stars about half a magnitude fainter. Going from 30 inches to 31 inches is a 6.8% increase, which would mean about a 0.07 magnitude gain.

Moral of the story: when considering upgrading think in terms of aperture ratio rather than number of extra inches. 50% is the minimum I'd consider, anything less in my opinion isn't worth the extra weight and bulk. I went from an 8-inch to a 12-inch and my next scope (if I ever get to that stage) will be an 18-inch.

Final point about the central star of M57: according to Wolfgang Steinicke's superb history of the NGC ("Observing and Cataloguing Nebulae and Star Clusters") the star was first reported by Friedrich von Hahn in 1800. He said that with a 12-inch reflector he had seen a central star in 1795, but now he could no longer see it. Hahn thought the nebula itself had changed. Possibly it was a spurious observation: the star is a challenge in 12 inches and I've never managed it with mine, though I can see stars to mag 15. Admittedly I've never tried very hard, maybe I should give it another go.

Steinicke adds that Lord Rosse found the central star "pretty bright" in his 72" Leviathan (first seeing it in 1848). Secchi and Schultz saw it using 9.5" refractors in 1855 and 1865 respectively. Vogler failed to see it using a 27" refractor in 1884, Keeler saw it with the 36" Lick refractor in 1892. It was first photographed in 1886 using a 10.25" reflector.

Brian Skiff of Lowell Observatory reckons 8-10 inches as minimum required aperture and has an interesting page about it.

http://www.hacastronomy.com/m57.htm

Edited by acey

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I know people who have seen the central star with a 12.5" reflector but it gets easier as aperture increases (as you might guess). I believe 10" is the realistic minimum aperture.. The difficulty is poor contrast because you're trying to see a star against a background of nebulosity. You therefore need high power to see it. Think >300x. So you'll need seeing that can support that power. If the seeing is bad then the star will blur into the surrounding nebulosity and you're not going to see it. You need to study the object over a long period in order to get a sighting. If the seeing is bad then no aperture will be able to bag it.

There have photometric observations which indicate that the central star is not variable. It's magnitude 15.1. To check that the instrument is able to pull that in you can use this reference image: http://www.cloudynights.com/ubbthreads/attachments/4289596-M57MagSequence1.jpg

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I was reading something from one of the guys in the States who works at OPT, he claims to have seen the star in the middle of M57 with a 12 inch Dob, but one that had been re-polished to a very high PV, I think it was 1/14 PV, something crazy, still that America, if it can be done then why not ! :shocked:

Alan.

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You will notice that in the limining magnitude calculator, much depends on the expertise of the observer. under mag 5.5 skies it states that a novice could see mag 13.6 objects with my C8, an average observer 14.2 (which is often listed as limiting magnitude for a C8), and an expert 14.9. I have gone down to mag 14.2 from my suburban garden (mag 4.5-5.0 skies), so this chimes with my experience (at least, I think I can by now count myself as an expert ;) ).

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