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Old 07-04-2014, 08:46 AM
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Varangian (John)
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The elusive (for me) Centaurus A

Hi all,

I'm having a problem and it's frustrating. I've been looking for weeks for Centaurus A without any luck at all. I'm using a push to azimuth setting circle system on my dob with a wixey altimeter and I KNOW I am in the right location because the co-ords of other targets (Omega Centauri etc.) I'm using are spot on - so I'm definitely looking in the right area of sky.

The main constraint at this stage is that I've been searching on a new moon from suburban Melbourne (approx 10km north of the city) and while LP washes out faint objects at 7.5mag I should be able to pick the Hamburger up (shouldn't I?)

I'm using averted vision and I'm being patient (whole glass of wine patient) with no luck... With my 12" Dob I've used both a Superview 30mm 68d and a ES 14mm 100d without any luck.

Does Centaurus A demand a dark sky site? I'm finding it hard to get away at the moment with two young kids. Should I just shelve this target until I can get out of town? Tips welcome....
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Old 07-04-2014, 09:24 AM
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jenchris (Jennifer)
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I'm niot sure, but I know even my long exposures were very faint and I had to add them in DSS before I got a good look at it.
This was about 3 mins I think on ISO800
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Click for full-size image (centA_7550.jpg)
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Old 07-04-2014, 09:31 AM
astro744
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Do you have a good star chart? If so star hop with your low power eyepiece until you get to the area where it should be. Work in triangles starting with the brighter stars and look for star patterns on the map and then see if you can spot the same patterns in the eyepiece at low power.

If you are using a Newtonian then the view in the eyepiece will match the star chart (rotate as needed). There is no reversal of the image.

Centaurus A looks like a hamburger from side on and is not too difficult but maybe fainter than you are expecting. Once you see it you'll wonder how you missed it.

Note your field of view with your 30mm/68deg eyepiece is approx 1.3 degrees in your 12"/1500mm 'scope.
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Old 07-04-2014, 09:45 AM
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mental4astro (Alexander)
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Following on from astro744: Centaurus A is a big DSO. The curse it has is the 7mag quoted is its integrated magnitude. Being so big, it ultimately has a low surface brightness. Trying to spot Cent. A from light polluted areas can be one of the most frustrating exercises there is, no matter the aperture! For me, it was easy to spot it with an 8" scope 20 years ago from inner Sydney. Today I'm lucky to spot it with my 17.5".

Funny thing though, you mentioned dark skies. I've seen Cent A from a dark site through just a 6X30 finder!

Here in lies the biggest tip I can give you - drop your magnification, and DON'T look directly for it if you are in the big smoke. You need to slowly pan the field and rely upon your peripheral vision to pick up the faint extended glow initially. As soon as you look for it directly you won't see it. Once you've caught a glimpse of something suspicious picked up out of the corner of your eye, you've got it. Change to a medium power eyepiece and start a slow pan again for it. The two lobes of Cent A will be a faint glow. This galaxy can be one of the most frustrating objects to spot from the city. Out in the sticks, no problem.

Be patient. Centaurus A is a real challenge from the big smoke now. M33 is pretty much impossible, even though it is as bright, it is larger, and its surface brightness is abysmal as a comparison. But from a dark sky, and with perfect conditions, I've also seen M33 naked eye! Two real brain teasers.

Mental.
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Old 07-04-2014, 10:14 AM
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Quote:
Originally Posted by mental4astro View Post
Here in lies the biggest tip I can give you - drop your magnification, and DON'T look directly for it if you are in the big smoke. You need to slowly pan the field and rely upon your peripheral vision to pick up the faint extended glow initially. As soon as you look for it directly you won't see it. Once you've caught a glimpse of something suspicious picked up out of the corner of your eye, you've got it. Change to a medium power eyepiece and start a slow pan again for it. The two lobes of Cent A will be a faint glow. This galaxy can be one of the most frustrating objects to spot from the city. Out in the sticks, no problem.

Be patient. Centaurus A is a real challenge from the big smoke now. M33 is pretty much impossible, even though it is as bright, it is larger, and its surface brightness is abysmal as a comparison. But from a dark sky, and with perfect conditions, I've also seen M33 naked eye! Two real brain teasers.

Mental.
A faint smudge in a GSO 20mm Superview is how I found it in the 10".
It is faint in LP skies but it can be done. Find the asterism correctly then be patient and slow scan visually till you get that first glimpse.
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Old 07-04-2014, 12:40 PM
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Forgot to mention what Brent has alluded to when I said swapping to a medium power eyepiece. Once you've spot your suspect patch, upping the magnification increases contrast, so seeing Centaurus A should be a little easier. You might even like to increase the magnification further to see how the contrast goes again. At a dark site this is a good technique, and a surprising one too. Worth trying in the city too.
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Old 07-04-2014, 01:00 PM
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I can spot it in my 9x50 finder scope from my light polluted Wyong backyard.
BUT only when the moon is not around.

It is an easy star hop exercise.

From Beta Centaurus (Hadar, pointer closer to Crux),
move towards Omega Centauri.
Halfway there is another bright star (epsilon centauri).
Then the distance from that to Omega Centauri, move that again in same line,
and there it is - in the finder scope - Centaurus A

(Possibly easier to spot in the finder than the eyepiece)
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Old 07-04-2014, 03:26 PM
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Varangian (John)
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Quote:
Originally Posted by jenchris View Post
I'm niot sure, but I know even my long exposures were very faint and I had to add them in DSS before I got a good look at it.
This was about 3 mins I think on ISO800
Yes I think it is fainter than I thought and will present more of a challenge than originally anticipated.

Quote:
Originally Posted by astro744 View Post
Do you have a good star chart? If so star hop with your low power eyepiece until you get to the area where it should be. Work in triangles starting with the brighter stars and look for star patterns on the map and then see if you can spot the same patterns in the eyepiece at low power.

If you are using a Newtonian then the view in the eyepiece will match the star chart (rotate as needed). There is no reversal of the image.

Centaurus A looks like a hamburger from side on and is not too difficult but maybe fainter than you are expecting. Once you see it you'll wonder how you missed it.

Note your field of view with your 30mm/68deg eyepiece is approx 1.3 degrees in your 12"/1500mm 'scope.
Thanks for that. I've got Wil Tirion's star charts laminated as individual A3 sheets - may have to get them out.

Quote:
Originally Posted by mental4astro View Post
Following on from astro744: Centaurus A is a big DSO. The curse it has is the 7mag quoted is its integrated magnitude. Being so big, it ultimately has a low surface brightness. Trying to spot Cent. A from light polluted areas can be one of the most frustrating exercises there is, no matter the aperture! For me, it was easy to spot it with an 8" scope 20 years ago from inner Sydney. Today I'm lucky to spot it with my 17.5".

Funny thing though, you mentioned dark skies. I've seen Cent A from a dark site through just a 6X30 finder!

Here in lies the biggest tip I can give you - drop your magnification, and DON'T look directly for it if you are in the big smoke. You need to slowly pan the field and rely upon your peripheral vision to pick up the faint extended glow initially. As soon as you look for it directly you won't see it. Once you've caught a glimpse of something suspicious picked up out of the corner of your eye, you've got it. Change to a medium power eyepiece and start a slow pan again for it. The two lobes of Cent A will be a faint glow. This galaxy can be one of the most frustrating objects to spot from the city. Out in the sticks, no problem.

Be patient. Centaurus A is a real challenge from the big smoke now. M33 is pretty much impossible, even though it is as bright, it is larger, and its surface brightness is abysmal as a comparison. But from a dark sky, and with perfect conditions, I've also seen M33 naked eye! Two real brain teasers.

Mental.
Thanks Alex, that sure has put things in perspective. I as always appreciate your logic and approach on this. I need to take a deep breath.


Quote:
Originally Posted by Allan_L View Post
I can spot it in my 9x50 finder scope from my light polluted Wyong backyard.
BUT only when the moon is not around.

It is an easy star hop exercise.

From Beta Centaurus (Hadar, pointer closer to Crux),
move towards Omega Centauri.
Halfway there is another bright star (epsilon centauri).
Then the distance from that to Omega Centauri, move that again in same line,
and there it is - in the finder scope - Centaurus A

(Possibly easier to spot in the finder than the eyepiece)
Perhaps a better approach. I am very familiar with this part of the sky and especially with Omega Centauri so may have more luck this way than relying on the push to.

Thanks to others for their comment(s)
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Old 07-04-2014, 04:02 PM
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Quote:
Originally Posted by Varangian View Post
Thanks for that. I've got Wil Tirion's star charts laminated as individual A3 sheets - may have to get them out.
Tirion's Sky Atlas 2000.0 is an excellent atlas and highly recommended for beginners to intermediate. I have used the original colour deluxe version with stars to mag. 8.0 for many years and it is still the atlas I take outside most often. (The second edition has stars to mag. 8.5). If I need a few more stars I use Uranometria. I do not use electronic maps at the telescope since even in red light night mode they are still too bright ruining any chance of seeing anything faint.
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Old 07-04-2014, 09:08 PM
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Hi John,
I have seen CentA from the burbs in Melbourne with my 10" Newt and a 28mm ES 68º eyepiece + ES Coma Corrector. Can't help with the finding as I used GOTO. As Astro744 said once you see it you'll wonder why….well that was me, spent a couple of years trying with GOTO…. but in Melbourne I think it comes down to local conditions. I saw it on a good night, that comes around rarely in Melbourne, maybe 2-3 times a year…. and it was just about overhead which helps.
Out in the dark I can see it naked eye, as long as there is no moon around, in the scope its magnificent and fills the 28mm eyepiece.
There is no hope seeing it in the city with my 6" refractor.
Matt
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Old 07-04-2014, 10:49 PM
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Hi John,
I have seen CentA from the burbs in Melbourne with my 10" Newt and a 28mm ES 68º eyepiece + ES Coma Corrector. Can't help with the finding as I used GOTO. As Astro744 said once you see it you'll wonder why….well that was me, spent a couple of years trying with GOTO…. but in Melbourne I think it comes down to local conditions. I saw it on a good night, that comes around rarely in Melbourne, maybe 2-3 times a year…. and it was just about overhead which helps.
Out in the dark I can see it naked eye, as long as there is no moon around, in the scope its magnificent and fills the 28mm eyepiece.
There is no hope seeing it in the city with my 6" refractor.
Matt
Are you sure you are talking about Centaurus A (NGC 5128 see http://en.wikipedia.org/wiki/NGC5128 )? In your scope a 28mm 68deg EP will give a field of about 1.5deg. Not sure it is that big!!
And naked eye observation is possible, but very hard to do.

Malcolm
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Old 08-04-2014, 05:18 AM
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John
I can see it in my 50mm finder, but you need a dark sky.
Try using a pair of binoculars.
Cen A is 4.5 degrees north of omega Cen.
The attached wikisky org image shows omega Cen and Cen A.
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Old 08-04-2014, 06:25 AM
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Originally Posted by barx1963 View Post
Are you sure you are talking about Centaurus A (NGC 5128 see http://en.wikipedia.org/wiki/NGC5128 )? In your scope a 28mm 68deg EP will give a field of about 1.5deg. Not sure it is that big!!
And naked eye observation is possible, but very hard to do.

Malcolm
Hi Malcolm,
I'm going on memory from the last time I saw it, which was at a dark site a year and a half back. It was a really good night. I might have changed eyepieces to get a shorter FL, to get it to 'fill the eyepiece' but can't remember doing that highly probable as I do change eyepieces a lot. Naked eye it is a very faint smudge.

Matt
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Old 08-04-2014, 06:57 AM
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Hi Malcolm,
I'm going on memory from the last time I saw it, which was at a dark site a year and a half back. It was a really good night. I might have changed eyepieces to get a shorter FL, to get it to 'fill the eyepiece' but can't remember doing that highly probable as I do change eyepieces a lot. Naked eye it is a very faint smudge.

Matt
You're doing well to see NGC 5128 (Centaurus A) unaided as it is 7 mag. with that light spread over 18' x 14'.

http://messier.seds.org/xtra/ngc/n5128.html

Nearby NGC 5139 (Omega Centauri) is 3.68 mag. spread over 36.3' and fairly easy unaided.

http://messier.seds.org/xtra/ngc/n5139.html

The difference between 3.68 and 7 is a factor of 21.3x. (2.512^(7-3.68)). This is referring to a point source. The calculation for extended objects is a little more complex.
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Old 08-04-2014, 07:10 AM
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Very interesting thread. I observed Cen A a month ago. I found it fairly easily under rural skies by going to Omega Cen & then just searching the general vicinity at 15x with a 4mm exit pupil. Altitude of nearly 90 degrees was the hardest bit rather than visibility. I agree with Alex & others in that low power seems more conducive to observing this object. And the panning movement did help tremendously. But I can see why it's a challenge from the city or suburbs. I will give that a go next time the opportunity arises, just out of interest.
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Old 08-04-2014, 08:23 AM
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A very interesting thread, I agree. I can usually ‘see’ Centaurus A from my backyard, 5km from Adelaide CBD, with my 12”DOB but it took me about 3 years and a GOTO to find it in Adelaide. Like Alex et al, lower power and panning definitely help as does having found it once. I had/have no luck if I use my 150mm MAK!
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Old 08-04-2014, 04:49 PM
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Quote:
Originally Posted by MattT View Post
Hi Malcolm,
I'm going on memory from the last time I saw it, which was at a dark site a year and a half back. It was a really good night. I might have changed eyepieces to get a shorter FL, to get it to 'fill the eyepiece' but can't remember doing that highly probable as I do change eyepieces a lot. Naked eye it is a very faint smudge.

Matt
That makes more sense, I was worried you may be giving the impression it is a huge object, while it is large for a galaxy, it is still fairly small and small scopes it appears even smaller as the halo fades gradually.

Well done seeing it naked eye. I once thought I had caught it on a really dark night 2 years ago at Snake Valley, never since. It is a really hard test!

Cheers

Malcolm
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Old 09-04-2014, 11:53 AM
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Well what I have established from the thread is that if you are patient and work hard enough you'll get the reward. Is it just me or can you become REALLY obsessed with a particular target?

I'm really keen to get this one in Melbourne but now have to wait near new moon. I have a pretty good idea what I'm looking for now and will attempt to go with the low mag / pan approach. I'll keep my focus on this one for a while and if it all gets too much I guess it's up to Heathcote I go.

Glen I'm assuming CentA is the bright blotch above Omega Centauri in the image you supplied?
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Old 09-04-2014, 09:51 PM
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Answering for Glen, yes, Centaurus A is the bright blob above Omega.

To help inspire you, last night I was doing an astro viewing night at a high school. Had everything going against me: thin cloud across the entire sky, dew, and terrible light pollution that was heightened by the thin cloud, and a first quarter Moon.

But, despite all this, just by star hoping through an 8X50 finder, I was able to nail Centaurus A on a C8 with a GSTAR camera coupled to it. Just by using the finder! Did the Sombrero next in the same way too, . Both were totally invisible through the finder,

Mental.
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Old 10-04-2014, 03:00 PM
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Thanks Alex, I have been away.
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