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29-02-2012, 09:49 PM
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Highest Observatory in Oz
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Join Date: May 2006
Location: Canberra
Posts: 17,176
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Seeing and what it can do to your images
There have been some discussions around seeing and how it can affect the clarity of images so I thought this would be a nice illustration of what can happen if your seeing is bad even at medium focal lengths.
Both images taken with Starfire 152 EDF at F7.5 using Starlightxpress SXVH9 CCD, same site (Canberra ACT 780m ASL), same photographer, same mount, with only basic processing and esposure times were similar too, plus careful attention to focus was used (the imager is anal about this) in both cases.
Image on the Left was taken in very poor seeing
Image on the right was taken in quite good seeing
Even at the relatively modest focal length of 1140mm - the difference is remarkable!
Here is the comparison
So seeing is everything (as is focus) as far as I am concerned, here in Newcastle the left hand image is easy to take most nights but the right hand image is a rarity indeed (if ever) 
Mike
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29-02-2012, 09:57 PM
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Mostly harmless...
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Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,716
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Nice comparison Mike - all too easy for us to gloss over these essentials while worrying about collecting enough data.
You know, those pictures are showing some real potential. This might be an object you should perhaps spend some more time on.... 
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29-02-2012, 09:58 PM
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Galaxy hitchhiking guide
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Join Date: Dec 2007
Location: The Shire
Posts: 8,111
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Quote:
Originally Posted by strongmanmike
...So seeing is everything (as is focus) as far as I am concerned,...
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Sorry Mike , while I entirely agree, you have not discovered fire here.
The Pro's have been putting fast optics on mountain-tops for decades........ I suspect it was not because they liked the fresh mountain air 
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29-02-2012, 10:08 PM
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Highest Observatory in Oz
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Join Date: May 2006
Location: Canberra
Posts: 17,176
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Quote:
Originally Posted by Peter Ward
Sorry Mike , while I entirely agree, you have not discovered fire here.
The Pro's have been putting fast optics on mountain-tops for decades........ I suspect it was not because they liked the fresh mountain air |
 Never thought I had but unlike Rob, clearly you seemed to have missed the point of the post Peter.
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29-02-2012, 10:11 PM
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Highest Observatory in Oz
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Join Date: May 2006
Location: Canberra
Posts: 17,176
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Quote:
Originally Posted by RobF
You know, those pictures are showing some real potential. This might be an object you should perhaps spend some more time on.... |
  
Eureka! Done!
Ta ta ta daaa!
...sorry 
Last edited by strongmanmike; 29-02-2012 at 10:25 PM.
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29-02-2012, 10:15 PM
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Registered User
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Join Date: Aug 2009
Location: Waitakere Ranges, New Zealand
Posts: 2,260
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Great comparison there Mike, very nice. Do you have the FWHM values for the two images, that would be interesting to know?
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29-02-2012, 10:21 PM
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Highest Observatory in Oz
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Join Date: May 2006
Location: Canberra
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Quote:
Originally Posted by SkyViking
Great comparison there Mike, very nice. Do you have the FWHM values for the two images, that would be interesting to know?
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No unfortunately not, all I remember is that the night of the left hand image I limited it to 2min exposures I think to try and by pass some of the crap seeing (in vain of course) and that I was swearing a bit  The right hand image was 6 X 5min Lum and then from memory something happened, maybe cloud or an equipment failure I can't remember  I didn't get any colour for that one.
Mike
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29-02-2012, 10:30 PM
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Galaxy hitchhiking guide
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Join Date: Dec 2007
Location: The Shire
Posts: 8,111
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Quote:
Originally Posted by strongmanmike
Never thought I had but unlike Rob, clearly you seemed to have missed the point of the post Peter. |
Please don't get me wrong.
Dr Alan Holmes (aka SBIG) wrote a good piece (yep on SBIG's website somewhere) on how amateur gear could rival Pro mag limits *only if* their seeing was better (he'd just got back from Paranal)
...but if you are not saying even in modest apertures (less than about 150mm) seeing effects CCD images most nights.......
...well...I guess I have missed the point.
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29-02-2012, 10:34 PM
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Highest Observatory in Oz
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Join Date: May 2006
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Quote:
Originally Posted by Peter Ward
Please don't get me wrong.
Dr Alan Holmes (aka SBIG) wrote a good piece (yep on SBIG's website somewhere) on how amateur gear could rival Pro mag limits *only if* their seeing was better (he'd just got back from Paranal)
...but if you are not saying even in modest apertures (less than about 150mm) seeing effects CCD images most nights.......
...well...I guess I have missed the point.
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Not sure why all the cryptic code is necessary  I really don't think the point of my post is that complicated
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29-02-2012, 10:44 PM
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Registered User
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Join Date: Jul 2005
Location: Melbourne Australia
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106mm aperture same object 2 nights apart. Both focused with focusmax
http://www.dslrfocus.com/gallery/seeing.gif
Just give up and go inside when it's that bad.
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29-02-2012, 11:13 PM
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Galaxy hitchhiking guide
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Join Date: Dec 2007
Location: The Shire
Posts: 8,111
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Quote:
Originally Posted by strongmanmike
Not sure why all the cryptic code is necessary I really don't think the point of my post is that complicated |
You have given a graphic example of an obvious truth....and that's great. 
Chris has (kindly) shown an similar example of (as I had shown on a commercial web site....that I won't point to in respect to the other Mike...! ) well, about a decade ago (OK, I'm starting to feel like a grumpy old bugger )
Yes..... Seeing affects FWHM's ....so I'm now puzzled (I ask myself "why" more fequently in my dotage )
I don't see any other point. Yes, seeing can be awful and degrade images from otherwise perfect optics.
Was there something else I missed ?
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29-02-2012, 11:56 PM
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Mostly harmless...
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Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,716
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I'd be interested to know if/how people quantitate their seeing while setting up an imaging run. To be honest I'm not sure I'd recognise terrible seeing (you dont always have a great image from your equipment to compare with).
Does anyone have a quantitative rather than visual estimate way of doing this?
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01-03-2012, 12:00 AM
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Registered User
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Join Date: Jan 2012
Location: Melbourne
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Hi Peter ,
Thanks for posting those 2 examples.
Your Starfire 152mm has a Dawes limit of 0.76 arc seconds.
Seeing would normally be only about 2 arc secs.
I think top seeing conditions would seldom be better than 1 arc sec.
I have noticed that seeing is much better at higher altitudes
compared with sea level.
It would be interesting if you re-did the pic again at high altitude.
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01-03-2012, 12:16 AM
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Registered User
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Join Date: May 2009
Location: Para Hills, South Australia
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Great example Mike, currently my seeing is really bad at the moment so it is a good comparison not the worry so much about my images.
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01-03-2012, 04:00 AM
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1 of 7 of 9
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Join Date: Sep 2009
Location: Perth
Posts: 1,968
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Quote:
Originally Posted by Peter Ward
I don't see any other point. Yes, seeing can be awful and degrade images from otherwise perfect optics.
Was there something else I missed ?
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Peter, I dont think you have missed anything  ,
I, for one, who has not a set up like you or Mike, can use the comparison as a guide. There are so many factors that can affect AP and the pics give an ( slight)insight as to what to expect when imaging in good and bad seeing....
Bartman
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01-03-2012, 07:36 AM
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Registered User
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Join Date: Feb 2006
Location: Sydney
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The longer the focal length the more the seeing will wreak havoc. I did not know what was happening with my scope last year during a week of particularly bad seeing.
It would go out of focus from one focus subexpsoure to the next without changing the focus at all.
It is also highlighted when you use a small chipped camera as that creates a digital zoom effect and magnifies the seeing more.
So the moral of the story is widefield and large chips to be less affected by the seeing (also larger pixel sizes rather than smaller).
Greg.
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01-03-2012, 08:12 AM
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ze frogginator
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Join Date: Oct 2007
Location: Sydney
Posts: 22,062
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The difference in the two shots is quite striking. Sometime it's just not worth doing long FL work. Stick to lenses when the jetstream's up 
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01-03-2012, 08:20 AM
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Highest Observatory in Oz
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Join Date: May 2006
Location: Canberra
Posts: 17,176
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Quote:
Originally Posted by gregbradley
The longer the focal length the more the seeing will wreak havoc. I did not know what was happening with my scope last year during a week of particularly bad seeing.
It would go out of focus from one focus subexpsoure to the next without changing the focus at all.
It is also highlighted when you use a small chipped camera as that creates a digital zoom effect and magnifies the seeing more.
So the moral of the story is widefield and large chips to be less affected by the seeing (also larger pixel sizes rather than smaller).
Greg.
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Greg this is purely an illusion of course, the detail in the image is affected exactly the same by the seeing and it is really only the choice of file size you choose to display that causes this apparent difference. When you have a wide field compared to a narrower field at the same arc sec/pix and then display them at the same file resolution the affects of the seeing on the two images will look identical.
Mike
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01-03-2012, 08:30 AM
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1 of 7 of 9
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Join Date: Sep 2009
Location: Perth
Posts: 1,968
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Quote:
Originally Posted by strongmanmike
Greg this is purely an illusion of course, the detail in the image is affected exactly the same by the seeing and it is really only the choice of file size you choose to display that causes this apparent difference. When you have a wide field compared to a narrower field at the same arc sec/pix and then display them at the same file resolution the affects of the seeing on the two images will look identical.
Mike
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Awe Mike ...... now you have just completely confuzzled me ......
back to google I go to figure out what you have said....  maybe Wolfram Alpha will be able to decyfer what you said there....
I'll maybe  nah forget about it......
One day I'll get there 
Bartman
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01-03-2012, 08:49 AM
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Highest Observatory in Oz
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Join Date: May 2006
Location: Canberra
Posts: 17,176
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Quote:
Originally Posted by bartman
Awe Mike ...... now you have just completely confuzzled me ......
back to google I go to figure out what you have said.... maybe Wolfram Alpha will be able to decyfer what you said there....
I'll maybe nah forget about it......
One day I'll get there
Bartman |
Yeh?...sorry
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