Hmmm... Think I prefer the first version myself, probably because it has fewer saturated areas, and thus shows a bit more of the subtle detail. (Refers back to this thread (http://forum.orpington-astronomy.org.uk/index.php?topic=2760.0).)
(Note: This discussion sub-thread started with my asking the following question...)
Are the crosses through the stars diffraction spikes from the scope's spider, or are they an artefact of the receiver or the processing?
Processing, Rick
Quote from: Mike on Nov 27, 2007, 09:27:02They are artificially added in using Astronomy Tools.
Erf. :( Not good. If they're an un-avoidable side-effect of the processing, then choosing options to minimise them is to be preferred. If they're
added deliberately then I'd go further, and say it's a bad tool.
I don't agree with too much processing, Rick. I mean, people like Nik Syzmanek & Hubble, also, nearly recreate their images with their great knowledge & application of effects & enhancements with programs like PhotoShop.
I think a spike is very minimal, I would rather not use false colour, & prefer that the image is 99.5% natural.
I'm with Rick. Processing is essential to get a good image, but diffraction spikes are an anomoly due to your particular optics and are to be removed where possible. Sure, they can look nice if used in the right way, but to me, unless you are specifically doing an image of a single star, they distract from the main object in the image.
Processing is a dark art that I have yet to get involved in heavily myself and a lot of images are ruined by over processing. i've seen some great images that are noisy or have artifacts that have been ruined by over-processing to get rid of the noise or artifacts or in an attempt to sharpen the image, when it would have been better of left alone.
umm... I could see this thread developing into a very long one......!
My thoughts on this are that if you want an accurate image as possible then obviously the addition of any artificial data such as diffraction spikes is a NO-NO. However, in some instances the addition of a diffraction spike adds to the overall image (it doesn't work for all subjects but certainly helps with some) - it kinda gives it the wow factor to an otherwise flat image.
Fay - you need to be very careful when adding the spikes - I think on your image just spikes on the bright star above the bubble would have sufficed. If you look closely you can see spikes on other stars & the actual centre of the bubble as well. If after you run the script in Astronomy tools & add as a layer on top you can go in & manually delete all the spikes you don't want.
It's a great image by the way - colour would really enhance it it. Good to see you posting so many images.
John
Quote from: Space Dog on Nov 27, 2007, 14:28:07I like it, I also remember John and Mike posting images with spikes????
Guilty! I used a cross of cotton thread over the end of my ED80 method. Though the spikes were small and subtle and I mainly did it to obtain better focus.
I quite like diffraction spikes, but they really are a sign of a coffee table image. Trouble is, if I see a refractor in the kit list and spikes on the stars I get suspicious...
I don't get that sort of spike from my newt as the secondary is held on a bar, I think I might put another bar across the other way to even it up. When I start imaging through it, anyway. (need a guidescope with an FL over a metre I think...)
Genuine diffraction spikes are a fact of life. Certain optical arrangements will produce them. They're real optical phenomena, and, to mis-quote Scotty, "We canna' change th'laws o' physics, Capt'n!". ;)
If you have an accurate idea of exactly what diffraction signature your particular optical system produces, and you have sufficient compute power and the right software, then you might be able to remove most of the diffraction effects from an image. To do this accurately you'd probably have to do it frame-by-frame before stacking, though.
So for the most part we're going to be stuck with them whether we like them or not.
Now, if a processing system were to add fake diffraction spikes to make an image look "better", that would be bad science.
Rick - I definitely wouldn't remove them if my optical system added them i.e. due to design of secondary holder & I have never heard of any software that can remove them...! I think that trying to remove diffraction spikes that are added due to optical system would be even 'LESS SCIENTIFIC' than adding diffraction spikes to images that didn't have them.....!
So Rick do your images have diffraction spikes or not...?
Cheers, John.
Personally I like the diffractions ;)
Tony G
Quote from: JohnP on Nov 27, 2007, 17:44:17I think that trying to remove diffraction spikes that are added due to optical system would be even 'LESS SCIENTIFIC' than adding diffraction spikes to images that didn't have them.....!
Hmmm... You'd have to take that up with the Hubble folks, then. The algorithms they used to sharpen up the original camera's images (before they fitted COSTAR) are pretty much the same as the algorithms you could use to remove diffraction effects. They're all algorithms which take information that's been spread out one way or another and move it back to the position from which it came. They're not inventing fake stuff.
Quote from: Rick on Nov 27, 2007, 22:32:19
Quote from: JohnP on Nov 27, 2007, 17:44:17I think that trying to remove diffraction spikes that are added due to optical system would be even 'LESS SCIENTIFIC' than adding diffraction spikes to images that didn't have them.....!
Hmmm... You'd have to take that up with the Hubble folks, then. The algorithms they used to sharpen up the original camera's images (before they fitted COSTAR) are pretty much the same as the algorithms you could use to remove diffraction effects. They're all algorithms which take information that's been spread out one way or another and move it back to the position from which it came. They're not inventing fake stuff.
Hello,
Me again, the pre COSTAR images used de-convolution algorithms to remove as much as the optical effect as possible caused by the original mirror miss figuring. That is now fixed and has nothing to do with diffraction spikes.
The post COSTAR image we're fixed due to the corrective lens (Hubble glasses) which was put in front of the camera, or in their words "compensates for spherical aberration in Hubble's primary mirror".
All the images produces by Hubble contain diffraction spikes, see below, however depending on the brightness scaling used they are either really easy to see or quite difficult to see. Personally I don't think they are removed.
If the "the Hubble guys" did remove the diffraction spikes then they are about as scientific as Fay's image (nice image Fay, I now love the spikes, I like controversy). Hubble people bad, very bad people, evil in fact and should be stoned (rocks not grass) for producing non spiky images. What will they do next name teddy bears!
Anyway have a look here; you will see they don't remove the spikes from images, well unless they are for public release for pretty pictures, and only rarely.
http://hubblesite.org/
See http://hubblesite.org/newscenter/archive/releases/2005/36/image/a/ for a good example of Diffraction spikes on Hubble, post COSTAR. Also if you search for Diffraction spikes and you will see all the examples of them.
Also here's a good one http://imgsrc.hubblesite.org/hu/db/2004/28/images/a/formats/print.jpg
Chris
FIGHT! FIGHT! FIGHT! FIGHT! :twisted:
I was watching a Hubble/ESA disc last night & noticed the spikes on some of the images.
Quote from: Space Dog on Nov 29, 2007, 10:38:33
Me again, the pre COSTAR images used de-convolution algorithms to remove as much as the optical effect as possible caused by the original mirror miss figuring. That is now fixed and has nothing to do with diffraction spikes.
Deconvolution is a process by which a system's impulse response can be removed (up to a point). The diffraction spikes are part of the system's impulse response. The way deconvolution was applied to the pre-COSTAR Hubble images was by using a desired impulse response as an end target. There are other ways deconvolution can be applied. There are also a great many other signal processing algorithms which can be applied to enhance an image, and the Hubble team have made use of quite a few of them. They've even done a fair bit of the original research into them. Some are more successful than others.
None of this makes the addition of fake "diffraction spikes" to an image which didn't have them originally any more acceptable.
(Moderator note: I'm going to split this discussion out of this thread because it bears very little reference to the original thread.)
(Moderator apology: Sorry, I had to lock the thread long enough to split the topic. Normal service is now resumed.)
Quote from: Fay on Nov 29, 2007, 11:58:43
I was watching a Hubble/ESA disc last night & noticed the spikes on some of the images.
Those are genuine artefacts of the optical arrangement. Generally you don't want to remove them because they tell you something about the nature of the object. The Hubble folk know this.
Actually, deconvolution (restoring) is an algorithm-based process used to reverse the effects of convolution (mucking up of) on recorded data. In the case of the Hubble scope the convolution of the final image was caused by the miss figuring of the primary. Therefore, the Hubble guys wrote bespoke deconvolution routines to restore the picture back to what it should have been given a perfect mirror, or as near as possible anyway.
Have a look at http://en.wikipedia.org/wiki/Deconvolution under "Deconvolution and Optics", it points out that a) you need to know what the reference point of the original image should have been and b) the routine needs to be specific to the source data for it to work well.
This has nothing to do with the Hubble guys removing diffraction spikes, if you look at the images of Supernova 87a (I think that's what it was called) the early pre-COSTAR images did not have well defined diffraction spikes, but post COSTAR they did. This does not meant they removed them in the past; just that the routines written could not restore the images 100%.
Quote thingy "Deconvolution is a process by which a system's impulse response can be removed (up to a point)." I assume you agree with the above with this? If not what does it mean?
Quote thingy "None of this makes the addition of fake "diffraction spikes" to an image which didn't have them originally any more acceptable."
To whom, just because I like my other half it does not mean that you do? I believe someone probably wiser than both of us said "Beauty is in the Eye of the beholder", someone else said "don't over criticize other peoples work until you have tried it yourself" (me yesterday to one of my coders) I don't think Fay's image was meant to be technically significant, if it was then Flats, Bias no filters etc would come into play, its just a pretty picture with a bit of added bling!
Mike, is that ok? FIGHT FIGHT FIGHT!!!!
Well I'm actually learning quite a lot about deconvolution/ convolution - never understood any of it before but starting to make a little sense.. I'm looking forward to when someone starts talking about my best mates Lucy-Richardson & Maximum Entropy...... :-)
John
Quote from: Space Dog on Nov 29, 2007, 12:39:40
Actually, deconvolution (restoring) is an algorithm-based process used to reverse the effects of convolution (mucking up of) on recorded data.
Yep, that's a very good way of describing it. I was trying to say exactly the same thing, but fell into the trap of using the terminology folks at work would use.
Diffraction spikes are one of the ways an image gets "mucked up" by an optical system. However, they can be very useful. The Hubble folk knew what sort of diffraction pattern (spikes and all) they were expecting to see from the pre-COSTAR scope. They looked at the pattern the scope was returning, and tried to produce an algorithm which would take that "mucked up" diffraction pattern and bring it back into sharp focus. The trouble is the algorithm is very sensitive to noise, so it's not possible to do the job perfectly. That's at least part of the reason the pre-COSTAR diffraction spikes are a bit fuzzy.
Back when the first amateur astronomers to experiment with digital imaging were presenting their first results, there was a short talk at the BAA about the possible ethical problems digoital imaging might pose. The biggest fear was that it would be easy to add fake detail to a digital image; certainly much easier than it is to add fake detail to a photographic plate, and harder to detect. These days we rely entirely on the integrity of our observers to present an accurate image of the phenomena they've observed. Adding fake diffraction spikes is a very minor deception. Folks who've used the Astronomy Tools software may know it has a variety of "add spikes" options, and may recognise the effect in others' images. I didn't know, and didn't recognise the effect for what it was. I was fooled into thinking it was a genuine effect. Others who come to look back on the images years from now may not know, or may have forgotten, and may end up being fooled too. As I say, it's a very small deception, but it is a deception, and to do good science we should avoid even the hint of deception.
Quote from: JohnP on Nov 29, 2007, 13:44:39
Well I'm actually learning quite a lot about deconvolution/ convolution - never understood any of it before but starting to make a little sense.. I'm looking forward to when someone starts talking about my best mates Lucy-Richardson & Maximum Entropy...... :-)
John
Next week mate after half a bottle of whiskey.
Quote from: Space Dog on Nov 29, 2007, 12:39:40Mike, is that ok? FIGHT FIGHT FIGHT!!!
Hey all I said was ".....Not sure about the Christmas Card stars you've 'created' there though......."
To be honest, I'm still completely boggled that the author of Astronomy Tools thought that options to add fake spikes would be a good idea. In a paint package, where you go to let your inner artist is have a play, weird visual effects like spikes, lens flares, halos and such can work wonders on fantasy scenes, but they really shouldn't have any use when you're trying to enhance an image to bring out the real detail hidden in the raw data (which, as I understand it, is the aim of Astronomy Tools (or is it?) )... :roll: :roll: :roll:
Still after all that, I personally still like the diffraction spikes ;)
Tony G
Rick... can't believe you are still going on about this...... Just FYI here is what the author of Astronomy tools Noel Carboni (a brilliant astrophotography by the way- some of the best images on the web) says about them...
Star Diffraction Spikes - High-end Ritchey-Cretien telescopes (like the Hubble) make them because of the cross-shaped vanes that hold the secondary mirror in position... Some folks make them by tying strings across the aperture... One thing's for sure - they look cool! And the highest quality astroimages often have them. I own a Schmidt-Cassegrain telescope that doesn't make them optically, so I decided to craft a way to make spikes digitally. These spikes are created proportionate to the size/brightness of the stars and are the color of the stars, so they look very natural.
If your telescope doesn't make diffraction spikes and you want them in your images - or you'd like to enhance the ones you get optically - I've included four different actions for making them. You have the option of adding them to your images in the quantity you want, from spikes on even the smallest stars to spikes on only the biggest, brightest ones.
Tony - I'm with you.....
John
Aren't we missing the point here ? If we're out to produce pretty pictures (aren't we ?) then we adjust the colours by eye so they look pleasing (that means subjective in my book). Doesn't adding diffraction spikes come under the same heading i.e. a personal thing.
At the end of the day it would be difficult to do much real science with any of my images - I don't use Johnson or any other astrometric filters or even perform proper calibration. As for knowing the set point temperature, does anyone know what their CCD cools down to or even whether it's stable from image to image ?
Personally I like the spikes, the image looks better for them.
Just my penneth ...
QuoteQuoting Noel Carboni on diffraction spikes: I own a Schmidt-Cassegrain telescope that doesn't make them optically, so I decided to craft a way to make spikes digitally. These spikes are created proportionate to the size/brightness of the stars and are the color of the stars, so they look very natural.
That's exactly why I have a problem with it. It's not revealing something that's there already. It's adding something that wasn't there before. Once you start down that road, where do you stop? Will you sneak in a fake comet, supernova, TLP, or some such? I hope not, but once I know you've added something that wasn't there to an image, even if it's as innocuous as a fake diffraction spike, then you're sowing the seeds of doubt in my mind. That's why I am still going on about this. I want to know whose images I can trust. One thing's for sure, having read that quote I'll certainly trust Noel Carboni's images a lot less in future, however pretty they look.
Quote from: RobertM on Nov 29, 2007, 22:02:22
Aren't we missing the point here ? If we're out to produce pretty pictures (aren't we ?)
Are you really investing all that cash and effort in kit and imaging time just to produce pretty piccies, without even the slightest hope that maybe one day you'll image something nobody else has yet seen?
Quote from: RobertM on Nov 29, 2007, 22:02:22then we adjust the colours by eye so they look pleasing (that means subjective in my book). Doesn't adding diffraction spikes come under the same heading i.e. a personal thing.
Colour balance is very subjective. Different screens show colours in different ways, and peoples' eyes see colours different ways too. I even see some colours slightly differently depending on whether I use my left eye or my right. So for astro images you (hopefully) note which filters went into which channels and let folk decide whether the colour mapping works. Feed the red into green and the green into red if you like. The underlying image data remains the same, though it'll look a little weird. Sometimes weird colour mapping is just what you need to reveal some otherwise invisible detail. So no, adjusting the colour balance isn't the same sort of thing.
Quote from: RobertM on Nov 29, 2007, 22:02:22
At the end of the day it would be difficult to do much real science with any of my images - I don't use Johnson or any other astrometric filters or even perform proper calibration. As for knowing the set point temperature, does anyone know what their CCD cools down to or even whether it's stable from image to image ?
One thing with astronomy is that you never know when something unexpected and spectacular might happen. Just suppose you were out imaging one night. A week or two later there's a headline on the news saying something like "Spectacular supernova in...", and you realise it might have been in one of the images you made that night. Let's say you'd used "add spikes" on the final image, and thrown the raw frames away because it was "just a pretty picture" and you'd got it as pretty as you wanted. One of those spikes just happens to obscure the supernova's position, so you figure you saw nothing. Who's been cheated? Suppose instead you realise you do have a pre-discovery image of the supernova. Your image would be of real scientific value even if you havn't got the whole set-up calibrated to N decimal places, provided you have a few basic notes (like exactly when and where you took the image) and you've still got the raw frames or you've not mucked the final image about too much.
Think it'll never happen to you? I was out photographing the southern sky from Kenya about 8 hours before SN1987A in the LMC was discovered. I didn't quite manage to get the LMC into my camera's field of view that night because there was a tree in the way and I didn't quite stay up late enough...
Quote from: Rick on Nov 30, 2007, 01:00:47.....Let's say you'd used "add spikes" on the final image, and thrown the raw frames away because it was "just a pretty picture" ......
Oooh that's a big no no, even if you are just making pretty pictures. Despite the small number of images (approx 25?) I have posted, I have taken 2691 FITS frames over the years. Like with any photography, the majority of your pictures don't get used in the end as they are no good. However, i've still kept them all in case I come across a better way to process, etc. in the future, plus, as you say, there may well be some useful astronomical data in there.
I think a lot of the issue is around pretention (and I don't use that word in a negative way). I am realistic that my photos (if they happen :cry: ) are not created with the care and attention that images taken for scientific purposes demand. They are more an enhancement to my ability to observe. In my back garden, which is about as dark as Sidcup gets, I get the slightest hint of some objects, but they're the bright Deep Sky objects. A CCD image is a bit like the consolation prize, "look what you could have won".
But, as Rick says, there is the slight, but real chance that an image may have real scientific value, and then the raw data becomes scientifically interesting, and it's technical shortcoming can be compensated for.
However, in the meantime, I like to create images that give an impression of what is up there to see, often to show non-observers (like my wife who really doesn't understand the cost and extreme cold our hobby involves sometimes). That's what I mean by "Coffee Table Image". In that case, a small amount of scientific "dishonesty" to make an image look nice is acceptable. I don't have astronomy tools, I might one day, and I don't know if I'd use that particular one, but I think if it makes an image look nicer, and that's my aim for that image, then I might use it. (Ooh, lots of commas. It's like a camper van convention)
But, as Mike said, raw data is just that, and is always retained...
Rick - As an astrophotographer (not sure if you take images) you never throw raws away. The thing that inspired me was seeing all those wow images in the glossy mags etc... & yes at the end of the day my number 1 priority is to produce a 'pretty' image of some distant object - no way do I spend hours scanning my raws identifying all the stars etc. for some undiscovered object. Obviously, if something jumps out I will investigate further by that's about the level of science that I do & I believe that is true of a majority of imagers. If I wanted to present my images in a scientific manner then I would just post the raws (which I don't think would appeal to many people) - if you think about it even doing a simple curves adjustments is manipulating the data as you are changing the brightness of one area unproportiuonally to another.
QuoteAre you really investing all that cash and effort in kit and imaging time just to produce pretty piccies
YES - I'M AFFRAID SO..... & I'm also planning trips to south of France next year so I can get even PRETTIER ones :-)
John
OUCH!!
This is all your fault Fay.
This subject seems to have caused a verbal war between a few members, which I believe forums are for, so we'll see what happens when I add diffreaction spikes to my next Moon image. :lol:
Tony G
PS I still personally like the diffraction spikes still.
I saw Rodney Marsh moon on I'm a celebrity, I have no desire to see yours. Diffraction spikes or not. Sounds a bit eye watering, to be honest...
Tony, I am imagining your Moon with spikes!!!!!!!!!
Well, I think that when doing astrophotography as a hobby & you use any sort of enhancement, be it curves, levels, filters RGB Ha & the convoluting ones etc etc. it is false, but how else can us humans get an idea of the shapes & different textures of space?
A lot of things are a personal preference & some of the professionals go a bit too far, using so many tools & colours. Personally I think you have to pull the reins in a bit, not because I can't do it as well, but you have to be a bit realistic & allow objects to be seen as they most probably are & not defininitely as they are not, if you know what I mean.
If imaging for scientific reasons, they still enhance to bring out the features that they may be studying at the time so as to get a better knowledge of what they are looking at, but that is ok as it is done for a different reason.
Have the stags unlocked horns now??
Wow, I thought I was the only very opinionated person here who was publically verbal about astronomy.
I think this thread has been great for two reasons: -
1. It's been a very healthy discussion with regards to what people like and dislike.
2. I think we have definitely identified that people in the society are very passionate about imaging, especially for pretty pictures in this case.
One thing that is at the back of my mind though and why I kinda lost it a little earlier this week is that when Fay (the root of all evil as she will now be known) put her image up on the forum, the feedback given to the image was a little harsh and probably non considered. By this I mean that rather than expressing an "it's nice" or "it's crap" the feedback started to go in the reason for the production of the image, and why it should be done i.e. Science and not for Fun.
Fay and many others here spend £1,000's of buying kit to take images for fun, not science. Therefore a preference and pride in the presentation of your images is important to the person taking them and also to the audience and therefore the feedback should normally be proportionate to the objective of the Image. I has a real issue with some of the earlier comments made in the thread because it seemed that what was deemed to be the reason for taking pictures for member A was not acceptable for member B. And if I was to be brutally honest member B was overly pretentious with their views, and tried to cover it up by techno babble.
I have no doubt that at some point in time, some of us will get into some science in the future, when I don't know because frankly it can be very dull; may I quote "here is a picture of the wall facing east in my back garden" the speaker in this case would be considered by some members of the society and being technically correct in what they we're doing i.e. good science. Personally I thought these kind of people really need to be left in a dark room, because they can turn people off astronomy so quick it's unreal. Sorry I digress.
I suppose what I'm really saying here is that any effort to photograph the night sky is good, I remember Paul W image of McNeil's object as a big bonus a few years ago during DSC.
I know that John P has had I think 5 images published in Astronomy Now etc. Fay is the most committed imager out of us all. And the new members who have come on board recently have mainly been because of the work of the imagers, and the welcome environment DSC and other such events have provided. Even our chairman (all hail the chairman) has got hooked. We cannot put a message out to people that we are here only for science, I'm sure probably half the society are, and the other half are not and that's the truth.
So it might be a good idea to park this chat for now, personally I'm more than up for an on-line argument about the types of calibration needed to provide a 99.9% flat and accurate image for photometry or even a discussion on counterbalancing the chip dynamics against the recorded ADU's within spectra to give you an accurate spectrograph (and I'm 100% Robert M would win this discussion). Or the merits of telescope design and post processing techniques. But this was only a bloody picture.
So in summary, I look forward to seeing more images form the members, with our without diffraction spikes. I also look forward to seeing more science, we could actually put the Raw Calibrated FIT's up onto a share if other members want to analyse them?
But most of all I look forward to the skies clearing so we can all get out and use £1,000's of kit to do something the professionals do better, but maybe that's the point of being an amateur? Or am I missing something?
Woof!
Fay - Especially for you....... :-) This image is natural in everyway..... :-) John
(http://homepage.ntlworld.com/john.punnett/graphics/oas/stag1.jpg)
John,
Is that part of the Deer Lick Group NGC7331?
Chris
Wow, Chris, you reminded me of Martin Luther King there & what a lovely stag full of tetesterone!
(think that's the correct spelling, sounds like a pasta!
Quote from: Fay on Nov 30, 2007, 15:14:28
Wow, Chris, you reminded me of Martin Luther King there
May I ask why?
Well, it was a really powerful post, & thanks for your kind words.
Quote from: Fay on Nov 30, 2007, 15:40:00
Well, it was a really powerful post, & thanks for your kind words.
Thank you and that's a fiver I owe you :-)
Apologies if the words 'pretty pictures' offended anyone, I well know the amount of effort (and grief) that's required to take just one ! As Chris has so rightly pointed out, it's the thrill and enjoyment of seeing the Universe in all it's glory however we want to see it, but we all still dream of discovering the undiscovered.
"There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened."
- Douglas Adams
(http://farm3.static.flickr.com/2138/2076225419_0df16decee.jpg)
Its not brilliant, but I still like it. ;)
Tony G
I love it, Tony, you put them in just where they should be!!!!!!
Quote from: Fay on Nov 30, 2007, 14:13:31Well, I think that when doing astrophotography as a hobby & you use any sort of enhancement, be it curves, levels, filters RGB Ha & the convoluting ones etc etc. it is false, but how else can us humans get an idea of the shapes & different textures of space?
All that processing has been worked out to enhance the information in your original images. There are good reasons for taking those flats and darks as well as the actual images, and doing all that tedious manipulation. If you do the work well then you'll produce an accurate image that's a true representation of your subject within the limits of your imaging equipment.
Quote from: Fay on Nov 30, 2007, 14:13:31A lot of things are a personal preference & some of the professionals go a bit too far, using so many tools & colours. Personally I think you have to pull the reins in a bit, not because I can't do it as well, but you have to be a bit realistic & allow objects to be seen as they most probably are & not defininitely as they are not, if you know what I mean.
Yep. Get as much out of the image as you can, while being sure that everything you've drawn out of the original is actually real. If you do the work well then the image will be both pretty and a good scientific record.
Quote from: Fay on Nov 30, 2007, 14:13:31If imaging for scientific reasons, they still enhance to bring out the features that they may be studying at the time so as to get a better knowledge of what they are looking at, but that is ok as it is done for a different reason.
You can most definitely enhance an image in different ways. Take a look at the work Mark did on his Comet Holmes images in order to pull detail out of the comet's coma. The surrounding background went to pieces, but that was a price worth paying to pull the coma detail up out of the gloom.
I would very much like to see what your image would look like without any added spikes.
I'm used to seeing diffration spikes on astronomical images, and I use their presence to work out what I'm looking at. Take a look at Friday's APOD:
http://antwrp.gsfc.nasa.gov/apod/ap071129.html
The image was taken using a Takahashi Epsilon 180ED. (See http://www.astromart.com/articles/article.asp?article_id=571 fo a review of the instrument.)
Even in the small-scale version on the main APOD page you can see that some of the stars have quite sharp diffraction spikes while others do not. Look a little more closely at the image, and you'll find that the reason is that some of the stars are being obscured slightly by dust. Notice how they're the ones with less well-defined diffraction spikes. So whether a star shows diffraction spikes or not can be used to work out a little about whether you're seeing it directly, or whether its image is being blurred a bit by dust. Now, if you have a look at the review page you'll see an image including a globular cluster and a bright star. Note the prominent spikes on the star. So genuine spikes can tell us whether an object is stellar (at least within the resolving power of the telescope, which is probably better than the resolution of the imaging chip).
If you try to add spikes during processing you've only got the resolution of the imaging system to work with, and you have to try to work out which objects are stellar (and so might have spikes) and which are diffuse (and wouldn't have them). With the best will in the world the processing software isn't going to get it right all the time, and it's got less to go on in the first place. It's also, inevitably, going to mess the image up a bit. When folk look at the image, and they don't know the spikes have been added during processing, they could be fooled into mis-interpreting it (as I was when I first looked at your image), and that's the main reason why I think that adding spikes during processing is a bad thing to do.
Quote thingy "So whether a star shows diffraction spikes or not can be used to work out a little about whether you're seeing it directly"
Rick, nope, what is happening here is the stars with smaller diffraction spikes are dimmer, granted they could be obscured by dust, or the might be less bright, you only know this by checking the stars ADU's.
You could however measure the dark points of the sky and then compare it to the dust cloud and check the difference. In the image you have used as an example the dust clouds are a lot brighter than the sky background. And where the dust is there in abundance there are no stars.
Quote thingy "If you try to add spikes during processing you've only got the resolution of the imaging system to work with, and you have to try to work out which objects are stellar (and so might have spikes) and which are diffuse (and wouldn't have them). With the best will in the world the processing software isn't going to get it right all the time"
Wrong, this is actually piss easy, in optical system diffraction spikes are caused by the interference of the secondary supports or fluids in scope systems in relation to the brightness of the object and exposure duration. If we assume there is an ADU level where spikes are caused and visible, using that ADU value we could very easy define which stars would cause diffraction spikes and highlight them. If I took an image and said all stars over 40K ADU have spikes and 1/3rd of the brightest stars on my images have them this proves they have an ADU value over 40k. I could even write a routine to put a sliding scale on the application of diffraction spikes based on the ADU level and the size of the spike and simulate a scope, as with Mr Carbone.
You have given us a brilliant example of someone spending £8k on his imaging kit to take good photo's and techno babble again, well done.
Guys,
I'm going to lock this now. Time to agree to differ.
Ian