Well after doing DS images for a little while now I have found out I am doing my flats wrong, which may account for some of my poor results compared to the length of exposure.
I understand the principle about what it does and the orientation of all lenses etc. What I am a bit unsure about is how to achieve this 1/2 bucket, and am reading conflicting info.
I have read that I should take the flats at 100 ISO in one place, and in another I have read it should be the same ISO as my actual images which is normally 800 ISO. Also
QuoteMetering mode to average.
(This ensures that the camera will expose for the entire illuminated field).
Just wondered what those using DSLR thought about ISO and Metering Mode.
Think I am going to make myself a light box.
Carole
I've assumed that the object is to see any grot in the optics so that would mean leave iso same as image, just reduce exposure time so you can actually see the grot.
Sitting back waiting to be shouted at
It shouldn't matter about the ISO or shutter setting; flats are all about removing artifacts of the optical system. the electronics are dealt with by the Darks and Bias.
At the risk of invoking an argument on flats, you should really do your flats with a peak at 2/3rds and not half. The result will be cleaner and better able to take account of any severe vignetting. Since the sensor will still be in the linear part of it's range you are better off using the available signal to improve the signal to noise ratio.
Robert
I agree with Robert.
Also, since ISO does not matter, use ISO 100 to give the cleanest (i.e. least noisy) flats.
Mark
QuoteI've assumed that the object is to see any grot in the optics so that would mean leave iso same as image, just reduce exposure time so you can actually see the grot.
Well so had I Mick, which is what I had been doing, but as no dust or vignetting had shown up at all on my flats so far I need to find out what I am doing wrong.
I've found an article describing how to do it which I tried yesterday and the flats were more like what they should look like, but the flats had to be stretched quite a lot to show any grot. This same article says do them at 100 ISO which is what I did yesterday, but I've also read elsewhere that it should be at the same ISO as the imaging subs, so now I don't know which is right.
Currently building a light box.
Just read your posts Robert and Mark before posting this. Thanks, that's cleared that up.
Carole
QuoteAlso, since ISO does not matter, use ISO 100 to give the cleanest (i.e. least noisy) flats.
you should do your flats at the base ISO of your camera, which is either 100 or 200.
this is normally the lowest setting that is not Lo.
My Nikon is 200 but i have a 100 ISO setting which is shown as Lo1, this is achieved in the same was as the higher settings,
i.e. changing the amp gain.
Quotewhich is what I had been doing, but as no dust or vignetting had shown up at all on my flats so far I need to find out what I am doing wrong
lets think about this.
Your camera is probably between 12-8Mp
unless you have a huge lump of crud in the optical system then you are not going to see this unless you magnify your image to 1:1
and then look across the whole of the image, bit by bit, so it is taking the image of the dust.
As for the Vignetting, as you said it is there but the difference is not normally noticeable unless it really is serious,
but as you change the values it then shows up.
so your flats are working, just you cant see
the bits subtle difference the computer can.
Mac.
It's very difficult to see dust on a flat unless it is an extremely severe example.
To see the dust in your flat do the following:
Take the flat and perform a Guassian blur of around 20-50 pixel radius.
Subtract from the original flat (or use Photoshop difference layer with opacity around 90%)
All the big dark blobs that appear are bit of dust.
Mark
Thanks for your help every-one I think I know where I am going now.
I think my original flats were over-exposed as there is no crud on them at all whichever way I stretch them, if any-one with more expertise than me wants to examine the previous Masterflat it's in my dropbox on:
http://dl.dropbox.com/u/5730788/MasterFlat_ISO800.tif
The ones I did yesterday following new instructions do have some dust particles and vignetting showing on them after a fair bit of stretching, so I hope I'm now heading in the right direction.
I finished the light box and tried it out. I works OKish, need to experiment a bit more, the flats I did yesterday with the home-made diffusion mask into the daylight sky worked better.
This is the one I took yesterday after a lot of stretching to show the dust particles and vignetting.
(http://www.caroleastronomy.webspace.virginmedia.com/_wp_generated/wp8c61a815.jpg)
QuoteThis is the one I took yesterday after a lot of stretching to show the dust particles and vignetting.
See it is working.
You dont need to see the image you have stretched.
All you need to know is it contains the information about the crud,
Same as the bias images, they will be black, but unless you stretch them to kingdom come
you wont see the information contained within (which you are not supposed to see.)
Might update the RTFM post to include these in a little more detail.Mac
The best way to produce a flat is to imge the dusk sky (tonight was perfect) because it is very distant and diffuse.
Next best thing is a diffusion plate over the end of the scope.
A home made light box is a thing of last resort - unless reasonably sophisticated it will never provide even illumination.
The only comment I have about your excellent flat produced yesterday is that the bright patch is not central in the CCD. This almost certainly indicates that your camera is not sitting centrally in the light path. Could be a drooping focuser or a screw not tightened properly. Whatever it is, it probably means that the camera is not "square on" and is likely to cause a little added distortion to stars at the extreme left or right. This might be linked to the chromatic aberration you were seeing on stars central in your field - maybe both the focal reducer and camera are not square on.
Opps I thought you meant this http://tinyurl.com/33j62qu
Quotethe bright patch is not central in the CCD. This almost certainly indicates that your camera is not sitting centrally in the light path.
Yes I noticed that.
I'm sure all the screws are tight.
I'll have another look at it.
QuoteYou dont need to see the image you have stretched.
Yes I thought that might be the case, but I just wanted to see the crud for reassurance that the flats were taken OK.
QuoteOpps I thought you meant this http://tinyurl.com/33j62qu
:lol:
Carole
QuoteOpps I thought you meant this http://tinyurl.com/33j62qu
I bet there's a lot of crud in those flats...
Come on, Chris, you know better than that. The http://tinyurl.com/33j62qu link clearly shows a STACK of flats or MASTER flat :D
Mark, I'm interested in your dislike of light-boxes. I was considering making one of these, largely because in their image processing book, Richard Berry and James Burnell state "....light-box flats are by far the best for amateur astronomers..." However, your method clearly works very well. Could you put a little more flesh on the bones please?
Quote from: PhilB on Oct 26, 2010, 18:58:45
Come on, Chris, you know better than that. The http://tinyurl.com/33j62qu link clearly shows a STACK of flats or MASTER flat :D
Sir,
With refrence to your comment above I would at this point like to say that this is indeed a single flat frame, however, it has very high signal to noise ratio.
So ner!
Yours Truly
Mr Trellis
:P :P :P
Quote from: PhilB
Mark, I'm interested in your dislike of light-boxes. I was considering making one of these, largely because in their image processing book, Richard Berry and James Burnell state "....light-box flats are by far the best for amateur astronomers..." However, your method clearly works very well. Could you put a little more flesh on the bones please?
Phil,
I've never read any books on astrophotography or processing - I prefer to develop my own theory as I go along :-)
As far as flats go, I have discovered by experience that taking an image of the uniform zenith of the sky shortly after sunset or before dawn produces a much better flat than the 2 other methods I have tried:
a) Putting the telescope right against a TV screen (or PC monitor) displaying a uniform white screen
b) Photographing a TV screen displaying uniform white from the other side of the room.
Unless you build a sophisticated light box then the illumination won't even be uniform - so it probably performs less well than a TV screen in close up. So from worst to best I would rate the techniques as follows:
Worst (1) DIY Lightbox
(2) TV screen close up
(3) TV screen from a distance
Best (4) Uniform sky
However I will admit though that a lightbox is very useful as a portable device in the field (literally!) especially when you have to make multiple flats for the multiple filters used.
So why do (1) and (2) produce the worst flats? I think the answer is that the illumination is far too close to the scope's optics and they allow lots of "stray" light to enter the optics at all sorts of weird angles and reach the CCD in ways that would never happen in a real imaging session.
How often do I take flats for my DSLR? Only when I notice that a particle of dust has appeared or disappeared in the frame. In practice this only happens every few months.
Mark
How about ELS.
http://www.w-co.co.uk/el.php (http://www.w-co.co.uk/el.php)
http://uk.farnell.com/pacel/a4-blue/el-sheet-a4-blue/dp/142116 (http://uk.farnell.com/pacel/a4-blue/el-sheet-a4-blue/dp/142116)
Quote from: Mac
How about ELS.
Thanks! A DIY lightbox made from an ELS is as good as a TV screen close up.
One other thing I should have said is this: if you use a small chip CCD then the method of creating a flat is not very critical. But for DSLRs or and larger format CCDs then it becomes more critical.
Mark
Why would ELS not be as good as a sky?
I now have an els panel and it works very well, better than the sky/diffuser combination I was using before.
Thanks for you interesting reply, Mark. I shall experiment with this.
Quote from: Mike
Why would ELS not be as good as a sky?
As I said earlier, if the ELS is very close to the scope, I think it allows lots of "stray" light to enter the optics at all sorts of weird angles and reach the CCD in ways that would never happen in a real imaging session.
Another way of looking at it is that using the sky, an "in-focus" image of the sky arrives at the CCD and so it has the same light distribution across the CCD as the "in-focus" sky background during a real imaging session. But using an ELS, an extremely out of focus image of the ELS arrives at the CCD so it almost certainly has a different distribution of light intensity.
To prove it mathematically would involve performing an integration of light flux at various pixel positions on the CCD. I'll give it a go sometime.
QuoteTo prove it mathematically would involve performing an integration of light flux at various pixel positions on the CCD
:o
See what you've started now :roll:
Mark,
Underrstood, but it still better to use flats than not if you have dust bunnies.
Also "How often do I take flats for my DSLR? Only when I notice that a particle of dust has appeared or disappeared in the frame. In practice this only happens every few months.", using your thought this means your own flats are faulty if you only take them periodically. This is because you will never match the focus and alignment of the camera every time out in the field. So your theory is sound but you yourself do not adhere to it, ha got ya!
Myself, again I don't take flats from here or France, no need if the signal to noise ratio is good enough, however if I was to do photometry I would do.
I'm with you Phil, thats a great book and the example in it are excellent.
Chris
Quotethe bright patch is not central in the CCD. This almost certainly indicates that your camera is not sitting centrally in the light path.
I've checked the screws and the position of the camera and flattener and they are all pushed in as far as they can go, so I can't see anything not sitting centrally. This makes me wonder if it is the telescope itself, could this be due to needing collimating as I notice my stars do not have spikes.
Carole
If you want spikes then stick a cross of string across the front of your OTA, but warning, some people may take offence !!!
But I would say it looks likely your collimation is out. Before you consider any nuclear option, you'll have to double check that the camera is sitting square in the focuser and theres is no sag in the focuser itself. A tell tale sign of a camera not sitting square is out of focus stars on one side of the image.
Collimation will only need to be adjusted if it noticably affects image quality and that may not be the case here.
Robert
QuoteQuote
the bright patch is not central in the CCD. This almost certainly indicates that your camera is not sitting centrally in the light path.
I suspect it might be the light source not sitting centrally on the camera
QuoteI've checked the screws and the position of the camera and flattener and they are all pushed in as far as they can go, so I can't see anything not sitting centrally. This makes me wonder if it is the telescope itself, could this be due to needing collimating as I notice my stars do not have spikes.
Is the focuser fully extended or wound in ? - Thinking the tension may be too loose
How do you collimate a refractor ?
You move the plane of the optics so that they are perpendicular to the normal, the same as you do with a reflector. Trouble is, not all refractors have adjustable cells. But, don't panic!! Having seen your light-box, Carole, I think it likely that Micks option one is accurate. The light source is not on the optical access.
Phil,
I didn't use the light box for the flat in question as I hadn't made it at that stage I put a white sheet of paper over the end of the scope (in the mask that I showed you which only holds the paper in place and does not go in front of the scope) and then pointed it at the daytime sky, so no chance of the light path
not being directed at the scope.
QuoteIf you want spikes then stick a cross of string across the front of your OTA
I don't particularly want spikes I just wondered whether that was another sign of the collimation being out as other people get spikes on the bright stars automatically and I don't.
QuoteIs the focuser fully extended or wound in ? - Thinking the tension may be too loose
The focuser is currently sitting at my "focus" position, and I have no idea how to tell whether the focusser has sag. Just wondering whether locking the draw tube (or not) might alter the light path fractionally.
What I will do is a Star test next time I am out and see what that shows both with draw tube locked and not locked and see if there is any difference. Though what I will do it it's out when it's locked I don't know as I need to lock it to keep focus from slipping as I have experienced in the past.
Carole
Carole, sorry wasn't sure what image I was looking at. As regards defraction spikes, you have a real problem if you're getting them in a refractor 'cos they shouldn't be there!
Quote from: PhilB
As regards defraction spikes, you have a real problem if you're getting them in a refractor 'cos they shouldn't be there!
I agree with Phil.
Diffraction spikes are typically caused by the spider that holds a Newtonian's secondary in place.
If your refractor is producing them then it indicates that something, somewhere is obstructing the light path. Maybe something has come loose? Have a careful look at the objective lens and inside the optical tube to see if anything is amiss. Check for loose bits in the focal reducer and inside the camera. Give it a gentle shake and listen for any rattles.
QuoteDiffraction spikes are typically caused by the spider that holds a Newtonian's secondary in place.
:oops: Silly moi, I should have realised that.
I have just had a thought I wonder whether my "clip in" CLS filter has come loose, will have to check.
Carole
Quote from: MarkS on Oct 27, 2010, 16:33:08If your refractor is producing them then it indicates that something, somewhere is obstructing the light path.
Even very small obstructions affecting the circularity of the aperture can cause quite noticable effects.
I have just checked the CLS clip filter and it was in place. However I notice the filter is not circular, two opposite sides are squared off a bit. Ive placed them top and bottom. Everything seems to be in place, so have taken some more flats to see what I get this time.
Carole
Carole, the filter is rectagular(ish). The ccd is rectangular... They need to line up.
Duncan
Cycling home tonight I was thinking through the maths of why using a lightbox close to the scope produces an incorrect flat. I now know I can definitely prove it mathematically but its a bit complex and few people will understand it. However, I believe can demonstrate it with a simple practical experiment and almost everyone will understand it.
I'll carry out the experiment and post the results. Watch this space!
Quotebut its a bit complex and few people will understand it.
Post the maths as well as the results :D
Mathematician fuel on standby, I think this may be good.
I think I need to qualify my earlier assertion. It may well be the case that for a naked refractor, Newtonian or SCT then a lightbox immediately in front of the scope may give an acceptable flat. However, as soon as you add a focal reducer (which most people do) then a very close lightbox does not give a correct flat.
My argument proceeds by experimental demonstration followed by a thought experiment.
A focal reducer is actually a convex lens and it will produce an image of whatever you put immediately in front of the scope. So I put a computer monitor showing a stripy pattern directly in front of my Celestron C11 SCT. Hold a piece of paper well behind the focal reducer and you can see the image - note the sharp disc (the shape of the scope's aperture) the stripy pattern and the scope's central obstruction. There is also an outer ring - probably an internal reflection - but this forms no part of my argument. Note that a lightbox would not normally use a stripy pattern - I've just done this to make clearer what is going on.
(http://www.markshelley.co.uk/Astronomy/technical/flatdemo1.jpg)
Now as I move this piece of paper closer to the focal reducer the original image appears out of focus. So that this is an out of focus image of the part of the lightbox seen through the scope's aperture.
(http://www.markshelley.co.uk/Astronomy/technical/flatdemo2.jpg)
Continuing to move the piece of paper closer to the focal reducer, we reach the plane where the CCD would sit - at this point we have a very out of focus image of the lightbox. This is the image you are taking when you shoot the flat - the camera CCD sits within the central blurry bright disc.
(http://www.markshelley.co.uk/Astronomy/technical/flatdemo3.jpg)
Now for the thought experiment.
Remember we've just demonstrated that a flat is a very out-of-focus image of the lightbox as seen through the telescope aperture.
We now move the lightbox 10 or 20 feet in front of the telescope (and assume it is large enough to fill the scope's field of view). As the lightbox moves then so does the the focal plane of it's image behind the focal reducer - in fact the focal plane moves nearer to the scope. As before, we can place a piece of paper in this plane and see the image of the lightbox. As we move the piece of paper towards the scope, the image becomes out-of-focus as before. By the time we reach the plane where the CCD would sit the image is very out of focus, but not quite as out of focus as in the original experiment. So the flat that we shoot with the lightbox 10-20 feet in front of the scope is DIFFERENT to the flat that we would shoot with the lightbox immediately in front of the scope. This is the whole crux of the argument - the position of the lightbox DOES make a difference to the flat. An infinitely large lightbox at an infinite distance away would give yet another different blur in the plane of the CCD. This is the flat that we really want and it is best approximated by taking an image of a uniformly illuminated sky (usually near dusk or dawn).
To drive the point home further, here is a 2008 Horsehead using a lightbox flat:
(http://www.markshelley.co.uk/Astronomy/2008/horse_cls_16022008_t.jpg)
And here is a 2009 Horsehead using a flat created from the sky:
(http://www.markshelley.co.uk/Astronomy/2009/horse030108v4_t.jpg)
It took me a long time to realise why my flats didn't work properly!
Mark
Sorry, but I'm not sure that I agree. When you use a light-box to produce flats you're not trying to form an image of the light-box itself. Surely the purpose of the light-box or, for that matter, any type of illumination used for flat production, is to provide a uniform source of light with which to illuminate the telescopes internal components.This then enables things such as errors, marks, dust and other debris within the optical system to be photographed so that they may be subtracted from the final image. It seems to me that it's actually quite important that you should not be able to image the light-box when making flats. If you do then you've introduced an artefact into the flat image which isn't there in the light frames but the software will still attempt to remove it during processing.
I rotate my diffuser or now my electroluminescent panel for each flat and then stack the flats as median or average to avoid pattern and non-uniformity issues.
Phil, yes, I (almost) totally agree. A light box is an physical object that you are putting in front of an optical train. So an image of it will be created somewhere (as demonstrated by my experiment). The flat you create by using a light box is simply a very out-of-focus image of that light-box.
You have fallen straight into the trap I carefully prepared! My whole experiment was designed to prove that you should not create flats by sticking illuminated or diffuse objects of any kind, in front the scope!
I agree that when use use a light-box you are not trying to create an image of the light-box itself. But, unfortunately, like it or not, that is precisely what you are doing! A flat created in this manner is simply the wrong thing for correcting vignetting etc in your astro-images.
That's very interesting and I agree with you Mark.
Another angle on this is that when you are imaging a nebula or other very remote object the photons are travelling parallel to the optical tube by the time they reach it. Because of that they do not illuminate the inside of the tube, edges of optics etc, that may be in or near the light path. An artificial light source in front of the tube is very diffuse and illuminates everything at varying incident angles. It couldn't give you a perfect flat but for most optics and imaging rigs it's obviously good enough.
Hope that makes sense!
Robert
Mark, I think I might be starting to see the light. However, it's the first image in your experiment, where there are in-focus stripes, that worries me. When taking a flat the telescope would be focused on infinity and nothing placed at close range would be in-focus. This being so, I not sure that moving the image plane through what is a relatively short distance would have any noticeable effect on the out of focus image.
Robert, yup, the thought that the light beam is not parallel had occurred to me also. I suppose that you could get round it by collimating the light-box beam but that would require some awfully big optics and an associated price tag.
I obviously need to go away and make a few more mistakes, whilst keeping what's been said in mind, at which point all will become clear!
As a slight aside, most small professional observatories get away with dome flats rather than some light source shining directly down the tube and this seems to work fairly well. Evenly illuminating a flat surface is also easier though portability may be an issue :-?
For the lens I've found shining a diffuse light on to a white sheet on the side of the observatory (aka dome flats) works very well. It works specially well if the light source is far from the sheet to blurr any imperfections. If there are no dust bunnies then synthetic flats can do a perfect job especially if there are no strange gradients and that's what I tend to use for narrow band.
Robert
Quotewhen you are imaging a nebula or other very remote object the photons are travelling parallel to the optical tube
But other photons (not from the target) are approaching at all angles. Admittedly they will not have a direct path to the imaging device but they are still there.
Taking flats against a dawn sky etc. also has the danger of introducing unwanted artefacts such as gradients across the FOV due to moisture in the air etc.
I agree that using a lighbox you are imaging an out of focus lightbox.
Similarly using the sky, you are imaging an in focus variable/uncertainty.
I would have expected that a lightbox
only sufficient intensity to illuminate any debris would be a better option, at least it's controllable and consistent (hopefully)
I know the maths say different.
Phil's quote
QuoteI think I might be starting to see the light
Pun :cheesy:
BTW I am not going to say HIJACK ALERT!!! as this has invoked an interesting discussion.
Carole
Mark,
What you have actually done is move the focus point of the scope by moving the light source, this is normally caused when the light is not evenly illuminated.
Also a FR with one or not hase nothing to do with the quality of the flat, the fat should include the whole image train including, scope, FR, filters, camera, all at the same settings as per the origional image. What you have done is prove if you focus and move the focus you get odd results.
Hence my earlier comment about your camera flats, they are technically flawed.
I'm with Phil on this, all your need is an evenly illuminated flat, without the image train changing anf bingo it works, and I can prove this using maths from the flats I have used with some om older images where alfer applying my flats the image is somewhere in the region of 99.998% flat, thats good enough for NASA its good enough for me.
AND YOUR NUTS thinking about maths when cycling in london, god someone could have stepped off a bus in front of you.
Chris
P.S. Taxi for one!
Quote from: PhilB
Mark, I think I might be starting to see the light. However, it's the first image in your experiment, where there are in-focus stripes, that worries me. When taking a flat the telescope would be focused on infinity and nothing placed at close range would be in-focus. This being so, I not sure that moving the image plane through what is a relatively short distance would have any noticeable effect on the out of focus image.
Yes, the scope is focused at infinity but, yes, it still produces an image of the light box only a foot or so behind the usual imaging plane. Strange, but true. The experiment demonstrates a fact that is otherwise quite unintuitive. The experiment also demonstrates the very noticeable effect of moving the that image plane.
Quotegod someone could have stepped off a bus in front of you.
Chris could you please DROP THIS I am getting fed up with you keep having a dig every so often and it's not funny. I was badly injured by an idiot cyclist, it's not something to poke fun at and keep bringing up every so often. I find your remarks offensive and have so far bitten my tongue rather than have a public argument but since you persist in doing this you leave me no choice.
Carole
At the end of the day the important thing is to produce a flat that is "good enough".
I readily admit that in most cases a light box is good enough for most practical purposes - for instance, it works very well for my ED80 (a Refractor) and for my camera lenses.
However in my experience it did not work well for the C11 (an SCT) nor for the Tak (a Newtonian), so sky flats were required. The experiment demonstrates why this is the case.
Mark
Mark, I have done another set of flats which seem to have worked out OK using a mask over the aperture. The mask has a piece of white typing paper on the front and I can point this at the sky whether or not it is cloudy and raise and lower the ISO according to how bright it is.
I think this will work better for me than a light box and as I don't have to change filters during an imaging session as I am using a DSLR I think I am going to stick to this method, I can always do them the next day if necessary now I can leave things set up.
I still have the slightly off centre problem, and I will give this a try without having the drawtube extended just to see whether focus sag is a problem.
Carole
Thats really odd if its happening on the SCT and Tak, my Vixen was ok, hmm more test needed me thinks.
Chris
Quote from: Carole on Oct 28, 2010, 11:42:06
Quotegod someone could have stepped off a bus in front of you.
I find your remarks offensive and have so far bitten my tongue rather than have a public argument but since you persist in doing this you leave me no choice.
Carole
Hi,
Your right, that subject is probably a little long in the tooth, I'll park it. Also to be honest you don't need to bite your tongue either I'm a complete insensitive git and I know it.
If you ever fancy a public argument I'm game, maybe not tonight, I don't think the membership would be too impressed. Maybe next DSC.
Chris
QuoteIf you ever fancy a public argument I'm game, maybe not tonight, I don't think the membership would be too impressed. Maybe next DSC.
No thanks I prefer a quiet life.
Carole
Fight Fight Fight. :cheesy:
Behave, all of you..............
Otherwise there'll be some slapped legs :evil:
Mark,
I've had a good think and I'm not sure either
1. you're right and I don't get it
2. you're wrong
My issue is understanding the physics of the difference between a focussed image of a diffuse light source, and an out of focus image of an diffuse light source.
I shall think about it more when I'm in the pool tomorrow. Do you fancy posting your maths, see if I can follow it?
Ian,
The maths only exist in my head at the moment. I'll spew it out onto paper as explanatory diagrams at the weekend. Here's another attempt to explain the same thing.
In summary:
The focal reducer is the heart of this problem: the focal reducer screws up your flats. If you are not using a focal reducer there is no problem.
In more detail:
With your scope and focal reducer combination focused to infinity it will produce focused stars on your CCD imager. But at the same time (don't touch the scope focuser!) it will also produce an image of any object you stick immediately in front of your scope. But the focal plane of this image is a foot or so behind your CCD imager (This is what my experiment demonstrated). You can calculate exactly where it appears by using the lens formula and the focal length of the reducer. So what appears on your CCD imager is an out of focus image of this object.
Now if you move this object slightly further from the scope (don't touch the scope focuser!) - the focal plane of this image moves and what appears on your CCD imager is a differently de-focused image.
So if the object in question is an illuminated sheet, we have proved that moving this sheet changes the shape of the blur you record as a flat at your CCD. So one or both are not the "true" flat you require to correct your optical system.
Mark
Or think about it this way:
Draw a squiggle in black felt pen on the objective lens of your refractor. Illuminate the objective lens. The lens with its squiggle becomes an object for your focal reducer to create an image. Find out the focal length of your reducer and then use the the lens equation to find out where the image of this squiggle will appear.
Then puzzle about it for a while and await the subsequent flash of inspiration. :-)
Mark
Right, I couldn't find a black marker so I used silicon carbide scribe.
I'm now sat patiently awaiting a flash of brilliance. Just so I know, what's one like? ;)
As another mind exercise I've tried to remember the last time you were wrong about something. The best I could come up with was when you said "here, try some of this weird French drink".
What I'm now thinking about is the requirement to have something to establish is an image (squiggle, stripes etc) and you argument then becomes a discussion on the effects of faults in the lightbox design.
This therefore brings me back to where I was before. Is there a difference between a focussed image of a perfectly uniform diffuse light source that is of effectly infinite dimensions and one that is out of focussed.
Thinking harder time again...
Mark, your argument appears to boil down to: a light-box flat produces an out of focus image that is much further back behind the plane of the infinity focus than does a does a sky flat. In this case, distortion increases with distance, hence a light-box flat is more distorted than sky flat. Am I getting there?
Lets not also talk about wavelengths or this will never end.
Quote from: Ian
This therefore brings me back to where I was before. Is there a difference between a focussed image of a perfectly uniform diffuse light source that is of effectly infinite dimensions and one that is out of focussed.
Yes, there is a difference. Although your diffuse light source may be infinite, your imaging train has finite sized lenses, mirrors, baffles etc.
It is the finite sized lenses, mirrors, baffles etc. that cause the vignetting. The vignetting (which may have multiple causes) is what you are trying to measure with your flat.
As you move your infinite uniform diffuse light source nearer or further from the scope it does alter the raypaths to the CCD and the distribution of light on that CCD.
Mark
Quote from: PhilB
Mark, your argument appears to boil down to: a light-box flat produces an out of focus image that is much further back behind the plane of the infinity focus than does a does a sky flat. In this case, distortion increases with distance, hence a light-box flat is more distorted than sky flat. Am I getting there?
Yes - I do think you have just about grasped it!
Mark
Quote from: Rocket Pooch
Lets not also talk about wavelengths or this will never end.
I don't this will ever end anyway !
Quote from: MarkS on Oct 29, 2010, 08:54:26Quote from: PhilB
Mark, your argument appears to boil down to: a light-box flat produces an out of focus image that is much further back behind the plane of the infinity focus than does a does a sky flat. In this case, distortion increases with distance, hence a light-box flat is more distorted than sky flat. Am I getting there?
Yes - I do think you have just about grasped it!
OK, so the reason that the problem presents itself more aggressively when a focal reducer is present is due to the reducer stretching the focal plane and, thereby, stretching any distortions that may be present too. Hence, the light-box flat suffers more by this process because it's distortions are greater in the first place.
Oh no!
Brace yourself, Chris - I haven't got to my question about wavefronts yet :o
Hang on let me add to that.
What you are suggesting is that there is an imperfection caused before and after the focal reducer is added?
If it is present after and is more aggressive then the issue is irrelevant, a flat will still cure this if it is taken correctly with the components all in line.
Example a) I have some dust, it looks like this
O O O
O O
O
This is without the focal reducer, and say it has an effect of 5% deviation over against the light frames.
Example b) with the focal reducer it looks like this (smaller o's)
ooo
oo
o
I have reduced the image by 50% to get better S/N but the dust is not a 10% deviation over the light frames. But they have better signal, maybe more noise depending on your location.
If I take accurate flats for example a) or b) can fix both issues.
I still think the issue Mark has shown in the HH image might be because the systems were setup different, or the light box was poorly built.
Oh and just for fun light pollution
As in my setup on one of my images is say the flat has a deviance of 1% of the overall image and the image has good signal to noise ratio then the dust is mostly insignificant.
In a light polluted area and the defects are more significant in your original images due to high background noise adding to the dust effect.
See example below;
No flat
(http://farm5.static.flickr.com/4005/5125220045_529bb484ac_b.jpg) (http://www.flickr.com/photos/13007140@N05/5125220045/)
NoFlat (http://www.flickr.com/photos/13007140@N05/5125220045/) by chrissuddell (http://www.flickr.com/people/13007140@N05/), on Flickr
Flat
(http://farm2.static.flickr.com/1393/5125825318_f5342b849a_b.jpg) (http://www.flickr.com/photos/13007140@N05/5125825318/)
O3Flat (http://www.flickr.com/photos/13007140@N05/5125825318/) by chrissuddell (http://www.flickr.com/people/13007140@N05/), on Flickr
Flat frame
(http://farm5.static.flickr.com/4012/5125222379_50a3af2449_b.jpg) (http://www.flickr.com/photos/13007140@N05/5125222379/)
Flat (http://www.flickr.com/photos/13007140@N05/5125222379/) by chrissuddell (http://www.flickr.com/people/13007140@N05/), on Flickr
Quote from: PhilB on Oct 29, 2010, 12:19:03
Brace yourself, Chris - I haven't got to my question about wavefronts yet :o
Oh thats easy to answer.
I'm now beginning to think I might be wrong about this after all.
Whilst it's true that the focal reducer can form an image of the objective lens (or of the primary mirror), this fact might be a complete red herring :oops:
But, the most interesting thing in all this is it would be quite a good thing to do is write a paper on light pulluted subs and flats and non-light polluted subs and flats. The number of and why use them is always discussed.
Chris
Oh and Robert, before you say my image is out of focus it was Mark's fault for bringing that French S*&t to DSC.
Blimey, Mark could give Stephen Hawking a run for his money!!! :lol:
Mark, I will say you have a lot of nerve to say, you may be wrong about this after all...............
Still trying to get head around the effect of a lens on an extended diffuse light source, regardless. I think an experiment is in order :)
Mark, I hope you're not wrong, I would then have to worry that your luck had run out too... :(
:)
If I was wrong, at least it means I can now go out and buy myself an electroluminescnt sheet and do flats the easy way!
Or, there's a chance that you're right, but for the wrong reasons. This could run on until it challenges the legendary battery thread.
Aaah the battery thread................
Fond memories........................
And no maths :)
I just realised something as well and I do need to goto the garage get a book and then translate it, or scan it, and this will help explain it more.
But, the origional post from Carol said she could not see the dust etc. Now I can only assume this is because the magic 1/3rd - 1/2 half well depth of the CCD has not been reached, but from my point of view how the hell do you measure it on a DSLR, I have never used one.
I show you what I do,
When I take a flat I'm aiming for 1/2 well depth for the flat, I take an image through the whole system and look at it in the image processing software, Carol look at the 1st image its similar to what you see i.e. not a lot.
(http://farm2.static.flickr.com/1100/5125475163_78c6fd466e_z.jpg) (http://www.flickr.com/photos/13007140@N05/5125475163/)
Initial load (http://www.flickr.com/photos/13007140@N05/5125475163/) by chrissuddell (http://www.flickr.com/people/13007140@N05/), on Flickr
Carol this is an image with an Auto Streach function showing similar to you flat.
(http://farm2.static.flickr.com/1126/5125475589_0082bcb93e_z.jpg) (http://www.flickr.com/photos/13007140@N05/5125475589/)
Initial load streach (http://www.flickr.com/photos/13007140@N05/5125475589/) by chrissuddell (http://www.flickr.com/people/13007140@N05/), on Flickr
You will see a dialog on the screen showing the stats on the image, this is how I find out if the flat is any good, cool eh!
How do you do this with IRIS, or DSS and a DSRL?
Chris
I always check the histogram in a processing program for DSLR
Hi,
I just read DSS infomation about taking flats, how unscientific!!!!
I bet I could come up with a better way in err 30 second.
Chris
Quote from: Fay on Oct 29, 2010, 15:16:06
I always check the histogram in a processing program for DSLR
Ok thats good, and on the back of the camera it should be unsaturated and half way up, is that what you do?
Chris
yes, I have read that it is now suggested that you go over the half way mark on the histogram. I only have a small view finder, so look via processing program, or Irfanview or Canon dedicated software
I use the histogram in nebulosity or irfanview if I'm using canon utility.
Chris thanks for doing that, but I am not familiar with the statistics to be able to tell if it's half well depth.
I have been doing some experiments today which I have to analyse as yet. It occured to me that the dome of my POD is white and shows the light through, so I have done a series of flats at different ISO settings and am going to compare the results or indeed see if it worked at all.
Also you may recall in an early post I said that in some literature I had read it had said to do Flats at 100 ISO. Well it seems that DSS is warning me that the ISO does not match the light frames which I do at 800, so would this affect the stacked image?
I also took the "flats" with the draw tube pulled right in to see if this shows I have focus sag. So even if the flats are no good might confirm this at least (hopefully).
Duncan
Quoteor irfanview if I'm using canon utility.
I use Canon Utility but what is Irfanview?
Carole
A flat taken through the dome should work well (for a refractor).
Irfanview is an excellent program: http://www.irfanview.com/
Don't get too hung up on half well depth - just make sure no part of the flat is saturated.
QuoteI also took the "flats" with the draw tube pulled right in to see if this shows I have focus sag
Good idea.
Mark
Quote from: MarkS
I'm now beginning to think I might be wrong about this after all.
Whilst it's true that the focal reducer can form an image of the objective lens (or of the primary mirror), this fact might be a complete red herring :oops:
I've had another idea about this while cycling home last night! I need to do a couple more practical experiments; revisit the discussion and come up with a new explanation ...
Quote from: MarkS on Oct 30, 2010, 08:15:57
QuoteI also took the "flats" with the draw tube pulled right in to see if this shows I have focus sag
Good idea.
Mark
Hi,
I guess this is a test, but when you actually take flats to be applied do not move the drawtube, it should be in the same place as when you are imaging.
Chris
QuoteI guess this is a test
Yes Chris, just to see if the image is still off centre with the drawtube pulled in. I know you need to do proper flats at the same focus. same optics etc etc.
Will report findings.
Carole
ok
QuoteIt occured to me that the dome of my POD is white and shows the light through
OK, the results of my experiment shows that flats through the dome do work. It also shows that the centre of the image is still slightly off centre even with the drawtube completely closed. So that narrows it down a bit, not quite sure where I go from here but I have guiding to sort out as the main priority.
I am going to check flats "through the dome" again at normal imaging focus and make sure they work OK as well.
Carole
Carole, if your image is still off centre there is something not quite right.
It may be that the camera is not sitting central in the draw tube.
If you take an image - flat or whatever then rotate the camera 180 degrees, if the offset switches to the other side of the image, that would confirm whether or not the camera fixings are the problem.
Good idea Mick, will give it a try.
Carole