I was very happy with my webcam M33
(http://farm4.static.flickr.com/3194/2990654075_294bf785d5_o.jpg)
but things change with a bit more knowledge & better equipment
(http://farm4.static.flickr.com/3029/2929363734_88d4988c52_o.jpg)
Fay,
This is why I said about doing a talk at High Elms on the imaging day for people like me (novices), as you can see, the vast difference in the images in such a short period of time is fantastic, and can you imaging what they will be like in another couple of years.
Tony G
So how much of the difference is down to the optics, the camera, the processing, etc.?
Quote from: Rick
So how much of the difference is down to the optics, the camera, the processing, etc.?
Most of it, I would think ...
Quote from: MarkS on Nov 01, 2008, 21:46:22Most of it, I would think ...
:twisted: Talk-to-pedant-mode on! Thwap! :twisted:
How much of the difference is down to the optics?
How much of the difference is down to the camera?
How much of the difference is down to the processing?
How much of the difference is down to other factors?
;)
Oo-er!!!!!
As Mark says, Rick, all have a contributing factor.
If any one of the following was missing:
better telescope & camera
more knowledge in processing
aquiring the image with guiding knowledge
more patience
a better image would not be possible.
Fay
Fay - did you those 2Xm33 with the same scope.... i.e. ED80? if so telescope contributes 0%.... :-) John
Yes, you are correct, that does eliminate the telescope. Very observant John.
Rick I would think appart from the scope quite a bit is down to the camera and the guiding, but the main element is experiencing here, I have seen some very good web cam pictures and some dam awful 314l images.
I don't think this is something which could be easily quantified, although technically we could work out the camera specs and also the sky conditions and also the S/N.
But I can't be bothered...
ATIK 314L: Sony ICX285AL ExView sensor. 1392x1040 pixels, with 6.45um. Readout noise 4e RMS
Webcam: 640x480?
I think that the points I made relate to a general improvement, whatever image, in equipment & experience, when going from a basic setup to a more sophisticated arrangement.
The ED80 would not make a big difference in overall improvement alone, without the other considerations, as can be seen in the first image. I am not saying the second one is amazing, it is just a lot better.
Rick,
QuoteATIK 314L: Sony ICX285AL ExView sensor. 1392x1040 pixels, with 6.45um. Readout noise 4e RMS
Webcam: 640x480?
Agreed camera does make a big difference but at the same time what both Chris & Fay has said is true. Fay's progress in this hobby is unbelievable... You can have the best camera in the world but if you don't know how to polar align, know how to run two scopes & camera's simultaneously, track objects to sub pixel accuracy for mins at a time and of course process to extract all the data from those those photons then your images will still be crap....
You can also learn how to add diffraction spikes if you want as well....
John...
Quote from: JohnP on Nov 02, 2008, 13:33:41
You can also learn how to add diffraction spikes if you want as well....
Stoppit... :D
fay was it the sc1 or sc3 on the older image?
Don't think Fay had SC3...?
SC3
I stand corrected... I didn't know that you had a webcam with the SONY ICX424 AL chip in.... :-( sorry...
Quote from: JohnP on Nov 02, 2008, 13:33:41You can have ...
And, conversely, if your kit is poor, that's going to limit what you can achieve.
The difference between the two images is amazing. Likewise yours of M42 (http://forum.orpington-astronomy.org.uk/index.php?topic=3988.0). I was hoping folks might actually look a little deeper, think a bit about what makes a difference, and maybe share some of what they've learned. If the way the improvement was achieved is treated as a black art then how's anyone else going to learn anything from it?
Hi Rick,
There are a few things I noticed which made a massive difference in my images, the 1st was to have an actual cooled asto CCD, being able to Autoguid Properly and have optics at least F7.5 or faster, basically anything to get the signal to noise better. After that we can all argue over QE, Ha Sensitivity and scope designs, but that the finer point of the sport.
For example Atik 314L and ED80 F7.5 required four times the exposure than a Atik 314L and a 6" Newt at F5, but the newt's stars may have diffraction spikes, etc etc.
The second part, and this is where some people here have come on in leaps and bounds, is to understand the processing process, it really does not matter how good the subs are, if you don't combine them properly the result are crap, for example I have a perfectly good noise free set of LRGB of the packman, but I can't quite get it right yet and the image looks rubbish.
Oh yes and light pollution this has probably the biggest effect on LRGB than anything.
Chris
I'll dig out my 2004 M33 but here's the 2008, the only difference between fays and mine (taken on the same night, same conditions, same location) was Fay was shooting at F7.5 I was shooting at F5 and my subs were 10 minutes, I think Fays we're 5 minutes, I have 5x the signal to noise.
(http://farm4.static.flickr.com/3259/2907846726_80696c12f5_b.jpg)
We have to consider location as well, also seeing conditions, very important. Very nice M33 Chris
I think an SC3 mod was the replacement of the chip for a more sensitive one. I Can't remember which chip it was. I think they do mods different now.
Fantastic progress Fay, Im sure equipment helps a lot, but technique definitely has a massive part to play, a big revelation I've had is that I now don't go hopping about the universe looking at objects, I make a choice on what Im going to image at the start and stick with it. I was trying to see too much, but not getting enough data on any one object before, hence some of my images being a bit better as of late.
Yes, I tend to get excited when I see all the goodies up there & want to do more than one a night!!!!!!!
Looking at the quality of Chris's M33, you can see where his experience in guiding & processing make such a big difference to an image. I was having a problem with guiding & although sorted, it was not good enough, that night, for 10 minutes, whereas Chris's was fine.
Chris what F5 scope have you got?
For me the biggest problem has been noise, and im definitely noticing the difference by not hopping about so much and getting good long exposures (im looking to add to my M42 and horsehead this year, I'd like to get a full 12 hours on both) Im hoping to get around my S/N problems using the DSLR quite soon, not by jumping to a cooled camera (don't think im ready for that yet) but I've finally ordered a hyperstar and im hoping to get my sub times down to under a minute (probably as high as I'll be able to go with the light polution around here)
Chris, im just wondering, I saw your working out of the S/N of yours and fays kit, do you know how much light gathering power the 14" at F1.9 would have over the 120ED at F7.5 im using right now?
Don't know about light gathering but I'm sure it would be 100-200 times faster at capturing the same data. I feel you may have hit the nail on the head when it comes to faster imaging in LP skies, so it will be very interesting to see your results, especially with the V3 Hyperstar and DSLR chip combo.
The light gathering part is easy, its just the following equation (size of large scope/size of small scope) squared.
it should realy be area, but the 2*pi, cancells out on both sides.
which works out as (355mm / 100mm) squared = 12.6
So for the same F ratio you scope has 12.6 time the light gathering power.
The differences in the F stop 1.9 over 7.5 work out roughly the same as F2 & F8 (easier on the maths), so this is a ratio of 4.
Exactly it 3.9......
F2 = 1min
F4 half the light. twice the time, ect. (better depth of field)
so its roughly 12.6 * 4 which is about 50 times faster
So if you took a 1 min sub on the 14" at F1.9, you would need to take a 50min sub roughly on the ED120.
to give you a better example. if you took a 1min sub on the keck (Single mirror), and compared it to your hyper star.
(keck/14")^2
(10m/.355m) the light gathering power of the keck is ~800 time that of your 14"
Cant find any data about the F stop for the keck.
Chris, I see your M33 was taken thru Ollys Celestron Genesis. Is that a better scope than the Skywatcher?
Mac, you are so clever!
It was a Televue Genesis, bit more expensive than a Celestron. It was also Olly's kit EQ6 etc, and AA guiding, basically the same setup except the scope.
Hold on - I think we're in danger of confusing two concepts here and drawing the wrong conclusions.
For astrophotography, just like terrestrial photography, the ONLY thing affecting image brightness at the film/CCD is the F-ratio (i.e. focal length / aperture). Any two lenses with the same F-ratio will give the same brightness - this is precisely the reason why F-ratio is so important for photographers. When you put your camera on the back of a telescope you are simply treating the 'scope as a giant lens. So an 18mm wide angle lens at F10 will give exactly the same image brightness as a Celestron C11 at F10.
So what changes as we jump from 18mm to the the 2800mm of the Celestron? Magnification! A 1 minute exposure of the Orion Nebula taken with the 18mm (at F10) will have just the same brightness as the 1 minute exposure of the Orion Nebula taken with the C11 (at F10). But the C11 will have have a much smaller field of view i.e. vastly increased magnification.
As the F-ratio is reduced, brightness increases and the the signal-to-noise ratio of the CCD improves with it. Hyperstar is the logical extreme of this - taking the F-ratio down to approx F2. F2 will give an image 25x brighter than F10 (the square law applies) so vastly reduced exposure times can be used.
So, going back to a previous question, given two telescopes with the same F-ratio, does the aperture make any difference? The answer is that the aperture will affect the magnification in the final image but both scopes will deliver the same brightness to the CCD.
Mark
Rick,
I agree with Chris on this one.
Once we have eliminated the optics (which apart from collimation and focusing, you have no control over) the main issue for image quality is reducing signal-to-noise ratio (SNR).
The main reasons for noise in the image are:
1) Light pollution (street lamps or moonlight)
2) Dark current of the CCD (cooled chips deal effectively with this)
3) Read noise - this is an inherent characteristic of the combination of CCD and amplifier.
The best way to reduce SNR is to reduce the F-ratio (by buying a different scope or attaching things to your present scope e.g. reducers or hyperstar )
The other way is to use much longer exposure times - but then we have to start using guided exposures and this introduces a whole new set of additional technical problems.
To a certain extent, light pollution can be dealt with by using narrowband filters but the only real solution is to go somewhere dark.
Once you acquired good, noise-free raw data then processing this into the best possible final image is another art which takes a long time to learn. And processing is not free from it's own controversial issues. It depends very much on your goal. If your intention is to perform photometry or some other science on the final image then your processing sequence will be different to the one required to create a beautiful poster for you living room.
Mark
true but the two scopes in the question, were not the same F ratios.
Quotedo you know how much light gathering power the 14" at F1.9 would have over the 120ED at F7.5 im using right now
So if you work out the light gathering power difference, as this is only the difference in size of glass, it works out at 12 times the light gathering power.
So if the two scopes were the same F number, then the 120ED would take 12 times longer to record the same amount of photons, due to the size of the optics.
and the difference in F stop, as you said relects the brightness, but the two differences is a factor of 4, ~F2 against ~F8
if you compare like for like, then an F8 scope will take 4 times as long to collect the same amount of photons as a F2.
so you do have to take in to consideration the size of the object lens as well as the F ratio.
As for the image size, dont forget the focal length of a scope is its objective size * F ratio. The F number is normally fixed. F10, 6.3 ect
as the F has now been changed from F10 to F1.9 its focal length also changes.
The 14" would now be a 674mm focal length telescope. (355 * 1.9) where as the ED would still be (100 * F7) 700mm focal length scope.
ps
this is the maths for the keck. http://ceres.hsc.edu/homepages/classes/astronomy/fall97/Mathematics/sec15.html (http://ceres.hsc.edu/homepages/classes/astronomy/fall97/Mathematics/sec15.html)
Hi Guy's thanks for the explainations, sounds like I should be able to get my exposure times right down then, I was worried light polution would be a big factor at this F ratio, but since the mirror will collect more photons in less time, I guess this will all balance Itself out, I'll still be getting the light gathering power of a long exposure on my refractor even if Im only imaging for low exposure times.
Thanks
Daniel
:O)
Mac,
I think I'm beginning to see where our arguments differ. I think there may be 2 cases that need to be considered.
1) For an extended object (e.g. nebula, planet, moon crater) I still argue that the F-ratio is the only meaningful comparison between telescopes for astrophotography purposes. The inverse of the square of the F-ratio determines the number of photons hitting each pixel of the CCD. For instance an F8 scope will take 16x as long as an F2 scope to collect the same number of photons/pixel.
2) On the other hand, for an isolated point source (i.e. a star) the limiting magnitude seen through the eyepiece is definitely related to the size of the objective i.e. the light gathering power.
I need to think about this further ...
Mark
Another way of thinking of it Mark...
Pixel scale is inversely proportional to focal length so for a set focal length the light falling on one pixel is proportional to the area of collection (squares of the diameters or radii) of the objectives.
Assuming the 120ED and C14@f/1.9 have the same f/l then the ratio of exposure lengths = 350^2/120^2 = 8.5
Assuming 10um pixels for simplicity:
ED120@f/7 has an pixel scale of: 206265 * 0.010 / (120 * 7) = 2.45 arcsec/pixel
C14@f/1.9 has an pixel scale of: 206265 * 0.010 / (350 * 1.9) = 3.10 arcsec/pixel
206265 = number of arcsec in a radian.
The C14@f/1.9 will collect 3.1^2/2.45^2 = 1.6 times the background signal of the ED120@f/7
So all in all the C14@f/1.9 will be 13.6 (8.5 * 1.6) time faster or will need 1/13th the exposure time.
For point sources like stars, correct focus has almost as much bearing on signal strength (and therefor s/n) as anything else. At focal lengths of 1.9 the critical focus window will be tiny and I would think a robotic focus or bahnitov mask will be essential.
Think that makes sense.
I bought some acetate for printing out a Bahnitov mask just yesterday, right now Im using both a hartman mask as well as Live view to zoom in 10x for focusing. Actually, with the greater light gathering power I wonder how much more will show up in my live view now, it would be nice not to have to focus on really bright stars for a change.
Quote from: RobertM
Another way of thinking of it Mark...
[argument snipped by me]
Think that makes sense.
No, it doesn't make sense. If you apply that argument to two different camera lenses (instead of two differennt telescopes) then the result violates what every photographer takes for granted i.e. that equivalent F-ratios give equivalent illumination.
I'll try to come up with a simple explanation this evening ...
Mark
Robert,
I owe you an apology: you did arrive at the correct result i.e. C14@F1.9 requires 1/13th the exposure time of ED120 at F7
I was slightly confused by your workings and misinterpreted the result.
However, you can get the result much quicker by working directly with the F-ratios:
Ratio of exposure times = square(Ratio of F-ratios) = (7 / 1.9)^2 = 3.684^2 = 13.573
Mark
Mark,
Yes, sorry it was a but obtuse but I wanted to work it all out in my mind using a worked example. I was thinking about your last response and came to the conclusion that we were all talking about the same thing in different ways. Thanks for proving my calculations and for the very useful shortcut.
Daniel,
Good luck, you'll have one heck of a rig when you get that all going perfectly. Been tempted for a while with the Starizona system but I'll see how yours goes with the new model before making any decision.
QuoteIf you apply that argument to two different camera lenses (instead of two different telescopes)
I aggree perfectly, but dont forget, with a lens, the focal length of the lens is completly fixed.
as is the F number an f2 lens is always an f2 lens., you can make it an f10 or f22, by stopping it down..
but with a camera lens, you are not changing the focal length by changing the F stop.
a 50mm lens @ f8 is the same focal length as the 50mm lens @ f2, but the amount of light has been changed by the diaphram at the back.
a 300mm F2 lens will give you exactly the same shutter speed as a 50mm F2 lens, but the size of the front optics, is greatly enlarged, to allow you to do this.
The F stop has been changed by stopping down the amount of light by making the exit hole smaller, thus increasing the depth of field
and increasing the exposure time, you havent actually changed the f of the camera lens. The 50mm @f 8 is still f2, but its just been stopped down.
With a telescope, the focal length although fixed normally is the size of the optics * F stop.
so the celeston 14" F10 has a focal length of 355 * 10 = 3550mm focal length. normally.
as a lens it would be decsribed as a 3550mm f10
but fitting the hyper star changes the F from F10 to F1.9
so its focal length changes from 355* 10 to 355 * 1.9 = 674mm focal length
its still a 14" telescope, but its focal length is 674mm as against 3550mm.
as a lens it would be describled as a 674mm f1.9
for the ED120 as a lens it would be described as 700mm f7.5
so effectivly, your 14" would be a ~700mm f1.9 lens as against the 700mm f7.5
The problem is (which i've just noticed sorry), by fitting the hyperstar, you have converted your 14" 3550mm into a super fast ~700mm scope
so the difference in speed is just the difference in ratios of the f. which is about 8, as you have already taken in to consideration
the change in focal lengths and object lens size ect.
i think somewhere back in the thread, i've combined the light and the f together. :oops:
So
if you bought an F1.9 700mm scope, you would get the 14" + hyper star.
and if you went for the 700mm F7.9 you would get the ED120.
and if you wanted an F1.9 3550mm scope would need an objective lens about 1.8m
Quote from: Mac
if you wanted an F1.9 3550mm scope would need an objective lens about 1.8m
Sounds good - where can I get one?
QuoteI bought some acetate for printing out a Bahnitov mask just yesterday,
I assume that you are going to print it and then place it on the front of the optics.
Not too sure if the affect of the focus would be that critical,
but dont forget you are introducing an extra light path through the acetate,
so the focus would be
slightly different, then without it, (it might not even be noticable)
although if you removed the mask and replaced it with a clear acetate,
it would still keep the same extra light path.
is it me, or have we managed to combine three different subjects in this one thread?
http://en.wikipedia.org/wiki/F-number
I love this, all I did is went away for one day come back and look what happened.
So I assume we are all in agreement about the F no and S/N.
Shall we start on the pixel size and sensitivity well depth and also the ADU count?
John and I had a good chat about the following, this is a pop quiz.
If John and I take two images of a star using two different CCD's
John's has an ATIK ic16 QE68%
Mine a ATIK314L QE65%
Same scopes, same conditions, same night, same star & same duration of exposure, everything the same.
Why is my bright star at 64k ADU's and John's at 38ADU?
And how does this affect the price of fish?
Daniel,
I'm really looking forward to seeing your results with the hyperstar -it sounds like a brilliant setup. The learning curve may be steep and there are a few obstacles you may need to overcome:
1) At F2 focusing will be extremely critical as will alignment and collimation.
2) Light pollution might still be the main limiting factor because it will saturate the CCD very quickly forcing you to keep exposures short. But then again short exposures is its main purpose!
3) Related to the above, some filters will not work very well at F2 - the steep angle of the light cone can cause loss of transmission and colour displacements (I also noticed this when putting my CLS filter on the front of a F1.8 camera lens). Check out the manufacturer info for each filter you intend to try.
4) Will Hyperstar give pin sharp stars across the whole DSLR CCD (which is big) or just a central portion?
Good luck!
Mark
Quote from: Space Dog
If John and I take two images of a star using two different CCD's
John's has an ATIK ic16 QE68%
Mine a ATIK314L QE65%
Same scopes, same conditions, same night, same star & same duration of exposure, everything the same.
Why is my bright star at 64k ADU's and John's at 38ADU?
I guess you mean John's was 38k ADU?
You pose an interesting question. The similar quantum efficiencies (QE) should mean they both capture the same proportion of photons. I'm not familiar with the ATIKs but here are some possibilities for the difference:
1) The pixels may be bigger on yours so each pixel collects more photons
2) The amplifier gain may be different i.e. electrons/ADU may be not be the same on each camera
3) At 64k ADU it also sounds like your pixels were saturated i.e. the light reached the full well depth. Maybe both cameras had saturated and John's well depth is only 38k?
Not sure how the price of fish is affected - maybe it's the Observer Effect: the very act of observing the Universe changes its state ...
Mark
Surely light pollution/sky conditions in the differ enet locations would have a huge effect on ADU.
Same place, same time, same image, same type of scope same filters. Same star being images for the same duration of time by both cameras.
The observer was a small mirror carp.
In that case one of you had the ability to deflect or attract photons and therefore making less of them go down one tube compared to the other.
QuoteSame place, same time, same image, same type of scope same filters. Same star being images for the same duration of time by both cameras.
So if everything was the same including the price of fish and the mirror carp wasn't under the influence of some exotic fish bait, no focal reducer used and the star was in the center of the FOV. Is it possible that the fish had a dodgy eye and used that for focusing john's image.
In all seriousness, that might be why. If the focus was just a tiny bit different on each set-up (which it would have been) then the ADU would be slightly different.
Did you use the same carp for focussing both scopes?
Ah ha everything is not the same, the cameras are different, the carp is exactly in the middle of the two scopes and is in a cardboard box.
So there is a difference, what's the answer?
If the carp is in the cardboard box, with a vial of arsenic, with the lid closed, and a tree fell in a forest with no-body around, would you see a rainbow?
Ok, the Carp is in fact a Red Herring and has nothing to do with the images, but..
The cameras has similar QE give or take 5%, but they have different full well capacity, I think I got them backwards by the way.
So with the 314L to conversation between the photons collected in the CCD is of a greater scale than on the 16IC
Roughly the 314L has a 27k full well capacity, thats 27k photon to get to 64k ADU
The 16IC has a 30k+ (and its apparently more like 60k) therefore 27k photon get you approx 32k ADU
That's it.
So how about doing a talk on that and image processing ?
Because I fear most of the people in the room will fall asleep.....
I'm counting on that ;)
So both the well capacity and the gain are different for the two cameras ...
yup, John noticed this the other week, but that's fine they are different chips and designs.
So the point is, its difficult but not impossible to be objective about the differences in performance of the optical systems, unless we fully understand what we are imaging with.
So with Rick's originals question we could work out what the differences are, including the sky background noise, but it would need some reference data, flat, flat dark, darks, bias, subs etc for each camera. Also if we were using different scopes we would need to have an understanding of the optical differences, F Ratio.
The sky background reading v's surface ADU's in reference between two images as a ration of brightness could be worked out using a reference no variable star as part of the image, this can be done using photometry, then we could measure the background sky reading after Bias etc are taken off the images, and then scaling them according to the duration of exposure etc.
So it is possible, but only if we take the reference images.
I have a set of images on M66 & M27 with all the BIAS Durations, Optics, Flats and reference items known, so I could probably do the same with the 314L and the 8" RC and work out what the effective difference is, but that would mean two nights imaging, and its cloudy and the wrong time of year.
Personally I think we have all practised a little more and we're just better. We can focus better guiding is better and also the processing is better, and because we're not taking scientific images we don't do all the bits needed to do a proper evaluation. But subjectively one is a hell of a lot better than the other.
For example, in mine and Fay Les Granges images we have not taken any flats, but I know we should have if we were to do a proper evaluation of the image.
Also if Fay or I was to try to re-produce the same images here we would need to take flats.
So another pop quiz, then,
If we were to take two images of the same object with the same equipment one in france, background ADU Sky Count = 200, and one from my back garden background ADU Sky Count = 2400, why do I need to take flats at home and not in France? The surface ADU reading of the object being imaged is 2600 ADU's?
This time the images we're being observed by a small horse called Monty who is playing a flute.
Because with the lower ADU in France the conditions responsible for making things such as dust bunnies, etc. show up on the image are far less prevalent, plus, I am guessing, the background IR radiation will be less (due to less refelctivity from droplets in the atmosphere, etc.) and therefore the overall noise level will be lower.
Monty must be very small indeed to enable him to get his hooves to press the keys on that flute. Either that or he is playing a very large flute. Does the size of the flute affect the background ADU somehow?
To answer Chris's second pop quiz question regarding flats:
One of the main reasons I take flats (other than for dust bunnies) is to remove the effect of vignetting. Vignetting means that with a high background sky count (as in your example) it will not be the same count across the frame but will be greater in the middle than in the corners. This variation in the background sky count makes it tricky to remove. Applying flats will compensate for the vignetting and subsequently makes the background easier to remove (it is likely to then be a simple matter of gradient removal).
There is a problem with flats - lots of buy-to-let investors have lost money on them - an event that they thought as being as improbable as seeing a horse called Monty playing a flute.
QuoteIf we were to take two images of the same object with the same equipment one in france, background ADU Sky Count = 200, and one from my back garden background ADU Sky Count = 2400, why do I need to take flats at home and not in France? The surface ADU reading of the object being imaged is 2600 ADU's?
I'm guessing that it's something to do with the fact that in the image in France the S/N ratio is so much better (because background is a lot lower) so you don't need to process the image as hard to get to the finished image. With the one from home you have to push the image hard to extract the signal above the background so issues like dust bunnies & vignetting which lie down in the background become more obvious....
The thing that confuses me is that according to theory it is more important to take flats from a dark site than a light polluted site...??
John
I thought it was darks not flats?
I still prefer Fay's second image. :D
Tony G
Sorry mate - yes you are correct (smart Arse) - I meant to say darks from a dark site than a light polluted site...
John
PS - Tony - I hope you are following all this for the next image you take at DSC....
John,
Following all what? I was lost from about the third reply, and when I looked again, I thought that I had stumbled onto an Open University Course in mathmatics and advanced physics in F ratios, I logged off three times before I realised I was on the right site. :-?
Tony G