Saturn

[Fay]

Did my first planet for a couple of years, last night. Skywatcher Mak. Cass. 6", DMK 04M camera.

I am quite happy for a first attempt, but a palaver getting on chip & focusing etc. Clouds were coming & going & every time I put the barlow on, I had to take it off put eyepiece in, centre it, put camera in without barlow, centre it, put barlow in search for it!!!!!

[RobertM]

Fay, that looks fantastic, and the focus looks spot on.

Sounds like you need a flip mirror, wasn't Chris is selling one the other day ?

[Fay]

What's a flip mirror?

[mickw]

About £50 

http://forum.orpington-astronomy.org.uk/index.php?topic=3258.0

[MarkS]

Well done Fay.  I did get the telescope out last night but the conditions weren't as good as Tuesday so I gave up.

But you got a decent image!

What "shutter" speed were you using?  How many frames did you stack?

One tip: when you put the Barlow in, use the eyepiece to centre Saturn then swap the eyepiece for the camera as the final step.

[Fay]

Mark, I must confess, I was concentrating on technique & forgot to notice how many frames were used etc. All I know is they were captured at 30 fps.

Will take notice next time.

[JohnP]

Very nice Fay - came out very well. Looks like it needs some more frames to reduce noise & you have to do colour next time Good to see you are making use of the 6-inch at last....

John

[Fay]

Yes, I was quite pleased as it was the first, but know more of what to do next time.

[Carole]

Please could some-one explain "noise", (what it is and what it does to your images).  I thought it created hot spots and interference etc on the background sky, but I can't see any problems with Fay's, so I obviously don't know what to look for.

Thanks
Carole

[Ian]

noise in a general term for anything that shouldn't be there. In Fay's image, the disc of Saturn is a little grainy, and we know from hi-res observations that that grain is not actually there on the disc. Therefore it's reasonable to call it noise.

Noise can also be used to describe other artifacts, including diffraction spikes and so on.

If you can tell what's noise and what's not, an absolute measure of the quality of the data (image) is the ratio, SNR or Signal to Noise Ratio. The higher the better as that indicates lots of signal to not much noise.

Granted SNR is not often talked about regarding astro imaging, but you may have come across it in other applications, including how good a tv signal you're getting. Alot of digital TV tuners show the quality of the signal as the SNR.

[JohnP]

Hi Carole - there are loads of webpages on the net that help with this... here is one:

http://photo.net/learn/dark_noise/

Basically - noise makes an image look grainy - the background on Fay's image is fine but if you look at the globe it has a 'mottled' appearance - Don't get me wrong Fay's image is great but if she took & stacked more exposures the signal would increase & noise diminish so globe would have a much smoother appearance..

As an example have a look at this old image I took from 2003 (not a very good one)... you can see just a single image of Saturn & a stack of 250 - notice how smoother the stacked image is - The final image is still noisey but a lot better than just a single image..

http://homepage.ntlworld.com/john.punnett/graphics/archive%20images/saturn/saturn_2.html

HTH, John

[Fay]

Perhaps I went heavy on the wavelets.

[JohnP]

Fay - Could you explain wavelets for me please 

[Fay]

Don't be cheeky.

Saturn was 720 frames. Quality 85%.

I have just merged two AVI's total of 1263 frames 90% quality, but looks similar to me.

Should it be a lot more?

[Fay]

Does Registax tell you how many frames it has taken? I could not see it, only the total of what it was starting with.

[Mike]

Noise can also be used to describe other artifacts, including diffraction spikes ......

Oh god no! Not again !

[Carole]

Thanks Ian and John, will take a look at the website John suggested.  I have been trying to remember back to that talk Mike gave on Astro-Imaging at the members evening some time ago, and remember him talking about noise, but came away with a different impression of what it was. 

Mike any chance of a repeat performance?

A bit more leant.

Carole

[Rocket Pooch]

John,

Frodo has waveletts in his hair, thats about as much as I'm going to explain.

Chris

[MarkS]

Fay - Could you explain wavelets for me please 

Explaining wavelets - how to do this intuitively - now there's a challenge.  Maybe this should be in a new thread ...

Here's my attempt.

Wavelets allow you to increase definition and contrast in the final image when the original image has been degraded by:
1) Slight Gaussian blur due to atmospheric conditions
2) Slight defocus in the optical train
3) Central obstruction in the optics (e.g. using an SCT instead of a refractor)

It does this by allowing amplitude adjustments (amplification) of frequency components in the image. It is very analagous to using the equalisation controls of some household audio amplifiers - you can boost low, mid or high frequencies independently.

But why does boosting some frequencies improve the image?  The reason is that the three blurring effects mentioned above do so by attenuating certain frequencies in the frequency band.  Wavelets boost those frequencies back to where they should be.  Watching the universe through the atmosphere is a bit like recording a concert from behind a pair of curtains - you'll end up with a muffled recording.  But if you play back the recording with the correct settings on the equaliser then you can make it sound more natural again.  This is exactly what wavelets is doing to your prized image: the sliders in the wavelet application are behaving just like the sliders on your audio equaliser.

There is a fly in the ointment however - noise.  Your degraded image will incorporate the same amount of noise as an undegraded image.  So when you boost some frequencies you are also boosting the noise associated with those frequencies.  This ends up causing speckely/blobby effects right across your final image.  Going back to the audio analogy, when you boost certain frequency bands using your equaliser you end up increasing the backgound hiss through the speakers.  And this is where things become very subjective: just as various people can live with varying amounts of hiss as they play back their degraded audio recordings, so various people can live with various amounts of speckly/blobby noise in their final image.  There is no right or wrong here - improving definition and contrast will also increase the speckly/blobby noise  - each person makes their own compromise between the two.

Now, if your degraded image has low levels of noise then the frequencies can be boosted much higher before noise starts to become objectionable.  This is why we stack hundreds of frames of Saturn together - it improves the ratio of the signal that we want versus the noise that we don't want.

In theory you can also apply wavelet transforms to improve definition in your images of faint galaxies.  But generally speaking the signal to noise ratio of these images is much lower and you cannot boost the frequencies very far before the speckly/blobby effects become severe.

For the more technically minded amongst you, there is a good reason why it is very important that your focusing is as accurate as possible.  Wavelets can reverse mild amounts of defocusing but for more severe cases a defocused image, for certain frequencies,  contains phase reversals as well as attenuation.  These phase reversals can only be corrected by a more sophisticated technique - deconvolution.  There are whole families of these deconvolution techniques - Wiener, Richardson-Lucy (famous for the old Hubble images), Maximum Entropy to name but a few.  But now we are well outside the scope(!) of an intuitive explanation.

[Rocket Pooch]

sounds like decon to me ?

[MarkS]

Wavelets is definitely deconvolution.  I should've made that clearer.

[Fay]

No wonder John asked me to explain wavelets!!
I'm glad Mark took up the challenge, very well, before I could post the same answer!