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Messages - Bobinius

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16
General / Re: The problem of green in narrowband SHO palette
« on: 2019 August 05 08:08:39 »
Any method you choose to create your final colour image from your NB data is perfectly valid.

After all, you didn't start with 'colour' in the first place.

You are free to use any processing steps you wish to get the image you desire - and there are no requirements to 'fill the Green channel' with data whatsoever.

Food for thought?
h
I agree that the color assignment is arbitrary and the SHO palette is there mainly for contrast reasons. Once you assign Ha to green, the Ha data is Green ( I was talking specifically about the SHO palette). If I am trying to take a real picture of a celestial object, that's representative, am I not going to lose data and misrepresent that object if I clip the green data? Should this difference in intensity be compensated by longer total exposure for O3 and S2 for example?

17
General / Re: The problem of green in narrowband SHO palette
« on: 2019 August 05 08:03:49 »
The green should not actually be there or is required, however, it is the way the different channels react.....its very complicated, Vincent Peris did explain it to me with lots of diagrams but it was 5 years ago and I have forgotten, basically you should not have any green in a narrowband image, unless, of course you want some.....that's all up to you.

To remove red use color masks then curves on the red channel.

Thanx for the video, very useful as I did not use this mask technique for processing. Probably the mask is more flexible than SCNR. SCNR is really fast for removing the magenta around the stars (due usually to the O3 filter bloating): you just invert the image and apply a SCNR at 1, that's it.

The idea with the red is that we usually do not decrease the reds for example in a HOO image (or do you?) It would be analogous to decreasing green in SHO. How do you know IF it should be removed or HOW MUCH to remove? (other than esthetic criteria, "I like it more this way" kind of justification).

18
General / Re: The problem of green in narrowband SHO palette
« on: 2019 August 04 15:51:26 »
Thanx, I'll check it out.

But why do you think that some green should be removed in the first place? (We may like more or less green, but since it is Ha, how do we decide ?) I feel it is like taking out green from an apple to make it look more yellow when it is actually green. I don't know of processing steps that take out red from a HOO palette (when it is the same Ha signal).

19
Took some image with my C14 Hyperstar + EOS 60Da (32x30s IRRC) and trying to take the drizzled and pcc'd linear image through deconvolution/denoising and stretching. I think I am reasonably proficient with star masks now (I do miss a few double stars....seems like a lot of trouble to refine to get those last bits, is this typical?) and tried to create an appropriate deringing support as advised here:
http://www.pixinsight.com/examples/M81M82/index.html

No matter what I do, I can't seem to to improve the image with deconvolution.  I can see "too much" where it looks like it is being stretched around noise or what looks like next to nothing. I can't see the same sorts of visual contrasts that are in the tutorial.  An important issue:

When I create the external PSF, the star profile is not round; a brutally honest reminder that the collimation is off (I've been trying to fix, but having a hard time).  Does the poor collimation affect how deconvolution will perform? E.g. Will it try an model collimation error in the underlying model that is based on atmosphere?

Are there any other image issues that stand out? Note I only correct with bias and flat due to the way the Canon does darks.


I've included masks in the zip file (1G):
https://drive.google.com/open?id=1tA7bES-s9rlv09VReQBoYFz6qVgUcDxv

Edit: Added a screenshot of a blind mask stretch and HDRMT.  Nice to see that there is some comparable detail in the data...I wouldn't know much more than to do pre/post denoising in the dark and a little bit of saturation and brightness adjustment.  Should I be able to do much better with this data? I swear I see a bit the IFN when I compare to other pictures, but it is quite faint and not sure if I can successfully pull it out.

I I understand correctly, you are expecting some spectacular change but you don't see any difference after deconvolution? The change is rarely spectacular but it can increase the details in the nebulosity. However, in order to see it you need to use a preview with a small nebular selection (maybe you do that already) and some stars; if your field is too wide it is hard to see the difference in the details. I usually use ctrl-shift-z on the preview to rapidly shift between before/after. You did not tell your parameters for deconvolution, how many steps, etc. I usually start with 20 iterations and try to correct progressively for the dark rings artifacts. Eventually increase the iterations after as much as possible in order to keep the convergence (you will have to also increase the global dark ring protection once you increase the iterations).

The PSF not being round is not wrong, it reflects the stars that were entered in the equation. Your whole image was exposed to the same collimation/tracking/flexion problem. Hope this helps.

B

20
General / The problem of green in narrowband SHO palette
« on: 2019 August 03 01:24:43 »
Hi guys,

I was wondering what would be the correct and scientific way of dealing with green after combining the channels/masters in narrowband images using the SHO Hubble palette. Since we assign the Ha data to the green channel, when we use SCNR to decrease the green aren't we actually clipping Ha data? While this is normal when using an OSC camera with 2 green pixels for the Bayer matrix, it seems incorrect for narrowband.

The problem is that in order to obtain for example a correct blue colour that represents the Oyxgen, I am almost always obliged to decrease the green using SCNR. I generally try to use equal total exposition times for the 3 channels, but even after stretching the three channels in order to obtain equal median pixel values using Statistics, the green is still too strong and unbalanced. That is the case even when I have less total exposition for Ha! Or when I combine the images in Linear using linearfit. Astrophotographers using equal duration exposures still present the classical appearance of the nebula, with less green than 'normal'.

The question is, am I doing something wrong? I like Pixinsight especially for its scientific way of processing images, so I don't want to end up cosmetising things in order to obtain the result that I am looking for by clipping data and misrepresenting reality.

Thanx.

21
General / Re: DYNAMIC VERSUS AUTO BACKGROUND EXTRACTION
« on: 2019 May 01 00:40:25 »
I'm a PI newbie, I have found that in some cases ABE works a lot better than DBE, my understanding from Warren Keller's book is that ABE is preferable when there is a lot of background sky available, whereas DBE is better where there is a lot of nebulosity, so far my experiences confirm this.

I confirm that. ABE works really well when your target is a galaxy and you have plenty of uniform sky available. I just shot M101 without an antipollution filter, applied DBE using Generate in order to have lots of samples available and it did not remove the glow completely. Applied ABE after and everything disappeared (I mean the gradient  :D ).

Most of the time when ABE is applied to complex structures like nebulae with variation in dust brightness which can be close to sky glow/background the results are most of the time catastrophic (lots of parameter testing needed to differentiate between the two), imho. DBE allows you to really see and control what's happening.

22
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 26 10:53:40 »
Ok, thanks!

23
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 25 11:50:51 »
Thanx guys. I'll try to shoot this weekend if the weather alows and test the settings.

Niall, I gues the bit difference explains the mono subtraction problem for the final integration image. I try to work with RAW fits. Good point about the bias frames.

Do you use the same gain when taking flats or the minimal/unitary gain when shooting with CCD cameras? I fail to see the point of shooting with minimal gain (advocated by some) since the spot/vignetting on the light frame are produced when shooting with another gain.

24
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 24 13:27:38 »
Yeah I leave the filter on. I'll have to experiment with the settings and some flat darks to see how it goes.

What do you guys use as target for the flat histogram? I saw multiple recommandations on the web of around 20-25 k ADU, for a 16 bit image (not my camera) that's lower than the 50% for 65 k (which would be around 32 k). So should the intensity be lower than half? APT has a nice flat aid where you introduce your ADU target, I intend to use APT for acquisition (works really well for DSLR).

25
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 24 11:58:01 »
Quote
The settings that seem to work best are when I shoot them in RGB

You should be shooting all your frames the same way you appear to ve shooting your Lights - i.e. in RGB mode. You should not deBayer until you have fully calibrated all of your (still in grey-scale) Lights - just before you Align and Combine the Calibrated Lights. You should only expect to see the beginnings of a colour image if you unlock the channels duriing STF.

Hope  this helps.

Thanx Niall, that's what I'm trying to do (and since I use the batch it does it on its own). SharpCap tells you however to shoot the flats in Mono because RGB flats could alter the white balance. I was wondering if that really holds.And their Flat script is creating a mono flat.  I am having the impression that my images with the Altair Astro 294C have less blue in the galaxies compared to other images. Or could the Baader filter have an influence? There's something fishy about these flats...

26
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 24 11:39:09 »
It seems that you're doing the right thing. What exposure times do the flats have? How do you create the master flat? Do you use filters when imaging? If yes, do you take flats for those as well?

Lots of questions, I know :)

I am asking myself the same questions : ). I use a LPS filter UHC-S from Baader which stays on all the time.
The exposure is the one needed to bring the histogram close to the middle, usually between 1-3s.
The master flat is created by the batchpreprocessing script. Or I created by respecting the protocol from Warren's book, Inside Pixinsight, multiplicative normalization when integrating, calibrating before with the bias and master and enabling optimization.

For my latest preprocessing image I made the masters manually Bias, Dark and Flat and integrated using batchpreprocessing without obtaining the Starburst.

Why would optimizing the darks for the flat calibration fail to subtract the Starburst?

I imagine that the capturing program writes the duration of  the exposition in the fits file. Could it be a problem at this level? Can I check somehow the parameters in the fits file?

27
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 24 03:57:49 »
Well, the flats are still an issue with the CCD for me, I'm still trying to find the optimal way of taking them. I am using an Aurora flat field. The settings that seem to work best are when I shoot them in RGB (I have a colour camera 294C), same gain as the lights and the histogram is balanced at 50% for the green/blue channel, while the red is to the left (the flat field is predominantly blue).  I tried mono with the histogram in the middle (SharpCap uses 8 bit for those) using the same gain but the results on the final image are catastrophic and the vignetting/dust is actually increased.

28
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 23 13:59:22 »
Darks, the usual way. usually in the same session with the cam cap on, same gain same temperature. Some dark series the next day, same temp and gain. Starbust does not seem to depend on that

29
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 23 13:16:08 »
Good point. Those where stretched images, so I redid a histogram transformation using the same STF parameters. They look more similar in noise and no Starbust. I can still see some vignetting.

Background: 0.0081for optimization and 0.0085 for no optimization

30
General / Re: Dark frames are not correctly subtracted
« on: 2019 April 23 11:35:43 »
Thanx Niall and Rob. Maybe the flat dark will work.

Here is a comparison with "optimize darks" enabled and desabled in the batch process. The left image has unoptimized darks and the right one optimized. The Starburst has disappeared, but the image looks really noisy (even though the Standard deviation of the background is slightly lower!). I would not want the process to impact the quality of the final integration. Last night I did the process on another shot by doing it manually for the Master flat with dark optimization, and no optimization for the batch integration process. It worked without Starburst.

Could the method of taking flats influence the subtraction process?

PS: The integration still has vignetting so I am wondering if the flats are correctly subtracted... I'll have to try without flats at all.


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