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New member seeking help / advice with astrophotography / GIMP


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Hi All,

I'm new to the forum, although thanks retrospectively for all the useful advice in various topics!

I recently upgraded my 6" dob to an 8" Skywatcher 200PDS with EQ5 mount in order to try and get in to astrophotography (but still have a reasonable set-up for visual).

Have been really happy with the new kit, on the rare occasions we've had clear skies this past month! Visually it's a huge step up from the 6" & I've been really pleased with the photos I've taken, compared to the ~2sec exposures I could manage with the dob (OK, nothing compared with all those professionals out there, but as a beginner my expectations were very low!!!).

I should mention here that I'm currently using a smartphone (P20) through a 24mm 68deg ES eyepiece & using a coma corrector + ES UHC filter. DSLR is next on the list when my bank account has recovered...

I've been 'focussing' on M42 as an easy winter target, 30s at ISO800 & have finally (after procuring a Bahtinov mask) got pretty reasonable images.

So I decided to try out stacking (DSS) & processing (GIMP). I took about 15 shots + flats, darks & bias frames. Admittedly my flats were poor, I used the white T-Shirt approach, illuminated with my power supply torch, but it's got a severe radial gradient & looks kind of green! Anyway, I stacked the images, initially with the flats, but later without & uploaded the TIFF in to GIMP.

I've watched several tutorials, but so far, I'm unable to get anything near as good an image as the original single frame from the camera! :-(

Stretching the image brings up all the background noise & I just can't seem to get rid of it. I've subtracting a colour selection approach & also tried the despeckle approach (both from following youtube tutorials), but neither were successful in removing the background noise & end up taking a lot of detail away from the image itself.

Sorry, getting to the point, I wonder if there are any experts on GIMP that can highlight a different approach? I'm unsure if the problem is with my RAW data, or with my distinct lack of experience with the processing software! It just seems strange that the original phone image itself is 'reasonable', but can only make it significantly worse with additional processing!

Sorry for the long post! Any suggestion or advice is greatly appreciated.

Many thanks,

Rob

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Hi, I'm only my smartphone through the eyepiece at the moment. It's a Huawei P20pro, so reasonable as far as smartphones go, but for sure it's not equivalent to the DSLR approach...

I took about 15 images + similar quantity of darks, flats & bias shots. As I mentioned, the flats were suspect, but the darks (same ISO / exposure with dust cover on) & the bias (same ISO, fastest speed, dust cover on) frames were good (I think)...

Thanks,

Rob 

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Hi

AFAIK, Huawei raw produces a .jpg along with the raw file. Are you sure you are working with the latter?

Cheers

 

Edited by alacant
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Would you be able to upload the unprocessed stacked image? I'd happily have a go at processing it, and I'm sure a few others would too.

Disclaimer: I'm by no stretch of the imagination an expert with GIMP, but have played around with it quite a lot over the last few months for astro image processing.

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1 hour ago, happy-kat said:

no light getting in.

That could be it. I don't think the 'phone makes a lightproof seal with the eyepiece; the bias and dark -at least- would have to be taken with the 'phone wrapped/isolated/darkroom.

Edited by alacant
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Hi All,

Many thanks for your comments. I've attached the TIFF, would really appreciate a quick opinion if it's the data itself or just my inability to process it! I've also attached the JPG phone photo for reference.

Ha, yes, definitely used the RAW files for DSS, I did initially load all the JPG's as I didn't realise the phone stores both, with the RAW files being in a separate folder.

Regarding the 'dark / bias frames', yes, could be, I did only put the dust cap on the telescope so it's possible light was getting in around the eyepiece.

If it's the data, then I will put this exercise on hold until I can get myself a DSLR. Was really just trying to get some practice with the smartphone...

One thing I have realised is why M42 is suggested as an easy target. It's 'like the moon' compared with other nebulae! I took a similar 30s shot of the Horsehead (IC434) over the weekend & wondered if I'd left the dust cap on!

Thanks again,

Rob

IMG_20210203_213351.jpg

master2.TIF

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Hi

Excellent. Take proper flat frames and you're there:)

 I think that the way 'phone camera sensors are going, we're about to see a change in opinion as to their use in astrophotography.

Sorry, no time to have a go at the vignetting -which flat frames would cure anyway- but you'll get the idea.

ss1.thumb.jpg.b5cd881020fc4bdd0217742c4f205e6c.jpg

 

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Hi Alacant,

Fantastic, thanks so much for taking time to look at the data. So it's 'in there', just my lack of processing experience huh!

Can you give me a quick summary of what you did? I see you've stretched the colours, but how did you handle the background subtraction?

I guess I need to watch more tutorials. If anyone has any good suggestions please let me know. Seems like there are different approaches to processing even within the same software.

Thanks again,

Rob

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25 minutes ago, Robculm said:

background subtraction

I used overlay mode but there are far more elegant ways to do it.

Perhaps best to use GIMP (or better DarkTable) simply to add finishing touches after you've done the heavy lifting in e.g. Siril. The latter gives good calibration and stacking solutions too.

There's probably a lot more to be had in your current image. Correct calibration with flat frames would enable you to use the full frame. Or maybe that's a limitation of using an eyepiece  Did you try without the eyepiece? It would probably be better that way.

HTH

Edited by alacant
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Thanks again for the advice. Will work on a better method for the flats & experiment some more in GIMP now that I know the data in the master is OK! Will also explore Darktable & Siril, thanks for those suggestions.

Apparently 'prime focus' isn't possible with a smartphone, maybe because it has it's own fixed focus lens. A DSLR is definitely my next priority.

Cheers,

Rob

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Hi again,

I managed to retrospectively generate some flats & re-stacked everything in Siril, definitely helps :-).

I know M42 is tricky because of the high intensity in the centre & I've seen some reference to people blending shorter exposures etc...

But on the edit you made, I notice you managed to stretch the image & bring out the colours without blowing out the centre!

I'm really struggling with that, as soon as I get a decent width to the colour stretch, I lose more detail around the centre (in particular there are 3 stars you've managed to maintain to the right of the centre).

Can I ask how you managed to stretch to that extent without losing those?

Many thanks,

Rob 

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Ha, I didn't realise it made any difference. But playing around with that, I notice that I've just been pulling down the high point, not touching the mid point...

Looks like pulling down the mid point helps keep the detail 🙂 Will play some more with this!

I guess this is the problem with making adjustments but not really understanding the concept of what it's doing!

Will keep at it...

Many thanks,

Rob

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1 hour ago, Robculm said:

how you managed to stretch to that extent

Hi

easy: Select only the outer regions for stretching.

better: Make 2 layers with the a stretched layer over a non-stretched layer. Mask the top layer. Then, using a fuzzy brush, erase the mask where it's over-done.

best: move away from the 1990's stretch/layer/hope-for-the-best stuff. Try contemporary algorithms instead. After you've been moving sliders around on a graph for a few hours, destroying it's linearity from the word go, StarTools comes as a breath of fresh air.

I'll leave the building right away.

HTH

Edited by alacant
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image.png.7ef3d7450a0ca19c6e857db66e2d995b.png

This is where I'm getting to after more experimenting. I can get a reasonably broad stretch looking at the histogram, but there's still no colour showing in the image! Any idea what I'm doing wrong?

Yes, StarTools looks like a good option, will explore that. I see there's also the option in Siril where you can specify what the image is and do photometric colour calibration, but it won't run with this image, I guess again because it's a smartphone, so the data is not as 'linear' as it would be with a DSLR...

I'm thinking I should give up on this for now until I get a DSLR, then the raw images should be more representative and it should be easier to follow the various youtube tutorials etc, as I will be starting out with a similar image!!!

Cheers,

Rob

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15 minutes ago, Robculm said:

photometric colour calibration

Ah, Siril. A much better choice:)

You need a linear image; a stretched image from a dslr will not calibrate either. Just use the colour calibration:

ss1.thumb.jpg.4db892891468b5de8f6a45cc64fa7757.jpg ss2.thumb.jpg.d2b30101711634c578737300c9f0af0b.jpg

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362251578_resultwithmoresirilediting4colouronlynebulasaturationenhanced32bitgimp.thumb.jpg.9a72c5d1822f346e2e2a93ab5454239c.jpg

Pretty happy with the result now. What do you think?

At least there's some colour now & most importantly I can see a definite difference in the final stacked & processed image compared to the original single shot!

Thanks again for all of your advice.

Cheers,

Rob

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Yep! I bet it feels good to make this much progress.

The GIMP is a terrific pixel-level editing program but astro has some special challenges that respond to purpose-built software. I've actually done some deep-sky images start-to-finish in Photoshop, stacking and all. It's possible, but it's like using a hammer to smooth wood before painting it -- just not intended for the task.

In addition to calibration and integration, astro software like Siril (or Astro Pixel Processor, PixInsight, ASTAP...) offers functions such as gradient reduction and background correction (to get rid of light pollution effects), and can do an automatic stretch with a single button press that gets you almost there.

One note about sub-exposures and integration time: longer integration time reduces noise (not just the "grainy" appearance, but lots of other problems with the image) in a square-root curve. So to halve the noise, you need four times the total exposure time. To cut it to 1/4, sixteen times. You can see that for short exposures, you get a lot of bang out of adding your first few sub-exposure bucks, but after that it's a game of diminishing returns. Basically this means you should go crazy on the number of subs you take, especially if they're short.

M42 is a wonderful target, in that a beginner can get a really beautiful, satisfying result like yours, but there are just endless challenges to master for this one object. It has a very high dynamic range, so that you need to do something about not blowing out the central Trapezium if you want to capture the delicate, dim nebulosity out at the edges. It has both emission and reflection components. It has an incredible range of color. And, as you note, it's bright and big enough to find!

Edited by rickwayne
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Ha, yes, there's certainly a lot going on there, it's definitely a great target to experiment with.

After reading some of Roger Clark's (ClarkVision) articles, I guess my colours are all over the place, pink & blue is good, orange is bad! Will need to work on that...

Have just ordered a DSLR, so very excited to get started with that in the coming weeks... 

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