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M31 with 6" f/4 Newtonian at f/3: Test of the nexus 0,75x reducer

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#1 hobbyknipser

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Posted 23 September 2022 - 04:08 PM

Hi, all,

 

I once treated myself to the relatively expensive starizona Nexus 0.75x for Newtonians, for my 6" f/4 and 8" f/4 Newtonians.

Here is a result (nearly full field), unfortunately in the first night without any filter (I forgot the uv-ir filter) and the second night with the Optolong l-pro. With the Canon 77da, 2h without filter and 2,5h with l-pro, so 4,5h at f/3, with my approx. Bortle 7-8 sky.

Just adjusted to f/4 and then simply plugged the reducer into the OAZ. It's a gamble that the stars appear tolerable throughout the frame. But I'm still satisfied with the reducer, I can recommend it. I had imagined the result to be even worse. M52's test image (only 30 sec) was even better, but I didn't want to capture this object.

You could already tell that rays from other light sources are now entering the field of view. I couldn't completely remove the gradients with Graxpert, it's quite complex with the bright sky. There was nothing to be done with fitswork either. Not dithered and just made flats. A little softened and sharpened at times. That's the most stretched result, which might be overkill. tongue2.gif 

 

M31_38x3min_38x4min_6zoll_f3_450mm_oF_lpro_22_230922_gesges31292.jpg

 

cheers

Andreas

 

 


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#2 hobbyknipser

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Posted 23 September 2022 - 04:22 PM

...and this is the result of 2h without any filter:

 

M31_38x3min_6zoll_f3_450mm_oF_220922_ges41295.jpg

 

greetings

Andreas

 

 


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#3 hobbyknipser

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Posted 23 September 2022 - 04:34 PM

...and here is a version, not so much stretched, with the 4,5h:

 

M31_38x3min_38x4min_6zoll_f3_450mm_oF_lpro_22_230922_gesges11294.jpg

 

the second night the bad seeing was only of about 4-5"; never mind, better than doing nothing. The gradients were again in the same places as before, strange. Is there already an IFN near or at M31 or is the same observation site responsible for this?

 

best

Andreas


Edited by hobbyknipser, 23 September 2022 - 04:41 PM.

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#4 Nemo51

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Posted 23 September 2022 - 04:38 PM

Which scope or scopes?  (Manufacturer; model)



#5 hobbyknipser

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Posted 23 September 2022 - 04:51 PM

Hi, Nemo,

 

GSO optics 6" f/4, steel tubus.

That's the brute method to eliminate the gradients, fitswork: background variable:

M31_38x3min_38x4min_6zoll_f3_450mm_oF_lpro_22_230922_gesges4eben11293.jpg

 

cheers

Andreas

 


Edited by hobbyknipser, 23 September 2022 - 05:36 PM.

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#6 Astrojensen

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Posted 24 September 2022 - 07:03 AM

The "gradients" you removed were not gradients at all, but real objects... There's a lot of IFN near M31. 

 

 

Clear skies!

Thomas, Denmark


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#7 hobbyknipser

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Posted 24 September 2022 - 12:27 PM

Hi, Thomas,

 

thanks for the info!

I've read that there is IFN in the top right of M31, but I have the bright areas on the bottom left, too. Those are more likely to be gradients...undecided.gif

 

best

Andreas



#8 Astrojensen

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Posted 25 September 2022 - 01:54 AM

I wouldn't be so sure of that... 

 

https://www.astrobin.com/lqikyi/0/

 

https://stargazerslo...dfe43e40c3e.png

 

https://www.astrobin.com/9a6nbh/BU/

 

There's a lot of extremely subtle stuff floating around M31. Tiny differences in sensor efficiency and processing can easily account for some variability in appearances.

 

 

Clear skies!

Thomas, Denmark



#9 hobbyknipser

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Posted 25 September 2022 - 07:33 AM

Hi, Thomas,

 

thanks for the links!

Yes, there is faint IFN all around the galaxy, but I only see small brighter areas with definite shapes. This looks to me more like gradients to be eliminated. I don't think, that it is possible to show IFN in an Bortle 7-8 area with f/3 and only 4,5h.

 

greetings

Andreas


Edited by hobbyknipser, 25 September 2022 - 07:35 AM.

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#10 vidrazor

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Posted 25 September 2022 - 05:02 PM

I hate to be "that guy" but there's something terribly wrong with your renders. How are you processing your data? You seem to have eliminated most of the subtle data in the galaxy, it just seems to flatten out to brownish gray. A look at general M31 renders on Astrobin will show what I mean. Granted not all renders there are necessarily great, but in some examples, like this one, you should see what I mean. You're also missing a lot of color, both in the galaxy and the stars. How are you processing this?


Edited by vidrazor, 25 September 2022 - 05:34 PM.


#11 17.5Dob

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Posted 25 September 2022 - 05:31 PM

The "gradients" you removed were not gradients at all, but real objects... There's a lot of IFN near M31. 

 

 

Clear skies!

Thomas, Denmark

There is IFN "near" M31....just not in the OP's photo....



#12 asanmax

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Posted 25 September 2022 - 08:10 PM

Nice to see good stars across the frame!

I just got a Nexus and will be testing it with a 10" quattro tonight. I hope it turns out well.



#13 hobbyknipser

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Posted 26 September 2022 - 05:33 AM

Hi, vidrazor,

 

all o.k. no problem! Thanks for your comment! waytogo.gifsmile.gif I'm thankful for your tips.

I worked on the two nights separately, first the 2 hours without a filter, then the 2.5 hours with the l-pro. Then add the two results with fitswork. The individual images were processed as follows: first slightly stretched with fitswork until something could be seen, then gradXterminator, then further stretched again with fitswork until the galaxy was clearly visible, then I went to PS and increased the colors, further very slightly stretched, later some parts of the image are slightly softened with Gaussian blur or sharpened with PS. Also tried to emphasize the dust areas of the galaxy. For the core: a bit light and shadow in PS. Then gradXterminator again. I used now starnet v2, and a starmask (with subtract in PS) too.

What strikes me again and again: there are hardly any yellow, orange and above all blue in the galaxy. That's why the galaxy keeps coming out in these weird colors. But the colors are there in the stars. I can't see that I've lost details, o.k. al little bit because of the blurring. I fear that I'm not able to correct the colors.

I tried to do a color calibration: with siril: get a histogram with blue left and right flanks, but the galaxy is even more red, the background is a bit bluer, but not the edge of the galaxy; I also tried REGIM, but again only a redder galaxy. But I have to admit, I didn't do the color calibration in the linear state like it's supposed to be done.

Maybe it's the heavy light pollution (Bortle 7-8) at my location that prevents blue in particular from coming out so weak in my gray sky. Then it surprises me, however, that the colleague in the neighboring sub-forum experienced imaging can work out the colors so well with Bortle 8.

 

About the reducer: despite guiding, the stars are mostly shifted by one pixel. I checked the adjustment before, with f/4 , no corrections necessary. Therefore I have to correct with the reducer everything by one pixel in both directions, not without the reducer. It is important how exactly, centrically, the reducer is fastened in the focuser with the 2 screws.

 

I think, in reality there is not so much blue at the edges of m31 as shown in most of the images in this forum, see this example, especially at the south edge of M31, taken in Austria under bortle 3-4 sky:

https://www.astro-fo...m-31-andromeda/

 

This is the first 2h file, RAW-SumStack with lights and flats, 38x3,1 min, without any filter: if you are interested...

 

https://www.file-upl...tosave.fts.html

 

and this is the stack of 38x4 min with the l-pro filter:

 

https://www.file-upl...n_lpro.fts.html

 

greetings

Andreas


Edited by hobbyknipser, 26 September 2022 - 09:10 AM.


#14 hobbyknipser

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Posted 26 September 2022 - 05:46 AM

Hi, Dave and asanmax,

 

thanks for your comments!

 

best

Andreas



#15 vidrazor

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Posted 27 September 2022 - 01:16 AM

I think, in reality there is not so much blue at the edges of m31 as shown in most of the images in this forum, see this example, especially at the south edge of M31, taken in Austria under bortle 3-4 sky:

https://www.astro-fo...m-31-andromeda/

This is the first 2h file, RAW-SumStack with lights and flats, 38x3,1 min, without any filter: if you are interested...

https://www.file-upl...tosave.fts.html

and this is the stack of 38x4 min with the l-pro filter:

https://www.file-upl...n_lpro.fts.html

greetings

Andreas

OK, well, what program did you use to stack these? You mentioned "RAW-SumStack with lights and flats, 38x3,1 min, without any filter" for the first stack, what are you referring to in RAW-SumStack? I take it you used no darks in this stack?

 

On your L-Pro stack, were there calibration frames used?

Both of your stacks have a lot of gradients, and as best as I can deduct, they are a combination of amp glow and light pollution, and needless to say, they made quite a mess of things. I think that combining your stacks made it worse.

 

Both of your stacks were warm, the L-Pro being warmer for some reason, and little blue recorded in the galaxy. I could get more M31 luminosity from the first stack, and more color from the L-Pro stack, inasmuch as it was overtly warm.

 

However I was able to extract color from the stars from both stacks.

 

The gradients made it just about impossible to get a clean transition to black at the edge of the galaxy. I had to jump some hoops, first in Siril, then in Photoshop to neutralize them. I then performed other contrast processes to extract detail in the dust lanes in Photoshop using a combination of levels and Annie's Astro Filters.

 

Below is the sum of your stacks. I processed the two stacks independently, then grabbed some M31 luminosity from the first stack and added it to the edges of the L-Pro stack. It is mostly the L-Pro stack below. Most of this data extraction occurred in Photoshop, and it's a long list of processes, so I won't go into them here. The image below is the final product between Siril, Starnet, and Photoshop.

 

So here's what I suggest. If you have either Siril or PixInsight (and I believe possibly also APP), you need to perform a gradient reduction per sub while stacking. You need to remove those gradients first before you can effectively address extracting proper color and luminosity for M31, but you need to remove them in the stacking process itself. That will be the most efficient way of addressing them. Once you've removed that obstacle, you should be in a better position to work on extracting the data from your captures.

 

Hope this helps. smile.gif

Attached Thumbnails

  • M31_2h30min_lpro.jpg

Edited by vidrazor, 27 September 2022 - 01:20 AM.

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#16 hobbyknipser

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Posted 27 September 2022 - 04:04 AM

Hello vidrazor,

 

thank you again for the great effort you put into editing my data! bow.gifapplause.gif

It was stacked with DSS and I only took flats for no filter and for the l-pro, no darks, bias, no dithering. The l-pro image is lighter because it is exposed for 4 minutes instead of 3 minutes at ISO 400.
You notice that it is not easy to create good colors with the data. I have neither PI nor APP, but DSS, SIRIL, Gimp, Sequator, PS CS4 from 2008 and fitswork. For the gradient removal I use the very good program graXpert. Otherwise it would look even more hopeless.

What I did, you can read again in #1 and #13. I used graxpert nearly as the first step to get rid of the strongest gradients. The images were taken over an industrial area, nearly without filter effect. I think under these circumstances is not more possible.

 

I'm still wondering, how it can be possible, that the colleague in the neighboring sub-forum experienced imaging can work out the colors so well with Bortle 8. See this link and then his result in #23! This must be nearly the same bad sky as mine...From where come the whole blue and the good colors in this image? This were normal 2 min exposures. Is it the astro camera or the Celestron RASA light-pollution filter that make the difference? What did I make wrong? grin.gif

 

https://www.cloudyni...asa-8-42-hours/

 

many thanks again for the effort, help and further infos!

Andreas


Edited by hobbyknipser, 27 September 2022 - 04:39 AM.


#17 vidrazor

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Posted 27 September 2022 - 04:48 PM

I suspect you can't get color because you have to process the crap out of these stacks because they have excessive gradients from light pollution and amp glow. There's no reason why you shouldn't be able to extract the full spectrum of color from any target, especially M31. The L-Pro and similar filters are not color friendly either, which is why I don't use them. Additionally, although your non-filtered stack came out mostly monochromatic for some reason, L-Pro is meant for nebulae and such, not full spectrum targets like M31, which is probably another reason why you're missing color in the L-Pro stack.

Although GraXpert is now embedded into Siril, it will use polynomial processing to remove the gradients per sub. Per sub gradient removal is what u need with this data. You would need to manually stack because you're missing calibration frames the script uses for the per sub gradient removal stacking.

Looking at your gradients, you need to shoot a full set of calibration frames for your camera. Also, shooting flats but not bias will not allow stacking to properly process your data. You need bias for flats. Too late for this set, but I would not skip a on a full set of calibration frames next time.


Edited by vidrazor, 27 September 2022 - 11:37 PM.


#18 hobbyknipser

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Posted 28 September 2022 - 09:18 AM

Hi, vidrazor,

 

thanks for the further infos!

Darks (Hotpixel, amp glow, at a DSLR?) and bias, that are very dark images and these will help against the bright color gradients?

More experienced and better photographers than me have already reported that their dslr's produced the same result after image processing with darks or without darks, i.e. the image processing whitewashed the darks.

I fear, nothing will help in my light-polluted area; only possibility: stronger filters and no galaxies).

 

best

Andreas


Edited by hobbyknipser, 28 September 2022 - 09:31 AM.


#19 T~Stew

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Posted 28 September 2022 - 10:18 AM

Hi, vidrazor,

 

thanks for the further infos!

Darks (Hotpixel, amp glow, at a DSLR?) and bias, that are very dark images and these will help against the bright color gradients?

More experienced and better photographers than me have already reported that their dslr's produced the same result after image processing with darks or without darks, i.e. the image processing whitewashed the darks.

I fear, nothing will help in my light-polluted area; only possibility: stronger filters and no galaxies).

 

best

Andreas

Bias has nothing to do with reducing your "bright color gradients" directly (nor darks) but they are required for proper flat calibration... its in the math. If the flats don't calibrate properly you could be left with gradients in addition to your "bright color gradients" that would hinder gradient removal software from working effectively. Gradient removal software does work well, but only with nicely calibrated flat images to being with, else the gradient is to complex.

 

"i.e. the image processing whitewashed the darks." That is not how it works, darks get subtracted from the lights, there is no whitewashing going on. Getting properly temperature matched darks with dslr can be difficult, and else can actually increase the noise in the images. And some cameras (I believe Nikons), do some internal dark noise suppression, so darks are ineffective, but don't just apply one thing you heard to all dslr, they do not all act identically. Some have amp glow, some do not, some are very noisy, some are not, so have banding, etc etc.


Edited by T~Stew, 28 September 2022 - 10:34 AM.

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#20 vidrazor

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Posted 28 September 2022 - 10:30 AM

Darks (Hotpixel, amp glow, at a DSLR?) and bias, that are very dark images and these will help against the bright color gradients?

More experienced and better photographers than me have already reported that their dslr's produced the same result after image processing with darks or without darks, i.e. the image processing whitewashed the darks.

I fear, nothing will help in my light-polluted area; only possibility: stronger filters and no galaxies).

Not all cameras can do without darks, obviously yours is one of them, because amp glow is obvious. You need dark frames. They should also be shot right after the light frames, so the sensor is at the same temperature.

 

Flat frames cannot be properly processed without bias frames. You need bias frames. There's no point in shooting flat frames if you're not shooting bias frames. Bias frames are the easiest thing in the world to shoot, and although you may hear conflicting opinions on this, I would also shoot them right after subs and darks, at the same sensor temperature. Your flats should be the last set of calibration frames that you shoot, after darks, then bias.

 

You can shoot in light polluted skies, you just need to properly shoot in them. Shorter subs are the order of the day in high Bortle skies. You need to keep your subs short and your subs histogram recording at 1/4 from the left. Your Nexus 0.75x is probably working against you because at f/4 your scopes are already relatively fast. You're collecting too much light too soon. I shoot between 30-60 second subs in Bortle 9 skies with f/6-f/10 scopes at ISO 400-800. Your ISO should be between 800-1600 for a Canon. You need more sub time in high Bortle skies for a given target than you need for lower Bortle skies, so you just need to a lot of subs, but you need to expose your subs correctly, and you need to shoot a full set of calibration frames.

 

Once you've shot a session with a full set of calibration frames, stack the data in Siril instead of DSS and use the script you see below. The script requires a full set of calibration frames. In addition to camera induced noise, it will remove or reduce environmental gradients (the actual light pollution) on a per-frame basis. Any residual gradients remaining after that, if any, can be dealt with more effectively with the embedded GraXpert gradient processor in Siril.

Attached Thumbnails

  • gradient.jpg

Edited by vidrazor, 28 September 2022 - 10:52 AM.

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#21 FrostByte

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Posted Yesterday, 12:45 PM

Hello vidrazor,

 

thank you again for the great effort you put into editing my data! bow.gifapplause.gif

It was stacked with DSS and I only took flats for no filter and for the l-pro, no darks, bias, no dithering. The l-pro image is lighter because it is exposed for 4 minutes instead of 3 minutes at ISO 400.
You notice that it is not easy to create good colors with the data. I have neither PI nor APP, but DSS, SIRIL, Gimp, Sequator, PS CS4 from 2008 and fitswork. For the gradient removal I use the very good program graXpert. Otherwise it would look even more hopeless.

What I did, you can read again in #1 and #13. I used graxpert nearly as the first step to get rid of the strongest gradients. The images were taken over an industrial area, nearly without filter effect. I think under these circumstances is not more possible.

 

I'm still wondering, how it can be possible, that the colleague in the neighboring sub-forum experienced imaging can work out the colors so well with Bortle 8. See this link and then his result in #23! This must be nearly the same bad sky as mine...From where come the whole blue and the good colors in this image? This were normal 2 min exposures. Is it the astro camera or the Celestron RASA light-pollution filter that make the difference? What did I make wrong? grin.gif

 

https://www.cloudyni...asa-8-42-hours/

 

many thanks again for the effort, help and further infos!

Andreas

I think you should try stacking the data again, but shoot a set of bias frames. As was suggested above, flat correction tends to over-correct if you do not use bias or dark flats to calibrate your master flat. I downloaded your two files, and to me that looks more like flat over-correction than a light pollution gradient. Or at least, over-correction in addition to a light pollution gradient. Especially with a fairly short integration time the light pollution gradient shouldn't change that much to create a circular pattern like that.


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#22 vidrazor

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Posted Yesterday, 02:22 PM

I fear, nothing will help in my light-polluted area; only possibility: stronger filters and no galaxies).

As FrostByte mentioned, try shooting some bias frames, even though they won't be at the same temperature, and reprocess your data. You only need about 20-50 frames, which will take you all of one minute to shoot.

I'm not entirely sure of this, but there is another script right above the one I pointed to in the image that I'll repeat here. If I'm not mistaken, that is a script to perform per-sub background extraction without darks. So if you shoot a set of bias (and Siril want to see a folder called biases for bias frames), you should be able to stack them in Siril performing background extraction on an individual sub basis. Although you can try it with both sets of data that you have, I would apply this to the unfiltered set and not bother with the L-Pro set. I'm not entirely sure this script loads with Siril, so it may require an online search, or I could upload the one in my folder somewhere.

 

Once you have the new stack, it may need further gradient removal, but you should have a much better stack to start with, and you can apply the GraXpert processor right in Siril. Apply photometric color calibration, and you can also try the new generalized hyperbolic transformation process to extract faint data while keeping your cores from blowing out. I assume you're familiar with Siril, but if not, this is a good starting point.

 

post-283822-0-94184200-1664380252.jpg
 


Edited by vidrazor, Yesterday, 02:24 PM.

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#23 hobbyknipser

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Posted Today, 07:48 AM

Hi, vidrazor,

 

thanks for the further infos!

I took 30 bias frames at 1/4000 sec.

Then I used siril for stacking the data without any filter. I couldn't find your scipt noDark BGR SW in the net. So I used preprocessing without dark.

withbackground extraction: not possible, error, no darks found.

This is the stack result:

https://www.file-upl...result.fit.html

Then I tried background extraction in SIRIL, which is nearly the same as the independant program gradXpert. Then in the histogram transformation the autostretch function, then color hue, reducing green.

Now I have a similar result as mine in the middle of my postprocessing. I must see, if something is better...

...... Yes, I must say: the gradients are a lot weaker and I could see only one light-gradient. Is this the stacking with SIRIL or the additional bias frames? smile.gif

 

That was my result at that point, only the data with no filter, not correcting the colors:

result_fits1_Gra61295.jpg

 

cheers

Andreas

 


Edited by hobbyknipser, Today, 08:39 AM.


#24 FrostByte

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Posted Today, 08:33 AM

That's a huge improvement! And it's possible to get some really nice color out of your stack with just a little bit of coaxing.

 

Here's a quick run-through with PI, but I'm sure you can do the same thing in Siril. I'll try to use software-agnostic terms in what I did. lol.gif

 

Remove light pollution gradients

Background neutralization

Manual color calibration (pick the core of the galaxy as the white reference and any starless background region as the background reference)

Neutralize green cast

Hyperbolic stretch via GHS (which you can do in Siril)

Linear stretch via GHS to set the black point

Saturation stretch via GHS

 

For about 10 minutes of processing time, this is starting to look like a really nice m31! Needs more work, but hopefully you can see the potential in this stack.

Annotation 2022-09-30 092706.jpg


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#25 hobbyknipser

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Posted Today, 08:43 AM

Hi, FrostByte,

 

you are really fast in processing! Great result!

I will try it with the l-pro, too and then combine the 2 results...

 

thanks and greetings

Andreas




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