12 replies to this topic

[mgCatskills]

This forum got me interested in Arp's Peculiar Galaxies, and I'm now pretty far along with the Astronomical League's Arp Observing Program.  I've learned from many of you how best to capture Arp's, and am truly enjoying using the Player One Apollo-M camera that Alpha+++ turned me on to. 

 

I thought it would be useful to start a thread where we share best practices for "Arp Hunting".  We could also trade info about interesting targets now in view.

 

Recently I've changed how I go about planning my Arp Observation, and thought I'd share it...  I'm finding it extremely useful, and hope others will as well.

 

For a while now I've always made a practice of looking at the Atlas before starting a capture.  The plates are all available on this website:

 

http://ned.ipac.calt...Arp/frames.html

 

When I'm planning to capture an Arp, I learned a while ago it's important to 1) understand how Arp categorized it, and 2) look at the plate he published.  That is often a clue to the features Arp cared about the most.  Early on, before I started doing this, I shut down some observations early, thinking I'd done a great job, only to discover that Arp really cared about a filament or some other subtle detail, which my observation didn't capture.

 

If you go on the website and click into the images, you eventually get to the "bigarp" image which is about 1500 x 2000 pixels, and reveals a lot of information.  You'll get to something like this:

 

It is a photographic negative, and it tells you a lot.  Of course, we're collecting positive images and much more experienced in making sense of them.  I've discovered recently that it's helpful to "print" the image into a positive.  This is easy to do in any reasonable photo editing software:  I use Affinity, but Photoshop or GIMP do the same. You load the image, apply the Invert function, and then use levels to taste to reveal details in the positive image.  Generally you push the black level up, the white level down, and push the gamma to darken the background further.

 

This is what it looks like in this case:

 

 

In my own Arp 273 observation, I learned two important things from this positive image:

1. What I thought was the galactic center was actually a star that just happens to align dead center.   Instead of a "double" center, the galaxy's single galactic center has been squashed upwards asymmetrically.
2. Looking at the negative, it appeared to my inexperienced eye that the spiral was intact.  Looking at the positive, I see that the bottom half of the spiral has been heavily disrupted.

So it completely changed my interpretation.

 

Anyway, I hope others find this useful and I'd love to hear about thing's you've discovered on your Arp journey.

Edited by mgCatskills, 06 November 2024 - 10:33 PM.

[roelb]

I find it hard to see better detail applying this method.

Probably I'm doing something wrong ("over-manipulation" histogram?).

 

Arp 273

6 SE @ F/5

ASI290MM

20 x 8 s = 2.7 min

 

ORIGINAL CROP

 

INVERTED by Affinity

 

"RE-LEVELED" the inverted by Affinity

 

[mgCatskills]

Hi, Roel...

 

Looking at youir images, it looks to me like you may have used the top level, thumbnail images which have a lot less resolution.

 

Make sure you click on the thumbnail to get to the larger image, e.g. 

 

http://ned.ipac.calt...big_arp273.jpeg

 

If you are using the big image, then all I can say is it does take a deft touch with the levels dialog.  Make sure you use the gamma as well as white and black levels.

Edited by mgCatskills, 11 November 2024 - 08:40 PM.

[Clouzot]

Conversely, we could use Sharpcap's mini histogram invert function for Arps - and more generally for hunting faint fuzzies - during the livestacking proper.

 

Oddly enough, I often find inverted images more amenable to insane amounts of stretching. I guess there's a perceptual phenomenon that makes noise more acceptable in highlights, which allows pulling out more detail from the background. The positive version of that stretch would be horrible.

(Arp 145 from early november, 12x180s, C6 f/8, ASI294MM bin2 gain 120, Bortle 4)

Edited by Clouzot, 12 November 2024 - 04:05 AM.

[mgCatskills]

Interesting.....

 

To be clear, my suggestion of inverting the negative Arp catalog images to positives is a planning aid... I sometimes find the negatives in the Atlas hard to interpret, and can gain new insights if I "print" them.

 

You're  saying that in some circumstances it make senses to capture an inverted image during live-stacking because you can stretch them more and faint fuzzies sometime show up better as dark dust.  Wow!  I didn't know that was an option. 

 

Just googled it and found that there is an invert button in the mini-histogram (the half black, half white circle icon).  I presume you can "save exactly as seen" the inverted image.

 

Wow!  Thanks!!!!  I will have to try that.

Edited by mgCatskills, 12 November 2024 - 06:00 AM.

[BrentKnight]

I've found that if you use the invert feature with an OSC, results can be a little strange.  I think it works best when only capturing in mono.

[BrentKnight]

While discovering something about an object yourself is always fun, I do think that to get the most out of an Arp session that you need to do your homework before going out to capture it.

 

We only have limited control during the live session, but when doing simple post, those details seen in Arp's photographs can often be brought out much better - if we know they are there... 

 

M32 I think is a great example.  The small companion of M31 all by itself doesn't look all that peculiar, but what Arp was interested in was the faint extended halo around it that seems to show the two galaxies are actually connected.

 

But... faint tidal tails are the most challenging, I think.

[mgCatskills]

I've found that if you use the invert feature with an OSC, results can be a little strange.  I think it works best when only capturing in mono.

 

 

While discovering something about an object yourself is always fun, I do think that to get the most out of an Arp session that you need to do your homework before going out to capture it.

 

We only have limited control during the live session, but when doing simple post, those details seen in Arp's photographs can often be brought out much better - if we know they are there... 

 

M32 I think is a great example.  The small companion of M31 all by itself doesn't look all that peculiar, but what Arp was interested in was the faint extended halo around it that seems to show the two galaxies are actually connected.

 

But... faint tidal tails are the most challenging, I think.

In theory I love the idea of seeing Arp "fresh"... in practice, the details on which he's basing his categories can be so subtle that you'll invariably miss them without a cheat sheet (i.e. the combo of his category and the Atlas image).

 

Especially as his 200 inch instrument has so much more resolution than we have... luckily we have digital CMOS cameras or it would be a complete non-starter.

 

On my website I do process all of images to bring out the details like faint tidal tails which are ever so subtle in the EAA legal versions. 

 

Something I did for the first time last week... I had a color image I'd captured without reference to the Atlas on perhaps the first weekend of my Arp journey. Then had to stretch it horribly just to recover a hint of the filaments that Arp was after. 

 

Anyway, I took a look at it over the weekend, found the original .png and converted it to grayscale, then processed it in Affinity.  Turns out it was much easier to bring out the filaments in B&W.

[Clouzot]

I've found that if you use the invert feature with an OSC, results can be a little strange.  I think it works best when only capturing in mono.

Indeed. You could desaturate the stack completely (saturation slider in the livestack histogram) before inverting then?

[BrentKnight]

Indeed. You could desaturate the stack completely (saturation slider in the livestack histogram) before inverting then?

I was more concerned about the colors.  An inverted color image does not create a monochrome inverted image.

 

Anyway, I took a look at it over the weekend, found the original .png and converted it to grayscale, then processed it in Affinity.  Turns out it was much easier to bring out the filaments in B&W.

 

For fine details, I think it makes more sense to just capture the Arp's in monochrome.  Converting a color image just wastes pixels.  There definitely are interesting color components to the Arp's, but most of the peculiarities don't require color to be seen well.  And not having to mess with color balance issues makes imaging so much easier.  I can see using color in the larger galaxies for sure, but most Arp's are tiny little things...

[mgCatskills]

I was more concerned about the colors.  An inverted color image does not create a monochrome inverted image.

 

Anyway, I took a look at it over the weekend, found the original .png and converted it to grayscale, then processed it in Affinity.  Turns out it was much easier to bring out the filaments in B&W.

 

For fine details, I think it makes more sense to just capture the Arp's in monochrome.  Converting a color image just wastes pixels.  There definitely are interesting color components to the Arp's, but most of the peculiarities don't require color to be seen well.  And not having to mess with color balance issues makes imaging so much easier.  I can see using color in the larger galaxies for sure, but most Arp's are tiny little things...

I completely agree that capturing most Arps in B&W is better.  Not M51, obviously, but most.  However, when I started this I only had an OSC camera and I was trying to rescue an existing capture.  It turned out I could do a better rescue converting the image to B&W and using the monochrome tools in Affinity than I'd achieved in trying to process it in color.  Just sayin'...   worth trying if you're stuck, I'd suggest.  By no means a standard workflow.
 

Edited by mgCatskills, 13 November 2024 - 12:14 AM.

[BrentKnight]

I completely agree that capturing most Arps in B&W is better.  Not M51, obviously, but most.  However, when I started this I only had an OSC camera and I was trying to rescue an existing capture.  It turned out I could do a better rescue converting the image to B&W than I'd achieved in trying to process it color.  Just sayin'...   worth trying if you're stuck, I'd suggest.  By no means a standard workflow.
 

I totally get that, I was just going with the theme of your topic - best practices for Arp's...

 

I may be a little strange, but I still like mono for even larger galaxies like M51.  I'm pretty happy with my mono M33.

 

I know you use Affinity, but when I captured this M33, when I ran it through Siril I tried using the Wavelets tool to bring out details.  It seemed to make some difference (but you had to be careful with the controls).  Arp 262 also seemed to do well with Wavelets.  I wonder if Affinity has similar tools you could experiment with...

[roelb]

chatGPT says following:

Affinity software (such as Affinity Photo, Designer, and Publisher) doesn't include native support specifically for wavelet decomposition, which is a feature typically found in specialized photo editing and retouching software like Adobe Photoshop (via plugins) or software specifically tailored for scientific or medical imaging.

However, Affinity Photo does provide some powerful tools that allow users to achieve similar results to wavelet decomposition through:

1. **Frequency Separation**: Affinity Photo offers a built-in "Frequency Separation" tool, often used in photo retouching. While this is not a true wavelet decomposition, it splits an image into high and low spatial frequency layers, allowing you to edit textures and colors independently, which is one of the key purposes of wavelets in image processing.

2. **Customizable Filters**: Affinity's filters, such as High Pass, Gaussian Blur, and other noise reduction techniques, allow users to isolate details and manipulate specific frequency ranges, somewhat mimicking the effect of a wavelet breakdown.

For true wavelet decomposition, a plugin or external tool would be required.