Galen Gilmore

On 22/10/2020 at 03:06, ollypenrice said:

In my understanding it has to demosaic first because colour must be attributed according to the Bayer pattern of that particular chip. If it aligned on the stars first the alignment would ignore the Bayer matrix and would stack a red pixel onto a blue onto a green, etc etc, and the colour information would be lost.

I'll need to watch the video again but I'm left shaking my head when I hear that there is only one best way to stretch data. The stretch is the most important intervention in processing and cannot be reduced to a single method for three reasons. 1) Every image is different. 2) An imager might have a specific set of objectives for a dataset. Sometimes this can be extracted at the expense of that. We prioritize what it is we want our image to show. 3) Some of the most effective stretching techniques work by blending different stretches simply because one stretch cannot do everything.  This doesn't mean I don't think Mark's stretch is an excellent one. It is.

Olly

I suppose you have to oversell it a bit in order to get clicks on Youtube. You guys seem rather familiar with Mark's Stretch, where as this video is my first introduction to it.

Also, I would always trust the word of my fellow SGL members over a video I found on Youtube, haha.

23 minutes ago, JamesF said:

No idea, to be honest.  But if it is then my first guess would be that the green photosites are more sensitive or have a wider bandpass than the red or blue ones.

James

So I talked to the guy who made the video, this is what he had to say about the inquiries:

Quote

First, denosaicing is done for me using DSS, where an rggb debayer process is used and always results after stacking with the green channel having a higher SNR. That is because of having two green photosites as opposed to one. This is a very common result in OSC AP.

Secondly after adjusting the red and blue channels to align with the green channel, what occurs is a large offset in the black point with no data there and thus the black point needs to be “set” back to the left and remove the offset. This doesnt affect the color balance as there is no data being changed, it is just an offset adjustment of the black point. I listed a link in my video to Mark Shelly’s website, he came up with this technique and he is the author of the Pixinsight arcsinh stretch module. It requires no offset in the black point, please read his explanation to understand it properly.

Hope this helps clarify. Also, “the proof is in the eating of the pudding”.

Cheers and thanks for watching,

AstroEd

 So take from that what you will. Again I'm not very experienced when it comes to all of the technical details, all I know is that the process produces some very nice results from what I have seen so far. But there is no perfect process for every situation, what is described in the video can probably be improved upon in one way or another, but I'm not the person to ask when it comes to that. 😅

3 hours ago, JamesF said:

There are some comments in that video that bother me...

The first is one about the green histogram being further to the right because there are twice as many green photosites on his colour sensor than there are red or blue.  I'm not sure I follow the logic of that.  Surely the green histogram would shift to the right if the green photosites counted more photons, or if more than one green pixel value were added into the demosaic process without any scaling of the values used (which I don't believe will happen).  I'd guess that the reason is actually that the green photosites are just more sensitive and count more photons?

The second is the moving of the black point in the individual channels before the stretch is done.  If the point of this particular stretch is to preserve the colour balance, then surely moving the black points beforehand defeats that goal because in doing so the colour balance is changed?

James

Im not knowledgable enough to really comment on everything that you mentioned, but isn't a green color cast after debayering normal on the ZWO asi294? 

And for the second point, I see what you mean, it does seem a bit odd. But surely it doesn't change color balance too much, considering you are eyeballing the colorbalance, and you are only changing the black point per individual channel by only 1 - 2 positions.

I doubt you would need anything too expensive. Many people just use a flashlight/torch, and I use my computer screen with a blank white screen displayed. Just make sure you are covering the aperture of your scope with a tightly pulled t-shirt or something similar so that the light is evenly dispersed. 

I just processed a picture I took a couple months ago because I only just got around to doing it. But I also just happened to stumble across an interesting YouTube video about a processing technique called the Arc Hyperbolic Sine Stretch.

I decided on giving it a go on M57, and all I have to say is WOW!

The technique did a great job of making the image feel very natural by preserving star color, and preventing them from becoming bloated.

If anyone else is interested in it, here is the video.

The image is about 1h 22m of 2m exposures from my SW 130PDS, and Canon 600D from my light polluted front yard.

I finally got around to doing some more stuff with the good 'ole 130PDS!

I used a new processing technique called the arc hyperbolic sine stretch to edit this image, and it has done absolute wonders to make the image feel more natural!
Here's the link to the video in case anybody is interested.

Looking good! I've been getting back into it as well. 

Here's M97 & M108 from Sunday, very impressed with how it turned out, especially during a full moon.

48 x 120s, ISO 800.

That's fantastic, my first definitely didn't look like that.

I don't have much else to post at the moment, so here is a reprocess of the Pleiades I took over 2 years ago. It really shows how much better I have become at editing my images, and how much detail you can really pull out if you do things right.

I wasn't sure exactly where I should put this, so I put it here. Mods feel free to move it if you feel it belongs somewhere else.

I have started a project to see if I can find an asteroid, comet, or dwarf planet using only my own equipment, and no prior knowledge of the locations of said objects. Mind you I don't want to discover one, that would be out of the reach of my equipment.

To document my progress I have decided to make a series of Youtube videos that shows what I am doing, why I am doing it, and what I am learning. It has been a while since I have done any astronomy, or been on camera, so I hope I'm not embarrassing myself too much with this, as everything I recorded was done in the moment with no script or anything.

Thanks for checking it out, if anybody has any tips or anything for me, feel free to tell me.

edit: I just realized I have a typo in the video, please ignore that...

Incase anybody is still on this thread, I have made a youtube video documenting what I did on the first night.

2 hours ago, Piers said:

Hello everyone!

I've just bought a 130PDS from FLO, there's going to be a little delay before it's delivered due to the ongoing "situation"

I've also ordered a Baader MPPC III  and a  2" SW Light Pollution Filter (https://www.firstlightoptics.com/light-pollution-reduction/liht-pollution-filter.html)

I now wonder if I should have bought a different filter.

I'll be using a Sony A5000 (APS-C sensor) and I'm in a rather inconveniently illuminated area.

In truth I just searched for LPFs and then bought the cheapest...which is probably a less then ideal selection method.

Would I be better off going with an Optolong UHC Light Pollution Filter(https://www.firstlightoptics.com/light-pollution-reduction-imaging/optolong-uhc-light-pollution-filter.html) or Explore Scientific CLS Nebula Filter (https://www.firstlightoptics.com/light-pollution-reduction-imaging/explore-scientific-cls-nebula-filter-1-25-2-inch.html) instead?

(I've read about the first 30pages of this thread, informative and very impressive images! I'll carry on reading, but I thought it was best to just ask )

I might consider holding off on the light pollution filter until you have started doing some basic imaging with it, as you might find you won't necessarily need it right away. 

Although I haven't used either filter, I have heard that some visual light pollution filters aren't quite suited to imaging, as they can upset the color balance quite a lot. But I'm sure others will provide more accurate and in-depth info than I can.

11 hours ago, Stu Todd said:

I think in order to have your discovery verifiable by the MPC etc, you will need to submit images and precise astrometry of your object.

Also, blinking different images is how nova outbursts, supernovae etc are discovered, so, you really need to take images. This isn't difficult however.

Stu

I'm not really planning on trying to discover any, that is likely out of the reach of my equipment. This is just a little project/experiment I'm doing to learn about this kind of astronomy.

1 hour ago, Merlin66 said:

To help identify variable stars and similar changing objects I use a solved image (astrometry.net). This provides a accurate reference frame for the image which can then be "blinked" against a detailed star map - I use the features of Cartes du Ciel.

Any "new" star - asteroid or comet will show during the blink as having no matching star image on the underlying star chart. You can set the search limit magnitude to make things easier...

Hope that helps.

 

Interesting idea, this actually removes the need to take multiple pictures of the same area of the sky, right? 

Thanks, I’ll try it.

Anybody know of a piece of software that could detect the change the occurred in an image? I'm doing a project to see if I can find an asteroid or comet without any prior knowledge of where said asteroid/comet is.

I'm not really sure where to look for something like this, so any help would be appreciated.

Also, I would like something that could run on Mac. But I'll run it on bootcamp if there is no other options.

Thanks.

19 hours ago, Craney said:

Check this page for what's hot and what's not.......

https://in-the-sky.org/data/comets.php

 

Thanks, was looking for a site like this.

Always thought it would be cool to image a comet, absolutely not clue how I should go about doing it though. Anybody got any tips?

Thanks.

Nice job! The colors are great.

1 hour ago, Adreneline said:

I like this a lot - especially the star colours.

Hope you don't mind but I took the liberty of plate-solving in Astrometry.net - you bagged yourself a serious number of targets in one image!

Excellent!

Adrian

Thank you! The star colors are one of the biggest improvements in this image compared to my other images.

33 minutes ago, alacant said:

Hi

So it maybe, but way above average in dedication and the true spirit of the art.

Superb shot. Well done.

Thank you! Despite my busy schedule, astronomy always finds its way back into my free time.

M35 Star Cluster in Gemini

Equipment Details:

-Skywatcher 130PDS

-Skywatcher EQ5, R.A. Motors, unguided

-Canon Rebel T3i, Astro Modified

Acquisition Details:

-30 x 2min Lights

-6 x 2mins darks

-20 Flats

Processing:

-Deep Sky Stacker (DSS)

-Photoshop

(Cropping out stacking artifacts -> leveling -> slight color balance -> small curve stretch -> remove vignetting using lasso tool -> crop again to put more focus on the clusters -> saturation and more color balance.)

I was really proud of how this turned out. It may not look like much to you, but for someone who has below average equipment, and only has enough time to image during the weekends (when there is no moon, and it isn't cloudy mind you), this is great stuff for me. I also have heavy light pollution to my west, which causes a terrible gradient on any target even slightly west from the zenith.

Thanks for looking!

M35 cluster in Gemini  |  60min total exposure.

Really proud of this one, especially because of how little time I get to image at any given opportunity.

Give me your opinions! Thanks!

edit: and yes, I know I've got trailing stars, but I really didn't want to give up any exposure time by dropping frames.

1 hour ago, Stub Mandrel said:

Do you mean the threads have stripped through over-tightening? They shouldn't wear out

You can ask a local garage to fit a helicoil insert for you which will restore the thread and make it stronger than before.

Or even buy a suitable set and do it yourself, cheaper than new rings.

<edit>I assume you mean the threads that fit the rings to the wedge?

I am talking about the threads which clamp the two halfs of the ring together, not sure if what you said still applies.

You're probably right about them being stripped, and not worn out.

Where could you buy a new set of OTA rings for the PDS? Mine have worn out threads, so I effectively only have one working ring.

That's what I was going to ask. The moon reflects light in all spectrums (white), which means that you can't simply block out the light with a filter. Unless I'm missing something here?