Mosaic Aquisition and Processing

Bob Harris

Not that all of the new PI scripts, processes, and techniques aren't confusing enough - but, in the middle of all this, I have chosen to take on the challenge of tackling some Mosaic Projects 2, 3, 4, and yes, 8 frame compositions. As I stumble through this now, I realize I have a few fundamental questions I should have considered sooner. 

1. I think I mistakenly chose to "center" the most interesting part of my mosaic. As I walk through the process now I suspect that this may not be the best choice - since that means (especially in a 4 panel mosaic) that there are multiple "seams" where the most interesting material is.  Am I looking at that correctly, and should I try to have my seams and overlaps in the less interesting parts of the collective image?

2. How important is it to have a similar number of exposures in each frame? Typically there will be more interesting frames - and some frames that might almost be empty. Do I need to spend a similar amount of time gathering info from all of the areas?? Or can I focus my time on the most interesting frames and still have the overall composition work well together?

FYI - I am doing almost 100% NB imaging (SHO), with a full-frame camera on a StellarVue 130 - and my subject matter can easily extend for many frames - but with significant intervening areas of dark sky.

Thanks for any feedback - and please feel free to add any other thoughts and considerations I might want to consider as I start exploring the Mosaic process.

And thank you Adam for your very informative tutorials on the subject.

cheers - bob

BlockHead

1. Yeah, generally you do not want your seams to run through good stuff- but sometimes this cannot be helped. If you use the right tools though..this isn't an issue. (I demonstrate with M78).

2. Number of exposure per frame is not critical IF you have a significant number in each. I mean that beyond a certain point you have minimized the noise enough that if you have 80 frames and 300 frames... you still will have a hard time seeing the difference in your final image. When you have small numbers of frames... it is easier to see the S/N difference between the regions. Our eyes/brains are pretty good at this. However, as long as you are minimizing the noise in the shortest frame set... all is good. So you do not need to match exposure times... but those empty frames..you do not want to skimp too much or you will be able to tell. With noise reduction tools of today (like NXT)- you can apply different amounts of noise reduction and likely get what you want.

The best tool (you did not mention it) is Photometric Mosaic. It will take care of the offsets and scaling between images and let you blend the overlaps better than anything else that exists. 

As I am sure you are aware..I have a course on this topic.

-the Blockhead

Bob Harris

BH - thank you for the prompt and informative response.  Attached is a screenshot of the current number of subs I have for a 4-panel Tadpole/Flame Mosaic.

I am currently still adding frames for Panel 1 and 4. These are all 600sec NB exposures.

FYI - I am pretty obsessive about getting lots of info. Typically, for my single frame compositions, I try to get at least 40 or 50 hours of subs. Sometimes I will take over 100 hours. I think it helps.............? However, applying the same thinking to my mosaics will quickly get me bogged down. Having said that, I currently have almost 100 hours of subs for this mosaic.

Again, I am shooting with an SV130 and a QHY600, from a home observatory in Snowmass Village, CO - Bortle 4 - 8,500 ft with clear skies yes, but above average turbulence.

Might you suggest a minimum number of subs - and perhaps an "obsessive" ideal number - for frames of less consequence??

My apologies if that is too specific a question.

cheers - bob

p.s. I stumbled on the "Blink2" PI process the other day. With all of these different mosaic frames to Blink and sort through, I have found it significantly superior to the regular "Blink". Others may appreciate it as well. I still wish you could delete files with this Process.

BlockHead

Uh oh...maybe I do not have the most up to date version...I thought I did. I have not seen the blink2 process.

 With 600s exposures with your aperture... the *minimum is probably around 50. (Or maybe 60 for 10 hours).

However, if you are going to skimp... you can make the ratios for SHO fit the object. If it is really bright in Ha... then concentrate more on OIII and S2. So you can definitely do less in Ha (minimum) and spend more time on the others.

So... tell me about Blink2...

-the Blockhead

Bob Harris

Blink 2 - not sure if your site will let me drop the exact link into this correspondence - but here it is in secret decoder language. Just copy to your registry and voila - a new Blink2 process icon appears.

pixinsight(dot)arcturus(dot)ch/blink2/

A friend suggested that it was written by Juan himself - so why isn't it the "official" version?

Blink2 contains 4 additional sorting options for the checked and unchecked boxes you may have created. It is not a huge deal but I find it much more intuitive (and easier for me anyway) to immediately move specific files to new or different directories. 

Additionally, and as I said before - Juan should just add the ability to "delete" files right here and now. It would save me from moving the files I want to keep to a new directory - then going back (in File Explorer) and deleting the files I have chosen to get rid of from the original directory. This process is confusing and prone to making mistakes. Why not simply Blink your subs - clicking "off" (better yet have the default be no boxes checked and you click "on" the ones you want to be deleted - which you can easily do with Blink2) - and then choose to "delete" all of the files that are checked and highlighted. No creating a new directory - no moving files - no need to go out to File Explorer and delete your bad subs - much less opportunity to accidentally move things to the wrong place or erroneously delete something you want to keep.

Sorry if that is TMI or confusing, but you did ask - if nothing else it should be in a thread titled Blink2.

On the original Mosaic topic - thank you again for the specific insights - very useful. Hopefully, my questions and your suggestions prove helpful to other readers as well.

cheers - bob

BlockHead

Oh... a third party script. Got it. I misunderstood... I thought you were speaking of something in the official release.

Yes, I have pestered Juan about the file deletion since day one (that is my day one in 2016).

-the Blockhead