M81 Last of my reprocessing flood

[RickJ]

I really need to reshoot this one on a good night when I can use 1x1 binning to advantage. Those nights are rare and I'd need two to get enough data. So this reprocess of an image from over 2 years ago will have to do. The original had purple arms but any attempt to fix that just made things worse. I didn't know enough about color balance at the time to fix it. I started from scratch for this version. Colors seem far more real to me. Though I ended up with less blue arms than I usually see this is what I'm getting so going with it. Think I decided to blue up the arms the first time and that's what got me into trouble. Holmberg IX was isolated during the luminance processing and processed separately. Color however was not treated separately. It's true relative brightness is much less than shown. Call it artistic license or cheating whichever you prefer. Again, this used the old, noisy, 5' subs I long since gave up on.

14" LX200R @ f/10, L=10x5' RGB=3x5', STL-11000XM, Paramount ME

I didn't do a small version, it just wasn't working within band width limits. Here's the link.
http://www.spacebanter.com/attachmen...tid=2394&stc=1

Rick

[paul f. campbell]

Hi Rick.
Your image is WOW, nice job. Clerar skies to you. Paul.

[andyschlei]

I'll accept the artistic license rationale. it is a beautiful image.

On the sub-exposures, shouldn't two 5 minute exposures combined have the same SNR as one 10 minute one?

[RickJ]

On the sub-exposures, shouldn't two 5 minute exposures combined have the same SNR as one 10 minute one?

No. System noise added by all the electronics involved. That noise is constant and doesn't increase with exposure time. It's constant with every exposure You want the exposure time to be high enough that background sky noise dominates over system noise. Several sites have calculators for this.
http://starizona.com/acb/ccd/calc_ideal.aspx

They say I need 45 minutes on a bad night and up to 75 on a good night due to my very dark skies. So even my normal 10 minutes is way too short but the space debris I run into with longer exposures is a pain to deal with so I have chosen 10 as a compromise. Also I need to refocus if the temp drops rapidly. Also while I have no need to guide a 10 minute shot I would have to guide a 45 minute one and I never get as tight of stars guiding as not guiding. Apparently it is chasing seeing. Sometimes I'll go longer with faint objects to further reduce noise but not often.

I didn't realize this as a beginner when I took this data.

With a 10 minute exposure the background may be 300 with the image running up to 12K. With a 5 minute exposure the back ground will be about 200 and the max reading 6K (not counting stars). I have a much easier time teasing a good image out of nearly 12K of data than out of 6K range. Adding rather than averaging doesn't seem to change my ability to get a good image out of the data. Maybe it is just system noise at play. I don't know. I just know, for me it works much better.

Rick

[bunker9603]

That is a really nice shot!

[JAICOA]

A Beautiful image it is Rick, Very nice processing on the colors. Welldone!