NGC2237 - The Rosette

[rodmichael]

This is my first attempt at the Rosette Nebula. Completed in time for Valentines Day (sent to my friends). I was reluctant to post it here. Not sure of the relative quality. Composed of 139 subframes, 240s each, LRGB.

Critique or suggestions welcome.

[RickS]

Hi Rod,

That's quite a recognizable Rosette with some decent detail.  It's a bit dark which is probably because you didn't have the SNR to stretch it too far.  IMO, the thing you need to do first is start getting some longer subs and reducing the effect of read noise.  If you're interested there are ways to determine the sub length needed to take sky limited subs for your set up and conditions - let me know and I'll pontificate further

Cheers,
Rick.

[rodmichael]

Hi Rod,

That's quite a recognizable Rosette with some decent detail.  It's a bit dark which is probably because you didn't have the SNR to stretch it too far.  IMO, the thing you need to do first is start getting some longer subs and reducing the effect of read noise.  If you're interested there are ways to determine the sub length needed to take sky limited subs for your set up and conditions - let me know and I'll pontificate further

Cheers,
Rick.

I would welcome additional thoughts regarding exposure length. I have struggled a bit with that issue.

Since I started using the Paramount MX+ last Summer, I had been using exposures of 300s to 360s. However, a friend in our local astronomical society had his own thoughts about the exposures I should be attempting and had done some calculations for me. Due to the very low focal ratio of my system (f=2.22), he had arrived at the conclusion that I should be using very short exposures, e.g., less than a minute for RGB subframes, a couple 10s of seconds for Luminance, and perhaps a minute or two for NB subframes.

I have tried LRGB exposures of that magnitude since on both the Rosette Nebula (NGC2237) and the Jellyfish Nebula (IC443), both relatively faint objects. I didn't have much luck when I tried processing because there just wasn't very much data. I used 60s/subframe for imaging the Pleiades (M45) with pretty good results. This attempt on the Rosette represents a move toward longer exposures (240s) again.

I am imaging unguided with the Paramount MX+ and it's "TPoint Supermodel" and "ProTrack" tools. I have experimented with exposures up to 10 minutes with good results and have settled on 6 or 7 minutes as my upper limit for now.

From your comments, I conclude that longer exposures and a larger number of subframes, e.g., for the Rosette perhaps 300s or 360s and 50 subframes for each color (4 to 5 hours per filter as opposed to the 2.3 hours I have for this Rosette), will improve my result.

[RickS]

Rod,

That's a pretty fast scope but the result seems like the subs aren't optimal.  Here's how to check...

The method I use to check my subs is based on some fairly simple maths.  To make sure that the contribution from read noise is less than 5% you want to make sure that the background value in your (uncalibrated) subs is no less than: 10*RN*RN/gain + pedestal

You can calculate the read noise (RN above) and pedestal with the DarkBiasNoiseEstimator script and a pair of bias frames.  RN is the temporal noise calculated by the script multiplied by the camera gain (0.5 is the default gain for the QSI683 according to the specs.)  Pedestal is the offset value calculated by the script.

As an example, let's say that DarkBiasNoiseEstimator gives you an offset of 900 DN and Temporal Noise of 24 DN from your bias files.  RN is 24*0.5 which is 12e-.  The background target calculation is 10*12*12/0.5 + 900 which is 3780.

Check a few of your raw subs for each filter (as they come from the camera) and look at the median value in a background area.  If the median value is around 3780 or more then your subs are sky limited.  If the value is less then they are read noise limited and you should think about doing longer subs.

Note that you'll need longer subs with R/G/B filters than with luminance to get sky limited.  With narrowband filters you may find that a sky limited sub takes a very long time.  I generally don't do sky limited subs in narrowband because with the systems I use it would take hours per sub.  I just live with the fact that my NB imaging is not optimal from a SNR point of view.

Hope that all makes sense.

Cheers,
Rick.

[rodmichael]

Thanks, Rick. Something to chew on and perhaps get to some objectively defined exposure targets. I appreciate your insights. I'll see what I come up with. (Not today).

Rod

[rodmichael]

FOR ANYONE WHO LOOKED AT THIS POST WITHIN THE PAST HOUR, IT HAS BEEN HEAVILY EDITED FROM WHEN YOU FIRST SAW IT.

Rick, I have checked the parameters you describe.

First I calculated my "Background Target" with the "Dark Bias Noise Estimator" script.

Offset = 262
Temporal Noise = 24
Background Target = 3142

With the PixInsight "Contrast Background Noise Ratio" script I have measured the background noise for several different uncalibrated frames taken with LRGB and Ha. I have attached a screenshot of the spreadsheet with the data and some graphic representations of the data.

Summary of Data: All my exposures appear to be substantially underexposed (Read Limited, less than the "Background Target" of 3142) for exposures up to 360 seconds for all filters except Luminance at 150s and 360s. I think I know a lot better now what I need to do.

I appreciate the help, Rick!!!

Thanks!!!

[RickS]

Hi Rod,

I'm not sure what you mean by K.O. number?  I measure the background value by drawing a preview in a suitable area and applying the Statistics process (mode set to 16-bit.)  The median value is the one I use.

Cheers,
Rick.

[rodmichael]

Hi Rod,

I'm not sure what you mean by K.O. number?  I measure the background value by drawing a preview in a suitable area and applying the Statistics process (mode set to 16-bit.)  The median value is the one I use.

Cheers,
Rick.

The K:O. Number is the number on the bottom bar of the PixInsight workspace that appears when one scrolls over an image. It is one of 3 numbers, the other 2 being x & y coordinates of the cursor location on the image (I think). I thought it was that number that I was supposed to look at. As I re-read your original message, I realized there must be a script or process in PixInsight for getting the median background noise. I went looking for it and found it. So the script I ended up using to get Median Background Noise was the "Contrast Background Noise Ratio" which is a script in the "Image Analysis" menu. The rest was just an Excel spreadsheet exercise (see my heavily edited note above).

Thanks, again!!!

EDIT: And now I just found the "Statistics" process you mentioned. I checked one 240s Ha frame with the Statistics process and it agreed with the script that I had used (812 vs 847, different previews and frames used).

[RickS]

Sounds like it has been a useful exercise, Rod.  Trying bumping up the sub times a bit and I'm sure you'll get better results.

Cheers,
Rick.