My first LRGB

[codemonkey]

This is also my first guided image, and one taken using my new (NEQ6 pro) mount.

By the time I got everything working well enough to even bother taking a test image, NGC 5128 was very low in the sky so I had to keep subs very short.

This is about:
* 20x90s 1x1 bin luminance
* 16x15s 2x2 bin red (forgot to turn on cooling)
* 13x15s 2x2 bin green (forgot to turn on cooling)
* 16x15s 2x2 bin blue (remembered halfway through to turn on cooling)

Processing is really rough too with noise and gradients, but I'm just happy to have gotten my first LRGB image no matter the quality :-)

[RickS]

Well done, Lee!

[BruceG]

Well done Lee - I am sure you have just taken a large step forward and one that will continue to challenge and reward you.
Well done!
Bruce.

[raymo]

Good start Lee. What is the red and green doughnut on the right hand
side of 5128? Is it dust?
raymo

[Amaranthus]

It's a great feeling getting the first LRGB combo, isn't it Lee? You chose a tough first subject too, so well done - look forward to seeing more!

[codemonkey]

Thanks guys! :-)

raymo: I think it's hot pixels that were present in the binned RGB data, but I didn't have any bias frames for 2x2 binning. I assume that you have to do different bias frames for binned vs unbinned...?

[Amaranthus]

Yes, different flats for each filter and different darks + bias for the 2x2 bins. But overall I find it more hassle to meld 1x1 and 2x2 that I generally just stick with pure 1x1.

[raymo]

I've no idea Lee, I don't do bias frames or binning.
raymo

[codemonkey]

Thanks mate :-)

I took a series of 100 darks that I applied to several sets of images and saw no noticeable difference in noise so I don't bother with them now, but I usually use bias. I'll have to take a set for 2x2 at least.

I'm not currently using flats either, but I know I should start doing that... I'm putting it off until I've got more important things ironed out.

[Amaranthus]

Darks have to potential to inject noise. Most dark current is expressed as hot and dead pixels, especially if you have a cooled camera, and you can use a bad pixel map to instead control for these - no noise injection then! That is what I typically do.

For consistent and hassle-free flats, get a light box. Brilliant things.

[Paul Haese]

This is actually pretty reasonable.

Some points to look at here:



refocus the telescope regularly. You are ever so slightly out of focus.
Do ensure you are doing darks. Dark subtraction is critical for most sensors. It might have been the method you are using to subtract the darks that impacted your data.
For colour try using median combine
Use sum combine for your lum data.

Keep at it, you have started an interesting journey.

[Rex]

That's awesome Lee. I can't wait to see it with more data. Congrats on your first LRGB.

[codemonkey]

I'm not sure what happened here actually... I've got some bad pixels in the luminance even when applying the bias alone, or the dark, or the dark plus the bias. Strange.

A light box eh? Ah, another thing to add to the never-ending wish list lol.



Thanks for the tips Paul, I really appreciate that! :-)

I'm using a short tube refractor with a moonlite focuser that was locked, so if it's out of focus it's because I stuffed it up in the first place I think :-)

I'm not sure how to sum the luminance data, can you possibly elaborate on that? I use PixInsight.



Thanks Rex! :-)

Here's another attempt at processing it. A bit better than the previous. Going to take some practice. Any additional tips on processing LRGB in PI are very welcome.

[Paul Haese]

Lee I don't use Pixinsight but I imagine there is a menu that has combining methods to stack data. Summing is a stacking method. Perhaps take a look around for that.

I also urge you to start looking at automated focusing too. It is much more accurate and will produce sharp data for you.

The focus while it might have been sharp to begin with, actually has moved throughout your imaging session. So you probably got it right from the beginning but the tube has contracted over the course of the session and hence why your focus has drifted. To alleviate this initially I recommend you use a bahtinov mask every 40 minutes or so to check focus on a bright star near the meridian. To get the scope back to the same spot again take note of the star field for your guide camera. Ensure your goto's are accurate and place the scope back in the same spot by nudging. It might take 10 minutes or so but will give you sharp detail a lot of the time. When you are ready you can go to automated focusing.

[codemonkey]

Thanks again Paul :-)

PixInsight has a wealth of options when stacking, but none of them are summing... well, if they are they're not clear about it. I'll look into it further though, thanks.

With regard to the focus change... don't construe this as me being argumentative, it's a genuine question, but how do you know I didn't get it wrong in the first place? I honestly didn't spend much time on it, not as much as I usually do. Is there a tell-tale sign in the image that suggests it shifted? Or is this simply physics and you know that given temperature changes it would have shifted?

I understand this to be an issue with reflectors but I didn't think refractors needed to be refocused like that so that's new.

Another thought; could it be due to the colour filters not being perfectly parfocal?

[Paul Haese]

Yeah fair question. Well, I like to assume you got it close to begin with. Though even if you got it wrong first up and it was slightly out then you need to know that all metal tubes will contract over time in the night air. This is the co-efficiency of linear expansion at work.

I am assuming again you are using a metal tube refractor and hence why I stated that you need to refocus often. If you have a carbon tube then there will be little if any expansion due to its properties.

It could also be that the filters are not par focal but they would not be too far out. The more likely scenario is that the scope has contacted a little and this has led to focus shift. In any event if you know that the filters are not par focal you should focus between filter changes.

It should be remembered though, that this is still a good result for your first LRGB image. Keep working at it. :thumbsup:

[codemonkey]

Ok fair enough. Thanks mate :-)

The refractor does have a metal tube... I should be getting an older CF GSO RC8 tomorrow though :D

The image is pretty average, I'm happy to admit that, I've done better. I'm always happy to hear constructive criticism, in fact I'd much prefer that to people telling me it looks good... I can't take that to the bank :-)