M57 - 1st try & issues (advice welcome)

[AstroGS]

This was my 1st ever attempt with M57 but, unfortunately the Ring Nebula core comes out as over-exposed and burnt after the integration. The individual frames seem to have enough detail but, the integrated image does not look correct. Did i over-do it with the exposure? Was it the filter or something else?

• EQ6R-Pro
• WO GT81
• WO 0.8x flattener
• Optolong L-Extreme
• ASI533MC
• AsiAir Plus
• Unity gain at -10C
• 23 x 600 sec subs
• Bias & Darks - did not use any flats

Tracking was excellent (0.45-0.55) and focus was also very good.

For integration i used both Pixinisght and DSS - no drizzle. Very similar results with the integrated image.

Sharing with you the integrated image + 1 subframe as an FYI. What would you advice went wrong here?

Light_M57_600.0s_Bin1_-10.0C_0007.fit M57_RGB_CROPPED.xisf Light_BIN-1_3008x3008_EXPOSURE-600.00s_FILTER-NoFilter_RGB_LN_Reference_.xisf

Edited July 11, 2022 by George Sinanis

[The Lazy Astronomer]

1 hour ago, George Sinanis said:

This was my 1st ever attempt with M57 but, unfortunately the Ring Nebula core comes out as over-exposed and burnt after the integration. The individual frames seem to have enough detail but, the integrated image does not look correct. Did i over-do it with the exposure? Was it the filter or something else?

• EQ6R-Pro
• WO GT81
• WO 0.8x flattener
• Optolong L-Extreme
• ASI533MC
• AsiAir Plus
• Unity gain at -10C
• 23 x 600 sec subs
• Bias & Darks - did not use any flats

Tracking was excellent (0.45-0.55) and focus was also very good.

For integration i used both Pixinisght and DSS - no drizzle. Very similar results with the integrated image.

Sharing with you the integrated image + 1 subframe as an FYI. What would you advice went wrong here?

Light_M57_600.0s_Bin1_-10.0C_0007.fit 17.27 MB · 2 downloads M57_RGB_CROPPED.xisf 93.44 MB · 3 downloads Light_BIN-1_3008x3008_EXPOSURE-600.00s_FILTER-NoFilter_RGB_LN_Reference_.xisf 103.57 MB · 2 downloads

It's not overexposed in the sense that it doesn't appear to be clipped, but nevertheless, I'd start by saying that unless you have a very dark sky, 600 is probably overkill - you could likely drop down by half or more. As an example, I typically use 30 - 60s for broadband and 3 - 5 mins for 6nm narrowband (I have a pretty typical suburban sky).

Second, no need for bias and darks, the bias signal is already contained within the darks. I've also seen people with this camera do it the other way around and ditch the darks and just use bias instead as there's no amp glow to worry about. I can't comment much further on that as I don't own the camera myself. If pushed I would probably have a preference for using darks only instead of bias only, but then I may be, uh, biased (no pun intended) by my current camera experience (294MM, so I've got serious amp glow!!)

I whacked it through a quick bit of stretching using the GHS script in PI, followed by a saturation boost - I've probably not done it particularly well, as I'm not currently on my processing PC, but on a laptop which I know for a fact has a poor display. Hopefully it's an indication at least. How does it compare to what you were getting?

[AstroGS]

5 hours ago, The Lazy Astronomer said:

It's not overexposed in the sense that it doesn't appear to be clipped, but nevertheless, I'd start by saying that unless you have a very dark sky, 600 is probably overkill - you could likely drop down by half or more. As an example, I typically use 30 - 60s for broadband and 3 - 5 mins for 6nm narrowband (I have a pretty typical suburban sky).

Second, no need for bias and darks, the bias signal is already contained within the darks. I've also seen people with this camera do it the other way around and ditch the darks and just use bias instead as there's no amp glow to worry about. I can't comment much further on that as I don't own the camera myself. If pushed I would probably have a preference for using darks only instead of bias only, but then I may be, uh, biased (no pun intended) by my current camera experience (294MM, so I've got serious amp glow!!)

I whacked it through a quick bit of stretching using the GHS script in PI, followed by a saturation boost - I've probably not done it particularly well, as I'm not currently on my processing PC, but on a laptop which I know for a fact has a poor display. Hopefully it's an indication at least. How does it compare to what you were getting?

I will definitely look into this again later tonight and I will try the GHS as well.

The only reason I’m using 600 sec subs is because, I’ve managed to get some really nice detail in the past and I have already a set of darks for this exposure time. I will most definitely need to get 3 and 5 mins darks as different exposure times will be required for different targets.

[wimvb]

1 hour ago, George Sinanis said:

The only reason I’m using 600 sec subs is because, I’ve managed to get some really nice detail in the past and I have already a set of darks for this exposure time. I will most definitely need to get 3 and 5 mins darks as different exposure times will be required for different targets.

600 s at unity gain seems high to me. To compare: I shoot L at 0 gain and 180 s @f/5.3 for all targets. RGB 240 s (a bit short, I could extend to 300 s) at the same settings. For 7 nm Ha I use 240 s at gain 200. All with the ASI294MM.

Also, I don't think you need different exposure times for different targets, unless you are imaging the Orion nebula, possibly the Andromeda galaxy. Cmos cameras have enough dynamic range, and you get the final results in post processing. What will determine exposure time most is level of light pollution. If you have vastly different light pollution in different directions, you may need different exposure times. But even then, you can get away with either an average time or simply by avoiding the worst LP.

[wimvb]

Here's my attempt at your data. It is a very nice image  of this small planetary.

Imo, the image is over exposed, but not for the obvious reasons.  The nebula is quite bright and the brightest stars in the image are close to over exposure. This in itself is not a problem, but when you do a background correction in DBE and Background Neutralization, you get false colour in the star cores. With a bright target such as M57, there is a risk that the nebula is also affected by this.

Mathematically, what is happening is this. The core of a bright star is all white in the image (in PI colour coordinates: (1, 1, 1)). This includes any light pollution. The background, due to light pollution, is a bit green (0.05, 0.1, 0.05). Background Neutralization tries to correct the background to, say (0.01, 0.01, 0.01) by subtracting the difference: (0.04, 0.09, 0.04). The star is affected by this and gets a core colour (0.96, 0.91, 0.96). It loses green, and gets a magenta colour. That's what you see on the left side in the above before/after image. The outer edge/halo of the star is not affected, so this gets is natural colour. Had the star been exposed below saturation, the light pollution would have been added, and the uncorrected star before Background Neutralization would have looked green. After BN, it would have its correct colour. If your images suffer from LP, you need to make sure that the main target (ic M57) will not get become too bright in the singel exposures. Over exposed stars can be corrected with the HSV Repair script in PI, but if the main target has pixel values above the correction threshold (default, 0.5), those pixels will also be "corrected" and can end up having a wrong colour.

Btw, stacking will not make a target brighter or lead to saturation. Pixel values are averaged. However, the noise will be less, and PI tries to use a stronger stretch in the Screen Transfer Function. This is all good, because the only purpose of the STF is to show what is in the image, especially in the darker parts where you'd want to see any defects or gradients. The default STF was never intended to be used as a permanent stretch anyway. If you want a more pleasing view during processing, you have to manually adjust the STF.

I rotated the image such that North is top. The very brightes stars and M57 suffer from reflection halos. This is strongest for the star on the right. The halo around M57 is not extended nebulosity, because it is of the wrong colour. The extended nebulosity is red (Ha), and the halo is green in the image.

Edited July 12, 2022 by wimvb

[Tommohawk]

FWIW I think that's a very nice image, especially Wim's version. I'd agree that 600sec subs seems excessive, but the data doesn't seem to have suffered/clipped as a  result - it looks great.

Also I guess any PN will be tough at that scale.

[AstroGS]

Thank you…. I came home late last night from work but, I will definitely go back to re-processing the image and try to get the best out of the data.