Messier 82

[Hughsie]

This was the second outing with the RC8 this time taking a look at Messier 82. 

In 2020 I captured this galaxy along with Messier 81 with my 4” refractor and when I purchased the RC8 I really could only think about pointing it towards M82 first. That starburst region is so dramatic.

This image was captured during one night and is presented in LRGB. Exposure time was 120s across each filter with a total integration time of 4.3 hours.

More data was captured earlier in the week but I had to bin it. Whilst the scope was guiding well I came across flexure for the first time and over the course of a night I watched my images drift consistently in one direction. The next day "Operation Tighten Everything" kicked in and that night I hoped I had solved the issue but sadly I hadn’t.

"Operation Acquire an OAG” was a challenge and solved with an eBay purchase. Then “Operation How The Hell Do I Use This Thing” started. Nothing new there!

The rest as they say is history and “Stella” as my RC8 is now known continues to throw out challenges. But when you can get this close to M82, I don’t really care.

 

Edited April 10, 2021 by Hughsie

[CraigT82]

That is a beauty all right! Really nice work. Which OAG did you end up with? 

[Hughsie]

40 minutes ago, CraigT82 said:

That is a beauty all right! Really nice work. Which OAG did you end up with? 

Thanks Craig.

I went with the v2 ZWO OAG and it came with the Helical Focuser too. I set it up during the day, focused my main image camera then adjusted the guide camera until it came into focus too.

[vlaiv]

Very nice image.

What this taken with ASI1600? If so, did you drizzle by any chance and why?

Instead of drizzle - you should really bin x2 your data.

 

[Hughsie]

24 minutes ago, vlaiv said:

Very nice image.

What this taken with ASI1600? If so, did you drizzle by any chance and why?

Instead of drizzle - you should really bin x2 your data.

 

Thank you Vlav.

I did drizzle with the ASI1600. What are the advantages of x2 binning over drizzle?

Edited April 10, 2021 by Hughsie

[vlaiv]

3 minutes ago, Hughsie said:

Thank you Vlav.

I did drizzle. What are the advantages of x2 binning over drizzle?

Drizzle should be (questionably) used when you under sample - to attempt to recover some of the resolution that optical system has that was lost due to too large pixel size.

It was developed for Hubble space telescope - which is outside of atmosphere and has F/24 focal ratio - most CCD pixels at the time were too large to properly record what the telescope is capable of resolving.

In majority of cases in amateur setups - you don't need the drizzle algorithm and one of the features of drizzle algorithm is that it lowers SNR (signal to noise ratio) as it reduces number of samples that end up stacked for each pixel. If you stack 20 subs - in normal way - you'll end up with 20 values for each pixel which you average. With drizzle - you literally drizzle pixels over larger area and as result each output pixel gets less samples - SNR is not improved as it could be with regular stacking.

Opposite thing of under sampling is over sampling - and with small pixels of modern cameras it happens much more often. Here you again loose SNR due to light being spread over more pixels than is needed.

With 8" scope and mount like EQ6 under good conditions - you can realistically hope to achieve something like 1.3 - 1.4"/px and not much more than that.

With ASI1600 and 8" scope natively, you are working at 0.48"/px - which is much higher resolution than your system can resolve (and atmosphere allows).

You can easily see that if you look at your image at 100% zoom level - where each pixel on screen represents one pixel in the image. When you do that, you should still see "pin point" stars. When I do that with your image I get this:

This star is by no means pin point. This is in part due to x2 drizzle - where you increased further sampling rate by factor of two making it 0.24"/px. If we observe what that star would look like natively - it will be like this:

Ok, looking better - but still not pin point like. This is close to 0.48"/px. Now look what the image looks like at 0.98"/px (or about 1"/px):

This is almost ok - but I would say still a bit over sampled. Realistically you have detail for about 1.3"/px  and that would look like this:

You can also see that spikes are very thin in this image rather than smeared thick lines.

In any case - binning will improve your SNR in similar way to using larger pixels - and has similar mechanism like stacking - stacking 4 images improves SNR by factor of 2 (square root of number of stacked subs). Binning 2x2 - does the same as you in fact again average 4 samples - 4 adjacent pixels are averaged to produce one "larger" pixel.

As a comparison, here is crop from one of my images that was binned to ~1"/px (also 8" RC and ASI1600). It is Ha image and due to narrowband filter used - it has tighter stars:

This is at 100% zoom level.

That sort of star sizes you want in your image. If stars are significantly larger - you are over sampling and in doing so - loosing SNR without any benefit of added detail in the image.

 

[Hughsie]

Thank you Vlaiv, that is a really helpful explanation. 

[Hughsie]

I have gone back and re-processed the image following Vlaiv’s comments. I can’t re-capture in Bin 2 but I have removed Drizzle Integration from the workflow.

Thanks again Vlaiv, really good feedback.

 

Edited April 11, 2021 by Hughsie
Forgot to apply SCNR

[vlaiv]

8 minutes ago, Hughsie said:

Thanks again Vlaiv, really good feedback.

Main question is - did you find it easier to process this way and were you able to pull out more (better SNR)?

As far as I can tell - it indeed goes a bit deeper - outer glow is more visible on second image, but I'd like to know your subjective feel while processing?

[Hughsie]

6 hours ago, vlaiv said:

Main question is - did you find it easier to process this way and were you able to pull out more (better SNR)?

As far as I can tell - it indeed goes a bit deeper - outer glow is more visible on second image, but I'd like to know your subjective feel while processing?

Hi Vlaiv, my observations are as follows;

• Drizzle integration was dropped from the workflow.
• Rather than undertake a Histogram stretch to move from linear to non-linear data, I used MaskedStretch with the background set at 0.10. This really helped try and keep control of the star in the bottom left hand corner.
• Noise reduction. Having combined RGB then L to RGB, the noise in the image was so much lower. I cloned the LRGB and ran TGVDenoise on it and compared with LRGB with no noise reduction and it was tough to call whether noise reduction was needed.
• Visually, the ‘north’ side of M82 in the new image is showing slightly more detail than the original. It's tough to see in png format but the original xisf side by side this new version shows more detail.
• I agree, the ‘glow’ from the galaxy is more apparent and smoother.

The stand out for me was the low level of noise by not incorporating DrizzleIntegration into the workflow. This made the image so much easier to work with. Secondary to this the larger stars appear more tighter.

John

 

[vlaiv]

1 minute ago, Hughsie said:

The stand out for me was the low level of noise by not incorporating DrizzleIntegration into the workflow. This made the image so much easier to work with. 

I've noticed that DrizzleIntegration is often used and I believe that one or more tutorials out there shown it being used and that is the reason why people use it - most just follow a good tutorial.

However, as you have seen yourself - that actually hurts the image if used improperly.

[Hughsie]

I have presented a few images here over the last two years and received ‘likes’ which is always nice.

This last 24 hours Vlaiv has presented a critique of my image far and above anything I have received so far which has challenged me to consider what I feel is the ‘norm’. Having acknowledged his assessment, I undertook my own review of on the points he raised and have learned that this is not the first time he has called out the demerits of DrizzleIntegration. 

From my own personal perspective I really welcomed this challenge. Spending hours in the dark freezing cold I always aspire to produce the best that my scope/camera can see. To be better requires good constructive feedback and Vlaiv has provided that over the course of this weekend.

Thank you Vlaiv.

[tooth_dr]

Very nice indeed.  Just currently processing some M82 data myself, and I agree it's a fantastic target that deserves a bit of focal length.

[NigeB]

1 hour ago, Hughsie said:

I have presented a few images here over the last two years and received ‘likes’ which is always nice.

This last 24 hours Vlaiv has presented a critique of my image far and above anything I have received so far which has challenged me to consider what I feel is the ‘norm’. Having acknowledged his assessment, I undertook my own review of on the points he raised and have learned that this is not the first time he has called out the demerits of DrizzleIntegration. 

From my own personal perspective I really welcomed this challenge. Spending hours in the dark freezing cold I always aspire to produce the best that my scope/camera can see. To be better requires good constructive feedback and Vlaiv has provided that over the course of this weekend.

Thank you Vlaiv.

Agreed, this is an excellent discussion and I've just been reading the previous analysis of drizzle integration by @vlaiv. Bookmarked this one for future reference. 

[Astro Waves]

This is bloody lovely!

I'm still trying to wrap my head around the technical aspects of AP and just been reading about binning and still not sure I fully follow, I'll definitely have to read through all the stuff above again to try and get a better grasp on it. I heard about Drizzle and was unsure what it was really. I had stacked some data earlier which I was super happy with (even though you might secretly think its a bit substandard) and did an edit and it was not bad but I wanted to do better so then noticed there was a script in Siril with drizzle so thought I would give it a go and oh my days, the quality difference was just mad! way more detail and clarity and I had to do so much less processing on the image with drizzle, I'll post it in a new topic as its my finest work yet (of my limited experience).

[michael.h.f.wilkinson]

22 hours ago, vlaiv said:

As a comparison, here is crop from one of my images that was binned to ~1"/px (also 8" RC and ASI1600). It is Ha image and due to narrowband filter used - it has tighter stars:

This is at 100% zoom level.

 

 

Question: Why would stars be tighter in narrow-band imaging in a reflector like an RC? I have seen the effect in my APM triplet to some extent, but it is not noticeable in my Schmidt-Newton (and that might show sphero-chromaticity)

[Clarkey]

4 hours ago, Hughsie said:

This last 24 hours Vlaiv has presented a critique of my image far and above anything I have received so far

Vlaiv has critically assessed some of pictures in the past and it is something I am always grateful for. I am relatively inexperienced and technically far from expert and this sort of help is really appreciated. I would add more constructive criticism myself - if I was a bit cleverer🤪

Nice M82 - that's as critical I can manage at 10pm!!

[vlaiv]

2 hours ago, michael.h.f.wilkinson said:

Question: Why would stars be tighter in narrow-band imaging in a reflector like an RC? I have seen the effect in my APM triplet to some extent, but it is not noticeable in my Schmidt-Newton (and that might show sphero-chromaticity)

Not all narrow band has this. OIII often has higher FWHM than Ha.

Atmospheric influence.

Shorter wavelengths bend more than longer wavelengths and Ha is at red end of the spectrum. Often, lunar imagers use narrow band filters in Ha to lessen atmospheric influence.

For this reason if usually creates very tight stars - simply not scattered around as much as shorter wavelengths.

[michael.h.f.wilkinson]

7 hours ago, vlaiv said:

Not all narrow band has this. OIII often has higher FWHM than Ha.

Atmospheric influence.

Shorter wavelengths bend more than longer wavelengths and Ha is at red end of the spectrum. Often, lunar imagers use narrow band filters in Ha to lessen atmospheric influence.

For this reason if usually creates very tight stars - simply not scattered around as much as shorter wavelengths.

So as I thought it is a wavelength rather than narrow-band effect