[RCC] Getting my SCT imaging going, what to do? - Forums

Topic starter

Mar 25, 2021

Hi!

Without telling too much of a story, I want to dive directly into the topic. After a long time of totally clouded nights, I took my Meade 8" ACF SCT out to capture the Whirlpool Galaxy. Unfortunately, the Moon wasn't my friend that night. The result of my work can be found here:

Messier 51, getting my SCT imaging going

I would like to hear your feedback on the image and maybe some recommendations that you might be able to deduce from the technical data provided with the image. I am just wondering, if there is a way to extract more (fine) detail from the object?

On first glance, I would assume that I "simply" (in the sense that this has the highest leverage) need to improve on the amount of collected data (total exposure time). Especially for the Luminance layer.
While I'd say that I've gathered quite some experience on shooting with short focal lengths, an SCT scratches at other limits, like having a resolution per pixel that is significantly better than typical or even good seeing.

There might be several things to add but I might leave it at that for now and come back to your comments or questions.

CS!

Björn

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andrea tasselli

Frequent contributor

Mar 25, 2021

Hi Björn,

Well, data is not a minor point, or even an important point. is the WHOLE point. No data no image. This said, at your pixel scale you should have gone at least for 4 hours in luminance. Did you add the colour frames to the luminance frame? If not do it. Secondly you wiped out any fine detail by an excess of median filtering. Do not do it. If you want detail you either accept noise or increase exposure time. Colour balance is a bit too skewed toward the red end and doesn't help with the overall rendition (too dark and gloomy). Background is splotchy but that is a side effect of low SNR. But the issue number one is the amount of data collected.

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Björn Arnold

Topic starter

Mar 25, 2021

Hi Andrea,

I know that the amount of data collected is not a lot. Since clear nights are scarce in the recent month's, I wanted to capture a color image not matter what. With the mono camera I should get used to the point that I might only capture Luminance during one night and the colors on a different occasion. That explains why I'm short, especially on L data.

How do you relate the pixel scale to the amount of luminance data to be captured?

The combination of the color data and the luminance is that I'm blending the luminance data as a luminance layer in Affinity. Hence tone and saturation stem from the RGB while the luminance comes from the L. However, I think I need to be careful with the RGB data as its SNR is rather low. I indeed went to the extreme for the post processing as you've seen.

I increased the saturation of the red tones and probably the overall color treatment might have been loaded towards the red due the moon shine. Probably, I unintentionally overcompensated the moon.

My overall question comes from the fact that I try to match my expectations and my results and yet I am not perfectly sure what realistic expectations for the image quality I should have by using this SCT. At least I would assume that for resolution, the boundary here will be seeing and optical diffraction and not, like for the short FL APO, the pixel size.

CS!

Björn

andrea tasselli

Frequent contributor

Mar 25, 2021

Hi Bjorn,

There is no hard and fast rule for how much data is enough data. Too many variables; sky quality, seeing, sensor QE, pixel scale, how well is the scope focused and so on. Given that we have (or had) a similar instrument I'd venture to suggest that the sort of image scale you are using isn't going to be very beneficial in your situation. Go with a 1" pixel scale or thereabout, which in your case should be bin3x3. For colour go for bin4x4 to get as good a SNR as you can while spending most of the time with the L frame. Try not to over-process. Keep everything a simple as possible in first instance: a mild histogram stretching followed by an adjustment in the luminance curve. The moon isn't going to change tonality if your basic processing methodology is correct and you have a good flat-frame procedure.For resolution you number 1 enemy is seeing and the number 2 is keeping a tight focus. Diffraction is not going to bother any of us unless we happen to live in some of the northern Chile deserts. Nice for astronomy but not for much else. Or on top of Manua Kea.

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Björn Arnold

Topic starter

Mar 25, 2021

Hi Andrea,

Ok, I thought you referred to a certain rule to get from pixel scale to overall exposure time.

Actually, I had been thinking about capturing 4x4 for the RGB. I should give that a try next time. Bin 3x3 gives weird results, pixels rows become shifted. Makes sense if the Camera doesn’t know how to pack uneven patches into an even image.

My typical observation with the moon and my DSLR was that there’s a blueshift. The blue channel was always significantly more exposed. Moon light should behave the very same as direct sunlight in the atmosphere. The scattering should make it appear blue.

Indeed, the levels adjustment should, as you say, give a good color balance. I‘ll check again tomorrow to see where the tone shift got in and come back about that.

Björn

Lynn K

Mar 27, 2021

Bjorn, are you using a light pollution filter. I think some them can shift things to the Blue.

Lynn

Björn Arnold

Topic starter

Mar 27, 2021

Hi @Lynn K ,

No, I'm not using any light pollution filter. This image was captured with a mono camera and LRGB filters.

CS!
Björn

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Bob Lockwood

Mar 27, 2021

Hi Bjorn,

Not sure if this helps, but have you considered a camera with bigger pixels?
Your image-scale is so small not to mention your FOV is only 12x18 arcmin? Over time I have stopped binning any object especially galaxy’s. In my observations binning reduces resolution and each time you bin it reduces the image size, so if you say bin 4x4 your image will be so small that when you enlarge it to what you would see if you didn’t bin, the image will look pixely. I think your image look pretty good considering the condition you had to deal with. With an 8” f/10 scope, I think bigger un-binned pixels would be your best improvement.

Bob

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Björn Arnold

Topic starter

Mar 27, 2021

Hi @Bob Lockwood ,

Thanks for your feedback!

I‘m using a reducer which brings my focal length to something of around 1500 to 1600mm, although I wished I could go down to 1300. Nevertheless, the pixel size of the camera is a compromise. Before, I shot with my DSLR which has 3,7um which is still to small for that focal length given typical seeing. For my refractor, the pixel size of 2,4um of the mono is perfect however.

Fortunately, the dedicated camera allows binning while capturing. Something the DSLR doesn’t offer. The mono has a 1” chip which smaller than the APS-C of the DSLR but no matter what binning I am using, the FOV only depends on the chip size and not on the resolution. For the RGB, where I can afford having a low resolution image, I would think that the 4x4 makes sense but not for the Luminance.
Shooting the L with native resolution is too aggressive. That would bring me to 0,3”ppxl, which is too high res for sure.

Overall, I guess I need to capture more data and especially my red channel suffers some exposure time. Should have exposed 90“ to 120“ for R to be well above noise.

Another thing I have trouble with is the color balance. I was quite successful to get the right balance for my Rosette nebula and the open cluster M37 by weighing the RGB channels according to the sensitivity and calibrating the sky background. However, in this case, this procedure leads to a strong violett touch of M51 which I find difficult to get rid of. Still I believe the Moon is the culprit here.

CS!

Björn

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Bob Lockwood

Mar 27, 2021

Björn Arnold:
this procedure leads to a strong violett touch of M51 which I find difficult to get rid of

Not familiar with the software you use, I only use Photoshop CC. They have an adjustment called selective color and while it doesn’t have an adjustment for violet, it dose have one for magentas that effaces only the violet-magentas in the object. I use it to tone down the violet halos around stars you get sometimes in SHO images. If you have something like that in the software you are using, you may try backing off on the magentas and see if that helps with the violet issue.

Bob

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andrea tasselli

Frequent contributor

Mar 27, 2021

Bob Lockwood:
Björn Arnold:
this procedure leads to a strong violett touch of M51 which I find difficult to get rid of

Not familiar with the software you use, I only use Photoshop CC. They have an adjustment called selective color and while it doesn’t have an adjustment for violet, it dose have one for magentas that effaces only the violet-magentas in the object. I use it to tone down the violet halos around stars you get sometimes in SHO images. If you have something like that in the software you are using, you may try backing off on the magentas and see if that helps with the violet issue.

Bob

That kind of selective colour adjustment won't help him. It needs to have the colour balancing done right first and tweaked afterward. He thinks the moon is affecting it and I'm wondering what the blue channel looks like...

Björn Arnold

Topic starter

Mar 27, 2021

Hi,

@Bob Lockwood I am using Affinity Photo which considers itself being an alternative to Photoshop. With its recent version, it also allows me to stack the subs. I know about the method you mention but as Andrea says: it's a fundamental issue that I would need to resolve.

@andrea tasselli if it's ok for you, I can send you a private message with a link to download the stacked images so you can take a look at the frames yourself? (as usual, a picture says more than a 1k words).

Just as a starter, here's the histogram for my Rosette nebula and the Whirlpool after combining RGB and doing an initial stretch (I'm using an ArcSinh). You can clearly see that the blue has a much higher background in this latter case as the Rosette was shot at 3% Moon.
As a note on the side: to balance the different channels, I increase the exposure (through an exposure adjustment) for the red channel by 1,25 and for the blue by 1,176. With this I made the experience that it restores white as white.

CS!

Björn

Histogram Rosette.png Histogram Whirlpool.png

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andrea tasselli

Frequent contributor

Mar 27, 2021

Hi Björn,

Sure, send along!

Andrea