Galaxies & CLS Filter

Hi Mark,

I think you can safely use the CLS filter. As you said, Galaxies are broad band emitters and hence produce signal also in the transmissive part of the filter spectrum. With the CLS you should be able to counter the Bortle 8 sky pretty well and produce more contrast for the galaxies. So you definitely should give it a try.

Regarding exposure time, I don't know if you take a more quantitative or qualitative approach. In my very beginnings, I followed the rule that the histogram "mountain" should be located about 25 to 30% away from the left (black) side of the histogram (which I call the qualitative approach nowadays). However, that's way too much of exposure time. I'm taking the quantitative approach (at least that's what I call it):
I determined the signal statistics of the dark frames. Then I do the exposures on the target, I'm checking that the histogram peak starts about 4 to 5 standard deviations above the noise means. You'd be surprised how quickly this happens. I would assume you even wouldn't have to increase exposure by a factor of 2 to achieve that.

Personally, I don't see a reason for drizzle. You've got a 18MP APS-C chip. With the given optics, you get about 1,4"/pixel, which is close to ideal.

CS!

Björn

Hi Mark,

Can you explain what you mean by better regarding your Pleiades?
Assuming that all the settings were the same, the camera will collect more light without the filter. The Plejades are a star cluster with a reflection nebula and hence emit in a broad spectrum, contrary to nebulae consisting of ionized hydrogen. It also depends on where the objects was located during the time of shooting. If it's rather high up in the sky, the effect of light pollution is lower and you might even be able to shoot without the filter. Therefore, it can indeed be that without the filter, the picture might be better.
Also, I personally tend to question the future effectiveness of the light pollution filters as more and more sources are LED lights that don't emit lines like the classical sodium-vapor street lights. And these LEDs are insanely brighter than the classical lights - don't ask me why we need to brighten up the night further.

You need to use the darks as a basis for the histogram. Although in many cases, the bias gives most of the noise signal, the long term exposure and hence the temperature add some noise. I've analyzed my EOS250D and most came from the bias but the exposure time cannot be neglected.
You need to consider the color channels separately as the sensor is less sensitive in red and blue compared to green. Also standard deviation isn't sufficient, you need to know the mean of the dark signal as well.

I am nowadays shooting mostly mono but this can be easily transferred to color. I'm just making up some numbers as I don't have the values for my DSLR at hand: assume the mean of the darks is 0.8 (on a scale of 0 to 255) and the standard deviation 0.4, my lower signal limit would be 0.8+3*0.4=2.0. Hence, for my lights all signal should be above 2.0. To make your life easier with the colors, compute the limit based on the "worst" color channel in the darks and set this as a limit for the "worst" color channel of the light, which is likely to be the red as it has least sensitivity. That would be my approach.
I hope my explanation isn't too confusing. Otherwise, let me know.

If you should reduce the exposure time to 25 instead of 45 isn't necessarily true. It depends: it you increase exposure time, you'll catch fainter detail. On the other hand you are more likely to saturate the stars and they will more easily blow up in post processing. Therefore, I am checking two things: First, are the stars saturating? If not, I usually increase exposure time until they are at the edge of saturating. In parallel, I keep an eye on  the histogram spike. If that's well above the noise (see above), I rather prefer shorter exposure times and make more subs as this also increases dynamic range and you can afford trashing subs in case of whatever (satellite, guiding error etc.).
For fainter DSOs you'll likely saturate stars before your histogram spike is above noise level. But that's how it is.

Regarding your resolution: you're right, your 2.46"/pixel is at the limit but you need to judge on the final image quality. Drizzle is done after the capturing, so you won't loose anything if you try both approaches and compare which you like better. Also, I don't see if you are guiding or not. If you're not guiding, the error will likely be larger than the 2.46".

I've taken a look at your gallery. They're nice images.
If I may recommend: you should take a look at your flattener. The stars in the images are a bit egg shaped at the edges. Either the flattener doesn't fit the optics or the working distance isn't set correctly.

CS!

Björn

Hi @Mark Germani ,- Re: M45. As you said, you're collecting much more light. So, for the explanation, I think my previous post should explain it. I agree with the emission nebulae.- Re: Darks. It's not chicken-and-egg. One thing I am doing, and maybe some people would strongly object, is that I use a bias and darks database. I'm not capturing darks and bias each session. I captured one set of bias frames and different sets of darks for some typical exposure times (60, 120, 180 seconds). The darks were captured at 20 degree Centigrade temperature. Usually lights are captured at lower temperatures so with my „hot“ darks, I am subtracting too much. However, the strongest effect is exposure time and not temperature and if I place my signal level above the noise level of the „hot“ darks, I’ll be safe.Now, I’ve now put some realistic numbers for the darks together: my 60s@ISO800@20degC give a mean intensity of 2.29 (always on a scale 0 to 255). with std. dev. 3.52, hence my lower signal limit would be 12.86. That’s about a 20th of the histogram! So, you could even go below the 1/4. However, don't start to make exposures of 26 seconds because the best fit of the histogram. Round a bit up to common exposures (like 30 or 60 seconds).
There's one thing to be careful about: sometimes, the horizontal axis of the histogram is log scaled. As far as I know, Canon shows a linear intensity scale on the cameras. Something one has to keep in mind!
Also the numbers above are for my EOS250D. Your 600D might have different values (but I'd expect not completely off).
By the way: the bias frames have mean 2.18 and std. dev. 3.05. You can see how little the time and hence temperature make!! Most of the nose is simply the read out noise of the amplifier.

I agree with you in the point that without the laptop you cannot make this assessment. Also, I made my analysis in the very beginning, after capturing the darks and work with rule of thumbs. However, it should give you a feeling that the cameras don't have so much noise as one would think and that this broadly repeated 1/3 or 1/4 rule is IMHO way too conservative nowadays. If you go from 1/3 to 1/6 you're still way above the noise but you double the number of lights and also can achieve a higher dynamic range and your less likely to saturate anything etc. etc. (I'm attaching two images to illustrate)

Long story short: No need to make a science out of it but the rules of thumb can be more aggressive than they're right now. Experiment a bit and find a suitable setting. I'm certain that after a while, you'll find that you'll place your histogram below 1/4.

- Re flattener: When I received my refractor, I hadn't ordered the flattener (you can see this at my M31 https://astrob.in/3t38ig/0/) and I also cropped the worst parts away.

CS!

Björn

Attachments:
Here's a sub of my Plejades (https://astrob.in/pre6bm/0/) and the histogram. The peaks are a bit below 1/16th and I would say I am slightly underexposed with that. What you can also see in the sub is that the brightest stars are only slightly saturated (red pixels). Certainly, there are images showing much more detail of the nebula area. To enhance this part, I personally will try to capture more subs as this will increase dynamic range even further.
Bildschirmfoto 2021-03-13 um 08.03.13.png Bildschirmfoto 2021-03-13 um 08.02.59.png

Hi @Mark Germani ,

As always in this hobby, so many variables & approaches!

It absolutely is, but I can assure you, as I've worked in academia and I'm working now in industry: there's even more variables and approaches and there's no right and wrong like black and white either . But as a hobby, we should be relaxed about it and enjoy the process of following these approaches and the learnings we make. I also had and still have nights, where I was a little disappointed about the outcome but when you do a retrospective, there'll be things you've learned. 
And most importantly, IMHO, it's not about chasing likes on AstroBin but about following a hobby and share some ideas with like-minded people and find satisfaction that the image that you hold in your hand was made by you through your work.

- Re "the actual topic": I'm also only shooting at ISO800 with my DSLR.  About your idea to use the CLS as a luminance layer: try it!
However, I wouldn't expect a very different outcome. What will happen? Assume you image without filter. And also assume you shoot without any light pollution. You'd get a "perfect" RGB image. So it contains all info. When you shoot the CLS image, you remove certain colors, like the yellow and orange tones. If you put this as a luminance overlay, you'll remove these colors from the image by setting their luminance to 0. It'll increase contrast. I'd say you wouldn't gain a lot. It's essentially the very same thing the CLS does when it's in the camera. That's my theory but why not try it. As I said: it's a hobby and if you don't break anything why not following ideas. 

It'd be different if you wouldn't shoot with a color DSLR but a monochrome camera. There, you would capture lower quality RGB channels, as it's "only" about catching the tone. For the luminance, one would use highest resolution and since it's mono all pixels are sensitive and detail will be improved. But one shouldn't underestimate the fact that with a mono you have to shoot the channels in sequence. Hence, mono cameras don't invalidate DSLRs in any form, IMHO.


I'm curious what results you will get!

CS!

Björn