Herzy

https://docs.google.com/document/d/1bG-1YK96-D0icRRkbd3xmruZznbGDf7QQu7zbk0_kao/edit
 

I wrote this as a a chapter in a much larger astrophotography guide a while back. It’s a guide to deciding on your optical tube/camera/mount/locations for your imaging session. I created several illustrations to help visualize my explanations.

I wanted to share in hopes it helps some other people interested

8 minutes ago, vlaiv said:

No.

CMOS sensors have much lower read noise than CCD.

Again - this does not explain how is 12bit ADC low quality and 16bit ADC high quality.

From astrophotography perspective - there is absolutely no difference. Let me show you in rather simple example.

Say you compare 12bit CMOS sensor that has 1.7e of read noise with 16bit CCD sensor with 7e of read noise. For the sake of argument - let's suppose that both sensors have FWC that matches their bit count and operate at system gain of 1e/ADU (that is not the case with CCD sensors as they often have less than 65K FWC where CMOS sensors more often have higher than 4K FWC and thus have variable gain).

Since there is 4 bits of difference between CMOS and CCD - it stands to reason that CCD can expose for x16 times longer than CMOS, right?

But what happens if we take 16 subs with CMOS and just add them up? Signal adds up - and yes, it becomes the same as one taken with CCD. All time dependent signals/noises will just add up. Only one that is not time dependent is read noise.

There will be 16 "doses" of read noise in CMOS - so let's add those up to see what noise level we'll get. Noise adds as square root of sum of squares so we have sqrt(16 x 1.7e^2) = 4 * sqrt(1.7e^2) = 4 * 1.7e = 6.8e

Hm, same thing as CCD, or still slightly better (6.8e vs 7e).

How is 12bit ADC inferior?

 

I wasn’t referring to noise by calling a higher bit depth ADC ‘superior’, I was referring to the color range. Higher bit ADCs have less color banding and less banding between brightness values, giving a smoother overall image. Does it not?

5 minutes ago, ONIKKINEN said:

Seems outdated by a number of years, and today things are opposite to what you wrote about CCD and CMOS.

This is the opposite of truth in today's camera market with low read noise and practically no thermal noise CMOS cameras becoming the norm, whereas CCD cameras have an abundance of both.

Here you have ignored all broadband targets, which in my opinion are more common targets than the emission nebulae you meant with that sentence. Broadband targets are the opposite of that, and in fact shine brightest in the green channel. Still, it has truth in it as not every pixel receives useful light from a target and resolution is lost. Just have a gripe with how you have ignored broadband completely.

This is also the opposite of truth and frankly not useful at all today.

CMOS sensors today have either native 16-bit ADCs or 14-bit ones, 12bit is rare and only sold in planetary camera format today (where this does not matter). Amp glow is also a thing of the past, as is nonlinear pixel sensitivity with the newest DSO cameras reaching 99% linearity.

CCD sensors have 3-5x read noise compared to CMOS, so also just plain wrong.

Not sure i would recommend any of this to beginner astrophotographers.

You’re correct about the broadband targets showing up in the green channel, and I should change the wording of that section, but my point was simply that the standard Bayer matrix has 50% green pixels, 25% red pixels, and 25% blue pixels. Later I compared that to a filter wheel over a monochromatic sensor, showing how every pixel is operating at maximum efficiency rather than being constricted by color filters.

I don’t claim this to be perfect, and I appreciate the corrections. It’s a very complicated topic. 

1 minute ago, vlaiv said:

I did not go thru all of the text, but have noticed few things.

First - all this data on DSLRs (and more) is available online from several sources.

Look here:

https://www.photonstophotos.net/

Next - you listed both cost and costly as being disadvantage of CCDs for some reason

Then - you listed "low quality 12bit ADC", "Non linear pixel sensitivity" and "Amp glow" as disadvantages to CMOS.

Can you explain what do you mean by low quality 12bit ADC? In what way is it low quality? Does it produce digital values that are somewhat lower quality than standard? Does it introduce some sort of error into ADC process (different than read noise)?

Which CMOS sensors have you found to have pixel nonlinearity?

Amp glow is term we use on CMOS sensors but it has originated with CCD sensors (there was single amplifier unit on CCD that would thermally "glow" and cause electron build up on one side of sensor) - which means Amp glow is not exclusive feature of CMOS sensors - and can be calibrated out.

Pixel non-linearity in a CMOS sensor refers to internal ADC’s performing separate conversions at each pixel site, whereas the CCD performs all operations through a single external ADC. CMOS sensors will have more uncertainty/noise due to this, but they will have lower readout times because not ever charge has to pass through the same ADC. 
Because the CMOS sensor has built in ADC’s at every pixel they are usually only 12bit, whereas CCD sensors can get away with higher quality 16bit ADCs.

Amp glow isn’t exclusive to CMOS, you’re correct. But it is more prevalent, due to the more compacted and complex circuitry at every pixel site.

I hope that clears some of it up

The Ultimate Fireworks:

          In this picture, we see the death-throes of a long-dead star. A star’s core is an environment with pressure and temperatures sufficient to ignite nuclear fusion. Nuclear fusion is a process where atoms are fused into heavier elements, releasing enormous amounts of energy. A star maintains stability between nuclear fusion and gravity with a delicate balance of nature, so vital to our existence. Hydrogen nuclei are fused into Helium, Helium to Lithium, Lithium to Beryllium, and so on down the periodic table until a critical point. After Iron, the fusion process loses efficiency, disrupting the delicate balance between gravity and nuclear fusion. In a fraction of a second, the star implodes, sometimes reaching 25% the speed of light in a spectacular supernova. The energy released from the collapse jets out the star's outer layers - dumping millions of tons of heavy elements into the cosmos. These fireworks often outshine the entire galaxy they live in. 

A New Chapter:

          In this picture, we see gas clouds shaped by the ferocious solar wind. Structures like these form from the remnants of countless ancient supernovae who sacrificed their elements and energy. While staring into the core of this nebula, you are seeing heavy elements coalescing into pockets of high density. The heaviest among them can be classified as protostars, kick-starting the life-cycle once again. Once these protostars reach critical pressure, nuclear fusion is again ignited. A new generation of stars are born, each hosting a variety of orbiting planets. I derive a deep sense of connectivity from the fact that every atom other than Hydrogen in your body was forged in the core of long-dead stars. Thanks to these stars, the universe is a fertile environment, brimming with the key ingredients for life. This gives me an almost spiritual sense of harmony with nature. Not only are we in the universe, but the universe is in us.

A Snapshot in Time

          Galaxies are some of the most awe-inspiring and grandiose objects nature has to offer. The Whirlpool Galaxy (M51), seen above, is nearly 23 million light-years from Earth. The light emitted by this galaxy endured 31,000 millennia  of a journey across space and through time before coming to a rest on my sensor. When staring into this image, you see a freeze-frame of many million interstellar collisions and explosions, birthing new generations of stars to replace them. You are seeing planets evolving, enormous gas clouds coalescing, black holes devouring, and potentially even civilizations happily living without ever knowing of human existence. When staring into the heart of this galaxy, you glimpse into the life of countless solar systems. You are witnessing cosmic evolution at the most colossal scale -  It is a snapshot in time

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Over the years I’ve been collecting my knowledge on Astrophotography. Here is a section I wrote. I hope it is helpful, though I don’t claim it to be perfect. Any corrections are welcome. I also created all of the illustrations myself. 

Would you mind posting the stacked file? I had a similar issue last night on my M51. Big dark spot in the middle.

1 hour ago, Stub Mandrel said:

Find out which of the two adapters is cheapest/easiest to source, then saw through it with a junior hacksaw until only a bit of metal is left, then bend it away from the other adaptor.

This is what I've been doing recently. I've been filing through the top t-ring, but i can never get all the way through I without damaging the threads of the adapter underneath.

I have already unscrewed the screws for the top part of the t-ring as suggested. It makes it even harder to get a grip on the metal to turn. It has been taking a WD40 bath for the past 12-ish hours. I'm hoping that might help. I tried the heating thing last night, but I only did it once. I'll try that again if the WD40 doesn't work. I don't think it is a left hand thread. When I get it out of the WD40 I will make sure, but I'm almost 100% sure that it isn't.

I'm thinking of taking it to some sort of camera repair shop to see what they can do for me.

13 hours ago, Physopto said:

I have not seen these parts but if they are not actually on the sope and not attached to any expensive optics or focuser then you could as a last resort put them in a bath of wd40 and leave them there for a day or two, then clean and try again. BUT only if not attached to lenses or expensive scope parts.!!!!!

One method that often can work is putting parts into an oven then cooling down a bit  then putting into a freezer. Do this several times as it can cause small expansion and ccontractions of the parts so as to eventually allow a small free movement, enough to let the parts seperate. 

Derek

I've already tried the WD40, but that oven and freezer thing might be a possibility. I've tried freezing them, but they are both made out of the same metal so they will contract at the same rate. Would that method still work if they are made of the same metal?
 

I think there may be some confusion. The adapters are not connected to the telescope. The adapters screw in to the telescope extension tube like the picture shows.

17 hours ago, Waldemar said:

If it does not matter to you if you ruin the adapter and save the scope, you could use a small chisel and a nylon hammer. place the chisel in a 45°  angle on the edge of the adapter and hit it cautiously with the hammer in the direction to loosen it.

While doing that have somebody hold your unmounted scope

Could you elaborate? I don't think I fully understand. They screw together, so if anything, that method should make it worse. It could mess the threads up and make it impossible to unscrew. Unless I'm missing something?

I hope the pics help.

Hayden

Just about a year later and I still haven't got these adapters apart. I've literally turned to the point of bleeding. I've tried the shoes approach as well. 

Celestron doesn't make the scope anymore, so they can't really help me. 

8 hours ago, Filroden said:

Well, I've gone and done a "Nige".

You can blame @FLO and their "January" discounts, but I pressed buy on a new Celestron AVX mount during the night. Seeing how much of my field of view I was losing to rotation was the final straw. I paused at buying an OAG as I still think I will be taking short (60-120s) exposures as that seems to suit the camera better. I will see how that develops and might consider guiding later (I have a camera I can already use). It also means I need to press pause on having a custom spacer made as I would need a shorter one if I got the OAG.

Unless the weather is kind tonight (unlikely) I'll probably not be taking more images on the alt/az mount but I'll still be following this thread with interest. I learn far too much from reading it still!

If all goes well I can see me selling the Evolution mount. It's a great mount and certainly helped me into imaging but I (hopefully) won't need two similar capacity mounts. And the thought of being able to image near the Zenith...

You've far surpassed whatever results I can achieve, but I still like to go back on my alt/az every once in a while for the ease of use even though I have an AVX. Good luck!

Has anyone been able to capture the comet out now? It's up in the morning or so I've heard.

Thanks. Ken's images makes the actual nebula look great, and Ian's has great detail in the dust. If I may, I might blend the two together. I did take darks (no bias), but I only took 9 so i thought it wouldn't make a big difference and didn't apply them. Also, I checked out the flats and there is that green gradient in it. I'll try to take another set, although it might be too late.

https://www.dropbox.com/s/30y7o9zhegesrdl/M42 - Stacked (DSS).fts?dl=0

Here is the Stacked fits file. You can layer a stretched version and a less-stretched version to preserve the core. If you want to give it a go, please do.

4 hours ago, Filroden said:

I suspect the layering has added noise and given you the impression of it now being blurred. I think there is some fine detail in the nebula but you probably need to mask the fainter details and potentially deliberately blur it more while leaving the brighter areas in their full detail. Not only will this draw attention into the detail but it will hide some of that noise

You have a strange colour cast that almost looks like it's a vignette. Have your flats introduced something?

I think it might be the flats because I took them in a rush (it was freezing out there). I also suspect the masking introduced noise. I will try combining the fully stretched file with one that only stretches the core. That way, there will be less noise.

5 hours ago, Nigel G said:

I just had a quick go at reducing the highlights and boosting the shadows, ( hope you  don't mind ) I think it would work better during initial processing,  there's a lot of nebula and dust in there ?

I have done this on my tablet so can't see if I have over done it until I get to my pc. 

It's a great image, I would love to have a go from a fits file.

Nige.

 

Don't mind at all! You made the faint dust really pop! It is a little oversaturated, but that's an easy fix. When I get home from school I'll link the full stacked file so you can give it a go. You'll see that the core is way overexposed. The layering helped a lot.

I took this on Saturday with the clubs SE mount and an AT72ED from a dark site. I can't get the core to look right. The nebula was so bright even in 45s subs that it appeared all white. I layered some old pictures I had over it to make it more visible, but it still is blown out. I also don't like the blurred feeling to the image. Seems like the detail is all blurred.

21 hours ago, Filroden said:

My first Ha sub. A single sub of 60s at 300 gain, 50 offset and -20C. No calibration, just a quick levels and curves stretch and no noise reduction. Going to go lie down now...

Actually, I've set my first sequence to be 30 x 60s Ha, 10 x 30s RGB and 30 x 30s L. Hopefully I can repeat that sequence a few times to add to my original data.

What camera are you using? That's incredible. It looks better than several hours of normal broadband imaging from my house, and it's only a single sub.

Nige, what are you going for?  are you going to keep moving towards the pleiades? Great image(s) btw!

As a follow-up of my horse-head questions, I'll show what I managed tonight. There were a TON of wispy clouds, as you can probably see from the light leakage from Altinak. Stars, no matter how bright, never spill out like that for me, so the clouds must have spread the light out. Not only that, but I would guess that the image was blurred significantly from them. Anyways, here is 24 minutes on the HH nebula. My frames from last night (with no clouds) were much crisper, but I didn't take many of them. I'm excited to try this out with a few hours of integration soon, this time with no clouds!

BTW, my AVX mount counterweight is nowhere to be found (how in the world did I manage to lose a counterweight.....??????), so I'm back in business with my SLT mount. Surprisingly, it works well with my AT72ED and handles the weight well, which I never thought it could.

3 hours ago, Filroden said:

Here's a 45s sub of mine along with a copy with a standard stretch. At full resolution you can see the HH in the unstretched sub too but it's not as obvious from the screenshot. I think because there is such a small difference in brightness, any light pollution will make it harder to see by eye but the data will still be there at 45s. It will just require a background removal to see it.

 

Interesting.... I stretched my files and it wasn't at all visible. Not to worry, though! It turns out there were some wispy clouds over where I was imaging because all of my pictures for that 15 minute period (including the only pictures I took of the horsehead) are washed out.