Our Beautiful Universe - Photos and Videos

[Drakkith]

@Devin-M what were you shooting with?

 

[Devin-M]

@Devin-M what were you shooting with?

It was a 2130mm f/14.2 Maksutov Cassegrain w/ Nikon D800 dslr fitted on a Star Adventurer 2i mount (slightly modified to go at least 3x over the weight limit).

 

[Drakkith]

Dear god, f/14.2?!
It'll be the heat death of the universe before you get a deep sky photo finished!
Get a focal reducer for that scope!

 

[Devin-M]

Dear god, f/14.2?!
It'll be the heat death of the universe before you get a deep sky photo finished!
Get a focal reducer for that scope!

I got this one of the core of Andromeda a couple days before with identical exposure settings...

 

[Drakkith]

I got this one of the core of Andromeda a couple days before with identical exposure settings...

M31 (Andromeda Galaxy) is amongst the brightest of the deep sky objects. You can get a decent picture with two tin cans and a string.

Seriously, my F/8 scope is something close to 4x 'faster' than yours and I still think it's too slow!

 

[Ibix]

You can get a decent picture with two tin cans and a string.

The bottom of a bottle, my optics prof used to say.

 

[Devin-M]

M31 (Andromeda Galaxy) is amongst the brightest of the deep sky objects. You can get a decent picture with two tin cans and a string.

Seriously, my F/8 scope is something close to 4x 'faster' than yours and I still think it's too slow!

My counterweight is a 600mm f/9…

 

[Devin-M]

Does anyone else think I've captured a very strange transient signal?

view in WorldWideTelescope

 

[Devin-M]

Here's a cleaner re-process... (ps this is a stacked image... 84x 90 sec (2.1hrs), 6400 iso, 2130mm f/14.2)

view in WorldWideTelescope

 

[Devin-M]

Mystery solved... it's not in the individual frames so it must be nothing more than a stacking misalignment.

 

[Devin-M]

Restacked... now you can see the nebulas... I think something very bad happened during the 1st stacking attempt...

view in WorldWideTelescope

 

[Drakkith]

Restacked... now you can see the nebulas... I think something very bad happened during the 1st stacking attempt...

Was this taken from a bortle 2 site?

 

[Devin-M]

Was this taken from a bortle 2 site?

Yes it’s in bortle 2 conditions just outside of Shingletown, California, USA. It’s literally the same image I already posted but restacked… I think some very bad stacking misalignment happened the first time but it was quite hard to notice for some reason. I think with 90 second exposures a lot of the individual shots come out fine but over time the field drifts in the viewfinder so first I cropped all the source TIFs to the same field of view and then I restacked them without any dark or flat calibration frames and finally histogram stretched the stacked TIF in Adobe Lightroom.

 

[Drakkith]

Okay. It just looks really noisy for 2 hours of data in a bortle 2 spot. But I don't have your equipment, so this may be entirely normal for your setup.

 

[Devin-M]

It’s not a cooled camera & shot at 6400iso instead of a more ideal 100-400iso.

 

[Drakkith]

It’s not a cooled camera & shot at 6400iso instead of a more ideal 100-400iso.

I don't use DSLR's, so I'm not familiar with ISO settings. Why shoot at 6400 instead of 400ish?

 

[Devin-M]

I don't use DSLR's, so I'm not familiar with ISO settings. Why shoot at 6400 instead of 400ish?

As I understand it, it narrows the dynamic range of the sensor but puts the minimum detected values within the range of detection of the 14bit RAW files (on the Nikon D800 body). (otherwise certain various analog-to-digital detectable values will be normalized to 0 on the raw files). Above 6400iso (as I understand it, on the D800), the improvements are entirely digital as opposed to improvements in the analog to digital conversion).

 

[Andy Resnick]

As I understand it, it narrows the dynamic range of the sensor but puts the minimum detected values within the range of detection of the 14bit RAW files (on the Nikon D800 body). (otherwise certain various analog-to-digital detectable values will be normalized to 0 on the raw files). Above 6400iso (as I understand it, on the D800), the improvements are entirely digital as opposed to improvements in the analog to digital conversion).

That is not my experience (I shoot with a D810). I always shoot with as low an ISO as possible (ISO 64) to maximize the dynamic range, especially color information. Shooting with higher ISO values only increases the amount of noise in my stacked image.

 

[Devin-M]

That is not my experience (I shoot with a D810). I always shoot with as low an ISO as possible (ISO 64) to maximize the dynamic range, especially color information. Shooting with higher ISO values only increases the amount of noise in my stacked image.

Right, increasing the ISO lowers the dynamic range.

narrows the dynamic range of the sensor

But there are only so many brightness values you can store in the 14 bit raw file.

When shooting a very dim object would you rather accurately store in the RAW file the brightest analog to digital values or the dimmest analog to digital values? Turning up the iso means you are more accurately storing the dimmest A to D values in the raw files but that’s why it also makes the ordinarily dim sensor noise more visible.

Essentially by turning up the iso I am “throwing out” the brightest values recorded by the sensor (like the brightest stars) by recording them as simply “100% brightness” in favor of more accurately recording the dimmest A to D values (like the nebulas) rather than “throwing them out” by recording the dimmest values to the 14 bit raw file as “0% brightness”

 

[Devin-M]

Essentially, if you shoot a long exposure dark frame at 100iso with the lens cap on, all you are recording is the sensor noise, and you will barely see any noise in the final image it will just look almost fully black.

Now shoot one at 6400iso with the same exposure time (also with the lens cap on). Now the noise itself has more dynamic range because it takes on a greater range of values in the raw file.

So increasing the ISO reduces the overall dynamic range the sensor is able to record by “clipping” the brightest parts of the image if you have some bright areas without the lens cap on but it will increase the dynamic range in the raw file of the dimmest objects.

 

[collinsmark]

That is not my experience (I shoot with a D810). I always shoot with as low an ISO as possible (ISO 64) to maximize the dynamic range, especially color information. Shooting with higher ISO values only increases the amount of noise in my stacked image.

A good rule of thumb for deep sky (not planetary) astrophotography is once you decide on an exposure time (usually determined by how long your equipment can maintain proper tracking, or how long, on average, you can go without a stray cloud messing things up), you should generally increase the gain (ISO) as high as you can go without saturating stars. This generally minimizes the read noise, measured in electrons.

Check your camera's sensor's specs to be sure for that particular sensor, but its almost always the case that higher gain (i.e., higher ISO) reduces read noise.

As @Devin-M mentions, increasing gain (ISO) sacrifices dynamic range, but it reduces read noise and increases resolution. So long as you're not saturating stars, you don't need the dynamic range anyway.

Keep in mind that whichever gain (ISO) you choose, you should take DARK and FLAT (and BIAS and/or DARKFLATs) frames with that same gain (ISO) setting.

Here's a spec sheet of a typical camera:

Note that when the gain (ISO) increases, the read noise in units of electrons, decreases. Also note that for this camera (above figure), there is a discontinuity in some of the curves right around a gain of 60, above and below different circuitry is switched in or out. This is why you should check the specs of your particular camera's sensor, so you know what to expect regarding different gain (ISO) settings.

Because higher gain means finer resolution (electrons per ADU), sub-frames with higher gain might show higher noise in units of ADU, but the signal is also greater in units of ADU. And actually, the signal to noise ratio is generally better at higher gain (higher ISO). That's why read noise is typically specified in units of electrons rather than ADU units, because units of electrons more closely tracks signal to noise ratio.

Just make sure you're not saturating stars. If you're saturating stars, lower the gain (i.e., lower the ISO).

==============

If you were like me, you might be saying, "Woah, now. That's crazy. I've done terrestrial photography, portraits, landscapes, etc., for years. And using higher ISO always makes noisier photos. Lower the ISO if you want less noise!"

The flaw in that argument is when doing terrestrial photography, the tradeoff is between ISO and exposure. If you increase the ISO you must correspondingly decrease the aperture or increase the shutter speed (i.e., decrease the exposure time). (Here, "exposure" is aperature $\times$ shutter speed). Increasing the ISO means you'll have to decrease exposure to avoid blown highlights.

Most of the noise in that case is ultimately the result of reducing the exposure. Fewer photons hit the sensor, increasing shot noise. Less signal is allowed to reach the sensor, so that decreases signal the noise ratio. That, and as previously mentioned, the read noise in units of ADU can increase due to the finer electrons per ADU resolution.

However, in deep sky astrophotography, there is no such tradeoff (planetary astrophotography is a different matter -- let's leave planetary astrophotography out of this discussion). For deep sky astrophotography exposure times are kept as long as you can, given your equipment and cloud cover. And there's almost no sane reason why would ever reduce your aperture. Thus there is no tradeoff regarding ISO and exposure.

So for deep sky astrophotography, the rule of thumb is once you pick your exposure time, pick your gain setting (ISO) such that stars are not saturated. There's usually no need to reduce the gain (ISO) much lower than that.

(And don't forget that your DARKS, FLATs, DARKFLATs, and/or BIAS frames must use the same gain (ISO) setting as your LIGHTs.)

 

[Devin-M]

Except the Nikon D800 series uses 14 bit raw files which is fairly good to great by DSLR standards but not necessarily Astro-specific camera standards, which might be 16 or 32 bit per color channel raw files… so if you keep the stars from saturating with a low iso setting on a dslr you might be sacrificing some of the dynamic range recorded of the nebulas which are much dimmer than the stars. Sometimes you’re shooting through a clip in narrow band filter which reduces the photon count drastically, and you’re going to edit out the stars anyway before compositing with an rgb image of those same stars so you might as well get even more dynamic range on the nebulas by sacrificing the dynamic range of the stars which you’re going to edit out with the dust and scratches filter for compositing with rgb.

 

[Andy Resnick]

A good rule of thumb for deep sky (not planetary) astrophotography is once you decide on an exposure time (usually determined by how long your equipment can maintain proper tracking, or how long, on average, you can go without a stray cloud messing things up), you should generally increase the gain (ISO) as high as you can go without saturating stars. [snip]

That's true- and with my setup, I saturate pretty quickly. Typically, the brightest stars within a field of view saturate between 6s and 20s exposure times (ISO 64).

Edit- I forgot to mention your comment "And there's almost no sane reason why would ever reduce your aperture.", because for me, there are at least 2 good reasons. First, (slightly) stopping down the aperture makes the images less susceptible to poor seeing conditions. Second, (slightly) stopping down the lens improves the images by decreasing aberrations.

 

[Devin-M]

That's true- and with my setup, I saturate pretty quickly. Typically, the brightest stars within a field of view saturate between 6s and 20s exposure times (ISO 64).

Edit- I forgot to mention your comment "And there's almost no sane reason why would ever reduce your aperture.", because for me, there are at least 2 good reasons. First, (slightly) stopping down the aperture makes the images less susceptible to poor seeing conditions. Second, (slightly) stopping down the lens improves the images by decreasing aberrations.

This was 9 shots * 5 minute exposures per shot @ 6400iso @ 600mm f/9 through a 6nm clip in narrowband filter on a D800, stacked and histogram stretched in Adobe Lightroom...

https://www.physicsforums.com/threads/our-beautiful-universe-photos-and-videos.800540/post-6464705

 

[collinsmark]

Edit- I forgot to mention your comment "And there's almost no sane reason why would ever reduce your aperture.", because for me, there are at least 2 good reasons. First, (slightly) stopping down the aperture makes the images less susceptible to poor seeing conditions. Second, (slightly) stopping down the lens improves the images by decreasing aberrations.

Yes, that's right, I'll clarify. If you're using a fast lens that's primarily designed terrestrial photography in mind, then yes, stopping down the lens a little can be a good idea. But if using a telescope designed specifically for astronomy (cheapy* telescopes possibly excluded), stopping down the aperture is pure sacrilege!

*(And to be clear, I'm not necessarily knocking cheapy telescopes. A cheapy telescope can still be better than no telescope, if you know what you're using. Case in point: chromatic aberrations don't matter when you're imaging in narrowband. [Edit: as in actual narrowband with separate SII, Ha, Oiii filters and a monochrome camera; not necessarily the filters designed for one shot color (OSC) like the Optolong L-Extreme.])

 

[Devin-M]

Case in point: chromatic aberrations don't matter when you're imaging in narrowband. [Edit: as in actual narrowband with separate SII, Ha, Oiii filters and a monochrome camera; not necessarily the filters designed for one shot color (OSC) like the Optolong L-Extreme.])

To save time you can do all 3 at once...

 

[Drakkith]

First, (slightly) stopping down the aperture makes the images less susceptible to poor seeing conditions.

My understanding was that this would only improve visual quality since you can take advantage of those few seconds where the turbulence along the incoming cone of light is minimal. Thoughts?

 

[Drakkith]

To save time you can do all 3 at once...

I'd do the same thing with my gear if I could afford it!

 

[Andy Resnick]

My understanding was that this would only improve visual quality since you can take advantage of those few seconds where the turbulence along the incoming cone of light is minimal. Thoughts?

Not exactly- the amount of image degradation caused by clear air turbulence is related to the aperture diameter. I have a copy of an excellent dissertation discussing/measuring this at my office, but I am currently 'snowed in' so I can't get you the reference right now... IIRC, the aperture diameter sets a cutoff to the relevant length scales of the turbulence- smaller diameter, smaller cutoff.

 

[Andy Resnick]

To save time you can do all 3 at once...

Yeah... I'm not going to do that. I was considering getting a filter or 2 for solar imaging, tho.

 

[Drakkith]

Not exactly- the amount of image degradation caused by clear air turbulence is related to the aperture diameter. I have a copy of an excellent dissertation discussing/measuring this at my office, but I am currently 'snowed in' so I can't get you the reference right now... IIRC, the aperture diameter sets a cutoff to the relevant length scales of the turbulence- smaller diameter, smaller cutoff.

Hmm. I'll have to check my source and get back to you.

 

[timmdeeg]

Recently I took this photo with my Sony A7III and 200 mm focal length. It is the result of 29 single photos stacked using the DeepSkyStacker and then processed. My first astro photo with this camera and I must say I was surprised what this little equipment - by far not a telescope - brings about.

 

[collinsmark]

Edit- I forgot to mention your comment "And there's almost no sane reason why would ever reduce your aperture.", because for me, there are at least 2 good reasons. First, (slightly) stopping down the aperture makes the images less susceptible to poor seeing conditions. Second, (slightly) stopping down the lens improves the images by decreasing aberrations.

My understanding was that this would only improve visual quality since you can take advantage of those few seconds where the turbulence along the incoming cone of light is minimal. Thoughts?

Not exactly- the amount of image degradation caused by clear air turbulence is related to the aperture diameter. I have a copy of an excellent dissertation discussing/measuring this at my office, but I am currently 'snowed in' so I can't get you the reference right now... IIRC, the aperture diameter sets a cutoff to the relevant length scales of the turbulence- smaller diameter, smaller cutoff.

In deep sky astrophotography, where you're taking sub-frame images of at at least tens of seconds, but more typically several minutes, stopping down the aperture won't help more than they hurt (other physical abberations aside, i.e., chromatic, spherical, coma and astigmatism -- stopping down the aperture might help for those). The seeing changes are just way too fast.

Ignoring the other aberrations, you'll get more bang for the buck by increasing the aperture to raise the signal further above the noise floor.

A smaller aperture might have a slightly better effect on your guiding though, maybe, where exposures are only a couple of seconds. But I'm talking about the primary imaging here.

====

Planetary. Let's talk planetary now.

For visual use, when viewing Jupiter, Saturn, Mars, Venus, the Moon or the Sun, they're pretty bright through a telescope. You can afford to stop down the aperture for visual use. As @Andy Resnick points out, the planet might seem a bit more stable in the eyepiece during bad seeing conditions if you stop down the aperture. And since they're bright enough to begin with, a slightly dimmer view can be acceptable.

But for goodness sake, don't stop down the aperture when doing planetary astrophotography. Yes, in planetary astrophotography, seeing is king for any given setup. Meaning the seeing conditions trump scattered clouds, haze, bad transparency, and light pollution. If you find yourself in bad seeing conditions, wait until the seeing is better. Try to do you imaging when the target is near the meridian where there's less atmosphere between your telescope and the target. But stopping down the aperture is conceding to failure.

Now assuming the seeing conditions are average or better, the benefits of a larger aperture are twofold.

1. Using lucky imaging techniques, the goal is to take as many short of exposures as possible within the limited timeframe (you can't let the target rotate for too long), and as many exposures as possible, with exposure times on the order of 10 ms or so, to produce individual frames just enough above the noise floor where your lucky imaging processing software (such as AutoStakkert!) can just make out planetary features. The individual images will be noisy -- very noisy, but just barely above the noise floor where planetary features are visible. Because of seeing, not all of these individual images will be good. But that's OK, because you throw out the bad frames (roughly half of them). The high noise (compared to deep sky sub-frames) is acceptable, because you're taking so many images that you can let the Central Limit Theorem come to the rescue. Having a larger aperture means you can reduce the exposure time, all else being the same, and increase the number of frames. Thousands of frames. Maybe tens of thousands of frames, if your camera is fast enough. Or another way of looking at it, having a larger aperture means surface details will be above the noise floor (for the same, short exposure time) in the individual frames when they would otherwise fall below the read noise.
2. Properly using lucky imaging techniques, and if the seeing conditions are pretty good to begin with, you can get images that are pretty close to your telescope's diffraction limit. Striving for anything less is akin to resigning to failure, in my opinion. And your telescope's diffraction limit is a function of the telescope's aperture.

I have a 10" scope that I love. But love alone can't make this scope compete with a quality 14" scope, particularly for planetary imaging, no matter how dear it is to me.

 

[DennisN]

Recently I took this photo with my Sony A7III and 200 mm focal length

Beautiful!

Sony A7III and 200 mm focal length

That's a great camera I'd like to have myself, but it's out of my price range.
Which 200 mm lens did you use, I wonder? (brand, type (a prime tele or zoom lens)?)

 

[timmdeeg]

Beautiful!

Thanks!
I took the image on MiniTrack LX3, purely mechanical with amazing accuracy.

That's a great camera I'd like to have myself, but it's out of my price range.
Which 200 mm lens did you use, I wonder? (brand, type (a prime tele or zoom lens)?)

I got the Sony FE 4/70-200 G OSS.

Yes it's a good camera which I bought second hand. It is ISO invariant above ISO 800 what I didn't know yet when I took this image with ISO 12800. Features like the focus magnification are very helpful.