Our Beautiful Universe - Photos and Videos

[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.

 

[timmdeeg]

Hi, I'm still a beginner, the Image of M31 above was my first trial (with some good luck).

I'm reading this and that and found to improve the image quality stacking is important and gathering light as well. I wonder supposed I want to invest one hour what's an optimization between those two strategies. One could e.g. take 10 pictures each exposed 6 minutes with ISO 1600 or 80 pictures each exposed 45 seconds with ISO 12800. Due to the ISO-invariance the visible noise doesn't change.
Lets assume that the dynamic range isn't an issue what do you think?

 

[Drakkith]

@timmdeeg Due to readout noise, longer exposures are usually better all else being equal. However, realistically you have to worry about saturation of brighter pixels, tracking and guiding errors, and many other little things that can affect longer exposures more than shorter. Bump your scope while taking 45 second exposures and you lose less than a minute of data at worst. Bump your scope near the end of a 10 minute exposure and you've lost nearly all of that time.

And if you're not autoguiding then you're probably limited to about 2 minute exposures at most, depending on the focal length of your scope and the quality of your tracking.

By far the most important thing is to simply accumulate photons, either through long exposures or through many shorter ones. 10 hours of 1 min exposures is probably better than 1 hour of 10 min exposures unless you have a very noisy camera.

 

[Devin-M]

I get good exposures about half the time with 90 second exposures at 6400iso (Nikon D800) at 2130mm f/14.2 tracked but unguided, but when I’m shooting at 300mm f/4.5 or 600mm f/9 I can consistently crank out 5 minute exposures at 6400iso, sometimes even 10 or 20 minutes with my sulphur narrowband filter but above 5 minutes at 600mm is risky territory unguided… you can lose a lot of frames to wind, tracking problems, planes, satellites etc.

 

[timmdeeg]

Ok, thank you for your comments. I will have to make my own experiences anyway but I think 30" to 60" depending on the object should be fine for starting.

 

[DennisN]

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

Very cool! Here are two reviews of the lens:

• Sony FE 70-200mm F4 G OSS Lens Review (from The Digital Picture, which includes some usage suggestions)
• Sony FE 70-200mm f/4 G OSS (SEL70200G) - Review / Test Report (from OpticalLimits, a detailed test)

Features like the focus magnification are very helpful.

Yes, I agree . I've got that too on my camera (Sony A6000). And the autofocus is great too, and also the autofocus tracking (tracking moving objects) is quite impressive.

Hi, I'm still a beginner, the Image of M31 above was my first trial (with some good luck).

I'm reading this and that and found to improve the image quality stacking is important and gathering light as well.

I've posted some good tutorial videos in this thread which you may find interesting, see my post #1,472.

The two videos in that post is from this youtube channel which has a bunch of various tutorials on doing astrophotography with cameras and lenses (with or without trackers):

• Nebula Photos (by Nico Carver)

Another channel with good astrophotography tutorials is this one:

• Forrest Tanaka (e.g. this video in which he shoots the Andromeda galaxy)

 

[Andy Resnick]

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

The document I refer to is "Astronomical Seeing in Space and Time", a dissertation by David Saint-Jacques, University of Cambridge (1998). He presents an unusually clear (!) discussion about atmospheric seeing conditions, and some of the results demonstrate (as best I can understand) that susceptibility to seeing scales as the aperture diameter d^(1/3).

 

[Andy Resnick]

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.

I think we are starting to talk past each other, and this discussion would be more broadly interesting if we focused on the essential aspect: refractive vs. reflective telescopes and the trade-offs. It's my impression that the folks that post here on PF fall into one of these two broad categories: DSLR folks are using refractors while the 'astronomy' folks are using reflectors.

For example, after accounting for the optical design differences between a finite-conjugate objective and afocal telescopes, my 400/2.8 lens is roughly equivalent to a 152mm refractor plus whatever accessories are needed (e.g. field flattener, focal reducer).

I'm fairly sure that none of the astronomy people posting here uses an actual refractor, which I find mildly interesting and wonder why. Similarly, while fast DSLR lenses are comparable to astrographs in terms of f/# and field of view, I don't think anyone here uses an actual astrograph.

 

[Drakkith]

I'm fairly sure that none of the astronomy people posting here uses an actual refractor, which I find mildly interesting and wonder why.

I have a William Optics refractor that I usually use. A good refractor is a joy to work with.

 

[timmdeeg]

Very cool! Here are two reviews of the lens:

• Sony FE 70-200mm F4 G OSS Lens Review (from The Digital Picture, which includes some usage suggestions)
• Sony FE 70-200mm f/4 G OSS (SEL70200G) - Review / Test Report (from OpticalLimits, a detailed test)

Yes, I agree . I've got that too on my camera (Sony A6000). And the autofocus is great too, and also the autofocus tracking (tracking moving objects) is quite impressive.
I've posted some good tutorial videos in this thread which you may find interesting, see my post #1,472.

The two videos in that post is from this youtube channel which has a bunch of various tutorials on doing astrophotography with cameras and lenses (with or without trackers):

• Nebula Photos (by Nico Carver)

Another channel with good astrophotography tutorials is this one:

• Forrest Tanaka (e.g. this video in which he shoots the Andromeda galaxy)

Thanks, very helpful for getting more knowledge. It's amazing, having been a convinced visual observer for more than 20 years - inspired by the Sony now things are changing rapidly. The desire to have more was too strong. Since a few weeks I have in addition a Photoline 102 mm FPL53 Doublet on Skywatcher HEQ5 and am trying to gain initial experiences.

 

[Andy Resnick]

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

Found another reference: Roggemann and Welsh, "Imaging through turbulence", CRC publisher, 1996- this book is quite authoritative. Their derivation results in the long-exposure PSF FWHM scaling as aperture diameter d^5/3, quite a bit worse than the other report.

Bottom line: larger aperture, more sensitive to seeing conditions.