Deep Sky Astrophotography (Gear Discussion)

[jrista]

Re: Deep Sky Astrophotography

Well, first, they aren't separate images. They are just crops of the same image.

The scaling isn't the same kind of problem in astro as it is in regular photography. The general rule of thumb in astrophotography is that you SHOULD be OVERsampling. You don't want your stars to be pixel size...you want them to be several times pixel size. The rest of the softness is due to a number of things...tracking error, polar misalignment (in my case, at the time, it was about 2' misaligned, or 1/30th of a degree, so not all that bad, really), seeing. Seeing refers to atmospheric turbulence that causes stars to wobble and jump around.

So, the image is exposed and scaled properly...exactly as it should be, really. With a longer focal length, I'll only be oversampling that much more, but that's a good thing. The more pixels I can pack into any given object, the better.

I was at f/4 ISO 400 for this series, although my exposure times differed. I took three separate sets of exposures, because the dynamic range in Orion Nebula is massive. The 30 and 60 second exposure sets were used to dim the core, which was indeed overexposed in the 120 second exposures. Additionally, mixing and matching ISO settings makes removing noise very difficult. Read noise levels increase as ISO drops, fixed patterns change, etc. meaning you need to use different sets of dark frames. However the semi-random and random noise contributions are also different, and when stacking images from different ISO settings, you usually end up with the worst common denominator in noise...thus noise is usually higher.

The best approach is to use a single ISO setting, at the same aperture, and only vary shutter speed. That minimizes the variables, and allows the intelligent aspects of stacking software (such as dynamic dark scaling) to work it's magic and give you the best results.

 

[jrista]

Re: Deep Sky Astrophotography

Truly MAGNIFICENT!

Thanks!

Not sure I should even open my mouth here as I'm new to this and indeed very uneducated in the subject. I have modified my iOptron skytracker to add rigidity to the base and allow my gimbal head to be mounted on a horizonatal plane. I removed the gimbal swing arm and mounted my 300 X1.4 and was able to get quite good shots with pretty accurate focus. Aligning Polaris was not too much of an issue but it did need tweaking. With the now horizontal orientation of the iOptron base the gimbal worked really well in allowing smooth balanced movement of the lens.

However, last week I tried 300 X2 and found getting focus to be tricky becasue of lens movement due to lack of rigidity. I will try again and then perhaps have to accept that it's impractical. I'm not fully convinced, but obviously what I'm trying to do is not what anyone who is serious about the stars would be willing to accept.

An even bigger problem is my ignorance of where to aim. Also I have not yet tried stacking. I did post a shot in the other thread but I guess it's no longer active, so here's a sample at 420, 30 sec. My remote timer release is in the mail, so that'll help. Otherwise I'm all ears.

Jack

It looks like you want MUCH longer exposures. I'd say four times as long, 120 seconds, if you can manage it. I would start at 300mm, and not use the 1.4x TC. The longer the focal length, the more demanding the whole system is going to be on stability. You can do quite a bit of amazing work at 300mm...that would be wide field, so you could, for example, image the entire heart nebula in Cassiopeia, or get a wide field image of Rosette. That big lens is going to be your biggest drawback with the SkyTracker...you will need to get it as balanced as you possibly can, and make sure your polar alignment is as dead on as you can get it. Also, make sure you are imaging wide open, or close to it. You want f/2.8, although that might result in funky star halos, f/3.2 and f/3.5 are probably going to be your best friends.

 

[jrista]

Re: Deep Sky Astrophotography

Here is Rosette nebula, in the constellation Monoceros (Unicorn), just to the east of Orion. Integration of 30x210s ISO 800 light frames, 30x darks, 30x flats, and 100x bias. Total exposure time 1h 45m.

I wanted at least twice the light frames, and it seems I could probably do much better with 60x480s ISO 400, rather than ISO 800. Next time I get the chance, which may be tonight, I'm going to give it another go, and see if I can get more dim nebulosity in the periphery.

 

[Jack Douglas]

Re: Deep Sky Astrophotography

Thanks Jrista!,

I'll get much more serious once the timer release arrives, hopefully in about a week. The moon has been too bright too. I have the stacking software downloaded but haven't tried it and don't know the process at the moment. Yes, once the gimbal is perfectly balanced it doesn't impact the tracker very much since it is so smooth - that'll be a positive. Of course I'll go beyond 30 sec as soon as I can do it with the timer.

The big problem for me is going to be knowing where to aim. Any good references on that??

For anyone that has purchased the iOptron skytracker, that wobbly base is an obvious design flaw. The external ring and 3 lock screws I added is just night and day better. I posted the picture on the other star thread.

Jack

 

[expatinasia]

Re: Deep Sky Astrophotography

Here is Rosette nebula, in the constellation Monoceros (Unicorn), just to the east of Orion. Integration of 30x210s ISO 800 light frames, 30x darks, 30x flats, and 100x bias. Total exposure time 1h 45m.

I wanted at least twice the light frames, and it seems I could probably do much better with 60x480s ISO 400, rather than ISO 800. Next time I get the chance, which may be tonight, I'm going to give it another go, and see if I can get more dim nebulosity in the periphery.

Amazing pic, jrista. How do you even know where to look? I had always thought you would need a telescope (and a big one) to take such images. Amazing.

 

[jprusa]

Re: Deep Sky Astrophotography

Here is Rosette nebula, in the constellation Monoceros (Unicorn), just to the east of Orion. Integration of 30x210s ISO 800 light frames, 30x darks, 30x flats, and 100x bias. Total exposure time 1h 45m.

I wanted at least twice the light frames, and it seems I could probably do much better with 60x480s ISO 400, rather than ISO 800. Next time I get the chance, which may be tonight, I'm going to give it another go, and see if I can get more dim nebulosity in the periphery.

Amazing, Thanks for sharing.

 

[AmbientLight]

Re: Deep Sky Astrophotography

Here is Rosette nebula, in the constellation Monoceros (Unicorn), just to the east of Orion. Integration of 30x210s ISO 800 light frames, 30x darks, 30x flats, and 100x bias. Total exposure time 1h 45m.

I wanted at least twice the light frames, and it seems I could probably do much better with 60x480s ISO 400, rather than ISO 800. Next time I get the chance, which may be tonight, I'm going to give it another go, and see if I can get more dim nebulosity in the periphery.

Fantastic work JRISTA and many thanks for all the information you've provided! I didn't expect such results were possible. This is great stuff.

 

[Northstar]

Re: Deep Sky Astrophotography

Here is Rosette nebula, in the constellation Monoceros (Unicorn), just to the east of Orion. Integration of 30x210s ISO 800 light frames, 30x darks, 30x flats, and 100x bias. Total exposure time 1h 45m.

I wanted at least twice the light frames, and it seems I could probably do much better with 60x480s ISO 400, rather than ISO 800. Next time I get the chance, which may be tonight, I'm going to give it another go, and see if I can get more dim nebulosity in the periphery.

Jrista...simply outstanding!

After reading through your posts I have to say that I'm impressed by your knowledge, and thankful for the time you've taken to "write a short book" on the subject in this thread! ;D

 

[Rienzphotoz]

Re: Deep Sky Astrophotography

Here is Rosette nebula, in the constellation Monoceros (Unicorn), just to the east of Orion. Integration of 30x210s ISO 800 light frames, 30x darks, 30x flats, and 100x bias. Total exposure time 1h 45m.

I wanted at least twice the light frames, and it seems I could probably do much better with 60x480s ISO 400, rather than ISO 800. Next time I get the chance, which may be tonight, I'm going to give it another go, and see if I can get more dim nebulosity in the periphery.

Jon, that is magnificent! Have you considered posting some tutorials on astro photography? I would love to learn how you make those images, they are just stellar.

 

[Jack Douglas]

Re: Deep Sky Astrophotography

Poor Jon won't have any time for photography if he keeps writing all these wonderful explanations for all of us. Maybe a suggestion on a great book for beginners - Jon - or anyone else.

Seems more than just me is wondering how to know where to aim. I suspect this is not as easy as it might seem.

Jack

 

[jrista]

Re: Deep Sky Astrophotography

Thanks Jrista!,

I'll get much more serious once the timer release arrives, hopefully in about a week. The moon has been too bright too. I have the stacking software downloaded but haven't tried it and don't know the process at the moment. Yes, once the gimbal is perfectly balanced it doesn't impact the tracker very much since it is so smooth - that'll be a positive. Of course I'll go beyond 30 sec as soon as I can do it with the timer.

The big problem for me is going to be knowing where to aim. Any good references on that??

For anyone that has purchased the iOptron skytracker, that wobbly base is an obvious design flaw. The external ring and 3 lock screws I added is just night and day better. I posted the picture on the other star thread.

Jack

Oh yes, the actual stand upon with the mount sits is probably one of the most critical factors, and is usually the limiting factor in terms of maximum capacity of the mount. A lot of midrange mounts ($1000-$3000) have capacities that top out at around 40-50lb, however that is usually because that's all the tripod or pier can handle before it begins to buckle. The mounts are often capable of handling a little more capacity than that if you place them on a sturdier tripod or pier.

As for knowing where to look, that's where an equatorial tracking GOTO mount is particularly handy. The GOTO mounts can be told to "go to" a set of RA/Dec coordinates, and they will. You have to have very good alignment for that to work...you need to have pretty precise polar alignment, and you need to align the goto feature itself by modeling the proper coordinates for known stars. Once you are polar aligned and have properly modeled, then you can use planetarium software (I use Microsoft WorldWide Telescope) to control the mount via ASCOM, and you can point at pretty much anything in the sky.

These days, I am now using plate solving. With either AstroTortilla+BackyardEOS, or Sequence Generator Pro, I plate solve, which takes a picture with my setup, and runs it through Astrometry.NET, which models the stars and DSOs in the image, figures out where you are actually pointing, then "syncs" the mount with a corrected model. It sometimes takes a few iterations of plate solving to fully correct the modeling of the mount, but once it does, pointing accuracy can often be within 50 pixels! (By default, pointing accuracy may be off by as many as a few tens of arcminutes on most mounts...accurate modeling is really what gives you good pointing accuracy, and that is usually only available on high end, $10,000+ mounts. I purchased Orion Atlas because it has EQMOD compatibility, which makes plate solving a very reliable option with SGP or AstroTortilla.)

I don't know if the SkyTracker has any computer control capabilities. If not, I'd check to see if it has any kind of control capabilities with a hand controller, as that may allow you to do some basic modelling for basic GOTO functionality. If you don't have those (and I suspect not, most of the ultra wide field mounts don't offer that, as most people are going to be using 14mm to 50mm lenses, in which case all you need to do is point it in generally the right direction), then your just going to have to learn the sky, and learn how to do iterative refinement. It doesn't take long, once you start spending time outside under the sky, to learn the positions of the key constellations and what the stars in them look like in a photo. Once you get that far, you eventually learn how to recognize when certain important stars for constellations are in the frame. From there, you can "star hop"...change where the mount is pointing little by little and "hop" from known star to known star until your in proximity to what you want to image, then you can basically do a spiral search, taking an image, moving, taking and image, moving until your framed the way you want to be.

I think your case is a little more unusual, as your using a 300mm lens on a SkyTracker. Generally those mounts are used for much wider field work...I'd say at most 100mm, and generally probably closer to 24mm to 50mm. At 300mm your probably at the limits of what that little mount is capable of. If you ever want to stick the 1.4x or 2x TCs on your lens, then I would highly recommend you bump up to the next level...like the Orion Sirius. That's a full blown Equatorial GOTO mount, and you could then (with an EQDIR cable, bought separately) have complete computer control over the mount...and you could plate solve, use planetarium software for pointing, and do full blown imaging like I am doing with BackyardEOS, Sequence Generator Pro, Nebulosity, etc.

 

[jrista]

Re: Deep Sky Astrophotography

Amazing pic, jrista. How do you even know where to look? I had always thought you would need a telescope (and a big one) to take such images. Amazing.

Thanks!

As far as finding things, I use Microsoft WorldWide Telescope (WWT) to point my mount. Before I do that, I "model" my skies with a plate solving tool, which can figure out the stars and deep sky objects in a photo by referencing indexes and doing spatial mapping and modeling. The plate solvers "sync" their model to the mount, after which I am able to point very accurately, usually within an arcminute or two, sometimes within arcseconds. So, all I really have to do is use WWT to find what I want to image, highlight it, and tell it to "slew" the mount. That's all there really is to it!

 

[jrista]

Re: Deep Sky Astrophotography

Fantastic work JRISTA and many thanks for all the information you've provided! I didn't expect such results were possible. This is great stuff.

Thanks! And your welcome. It's possible to do MUCH better than I have. I'm still a relative novice. Even with just a basic telescope and a DSLR, there are people out there who are more skilled and have gotten far more beautiful images than I have. I was actually surprised that the Canon 7D did as well as it did on Rosette...lot of hydrogen alpha (Ha) emission there, however most DSLRs, including the 7D, only pass about 15-20% of Ha wavelengths (it's only a 3nm bandpass). The fact that I was able to extract as much red nebula detail as this is pretty lucky.

 

[CarlTN]

Re: Deep Sky Astrophotography

Well, first, they aren't separate images. They are just crops of the same image.

The scaling isn't the same kind of problem in astro as it is in regular photography. The general rule of thumb in astrophotography is that you SHOULD be OVERsampling. You don't want your stars to be pixel size...you want them to be several times pixel size. The rest of the softness is due to a number of things...tracking error, polar misalignment (in my case, at the time, it was about 2' misaligned, or 1/30th of a degree, so not all that bad, really), seeing. Seeing refers to atmospheric turbulence that causes stars to wobble and jump around.

So, the image is exposed and scaled properly...exactly as it should be, really. With a longer focal length, I'll only be oversampling that much more, but that's a good thing. The more pixels I can pack into any given object, the better.

I was at f/4 ISO 400 for this series, although my exposure times differed. I took three separate sets of exposures, because the dynamic range in Orion Nebula is massive. The 30 and 60 second exposure sets were used to dim the core, which was indeed overexposed in the 120 second exposures. Additionally, mixing and matching ISO settings makes removing noise very difficult. Read noise levels increase as ISO drops, fixed patterns change, etc. meaning you need to use different sets of dark frames. However the semi-random and random noise contributions are also different, and when stacking images from different ISO settings, you usually end up with the worst common denominator in noise...thus noise is usually higher.

The best approach is to use a single ISO setting, at the same aperture, and only vary shutter speed. That minimizes the variables, and allows the intelligent aspects of stacking software (such as dynamic dark scaling) to work it's magic and give you the best results.

Thanks, I’m familiar with “seeing”. The unwanted, different noise at different ISO’s in your stacking makes sense. Like I said, I was just trying to make sense of why the bright stars aren’t more blown out than they are, in your final image. Obviously I don’t know much of anything about the technique. Can you explain “dynamic dark scaling”? You don’t have to use over 300 words, I don’t want to take up all your shooting time!

 

[jrista]

Re: Deep Sky Astrophotography

Jrista...simply outstanding!

After reading through your posts I have to say that I'm impressed by your knowledge, and thankful for the time you've taken to "write a short book" on the subject in this thread! ;D

Jon, that is magnificent! Have you considered posting some tutorials on astro photography? I would love to learn how you make those images, they are just stellar.

Thanks, guys! For all your kind words.

I'm happy to write some tutorials. I have an area set up on my blog at jonrista.com for that. I'd love to see more people get into astrophotography. I've been limited myself to just the moon and larger solar system objects, and maybe some of the larger nebula like Orion, until I got myself a good tracking mount. It's a complex form of photography, but if you like a challenge and like all the gadgetry and math and tinkering and experimenting that goes into astrophotography, you'll love it!

I'd like to get a little bit more experience under my belt before I do write any tutorials...some things I'm still learning and refining my knowledge of. Learning more about the software options, for one. There are a LOT of processing techniques I still need to learn, and some additional tools (like PixInsight). One of the biggest issues, one of the most difficult to deal with unless you get a $20,000 mount, is tracking performance. I'm somewhat "lucky" to be imaging at "only" 600mm...most telescopes are around 1600mm and longer, some of the larger ones are well over 3000mm, and with a barlow, you can get as long as 9000mm or longer!

There is inherent error in all tracking, due to imperfections or precision limits in gears and worms and the like. It's called Periodic Error. There are also sources of non-periodic error, such as seeing (atmospheric turbulence), flexure (the mechanical flexing of anything on the mount, including the tripod, the mount itself, the telescope and guidescope, etc.), wind, etc. A real high end mount, like the 10Micron GM2000HPS, which uses "absolute" encoders which track the absolute position of both the RA and Dec axes with extremely high precision, is basically immune to most of these sources of error. Periodic error, unexpected movement due to wind, even seeing effects, are delt with by the absolute encoding and built-in sky modeling in a mount like the 2000HPS. That sucker generally costs about $24,000 for a complete package, though.

Tracking issues on lower end mounts are usually delt with by "guiding". Guiding uses a secondary scope, usually smaller than the primary scope, along with a small video camera and special guiding software, to lock onto a specific star, model it's shape, identify the "centroid" (an identifiable center point that can be reliably found and regularly tracked), and send correcting guide signals to the mount to tell it to slow down or speed up relative to "sidereal rate". This can solve tracking errors that are primarily due to periodic error. If you use "Off-axis Guiding", you can also solve tracking error that might be caused by various sources of flexure (which pretty much every scope is going to have to one degree or another), slight movement due to wind, etc.

Tracking is probably one of the toughest things to learn about astrophotography, but also one of the things you have to tackle early on to get images like the Rosette image I last shared. You have to get tracking error, in terms of arcseconds, to an average level below your image scale (the relative size of a pixel in arc seconds)...for example, the 7D has 4.3µm pixels, and with a 600mm lens, my image scale is 1.48" (arcseconds)...so for ideal tracking, my RMS error needs to be ~0.7", about half the image scale. I have been able to get my tracking accuracy down to 1" to 1.2", but I haven't yet figured out how to consistently get it below that.

Once I do, I'll be more able to image things on a consistent basis, and I'll have more data to stack and learn processing techniques with. I hope to be there by summer, at which time I'll probably start writing tutorials on my site.

 

[jrista]

Re: Deep Sky Astrophotography

Poor Jon won't have any time for photography if he keeps writing all these wonderful explanations for all of us. Maybe a suggestion on a great book for beginners - Jon - or anyone else.

Hah! I truly wish that was my problem! My biggest problem, really the curse of astrophotographers everywhere, is clouds! I always have cloud cover. For example, tonight...clouds. Hence the reason I'm wasting my time here answering all your fine questions.

Seems more than just me is wondering how to know where to aim. I suspect this is not as easy as it might seem.

It is definitely a technical artform. I knew a lot about photography and the technology behind it, but once I started getting more into AP last year, since then I've learned a TON more, or at least my knowledge of things has become more refined. Everything has to be more precise and accurate in astrophotography. Everything is exaggerated. Everything needs more, more, MORE, MOAR!!!!! AAARRR!!!

It really just takes practice, though, like anything else. You don't necessarily need to understand all the technical and mathematical underpinnings. That knowledge helps, especially when you have to dig in and start dealing with the technical aspects of mounts and tracking error and guiding performance that make your round stars go eggy. The wider your field, though, the easier it is. Your kind of at a middle ground with 300mm...you have some challenges ahead of you, but given your maximum aperture of f/2.8, and the moderate focal length, you have some SIGNIFICANT advantages working for you as well. I think, once you get some good tracking beyond 30 seconds, and learn how to create a fully calibrated integration of your images, you'll be surprised at what you can do.

Only real word of advice I can give you now, is, get as many subs (individual light sub frames) as you can. Noise is the biggest single issue in astrophotography. It's why people will spend two, three, five nights imaging the exact same object over a period of weeks or months to get 50, 70, 100 images of the exact same thing...so they can reduce noise. Remember that dynamic range in a properly calibrated stack of images is FAR greater than that of any normal single photograph, and probably greater than that of most HDR photographs. Were talking 20+ stops if you integrate enough frames to reduce noise to the point of near-invisibility.

Star centroids consume the vast majority of that dynamic range....out of that 20-24 stops of dynamic range, all the nebula detail lives in the bottom few stops, and the vast bulk of post processing work is geared towards lifting it all out of the deep shadows and making it visible. All of the images I've posted so far looked entirely black with a salting of bright stars...I had to DIG and pull out the nebula detail out of the wicked depths...that's pretty much what astrophotography is...digging the faintest little bits of detail out of the lowest couple of bits of data in your final integrations.

I hang out on a couple of astro forums on the net. I've seen a lot of beginners posting their first images, and you can see little shreds of nebula here and there due to too few subs, lots of blown out stars, lots of egg-shaped stars due to bad polar alignment and poor tracking. If you want to start out the gate getting acceptable results, then the single best piece of advice you should follow, beyond tracking performance itself, is to maximize the quality of each and every sub. Don't stop at 20, or 30. If you have the option, point your setup at one place in the sky, at most two hours after it rises in the east, and let your system track it all night long, and take images all night long, until it starts to set in the west. Find the broadest horizon you can with the best, and darkest, view of the sky that you can. Track...and EXPOSE. That's the real trick. That's what most of the novices miss (and the one reason why I've been able to get decent results despite only having owned my equipment for less than a month.) You need to get the longest subs possible (without clipping stars, or at least without clipping them too much...exposure equals greater signal strength), and you need to get as many of them as you possibly can (total number of subs equals higher SNR, which means lower noise in the final integration). THAT will get you some awesome results, especially with your f/2.8 aperture.

 

[jrista]

Re: Deep Sky Astrophotography

Thanks, I’m familiar with “seeing”. The unwanted, different noise at different ISO’s in your stacking makes sense. Like I said, I was just trying to make sense of why the bright stars aren’t more blown out than they are, in your final image.

Well, as far as stars go....they are, stars. When it comes to the cutting edge of "low light" imaging, most of the more recent technological innovations that you can find over on sites like ImageSensorsWorld, are rated on their ability to see "in starlight". Starlight is in the "Fraction of a Lux" range as far as total illumination goes...they are exceptionally dim, and pretty much anything, including a half moon, can blot them out of existence.

At ISO 400, under 0.01 Lux, your sensor "speed" is actually quite slow...relatively speaking. In normal photography terms, ISO 400 is moderately fast. ISO 1600 is quite fast. ISO 6400 is ultra fast. In imaging, ISO 400 is probably more akin to "normal" ISO 50, and ISO 1600 is more like ISO 200 or 400. You actually have a lot of time to expose before your pixels saturate, even with a sensor like the 7D's, which only has ~20ke- at max saturation @ ISO 100 (and only around 6ke- at ISO 400!).

It should be noted that the very central part of most stars, the "centroid", is often clipped. When I image galaxies, which I usually do at ISO 800 because they are so dim, most of the larger and brighter stars are a little clipped in the center. But that usually isn't a big deal, because of the way you process. Processing is all about bringing up the shadows without modifying the highlights at all...your compressing a MASSIVE amount of dynamic range into 8 stops...so its lift, lift, lift, lift! In astrophotography terms, it's called stretching, but that is only because we usually start out with 32-bit floating point TIFF images, which are capable of storing dozens of stops of dynamic range...so we "stretch" our images initially, then convert down to 16 bit integer TIFF, then do some more stretching, pushing, and pulling with levels and curves to bring out the nebula detail.

Obviously I don’t know much of anything about the technique. Can you explain “dynamic dark scaling”? You don’t have to use over 300 words, I don’t want to take up all your shooting time!

Dark frames are intended to remove fixed pattern and dark current noise from light frames. In order for that to be possible, the dark frames must be created at the same shutter speed, ISO setting AND temperature as the light frames...since all three of those factors affect how dark current (which doubles/halves every 6°C) and fixed pattern noise present. Dynamic dark scaling is a newer technique in applying dark frames, that allows you to use darks taken within an acceptable relative temperature range to your lights. There are a few different specific algorithms, but the general idea is to scale up or down the noise levels in a master dark to match the temperature of each light frame as the dark is subtracted from them one by one.

It goes without saying that it's extremely difficult to create a library of dark frames that matches every potential light frame. You have a number of standard exposure times...30s, 60s, 90s, 120s, 180s, 210s, 240s, 300s, 360s, 420s, 480s, 540s, 600s. On Canon sensors, the two most acceptable ISO settings are usually 400 and 800...go below that, and read noise is too high, go above that and read noise doesn't shrink appreciably relative to the drop in max saturation. Some people use ISO 1600 anyway, and sometimes it's just plain necessary. That right there is a factorial of 13 shutter speeds and three ISO speeds. Throw in the range of temperatures from -10°C to 80°C, with a tolerance of 2°C, and you have a factor of 13 shutter speeds, three ISO speeds, and 45 distinct temperatures. It's practically impossible to create a dark frame library complete enough that you could match the right darks to any given light frame.

Hence the benefit of dynamic dark scaling. It'll let you create a library of darks spanning far fewer specific temperature levels. You might stick the camera in the freezer for -10°C and 0°C, in the refrigerator for 10°C, and maybe get some darks for 20°C and 40°C. Dynamic dark scaling can take care of the rest. Bitch of it is, you still need to get darks for these five temperatures for the same 13 shutter speeds and three ISO settings, but a least it's only five temperatures, rather than 45 temperatures!

 

[CarlTN]

Re: Deep Sky Astrophotography

Thanks, I’m familiar with “seeing”. The unwanted, different noise at different ISO’s in your stacking makes sense. Like I said, I was just trying to make sense of why the bright stars aren’t more blown out than they are, in your final image.

Well, as far as stars go....they are, stars. When it comes to the cutting edge of "low light" imaging, most of the more recent technological innovations that you can find over on sites like ImageSensorsWorld, are rated on their ability to see "in starlight". Starlight is in the "Fraction of a Lux" range as far as total illumination goes...they are exceptionally dim, and pretty much anything, including a half moon, can blot them out of existence.

At ISO 400, under 0.01 Lux, your sensor "speed" is actually quite slow...relatively speaking. In normal photography terms, ISO 400 is moderately fast. ISO 1600 is quite fast. ISO 6400 is ultra fast. In imaging, ISO 400 is probably more akin to "normal" ISO 50, and ISO 1600 is more like ISO 200 or 400. You actually have a lot of time to expose before your pixels saturate, even with a sensor like the 7D's, which only has ~20ke- at max saturation @ ISO 100 (and only around 6ke- at ISO 400!).

It should be noted that the very central part of most stars, the "centroid", is often clipped. When I image galaxies, which I usually do at ISO 800 because they are so dim, most of the larger and brighter stars are a little clipped in the center. But that usually isn't a big deal, because of the way you process. Processing is all about bringing up the shadows without modifying the highlights at all...your compressing a MASSIVE amount of dynamic range into 8 stops...so its lift, lift, lift, lift! In astrophotography terms, it's called stretching, but that is only because we usually start out with 32-bit floating point TIFF images, which are capable of storing dozens of stops of dynamic range...so we "stretch" our images initially, then convert down to 16 bit integer TIFF, then do some more stretching, pushing, and pulling with levels and curves to bring out the nebula detail.

Obviously I don’t know much of anything about the technique. Can you explain “dynamic dark scaling”? You don’t have to use over 300 words, I don’t want to take up all your shooting time!

Dark frames are intended to remove fixed pattern and dark current noise from light frames. In order for that to be possible, the dark frames must be created at the same shutter speed, ISO setting AND temperature as the light frames...since all three of those factors affect how dark current (which doubles/halves every 6°C) and fixed pattern noise present. Dynamic dark scaling is a newer technique in applying dark frames, that allows you to use darks taken within an acceptable relative temperature range to your lights. There are a few different specific algorithms, but the general idea is to scale up or down the noise levels in a master dark to match the temperature of each light frame as the dark is subtracted from them one by one.

It goes without saying that it's extremely difficult to create a library of dark frames that matches every potential light frame. You have a number of standard exposure times...30s, 60s, 90s, 120s, 180s, 210s, 240s, 300s, 360s, 420s, 480s, 540s, 600s. On Canon sensors, the two most acceptable ISO settings are usually 400 and 800...go below that, and read noise is too high, go above that and read noise doesn't shrink appreciably relative to the drop in max saturation. Some people use ISO 1600 anyway, and sometimes it's just plain necessary. That right there is a factorial of 13 shutter speeds and three ISO speeds. Throw in the range of temperatures from -10°C to 80°C, with a tolerance of 2°C, and you have a factor of 13 shutter speeds, three ISO speeds, and 45 distinct temperatures. It's practically impossible to create a dark frame library complete enough that you could match the right darks to any given light frame.

Hence the benefit of dynamic dark scaling. It'll let you create a library of darks spanning far fewer specific temperature levels. You might stick the camera in the freezer for -10°C and 0°C, in the refrigerator for 10°C, and maybe get some darks for 20°C and 40°C. Dynamic dark scaling can take care of the rest. Bitch of it is, you still need to get darks for these five temperatures for the same 13 shutter speeds and three ISO settings, but a least it's only five temperatures, rather than 45 temperatures!

Ok, never knew you were sticking the camera in a freezer. Is that on location at your lens and mount? Or do you rely on the cold ambient temperature, and somehow measure the camera's sensor temperature? I don't know how you'd do that. And isn't it freezing the battery, robbing it of charge? Or are you using some type of AC or regulated power supply for the camera?

 

[jrista]

Re: Deep Sky Astrophotography

Ok, never knew you were sticking the camera in a freezer. Is that on location at your lens and mount? Or do you rely on the cold ambient temperature, and somehow measure the camera's sensor temperature? I don't know how you'd do that.

All modern DSLRs have temperature sensors built in. It's how cameras know how to turn themselves off, or turn off live view, when they get too hot. The temperature is stored in EXIF metadata for each image, and the tool I'm using to build my dark library, BackyardEOS, sticks the temperature in the automaically generated filenames. So it's pretty easy to build a dark frame library that way.

The standard approach for taking darks is to take them right after your done taking your lights, on site. The idea there is that by doing it after the lights, on site, you create them at the same temperature. Problem with that approach is that temperature fluctuates, even when your taking your lights, and sometimes by a lot. I've had 8-12° swings during my light frame imaging, peak to peak. Once you start taking your darks, the same thing is likely to happen. So getting temperature-matched darks is very difficult. Since dark current changes by a factor of two every six degrees, a swing of 8-12 degrees in your lights makes it pretty difficult to match up darks. It's possible, but it usually involves dividing up your images into batches of equal temperature for calibration, which also need the darks to be divided up. You need at least 30 darks, preferably 50, to get good results, and it's pretty difficult to get that many darks of a single temperature.

And isn't it freezing the battery, robbing it of charge? Or are you using some type of AC or regulated power supply for the camera?

I use an AC adapter for my camera when I'm imaging. Imaging the night sky takes hours...I've had my equipment outside, tracking, and imaging for six to seven hours strait. Using batteries is impractical. As for temperature, my camera, my entire setup actually, has endured much greater cold than I think my freezer is even capable of achieving. When I first started in early February, there were a couple nights where the temperature at night got down to almost -24°F in the early hours before dawn, which is around -30°C. Even during the last week of February, we had a couple of nights where the temps were around -20°C.

I know a couple of places out under true dark skies, out on the plains east of the Rockies, where it is very flat and wide open (great horizon view, excellent for imaging). I haven't taken my gear out there yet, as I need a powerful remote power supply (I'm thinking of getting an ultra-quiet 55dB 2000W portable generator), but out there, during January and February, the temperatures can get even colder...and with wind chill, any residual heat that builds up because of the electronics will be constantly sucked out...so were not just talking cold, were talking deep freeze with active cooling! I was actually out there a couple years ago during our last Total Lunar Eclipse (I think that was Dec. 2010), and it was one of the coldest experiences of my life.

So freezing my camera in my freezer is actually rather mild. ;P

 

[CarlTN]

Re: Deep Sky Astrophotography

Ok, never knew you were sticking the camera in a freezer. Is that on location at your lens and mount? Or do you rely on the cold ambient temperature, and somehow measure the camera's sensor temperature? I don't know how you'd do that.

All modern DSLRs have temperature sensors built in. It's how cameras know how to turn themselves off, or turn off live view, when they get too hot. The temperature is stored in EXIF metadata for each image, and the tool I'm using to build my dark library, BackyardEOS, sticks the temperature in the automaically generated filenames. So it's pretty easy to build a dark frame library that way.

The standard approach for taking darks is to take them right after your done taking your lights, on site. The idea there is that by doing it after the lights, on site, you create them at the same temperature. Problem with that approach is that temperature fluctuates, even when your taking your lights, and sometimes by a lot. I've had 8-12° swings during my light frame imaging, peak to peak. Once you start taking your darks, the same thing is likely to happen. So getting temperature-matched darks is very difficult. Since dark current changes by a factor of two every six degrees, a swing of 8-12 degrees in your lights makes it pretty difficult to match up darks. It's possible, but it usually involves dividing up your images into batches of equal temperature for calibration, which also need the darks to be divided up. You need at least 30 darks, preferably 50, to get good results, and it's pretty difficult to get that many darks of a single temperature.

And isn't it freezing the battery, robbing it of charge? Or are you using some type of AC or regulated power supply for the camera?

I use an AC adapter for my camera when I'm imaging. Imaging the night sky takes hours...I've had my equipment outside, tracking, and imaging for six to seven hours strait. Using batteries is impractical. As for temperature, my camera, my entire setup actually, has endured much greater cold than I think my freezer is even capable of achieving. When I first started in early February, there were a couple nights where the temperature at night got down to almost -24°F in the early hours before dawn, which is around -30°C. Even during the last week of February, we had a couple of nights where the temps were around -20°C.

I know a couple of places out under true dark skies, out on the plains east of the Rockies, where it is very flat and wide open (great horizon view, excellent for imaging). I haven't taken my gear out there yet, as I need a powerful remote power supply (I'm thinking of getting an ultra-quiet 55dB 2000W portable generator), but out there, during January and February, the temperatures can get even colder...and with wind chill, any residual heat that builds up because of the electronics will be constantly sucked out...so were not just talking cold, were talking deep freeze with active cooling! I was actually out there a couple years ago during our last Total Lunar Eclipse (I think that was Dec. 2010), and it was one of the coldest experiences of my life.

So freezing my camera in my freezer is actually rather mild. ;P

How much cold can the shutter take? I had thought that temperature ratings for cameras, were meant to be a guideline for overall functionality, and not just the battery. I was out when it was "only" -3F here with my 6D at the end of January, taking wide shots over my oak tree. I was only out about 25 minutes though...the battery held up fine, didn't even have a full charge. I, however, was fairly cold...at least my hand was whenever I took it out of the glove! I wore wool insulated boots, there was about 3 inches of snow, frozen over. I can't imagine being in -30F ambient, with wind on top of that. At that point, I would rather just look at other people's photographs! (Especially if I'm not only not getting paid for the shoot, but not getting paid AHEAD OF TIME for the shoot...haha). Heck, even with the amount of knowledge and money it took for your setup...I'm content to just look at yours...and occasionally share my "all in one exposure" wider shots.