December 21, 2014, 07:49:00 PM

Author Topic: Deep Sky Astrophotography (Gear Discussion)  (Read 36654 times)

jrista

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Re: Deep Sky Astrophotography
« Reply #120 on: March 19, 2014, 05:57:33 PM »
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.
« Last Edit: March 19, 2014, 06:08:07 PM by jrista »

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Re: Deep Sky Astrophotography
« Reply #120 on: March 19, 2014, 05:57:33 PM »

jrista

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Re: Deep Sky Astrophotography
« Reply #121 on: March 19, 2014, 06:01:11 PM »
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

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Re: Deep Sky Astrophotography
« Reply #122 on: March 19, 2014, 06:06:16 PM »
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.

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Re: Deep Sky Astrophotography
« Reply #123 on: March 19, 2014, 07:47:31 PM »
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
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expatinasia

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Re: Deep Sky Astrophotography
« Reply #124 on: March 19, 2014, 08:31:16 PM »
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.
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Re: Deep Sky Astrophotography
« Reply #125 on: March 19, 2014, 08:54:25 PM »
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.

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Re: Deep Sky Astrophotography
« Reply #126 on: March 20, 2014, 03:57:12 AM »
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.

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Re: Deep Sky Astrophotography
« Reply #126 on: March 20, 2014, 03:57:12 AM »

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Re: Deep Sky Astrophotography
« Reply #127 on: March 20, 2014, 07:47:53 AM »
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

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Re: Deep Sky Astrophotography
« Reply #128 on: March 20, 2014, 08:45:25 AM »
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.
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Jack Douglas

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Re: Deep Sky Astrophotography
« Reply #129 on: March 20, 2014, 12:11:52 PM »
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.

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jrista

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Re: Deep Sky Astrophotography
« Reply #130 on: March 21, 2014, 02:46:32 AM »
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

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Re: Deep Sky Astrophotography
« Reply #131 on: March 21, 2014, 02:49:23 AM »
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! :)

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Re: Deep Sky Astrophotography
« Reply #132 on: March 21, 2014, 02:51:41 AM »
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.

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Re: Deep Sky Astrophotography
« Reply #132 on: March 21, 2014, 02:51:41 AM »

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Re: Deep Sky Astrophotography
« Reply #133 on: March 21, 2014, 03:05:24 AM »
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!

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Re: Deep Sky Astrophotography
« Reply #134 on: March 21, 2014, 03:10:52 AM »
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. 

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Re: Deep Sky Astrophotography
« Reply #134 on: March 21, 2014, 03:10:52 AM »