# Astrophotography With DSLRs



## lucasbellator (Sep 3, 2013)

Hy, fellas. 

So, I'm the proud owner of a Sony Alpha SLT 55V and I want to do some astrophotography without having to mod my camera.
My lenses are: a 18-55mm/f3.5 and a 75-300mm/f3.5 

I've seen that you must set the shutter speed to 10 or 15 minutes. But my A55v goes to 30 seconds only and the BULB mode. 

Do any of you happen to know how long the BULB mode goes? Or how could I set the bulb mode to keep going? 




p.s: As I am a newcomer to the forum, sorry if this post is in the wrong section.


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## Gavjenks (Sep 3, 2013)

BULB mode will go forever.  Or more accurately, I guess, it will go for as long as the battery lasts on your DSLR =P

Generally to keep it going for 15 minutes, you obviously don't want to hold the shutter button, since you'll shake the camera eight ways from sunday.  You need to buy a remote shutter that works with your camera and has a timer in it (they are cheap and usually go up to something like 99 days, etc.)


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## TCampbell (Sep 3, 2013)

This all depends on what you're trying to image... and how.

If the camera is on a stationary tripod and you're using a camera lens, then the exposure times are limited unless it's your intention to capture star "trails".   

As you leave the shutter open, the camera will not only record light from the objects you intend to capture... it'll also catch light pollution reflecting off the atmosphere.  A 15 minute exposure is _VERY_ long for a DSLR and unless you are doing this from a fantastic dark-sky location, the sky will begin to just become muddy and lose all contrast. 

To take longer exposures, you can either piggy-back the camera on top of an equatorially mounted telescope which is tracking at sidereal speed -or- you can use a T-adapter and T-mount so that the telescope becomes the lens for your camera.

I should warn you at this point:  This SOUNDS a lot easier than it actually is.  When using a camera through a telescope, the views are fairly narrow (the angular field of view is tight).  That means the TINIEST of vibrations will blur the image and I cannot stress that enough.  

I'm an avid astronomer, do some imaging (not that I'm especially good at it yet) and have friends who are fantastic at it.  What I've learned in the years that I've been doing this is that generally the astronomer spends more money on the telescope "mount" than do on the actual telescope.  In addition, for long exposure times (and 15 minutes definitely qualifies though I have friends who routinely take images where each of their "lights" is 45 minutes or longer) the mount needs to be extremely well aligned -- but even with that there will be some drift after 15 minutes.  So the scope needs an auto-guider (either an off-axis guider or a separate telescope acting as the guider.) 

They also typically using CCD imaging cameras rather than DSLRs and these are monochrome cameras and they're "cooled" because there's a relationship between chip temperature and the resulting amount of image "noise".  The camera is typically mounted to a filter wheel which takes a separate image with each filter... and the images are later processed and combined to create a false-color image of the object.

If you don't mod the camera, then the IR filter in the camera is going to cut a lot of light in the Ha band.  90% of all atoms in the universe are hydrogen and the Ha band is the strongest emission of light from those atoms.  When imaging, say, diffuse nebula, you want a camera that collects as much light in Ha and Hb as possible (Hb is generally never a problem though.)


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## DannL (Sep 3, 2013)

I recently made a small investment in some gear to accomplish astrophotography. Part of that included a Canon camera which required a wired remote control. I researched ebay until I found one that worked with the camera. Some remotes can be very basic, but I went with the digital programmable remote. They are not very expensive.


Astrobin may have images from similar cameras to your Sony. Astrobin Link


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## Judobreaker (Sep 3, 2013)

Gavjenks said:


> BULB mode will go forever.  Or more accurately, I guess, it will go for as long as the battery lasts on your DSLR =P



Not entirely true.
My D7000 will go up to 30 minutes but not longer.
This might not be true for every camera (can't find it for the D800 and haven't tried it yet either) but not all cameras go until the battery dies.


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## KmH (Sep 3, 2013)

Judobreaker said:


> Gavjenks said:
> 
> 
> > BULB mode will go forever.  Or more accurately, I guess, it will go for as long as the battery lasts on your DSLR =P
> ...


Yep! Many DSLR's have a Bulb/Live View/Video time limit so the image sensor does not overheat.
Heat will also add thermal image noise, be it from high air temperatures or camera electronics generated heat. 
Cameras that have complete magnesium alloy chassis dissipate heat better than all plastic camera bodies.

An electrically or manually driven mount is needed to counteract Earth's rotation during a long sky exposure. 
Equatorial mounts only need be driven in one axis as long as the mount's polar (right-ascension) axis is aligned with the Earth's axis of rotation. Alt-azimuth mounts need to be driven in 2 axis (altitude and azimuth) and thus require computer control for tracking.


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## Patrice (Sep 4, 2013)

Keith,

Your second paragraph should say ".... as long as the mount's polar _(right ascension)_ axis is aligned...."  Also, even computer driven Altitude-Azimuth mounts are not that great for long exposure astro photos of deep sky objects. Even though they track well they still suffer from field rotation which can only be countered via rotation about the earth's axis of rotation.


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## lucasbellator (Sep 4, 2013)

And what about the Photoshoping of the pictures?


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## KmH (Sep 4, 2013)

Yes. The pictures can be edited.


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## TCampbell (Sep 4, 2013)

And just for fun.... I just took this image last night.  This is M57... aka the "Ring Nebula" (A planetary nebulas created by a star which exploded and is now fading in the center.  This is believed to be the fate of our own Sun in about 5 billion years.)  The faint star in the center of the planetary nebula is now a white dwarf and will just continue to cool.  BTW, there is no relationship to planets and "planetary nebula".  The name for this type of nebula was given because the first astronomer to spot one thought it resembled a fading planet.

This was taken with my Canon 60Da using a Celestron C14 (14" f/11 Schmidt Cassegrain Telescope - natively about 3900mm focal length but this image was taken using a TeleVue 2x Powermate (like a 2x barlow but TeleVue adds some corrective optics which produce a better result than a typical barlow.)

The telescope sits on a Losmandy Titan mount with _very_ good equatorial alignment and extremely precise tracking.  The scope sits inside mounting rings that attempt to avoid any flexure issues.  

This is a 2 minute exposure.  The original image had a muddier background (caused by light pollution) and I used curves to try to reduce the background and help bring out the nebula.  In retrospect I should have tried a 4 minute exposure just to see how it would have worked.  But keep in mind the 60Da (it's the "a" that's significant) is an astrophotography camera -- essentially it's a modded camera (except it's modded by Canon so you're not breaking the warranty by tearing it apart to replace the low pass filter).  Had I been using a non-modded camera, I'd probably have needed to run this exposure much longer -- likely more like 6 minutes and at that point I'd have horrible background light pollution.

I'm headed up to a star party (Great Lakes Star Gaze) for the weekend, and if the weather cooperates hopefully I can get some images where the sky glow isn't such a problem.  Out of control light pollution is the bane of every astronomer.

Note the colors in the image.  A star is largely hydrogen gas so as it explodes that outer shell, the shell is also largely hydrogen atoms and they're going to release light following the Balmer series for spectral emission of hydrogen atoms... which means Hydrogen alpha (Ha) is the dominant red and Hydrogen beta (Hb) is the dominant blue -- a light blue / cyan color.   (Hydrogen also emits a deep blue and violet color but those are usually not nearly as intense and difficult to see.)  Most deep space objects are going to appear as red, blue, or a combination of red & blue specifically because they are primarily hydrogen.  An unmodded camera will only be able to detect about 1/3rd of the Ha as compared to a modded camera.  So had this been taken with an unmodded camera, you wouldn't notice as much red (you'd get some... just not as much.)


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## DannL (Sep 4, 2013)

That's good stuff, Tim. And a good shot also. I picked up the Celestron EdgeHD 800/CGEM about a month ago. Still getting accustomed to it's operation. I've piggy-backed the camera occasionally with a 55-250mm and it works perty good . . . considering the light pollution you've already mentioned. The maximum exposure so far with the camera attached directly to the scope has been about 2.5 minutes. I stopped with that because I knew in advance that the polar alignment was off. I really need to get a guide scope. I replaced my old 6" Orion dob with the Celestron. I swear that the visual with the 6" was much better than this 8".

Lucas might consider a *Barn Door Tracker*, also. 

Images of *Barn Door Trackers*.


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## TCampbell (Sep 4, 2013)

Thanks DannL.  Incidentally, you don't necessarily "need" a guide "scope" per se.  You can get an off-axis guider... a small tube with a little sliver of mirror at one edge that bounces light into a camera (think "T" intersection... most like goes straight through, but a tiny bit from the edge of the frame is stolen and bounced into the guide camera.  You still need two cameras, but just one scope.  Sometimes there's just nothing suitable in the frame to guide on and you need that second scope.

An Orion ST-80 (Short Tube 80mm) is a _very_ popular wide-field achromatic refractor used for guiding.  It's usually wide enough that you can find a suitable guide star (you can even get mounts that allow you to point it off-axis from your main imaging scope).  These are pretty inexpensive... about $100 for the optical tube and then you need a 2" extender to go back at the eyepiece to put the guide camera back far enough.  You do have to have a way to mount it.  I bought a dual-saddle mount with vixen-style saddles from ADM and then I just had to add the vixen-style dovetail bar on the bottom of my ST-80 and was good to go.

The Orion StarShoot auto-guider is probably one of most economically priced yet decent performing guide cameras I've used.  We did several tests before settling on it.  I think it was $279.  It's output assumes your mount has the industry standard SBIG ST-4 auto-guider port.  My Celestron CG-5 mount (and my Meade LX-200) have that port built-in.  My Meade LX-80 mount does _not_ have the port built in, but they sell a $39 accessory that adds the ST-4 port and plugs into one of the Aux jacks on the side of the mount.

PHD auto-guiding software is free and available for Mac & Windows alike (they encourage donations).  BTW, "PHD" stands for "Push Here Dummy" (the software is actually pretty easy to use... go find and watch a YouTube video that demos how it works and you'll be good to go.)  It's GREAT guiding software (requires a computer when you observe of course).  (HINT: decrease the aggressiveness setting when seeing is poor -- the guy who taught me to do this told me that setting aggressiveness to 85% is pretty good.  If it's too aggressive (e.g. 100%) then that means the scope will be "chasing the seeing".  In other words, stars will appear to wobble not because the scope is having tracking problems... but because the seeing conditions in the sky causes them to twinkle and appear to shift slightly EVEN if the scope is tracking them perfectly.  You don't want the auto-guider to try to "chase" that star because it didn't really move.

If you happen to own a Canon (and for some reason Canon EOS bodies are _vastly_ more popular than any other camera body for amateur astronomer astrophotography) there's a program called BackYard EOS that is _extremely_ popular for computer controlled image acquisition.  I use this and, on my Mac, I recently picked up a program called "Nebulosity" made by the same folks who wrote the PHD auto-guiding software.

To fight light pollution, you can get a "UHC" (Ultra High Contrast) filter to put in front of the camera.  Either a 2" screw-on filter that you can thread onto the camera nose-piece or, Astronomik brand makes a special filter that snaps into EOS bodies APS-C bodies (will not fit into a full-frame body) that snaps in rather cleverly (BTW, Astronomik's general purpose light pollution filter is called a "CLS" filter rather than "UHC" filter).  These cut light.... but they're very selective.  They basically cut all the emission bands created by all the popular street lights (sodium, mercury, etc. etc.) but don't block the major bands emitted by most deep-space nebulous objects.  This means the exposure times usually have to run longer (because overall the image is darker) but as you run the exposure longer, you get more of the deep-space object and much less of the light pollution (although your objects will have a VERY distinct color cast as a result of the select band blocking of the filter.)  OIII filters are extremely good for _some_ types of nebulae but they will make everything appear green -- just as H-beta filters are good for certain types of nebulae but will make everything appear blue.

The best way to deal with light pollution is to just get away from it... but that's become increasingly more difficult lately.  There's nowhere left for us astronomers to run that isn't already having light pollution issues.

I would be all for some iron-fisted regulation that flat out bans outdoors lights unless they are properly shielded for dark sky.  There's pretty much no valid reason to have lights that can be seen from above OTHER than for airports.  And no reason to have lights that can be seen from the side other than for nautical markers (and those aren't very bright.)  Everything used for street lighting and parking lot lighting has pretty much trashed the night sky in ways that are far far worse than most people could possibly even imagine.  It's all happened so gradually that unless there's a wide-spread (e.g. city-wide or county-wide) power failure that happens to come on a clear night with no moon, people have absolutely no clue how much is in the sky above them and they never even knew it was there.


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## DannL (Sep 4, 2013)

Thanks for the info, Tim. Based on your post, I've gone and ordered the Orion StarShoot auto-guider outfit. If it doesn't work out, I'm driving to Dearborn. ;-)

I needed one anyway. And they had the scope and camera with rings and rail in a kit for a good price.


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## Garbz (Sep 6, 2013)

lucasbellator said:


> I've seen that you must set the shutter speed to 10 or 15 minutes. But my A55v goes to 30 seconds only and the BULB mode.
> 
> Do any of you happen to know how long the BULB mode goes? Or how could I set the bulb mode to keep going?



You need to set the shutter speed as long as you can without blowing out stars or making skyglow unbearable. I don't use 15min anywhere near the city and have a tendency to shoot more 5min sub exposures in stead. A lot of people come into this thinking the more exposure the better and then can't figure out why their stars all look white. One of the big problems you'll have is with camera response to certain deep sky objects. As Tim said above the dominant emission from many nebula is Hydrogen Alpha and that sits in the near infrared band. Depending on the camera this can be quite bad as you can see on this example here with a D800 : NGC 7293 - Helix Nebula | Flickr - Photo Sharing! and this one here NGC7293 - Helix Nebula | Flickr - Photo Sharing! with a QHY10 (basically same sensor as the D200 but cooled and without an IR filter). The D800 picks up almost next to no Ha emissions. Your Mileage May Vary. 



lucasbellator said:


> And what about the Photoshoping of the pictures?



Processing Astro-photography is an art in itself. The bees-knees of image editing packages for Astronomy is PixInsight. That said there's plenty of people who produce amazing results with Deep Sky Stacker (free program for integrating your subs into a final 32bit exposure) and Photoshop. You just have to be really careful how you stretch images, saturate them, and ensure that all your initial stretches are done at 16bit or even 32bit (not sure if Photoshop supports the curves and levels tools on 32bit images).




DannL said:


> Thanks for the info, Tim. Based on your post, I've gone and ordered the Orion StarShoot auto-guider outfit. If it doesn't work out, I'm driving to Dearborn. ;-)



It should work out. It's a very capable camera, probably not sensitive enough for off-axis guiding but works a treat with both a 50mm finderscope and the ST-80 Tim just mentioned. Also remember that auto-guiding isn't a panacea for alignment problems. If the scope is misaligned and the exposures are long enough you'll start getting field rotation while guiding. I have no problem doing 10-15min exposures using the SSAG in a 50mm finderscope.


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