Nikon D80 with Telescope for Astrophotography

<p>A recent article I read in Sky and Telescope spoke on the subject of using digital cameras with telescopes for Astrophotography. I'm expressing in interest in taking photos of galaxies, constellations.<br>

Any thoughts on equipment to start with using my Nikon D80. Obviously a tripod. But beyond that. Where would a beginner go for information.<br>

Thanks</p>

 

<p>The two best ways to start off are to photograph star trails with the camera on a tripod, or piggyback the camera on a mounted equatorially driven scope. Start with wide to normal lenses and move to telephoto before shooting through the telescope. Everything gets harder very fast when you increase magnification, focus, polar alignment, PE in the drive, pointing accuracy.</p>

<p>To get galaxies, you're going to need to work through a scope, except maybe for Andromeda (M31).</p>

<p>Now, you certainly can go straight for the gusto as it were, but just prepare yourself for a big learning curve and some frustration.</p>

<p>Do you already own a scope or other astronomy equipment?</p>

<p>My husband has a spotting scope that we peered through and saw Jupiter at 60x power inlcuding it's four moons. At that time I was unaware that I could place my glass over to capture this and not really sure that I can. Have every intention of doing this at a good time. </p>

<p>You might be able to get something afocally, i.e., by just holding the camera up to the eyepiece as you were thinking. This can work with bright objects, like the moon, but Jupiter is pretty bright itself.</p>

<p>In terms of other equipment, if you have a budget in mind I could make some specific suggestions.</p>

<p>It's pretty tough to get good images, though star trails are not hard. The cheapest route might be to your camera with a lens and a powered Scotch mount, or a tangent arm such as the Astrotrak.</p>

<p>For the Moon, the D80 is fine, because the exposure times are quite short. Galaxies do not require high magnification (the Andromeda Galaxy is probably the easiest). BUT they require long exposure times, which means mounting the camera on a motor-driven, tracking mount.<br>

They also require DARK skies.<br>

The Sigma 30/1.4 lens would be perfect for a galaxy shot on a D80.<br>

Canon however has released a few DSLR's especially marketed to amateur astronomers because they provide LONG exposure and very low noise images. </p>

<p>Bill,</p>

<p>I disagree about the suitability of the Sigma 30mm for galaxies. M31 is by a wide margin the largest galaxy in terms of apparent size with its long dimension at just over 3°. The Sigma 30mm will have a FOV of 30° on the DX format D80, making the target only cover about 1/10th of the sensor. As another example, M81 comes in at about 1/2°, or 1/60th of the sensor size. You certainly would be able to record something, but you're not going to be able to see much of anything anything. Now, if the target is constellations or expansive star/nebula fields, then I agree that it's an excellent choice.</p>

<p>Also, digital makes the dark sky / long exposure thing not the problem it used to be, provided you are set up to do layers and levels in something like Photoshop. I have gotten good results even on fairly dim objects like the Horsehead Nebula in Bortle 6/7 skies with an 80mm scope by stacking 2 1/2 minute subexposures and using levels to reorient the background light pollution to a black point in the photo. It's still a multi-minute exposure, but it's not the multi hour single shot that you do with film, partly because you're not dealing with reciprocity failure. Plus, narrowband imaging with Ha or other filters just avoids the issue of light pollution, though at some cost of needed more stacked images.</p>

<p>As you point out, the real hurdle is tracking plus powering the DSLR for a long time. But, if you ditch the idea of a telescope and just use normal camera lenses, the image scale (in arc seconds/pixel) gets down to where nice shots are even possible unguided - here's a link to one of Barnard's Loop - http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2835082/page/0/view/collapsed/sb/5/o/all/fpart/1.</p>

<p> </p>

<p>My brain is still on vacation and I gummed up my math, so to clarify and correct.</p>

<blockquote>

<p><img src="http://www.wilmslowastro.com/software/images/formula05.png" alt="arcsec/pix = (pix size/focal length) * 206.3" width="355" height="41" /></p>

</blockquote>

<p>The D80 has square 6.1 micron pixels, which yields 41.95 arc seconds per pixel. M31 is 190 arc minutes long, so 190x60 = 11,400 arc seconds in length ÷ 41.95 arc seconds per pixel = 272 pixels in length, which is 1/10th the short width of the sensor and 1/14th the long side.</p>

<p>Also, as regards star trails.</p>

<blockquote>

<p><img src="http://www.wilmslowastro.com/software/images/formula21.png" alt="Star trail length in pixels" /><br>

where:<br>

F = Focal length of lens scope (trail length in same units as focal length)<br /> E = Exposure length<br /> T = Length of sidereal day in same units as exposure<br /> D = Declination of the star</p>

</blockquote>

<p>So using Betelgeuse (the top left star in Orion at a declination of 7° 24') as an example, and being lazy running it through a JavaScript calculator, a 1 minute exposure on a fixed tripod will create a trail 21 pixels in length. Which illustrates that I misread the info for the image I linked - that image was <em>unguided</em> but it definitely was tracking since the exposures were 4 and 5 minutes long. Unguided just means that he wasn't making tracking adjustments during the exposure.</p>

<p>However, 21 pixels is less than 1% of the short side of the sensor. It might be worth experimenting with short exposures.</p>

<p>"nice shots are even possible unguided - here's a link to one of Barnard's Loop"</p>

<p>I think it worth pointing out that in this context 'unguided' means using the telescope mounting's built in motor drive to track the starts, but without any human intervention to correct for drive errors. It does not mean a fixed (unmoving) camera!</p>

<p>Correct - that's what I was trying to clarify in my last post, as the first was misleading to someone not familiar with the details. The important difference is that a tracking device can be built simply and cheaply, whereas something capable of guiding is generally not.</p>

<p>This has been great advice. Thanks so much for all your input. I will continue to study further, the galaxies, constellations and nebulae before investing. I don't want anything that will sit and collect dust because of my lack of knowledge. BTW, I did come across plans for building a guide. I could start there until I've gained further knowledge.<br>

Thanks again.</p>

<p>I've used my D80 for astrophotography, it's not the best choice for any astro un-modified. Nikon started using a very strong hot mirror, with a cut off well above the IR level. For the best astro-photography you need to be able to get down to 680nm (very deep red), almost all emission nebula are hydrogen and glow that wavelength.</p>

<p>If you want to experiment though, with at least a 300mm lens you can get many of the larger nebula in the sky. If you have a way of guiding the camera, such as a computerized telescope, barn-door, or at least a motor-driven scope mount-combined with a small scope to see thru and keep track you can do some fantastic wide-field photography with something like a 35mm lens or 50mm lens etc.</p>

<p>For larger galaxies such as Andromeda, M101, M110, etc you will still need a telescope, a guide scope, a motorized mount and either a guiding camera or eyepiece with cross-hairs. You can expect total exposure time exceeding 100 minutes, iso 400, @ f/4. With digital, total exposure time is all the frames added together. To get detail, lower the noise, etc, you will have to take, for example: 10 frames @ 30sec, 20 frames @ 1 minute, 20 frames @ 2min, etc. Then taking dark frames separate for each set, subtracting them in the software and then aligning all the individual frames and adding them together, then align and combine each set. </p>

<p>Just for reference, Andromeda galaxy will fill a 35mm frame using a 1016mm telescope, so will the Orion Nebula. M101 barely fits within a 35mm frame using that size of telescope. With the D80, I was able to get a reasonable shot of the Sunflower galaxy thru the scope, about 20min exposure @ f/4, @ iso800, but not with much color and a lot of noise and amp glow. Targets like the North American Nebula in Cygnus require a short and very sharp lens with ED coatings, I wouldn't use anything bigger than 300mm lens. </p>

<p><a href="http://www.cloudynights.com">www.cloudynights.com</a> is the biggest source of info on the subject. I have a few examples in my port, if you have any questions feel free to contact me.</p>

<p>Jerry</p>

<p>Hmmm all these strange formulae, I've never used!<br>

You can quite happily photograph deep sky objects from a static tripod. The formula I've always used is 1000/focal length x cos d = t Where t = max. exposure time before star trailing and d is the declination of the object in the sky.<br>

So for the Andromeda Galaxy at approx. 41° Dec max time of exposure with a 300mm lens is 2.5 seconds.<br>

I took 60 x 2 second shots with a 300mm f2.8 and used "Deep Sky Stacker" a free program available online to combine the images. I also took 60 x 2 second dark frame shots to remove any hot pixels from the image - Deep Sky Stacker subtracts these images for you.<br>

I've used a Nikon D70s and a D3 for this and they both worked fine. The D70s needed more frames shot but still produced the goods at the end.<br>

I do have a driven equatorial mount (and a telescope to shoot through!), which allows far longer exposures, but enjoy seeing the results achieved by the minimum of gear carted out into the field!<br>

Quite acceptable results can be had with just the minimum of equipment:</p>

<ul>

<li>Tripod</li>

</ul>

<ul>

<li>Camera </li>

</ul>

<ul>

<li>Lens (telephoto for pinpointing galaxies and nebulae ... Wider angle for star field shots and milky way)</li>

</ul>

<ul>

<li>Use the timer on your camera to reduce camera wobble when taking a shot or a remote shutter release.</li>

</ul>

<ul>

<li>You are looking to keep your camera just slightly above ambient temperature, to prevent dew forming on the lens... and use a lens hood.</li>

</ul>

<ul>

<li>Deep Sky Stacker available here: http://deepskystacker.free.fr/english/index.html</li>

</ul>

<p>Good luck with your first attempts Pam.<br>

Astrophotography is a very enjoyable and worthwhile aspect of photography. I get a great deal of pleasure from it.<br>

Oh I forgot, one more thing, and probably the most important, you do need the darkest skies you can get to for best results... and of course cloud free!<br>

Clear Skies!<br>

Roger</p>

<p>Here's the link to the Andromeda Galxy image using the above 'Static Tripod' method... (forgot to include it)<br>

<em>It's not the best I've done but it does show that reasonable results can be achieved!</em></p>