Geminids - and astrophotography in general

<p>Attached is the only successful capture we had this evening. This was the first time I've hauled out the camera for any type of astrophotography, so I wasn't too disappointed, none of us even noticed this particular meteor when it passed (though we noticed a few brighter ones that were out of the frame). </p>

<p>It seems to me that there are myriad challenges when doing stargazing pics. I was wondering if anybody would share their techniques/equipment recommendations. <br>

The challenges (as I see from a few hours worth of shooting) are as follows:<br>

For broad field stuff, a UWA-WA capable lens seems a necessity, I was shooting at 15-24mm (on the 5D2), And never felt like I had the opportunity to go wide enough.<br>

Aperture. obviously, manually focused at infintiy (just before the stop), depth of field is galaxies deep, but I got the impression that I wanted to be shooting at f5.6+, not the f2.8 I was forced to shoot at. Is there any advantage to shooting in the f2 range? or does the loss of optical quality (and available glass), and DOF in that aperture range destroy the potential advantage of being able to open it up?<br>

Time: I was shooting at 20-30sec exposures, but even by 30sec, at 24mm, I saw star movement. Aside from getting a motorized following rig, is there any realistic/practical way to negate that, or are you just stuck doing <30 sec. exposures?<br>

ISO: I shot at 800-1600ISO, and on the 5D2, it was more than adequate, but I don't think I'd have chosen to go beyond that, and @f2.8, 30sec, and ISO 1600, I felt I was barely getting into the ideal range of the exposures. </p>

<p>So here it is, the one Geminid we got tonight! Hopefully some of y'all did better!</p>

<div></div>

<p>There are a ton of great resources available for astrophotography. I'm hardly an expert, but I have a couple of suggestions. </p>

<p>First, for most lenses, infinity is at the stop, so you were hurting yourself a bit by focusing just before the stop, and this is why you may have felt the need to stop-down a bit. The 24-70 should be plenty good wide-open, but as you have noticed, it really isn't fast enough to keep exposure short enough to minimize star movement. </p>

<p>Barring buying a wider, faster lens or a following rig, there is software that allows you to take multiple short exposures at really high ISOs and stack them so you get a surprisingly high quality image (they take into account star movement so everything is properly registered, though this does shrink the final FOV). This probably wouldn't have been terribly useful for meteorites, but I've never really tried. </p>

<p>I use a simple rule of thumb for magnitude 4 star fields. 4-4-4: f/4, 4 sec, ISO 400. Nice and simple to remember. The light pollution here won't allow for much more. I'm a bit envious of your dark skies. FWIW, your exposure is 6 stops brighter, and should capture magnitude 8.5.</p>

<p>Wow, 4-4-4? That seems like I wouldn't be able to capture much at all, but I didn't try to capture something that short... So capturing higher magnitude starfields (from 4-4-4 ;-) ) is a logarithmic scale? We are out in the boonies, but we are near sea level, and the air is usually laden w/ moisture, not ideal (and very bad at 'spreading' Savannah's light pollution)...</p>

<p>That meteor streak (on this monitor, I can barely see - for reference, it's directly above the dash in front of '5' about half->2/3 up the image) was far dimmer than most of the star points. If it had occured during a f4/4sec exposure (at iso400) I wonder if it would been much above ambient? Since it entire traverse is within the FOV, and it was (apparently) high enough to just graze the atmosphere (as it's trail appears graduated, and tapers the same)</p>

<p>As far as the lens choice goes, there's not much available in WA-> UWA @ wider than f2.8 -- most of the primes that wide only go there. obviously a fully manual lens would be fine in this application, So I wonder if anybody has any recommendations for even fully manual glass that is in the f1.8-2 range and wider than 24mm?</p>

<p>Attached is a section of the image that contains the meteor.</p>

<p>Oh yeah... ;-)</p><div></div>

<p>That's about what I expect to see. Looks about magnitude 1 or 2, but streaking across the sensor, so it prints somewhat dim.</p>

<p>Yes, star magnitudes are logarithmic, approximately base 2.512, or 5 magnitudes for 100x change in light. This compares to base 2 logarithms for photographic EV. 2^N = 2.51^M, for N f/stops and M magnitudes.</p>

<p>FWIW, I don't find the streaks at all objectionable when printed. It helps orient the view direction, and gives some hint of the exposure used. But maybe not everyone cares. ;)</p>

 

<blockquote>

<p>...First, for most lenses, infinity is at the stop...</p>

</blockquote>

<p>That's not a dependable assumption, especially with longer focal length lenses and zooms. The only way to find infinity focus is to actually check it. Thankfully that's pretty easy with most DSLRs as you can use Live View with maximum magnification on any reasonably bright star.</p>

<p>BTW star images (when tracking the stars) have a brightness related to the physical aperture of the lens in use, not the f-stop. Thus with a tracking mount a 24/2.8 won't record stars as dim as a 50/2.8 which in turn won't records stars as faint as a 300/2.8. This doesn't apply to extended objects, only stars which are effectively diffraction limited pinpoints at any magnification or focal length.</p>

<p><a href="../nature-photography-forum/00VGEm">http://www.photo.net/nature-photography-forum/00VGEm</a></p>

<p>This guy has been doing astrophotography for a while and has a lot of resources on line about it:</p>

<p><a href="http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM">http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM</a></p>

<blockquote>

<p>Thankfully that's pretty easy with most DSLRs as you can use Live View with maximum magnification on any reasonably bright star.</p>

</blockquote>

<p>Bob, My problem w/ that was that live view on the 5D2 defaults to a 1/30 exposure. I'm not sure what other options there are, as this is the lowest setting the unit allows for video (default) This is completely inadequate for capturing <em>anything</em> in a star field (and when the moon is out, starfields are drown out, at least here). Additionally, w/ max magnification, the starfield movement would limit the effectiveness of this strategy.<br>

I suspect that the best way to do it is to plan it out ahead of time, and perform an infinity focus check at a variety of focal lengths, and apertures <em>before</em> you are in a pitch black field, take notes, and refer to them in the field to get good results. Overall, I was happy w/ the sharpness I had at focusing to the stop, then backing off by hand to the 'infinity' mark.</p>

<p>I suspect there's a workaround, but I virtually never use LV, so all I knew then was that live view showed a 100% black screen (took me a few seconds to figure out what was going on, I'll admit, I checked for a lens cap...) - utterly useless for actually <em>doing </em>anything... I defaulted to 100% manual mode ;-)</p>

<p>Michael,<br>

we saw several others, MUCH brighter, unfortunately they weren't in the FOV, or the shutter was closed, I didn't expect to see this one, as we hadn't even noticed it w/ the naked eye. I was just thrilled to see it at all! When I looked t over a bit closer, I realized just how high it must have been, barely grazing the atmosphere.</p>

<p>Of course the other thing I just didn't expect was to be able to see so clearly the spectral differences of the different stars - In the one crop I can see samples of pactically the entire spectrum!. It might be because (as discussed above) I overdid the exposures, but I certainly was quite pleased by that aspect.</p>

<p>As far as the starfield movement goes, It gives me the feeling the whole image has been stretched - I haven't decided exactly how I feel about it just yet...</p>

<p>Marcus, just snap several photos in a series, slightly varying focus ring position (in MF, of course) while on tripod and pointing upwards. The true infinity focus will be when saved jpeg size (in Mb) is at maximum. Gafer's tape does then permanent lock.</p>

<blockquote>

<p>BTW star images (when tracking the stars) have a brightness related to the physical aperture of the lens in use, not the f-stop. Thus with a tracking mount a 24/2.8 won't record stars as dim as a 50/2.8 which in turn won't records stars as faint as a 300/2.8. This doesn't apply to extended objects, only stars which are effectively diffraction limited pinpoints at any magnification or focal length.</p>

</blockquote>

<p>Now that is interesting. I couldn't find a reference that says that, and I don't understand the physics well enough to work it out on my own. Can you explain further, or point to a reference that does? Thanks.</p>

 

<p>Michael - in an optical device like a telescope, rifle scope, or binoculars the diameter of the objective lens has everything to do with how much light the optic can gather. If you think of a camera lens like a telescope, then it is easier to see why the 300 F2.8s larger diameter objective can gather more light and show dimmer objects.</p>

<p>Hello, Mark. Yes, I understand that. (In your example, the 300 f/2.8 has an objective diameter of 300/2.8 = 107mm.) I was asking about Bob's observation that a star's image is a diffraction limited pinpoint. I follow this part as well, but only fuzzily. It's less than obvious that this should exclude the telescope's (or lens's) focal length from consideration. (Imaged brightness for diffracted pinpoints depend only upon aperture diameter, at every focal length.)</p>

<p>I can see that:</p>

<ol>

<li> focal length determines field of view; </li>

<li>imaged brightness for a moving pinpoint light depends how far its "trail"moves (or is scattered) in the image, </li>

<li>which in turn depends on the field of view.</li>

<li>Tracking the motion of the diffracted pinpoint reduces the "trail" to zero for every focal length. </li>

<li>Thus, the focal length has no effect on the imaged brightness of that diffracted pinpoint light.</li>

</ol>

<p>Is this all that was meant? If so, isn't it simply just the parallel nature of the distant light rather than diffraction that's responsible?</p>

<p>Also, I'm not entirely convinced that a distant star imaged at very long distance is necessarily diffraction limited. Doesn't diffraction require an occluding edge?</p>

<p>Any way, it's entirely obvious that a <em>focused </em>imaged of a non-moving pinpoint light that is NOT diffraction limited is itself a pinpoint on the sensor. Tracking it requires only that the image falls on the same "pixel". (Aside: Wouldn't this lead to color aliasing?)</p>

<p> </p>

<p>Here is a link on diffraction and camera lenses:</p>

<p><a href="http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm">http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm</a></p>

<p>Being diffraction limited in the camera and lens world means your lens / sensor combination has begun to soften the image due to the airy disk size in relation to your photo sites on the sensor. This happens when you stop down in an effort to increase DOF and it is dependent on photo site size, Bayer filter construction, etc.</p>

<p>In a telescope aperture is king; the bigger the hole to the sky on the end of your telescope the dimmer the objects you can see. I use the effects of diffraction in a point source of light (star) to collimate my telescope by purposely defocussing and tweaking the secondary mirror until the resultant airy disk is uniform in appearance. A GIS on collimating a Schmidt - Cassegrain 'scope can get you more info if you care.</p>

<p>A point source of light, like a star, will appear as an airy disk under high magnification in a telescope. Telescopes are considered diffraction limited when all the frequencies of light fall within that airy disk and no light is scattered outside of it by defects in the mirrors. Telescope optics (eyepieces and such) are considered diffraction limited when all light is focused within 1/4 wavelength at the same final focus point. In the world of telescopes this relates directly to resolving power. A good, diffraction limited scope will be capable of about 1 arc second of resolving power. It should be noted that seldom do you get better than 5 arc seconds or so of seeing due to atmospheric effects.</p>

<p>To get enough photons through a telescope to expose a piece of film or collect an image on a sensor is highly dependent on aperture. An 8" opening collects less light than a 12" and you can not get a sharp picture of a magnitude ten star w/ an 8" scope unless you can accurately guide the machine for a very long time. Another option is hundreds of images of the same part of the sky registered and stacked in software. The same goes for extended objects that are too dim to see through an eyepiece. Length of exposure is key there. If you lack aperture, you need time on target.</p>

<p>So, for your list:</p>

<p>1. True<br />2. Not really. It could be very bright, or very dim, and still show as a streak. This is simply because the earth rotates and the stars appear to move in the sky. Set you camera on a tripod and and take any lens and take a few minute exposure, the star trails will be all sorts of brightness and colors. Stars come in many colors, and this will show up in star trail shots.<br />3. Kinda true, the wider the field of view, the less apparent the motion and the longer it will take to become a visible streak. A rule of thumb many folks swear by is 1000 / aperture to yield seconds of exposure to avoid star trails.<br />4. True. However smaller apertures will require longer tracking time to get the same exposure brightness.<br />5. Wrong, see above.</p>

<p>If you have color aliasing, you most likely do not have a diffraction limited scope, you have set your camera / lens combo to a small aperture that caused a diffracted image, or your lens has chromatic aberrations. Telescopes, like lenses, may use exotic materials to try and ensure that all frequencies of light focus at the same point. The Cambridge in Colour link I opened with is quite good.</p>

<p>Tracking a point of light that is only a few arc seconds in size requires very expensive machinery... I was at Chaco Canyon several years ago w/ my telescope. They have an astronomy program there and they comped my camping if I let folks use my telescope. It was a blast. I have attached a picture of a guy I ran into there. He had just purchased this telescope, mount, and tripod. It cost several tens of thousands of dollars so he could move into astrophotography and track stars for long periods.</p>

<p>Whew! Longest post I have typed in many months. Hope it helps.</p><div></div>

<p>Thanks for that, Mark. I'll limit my comments to diffraction only, and then only in relation to Bob's earlier comment. Diffraction relates to both the aperture size (objective diameter) and its distance. In photography, the two are combined and expressed as a single value: an f-stop, the ratio of focal length to aperture diameter. Again, in relation to photography only, in my experience and with the mental model that I carry, the f/stop ratio is what sets image brightness, not simply aperture diameter. Is it so very different with telescopes, that changing the focal length but keeping aperture diameter the same has no effect on image brightness? That really would rock my world, as my understanding of optics and physics doesn't explain that.</p>

 

<p>A telescope can not really change its focal ratio. You can add a focal reducer, which is an intermediary lens and they sell two of them for my telescope. One can turn my F10 scope into an F6.3 and the other makes it F3.3. These optical enhancements will indeed make the scope 'faster' and result in a brighter image in the same time on the target. But these reducers, particularly the 3.3 are used for photography and not observations through an eyepiece. The 6.3 reducer can provide a brighter image for an eyepiece, but at the cost of some sharpness by adding more glass.</p>

<p>In a telescope the theoretical diffraction limit in radians is described by the formula 1.22 x wavelength (in Angstroms) / objective diameter (cm). There are 206,625 arc seconds in a radian. Theoretically speaking, my 20 cm scope could resolve about .07 arc second at a wavelength of 5000 A (conversion is 10 ^-8 cm / A). Well, that don't happen. :-) But diffraction limits are a function of objective diameter and wavelength, not distance. Telescopes wouldn't work very well if distance increased diffraction!</p>

<p>If you check the Cambridge link you will see they reference the wavelength of light as well in their discussion of diffraction in a camera lens. This diffraction formula is also valid in camera lens design btw. Trying to get all the colors of the rainbow focused on the same point is a big part of what drives expense in lens design.</p>

<p>A telescope is a lens in effect, but you have other items in the visual path and the sensor is your retina. A human pupil and the limitation it imposes adds complexity.</p>

<p>So no, a telescope does not violate the laws of physics and optics, so you can rest assured there. You do have to consider exposure time at a certain F stop though. As you make the F stop larger, you need to expose for a longer time to get the same brightness since you have reduced the size of the objective opening. In a sense you are correct in the F stop sets brightness thought, but without time it makes little sense in a practical application like photography.</p>

<p>A similar thing will happen when you introduce an eyepiece into the optical path of a telescope. A high power telescope eyepiece like a 9 mm will produce a magnification factor in my telescope of 2000 mm FL / 9 mm FL = 222X magnification, but the image will be very much darkened, and possibly of such low contrast it may appear very muddy. A 40 mm eyepiece (larger aperture) will magnify much less, approximately 50X, but will be brighter. You will get what is called kidney beaning though since the lens opening is so large; its a kind of vignetting in reverse. ;) 32 mm is the practical limit for my telescope in low mag eyepieces.</p>

<p>I think I have a feel now for what really winds your watch. ;) For myself, starting as long ago as when it was still fashionable to grind your own and scrounge cardboard tubes, I had thought on and off about building an 8" Dobsonian, Fast forward enough years to raise a whole family to adulthood, and I find 16" being debated sometimes as a not altogether unreasonable start point. Times have changed a little.</p>

<p>Thanks for helping me sound this out. Let's see if Bob comes back and clarifies or expounds further, but I'm perfectly good with where you left it.</p>

 

<p>I too would like to have Bob expand on his statement that a star is a diffraction limited point of light. I do not understand his statement (which is why I didn't directly address it.). ;)</p>

<p>I had hoped my ramblings would allow me to understand, but that was not the case.</p>