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#1522 2008-01-15 09:39 GMT |
I remember one of my friends father has a telescope that could track particular objects automatically, and I'm wondering if such a thing could be done with an SLR to get long exposures of an area of sky without the stars streaking in order to capture things that are not visible to the naked eye or even short exposures?
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#1523 2008-01-15 21:00 GMT |
There are T mounts for telescopes, and you can connect your SLR to it, You can get very long exposure, but you will get a lot of noise on digital, and on film too. There are limitations of the camera and limitation of the lens (telescope is the lens) and low light objects are not very easy to capture. Occasionally I saw some amature sky pictures with authomatic telescopes - the pictures are definetly better then naked eye, but ... Try it.
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#1524 2008-01-17 16:47 GMT |
to clarify a bit, the mount needs to actually move, turning your camera to track continuously the fixed point you are shooting. I'm not sure exactly but I seem to recall the maximum exposure being only a few seconds before you lose absolute sharpness.(without tracking)
As for noise. Well, noise in film isn't so much dependent on exposure duration. In fact in most cases the longer the exposure the less the apparent noise will be. Noise in film is caused by the actual shape of the light sensitive particles in the film. Which is why the faster films (more light sensitive) require larger light sensitive particles in order to capture more photons. Long exposures with film simple fill out, or 'activate' all particles that are possible. So, it's not actually noisier, just more exposed. Digital on the other hand is completely different. Unlike film which particles are it's matrix, digital's matrix is a grid. Noise is caused from extraneous static and misfiring of individual pixels occurring at a constant rate. The longer the exposure, the more 'errors'. Digital and film noise are completely incomparable. Digital noise amount is related directly to exposure duration, where film noise (grain) is only the roughness of it's matrix related to how light sensitive it is.
digital has it's place in long exposures. The key is a large sensor. The smaller the sensor the more likely you will get noise, simply because the pixels are so small. They need much more time to capture the same number of photons as a huge sensor's pixel which would likely have many times the surface area...In fact most astronomy is being done now with digital. Also, digital doesn't suffer (that I know of) from reciprocity failure. In color film the three layers CMY respond differently to light. The manufacturer usually calibrates them to be roughly similar within the standard exposure range which is around 1 second to 1/2000th.(depending on the film type) Anything outside this range will produce color shifts. This is usually expressed as curves, as the time extends past the acceptable range the response curves for each color diverge. This doesn't happen with digital, which makes the color response reliable regardless of exposure duration. (sensibly, black and white film doesn't suffer from reciprocity failure)
Without a tracking mount you are limited in what can be captured. So, your equipment is very much part of the equation. If you are putting a $200 canon rangefinder on a tripod expect complete crap. The sensor is about 1/2.5 "... that's pretty small. also the tiny lens has very poor light gathering capabilities. Now, if you have a canon 5D with a sensor measuring 35mm or 1.4" measured corner to corner diagonally. I'm sure you can imagine that the surface area is roughly NINE times greater. Then, with a pro lens. Say a 200 f2.8. the optics are better but more importantly it gathers several times more light. It's all about diameter with astronomy.
The end result is... if you don't have pro(ish) gear just stay away from the night sky. I tried it with my Canon G9. I took a tripod shot of a city. 10" exposure. It was a horrid puddle of mud. These cameras are just not cut out for low light work. My Pentax *ist on the other hand has given me decent stars HAND HELD. Also, one final note. Your location on earth is important too. Don't expect anything decent unless you are at least 100km away from any major light source like a medium sized town or a city.
I
"help, help, I got ions all over me!"
IONclad |
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#1525 2008-01-17 22:30 GMT |
Thanks IONclad, I'm well familiar with how digital works but not so much with film.
I thought it would be a matter of some kind of mechanized tracking mount but wanted to check if there were any other considerations before going any further. I have a Sony Alpha 100, which has an APS-C size sensor so that should be reasonable from the description above.
The next question is can I get a mechanized mount for a reasonable price for an SLR or should I just break out the soldering iron and mechanize a tripod? 
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#1526 2008-01-18 01:49 GMT |
Sorry Dewbius, just noticed this thread. If you want to go long exposure on the night sky, you need a tracking mount that can be "polar aligned" (aligned with the axis of the celestial poles). Today, amateur astrophotographers use very sophisticated computerised mounts to accomplish their tracking, and more usually they auto-guide on a "guidestar" to ensure precise tracking. Then you can reduce noise using low ISOs for longer exposures. Shots are taken at prime focus (ie using the telescope as the camera lens), or wider field just piggy-backed on a telescope. Multiple exposures are "stacked" which further reduces noise, and "dark frames" (and "flats") are taken & subtracted. After that, there are incredible, specialised, image-processing software packages used! And the images they get are often nothing short of astonishing!
That said, MyShots is not the place for that stuff IMO, and I would warn that astrophotography is the road to madness LOL. But you can track using fairly crude means to produce pleasing shots of the night sky. Simplest thing is a Barn-Door Mount - people make their own, quite basic. Rather than explain it all here, have a look at this:
http://www.petesastrophotography.com/index.html?mainframe=http://www.petesastrophotography.com/barndoor.html
If you want something a bit more 'technical' but not all that expensive, you can go with a cheap telescope & manual equatorial (EQ) mount and piggy-back your camera. See this (my blog, sorry  ):
http://southcelestialpole.org.au/blogs/caveman.php
Secondhand, manual EQ1 mounts can be picked up cheaply, and you don't need an expensive telescope (remember, the telescope is necessary to facilitate the tracking only). You don't even have to have the webcam or laptop shown in the above link - there are things called "illuminated reticles" which are eyepieces with illuminated crosshairs. You turn the manual controls to keep a star in the centre of the crosshairs, while your camera is exposing.
In summary - DON'T DO IT! 
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#1527 2008-01-19 02:07 GMT |
Rob_k , But I like the night sky .
I'm also not afraid of some light construction/electronics/computer control projects so I will no doubt build the basic barndoor style rig and then experiment later with the more sophisticated approaches .
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#1528 2008-01-19 08:08 GMT |
Presume the last post is from you Dewbius, so good on you! I'm sure you'll have a lot of fun with it, as I have! My problem is that I am afraid of "some light construction/electronics/computer control projects", LOL....
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#1529 2008-01-19 09:42 GMT |
Ahhh but its not the electronics or the mechanization/control circuits that are going to be hard. Its the *^%@ing 3d geometry to calculate the appropriate angle to move the camera by to keep in line with the earths rotation. The barndoor rig takes the ouch out of the math by moving the whole tripod base. thus avoiding the need to convert between the earths axes and the cameras axes.
If you know what that is all about.
Otherwise don't go there, madness is sure to follow. ow my brain hurts. :P
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#1530 2008-01-19 18:12 GMT |
I warned you Dewbius!
No, it's really quite simple - the earth rotates 360º in 24 hours, so you need to 'open' the angle between the clapperboards at a rate of 1º every 4 minutes to track. Doesn't matter where the camera is pointed. If you are using a manual screw to open the angle, you then calibrate it by finding how many turns of the screw effect this, and mark fine divisions on the knob (bigger the better!). For widefields, small intermittent adjustments will not affect the final image, so, for example, you might find you need to creep it at one of your scale divisions every five seconds....
Polar aligning the hinge is the hardest bit, especially in the S Hemisphere where we don't have a "Pole Star" (NHemis have Polaris to point at!). The hinge needs to be tilted to the degrees of your latitude (eg 30º if you live at 30º South) and pointed to true South - you can use a magnetic compass to do this, but need to take account of magnetic declination.
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#1531 2008-01-19 21:14 GMT |
I was thinking of a completely motorized rig that would handle both the compensation for earths rotation and the cameras aim with one mounting which then could be aligned at the pole and automatically do the rest, but then I realized that would be excessively complicated and a bit overkill.
Placing the barndoor rig on a platform that can be elevated and panned would allow pole alignment without needing to prop up the whole setup with bricks etc, and the mechanics would do the same things as the motorized setup without needing to do fancy geometry to combine earths rotation plus camera rotation into angle relative to the rigs base.
And who need fancy automatic setups when the manual mechanical one does the job more than well enough.
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#1532 2008-01-20 18:28 GMT |
hey, I found this!
http://www.astronomyboy.com/barndoor/
kewl!
Yeah, the APS-C should do the trick, but you may want to invest in some good noise reduction software too, like noise assassin (?)
I'm really looking forward to seeing the results. Oh, does the sony have 'bulb' setting?
Gratuitous Factoid: Bulb refers to a leftover setting for use with flash bulbs. Since their output was over a long period of time (due to the fact it was a chemical burn) one could never be sure how long they would last, thus one would open the shutter, flash the bulb, then close the shutter. Thus the vestigial setting "B" or "bulb" found on cameras that have never seen the light of a flash bulb. The irony is that I have used the bulb setting quite a bit, but not once for an actual antique flashbulb. hehe
"help, help, I got ions all over me!"
IONclad |
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#1533 2008-01-20 22:18 GMT |
IONclad, yes the sony does indeed have a bulb setting, Feature wise its similar to most entry level DSLR's like for instance a canon 400d. The only major difference I can see between the canon/nikon ones is that it has Image stabilization in the body rather than the lens. Other than that it has a bit more resolution and the kit lens had a bit more range in the focal length.
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Banned
Forum posts: 78
Age: 24
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#1662 2008-04-17 18:59 GMT |
very niggerish
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