[Nokturner]

By no means is this meant to suggest that the advice given is wrong - in fact I would like to study it myself. But I want to point out that in the K-3 and I think to a lesser degree in the K-50 - you can use interval composite mode to build images. The idea is a lot of shorter exposures combined. The one time I have played with it I used my Rokinon 8mm fisheye on a cloudy night. The movement in the clouds is intentional and expected.

Ah! An interresting view of the problem, the problem with no single "best" solution for all situations. What will happen when stacking a lot of short duration exposures, as I guess is the case with the metod you describe, is that the cameras read noise will build up significantly the shorter the exposures get. Some where there is a sweet spot for camera read noise contra the problems who pale up with longer exposures (longer exposures minimize read noise the longer they run). In theory, longer exposures are better noisewize. In practic shorter exposures are better with respect to tracking accuracy, foregrounf blur etc (there is a zillion factors that easily ruin longer exposures, exponentially the longer they get).

[Nickrs]

Hi jtkzoe,

Lots of info for you in this thread, I'm sure it is all very pertinent.

You did ask a question about the blurring of a foreground when using the O-GPS1 unit, the answer is yes it does because from the camera's point of view the sky is stationary and the foreground is moving, the degree of blur is directly related to the length of your exposure of course. Without astrotracer a wide angle lens should enable you to take exposures up to 30 seconds without significant star smearing and the K50 should give you good results up to ISO 1600. If you want to get some really interesting whole sky shots then a fisheye lens can come in useful.

My advice would be have a go with what you have before deciding whether you want spend money on extra kit, Sigma's 10-20mm wide angle zooms seem to be good value at the moment and are a multi-purpose investment (for interiors and landscapes too). The O-GPS1 is quite expensive but in use you do get your geographical position in your exif and in astrotracer mode it works well for exposures up to three or four minutes.

I found sky photography was a matter of trial and error to start with and be prepared for failures, getting the focus right was the most difficult thing initially.

Below is a photograph of the Milky Way taken a couple of days ago from our garden here in Wales using the O-GPS1 module on my K3 with my Sigma 10mm f/2.8 fisheye lens (ISO 800, f/2.8 110 seconds), you can probably see the trees are blurred a bit. I also include below a photo (without O-GPS1) of what can go wrong when you don't have time to set the focus correctly because the International Space Station makes an unexpected appearance , you may also notice that even at 10mm the stars are slightly elongated because of the long exposure (144secs).

Best of luck,

Nick





---------- Post added 08-02-16 at 09:34 PM ----------

Oh, I forgot to add that the bottom image was taken at f/4 so even a kit lens at 18mm, f/3.5, 120secs, ISO 1600 should get you started (especially if you have time to set the exposure and focus properly). To find your way around the sky I would recommend downloading a copy of Stellarium, it is widely used by both professional and amateur astronomers and even works on your phone (Stellarium).

Nick

[Nokturner]

I forgot to mention that the 25-30 second max exposure time with 24/1.4@1.4 in my previous posts refers to ISO 1600.

[aiki76]

I use the GPS all the time, usually exposing for 60 seconds or similar. The shot I posted above used it. With the same tripod setup, you shoot once with the astrotracer on, then you turn it off and shoot for the same exposure time using the bulb setting with the tracker off. This stills the foreground with identical composition and exposure levels. You can even re-focus on the foreground to make sure that it's as sharp as possible for the 2nd shot. Then I mask the still foreground over the movement-augmented foreground in photoshop.

I am not skilled with panorama stitching, and I don't need mega-resolution files. It's easier for me to take 2 shots where the only difference is the flip of a switch rather than taking 4 and trying to merge them.

At high ISO's, for the non-astrotracer foreground images, you can take multiple shots of the same scene and using image-stacking and averaging reduce the noise to very low levels as Nokturner described. I didn't want to touch on post-processing techniques since that's getting a bit far down the road for now.

Two different philosophies. As with the rest of photography, there's many ways to reach similar goals. The more you do it, the more your mind will develop its own routine and optimize your most logical solution.

Hi, a question: in the shot with astrotracer on, an object in the background will be bigger than in the shot with the astrotracer off, because in the first shot the object will be slightly moved-stratched. How do you manage this point during the merging?
thanks

[disasterfilm]

Hi, a question: in the shot with astrotracer on, an object in the background will be bigger than in the shot with the astrotracer off, because in the first shot the object will be slightly moved-stratched. How do you manage this point during the merging?
thanks

For a standard landscape astro shot, the stars are in the background, the distant landscape is in the midground, and the objects near to the camera are in the foreground.

When you have the astrotracer turned on, the background remains still while the midground and foreground experience sensor motion blur. With astrotracer off, the background stars blur due to their own motion, and you get still foreground and midground.

The setup in photoshop is very simple. I use the astrotrace-on layer as the base layer/background. Stars are still, everything in front is blurry.
Immediately on top of that, I add the layer with astrotracer off. With a stable tripod, everything is stacked exactly where it should be without resorting to computer layer alignment.

For the mid/foreground layer, I add a quick mask to this layer and dump it black. If you're not familiar with layer masks, it's a digital shortcut to make parts of a layer visible or invisible. When the layer mask is painted black, it's invisible. When it's painted white, it's totally opaque. Any value of gray between them is a corresponding value of partial opacity/transparency.

So you dump the layer mask to black. This makes the layer invisible. Then I paint using a white brush into the layer mask for all the foreground/midground objects that I want visible. If you paint too much, switch to a black brush, and you can make your layer mask neater or try it again. You can change your brush shapes for different sizes or hardness of edges.

The reason it's important to expose the non-astrotracer layer exactly the same (or barely different) than the astrotracer layer is that you want the images to be identical in terms of brightness. So when you're painting in the edge of where a mountain or horizon meets the skies, you can paint a little past the mountains without having to be pixel-perfect. The sky (as long as there aren't stars) will be exactly the same regardless of which layer is showing through.

Obviously where you have stars hitting the horizon lines, you need to be a little more careful.

When you're using a really, really long shutter speed with the astrotracer or a mid to telephoto lens, I guess you could get a scene where a perfectly level horizon tilts somewhat due to the sensor movement. In that case, paint in your mask on the still midground/foreground layer, then using the Nudge tool, just shift it up a few pixels.

It gets more complex if you've got tree branches close to the camera, because they may blur a little more. This is where (again) the value of matching scene exposures exactly between frames is very handy. You can over-paint a little beyond the limits of the still tree. As long as there aren't stars, you're painting in a few pixels of the moving sky to cover up the moving tree below, but it's dark sky. Without a star, you don't know if it's moving or not.

I sort of buzzed through that explanation and wasn't very succinct at all. If this isn't intuitive, I can take a screenshot at home of what I'm talking about, and hopefully that will illustrate where plain words come up short.

[Nickrs]

Hi @jtkzoe,

Just to illustrate that you can get started with your kit lens, the first photo is of Jupiter and at the very top of the image The Pleiades open star cluster peeping out of some cloud, it was taken with my Pentax MX-1 compact camera (ISO 1600, 2sec, f/2.5, 24mm). The photograph of the Aurora Borealis (in Iceland last Feb) was taken by my daughter with her K50 and her 18-50 kit lens. If the astronomy bug really bites then a whole new world of spending money opens up to you. :-)

Nick

[Sagitta]

In my opinion, finding a lens that resolves decently wide open (or close) is as important as finding a lens that is fast - ideally you'll find one thats a bit of both.

Out of the lenses that I own, my favorite/best 'astro lens' would be my Sigma 10-20mm f/3.5, simply because its fast(ish) and wide enough that unless I get really silly with my exposures the ground movement is negligible.

With the newer cameras out there, you can also shoot at higher ISOs without as much worry, especially if you don't plan to leave the mage at pixel peeping levels of viewing.

My second favorite lenses would be my Pentax M 50 1.4 (stopped down a peg) and my Samyang 85mm f/1.4 (stopped a peg). I also have a 35mm f/1.4 that produces amazing images, but its almost impossible to nail focus on the thing for astro shots which makes using it a study in frustration more than fun. When it delivers though, man does it deliver.


EDIT: May not be obvious but ultra wide will always trail on a shot, tracked or not because its just so dang wide. You can see that in the first image below - the edges show trailing where the center does not.

Sigma 10-20mm on the K3


Pentax M 50 f/1.4 on the K3


Samyang 35mm on the K-3 (no tracker here, but you get the idea)

---

Last edited by Sagitta; 08-02-2016 at 07:38 PM.

[Nickrs]

In my opinion, finding a lens that resolves decently wide open (or close) is as important as finding a lens that is fast - ideally you'll find one thats a bit of both.

Out of the lenses that I own, my favorite/best 'astro lens' would be my Sigma 10-20mm f/3.5, simply because its fast(ish) and wide enough that unless I get really silly with my exposures the ground movement is negligible.

With the newer cameras out there, you can also shoot at higher ISOs without as much worry, especially if you don't plan to leave the mage at pixel peeping levels of viewing.

My second favorite lenses would be my Pentax M 500 1.4 (stoppepd down a peg) and my Samyang 85mm f/1.4 (stopped a peg). I also have a 35mm f/1.4 that produces amazing images, but its almost impossible to nail focus on the thing for astro shots which makes using it a study in frustration more than fun. When it delivers though, man does it deliver.


EDIT: May not be obvious but ultra wide will always trail on a shot, tracked or not because its just so dang wide. You can see that in the first image below - the edges show trailing where the center does not.

Sigma 10-20mm on the K3


Pentax M 50 f/1.4 on the K3


Samyang 35mm on the K-3 (no tracker here, but you get the idea)

I agree with Sagitta above, both the Sigma 10-20s (f/3.5 and the f/4-5.6) are good quality lenses which can be used for all sorts of stuff, and they are not too expensive.
Nick

[Nokturner]

To my previous posts in this thread where I recomend a 24/1.4@1.4 and stiching a pano for field of view on APS-C, before a tracking device and slower but wider lens, I have to add that the depth of field will be extremely thin.

Landscape elements in the foreground almost certain has to be focus stacked because of this, almost certainly if they are closer to the camera than say 6-10 meters. With a wider lens (10-14 mm) and f/2.8 or slower, one don't need to do this (if one isn't super picky). My guess is that the Rokinon/Samyang 16/2@2 also will need some focus stacking by the same reason, but not to the same extent (don't own that lens).

Focus stacking can be as time consuming and tedious work as blending an untracked image with a tracked one (the mentioned size difference between unblurred/blurred or tracked/untracked elements will be a main reason for this). For a good result both methods requires quite a high degree of photoshop skills. Some images are easier, some with complex elements such as branches, long grass etc in foreground are a nightmare to stack.

My experience is that a f/2.8 or slower lens is a little bit too much for the APS-C sensor to handle low light wise without tracking for a good sky. As shown in this thread with pictures, they will be ok, but there are situations when f/2.8 isn't enough and panoing f/1.4 is a huge difference from f/2.8 single image. But panoing also need a three-way tripod head with deegre scales horizontally and vertically in front of a ball head for easy panoing without tillting the camera in more than one axis and to a controoled extent for overlap between shots (best is of course a dedicated pano head).

With this said, a 16/2 lens untracked might be the best compromise. Anyway, a tracking devise is fun and minimizes startrails. Also, one can do sky with a f/3.5 lens and even slower (but slower is not wery optimal and I don't recomend).

No one best solution for this problem and for all situations.

[aiki76]

For a standard landscape astro shot, the stars are in the background, the distant landscape is in the midground, and the objects near to the camera are in the foreground.

When you have the astrotracer turned on, the background remains still while the midground and foreground experience sensor motion blur. With astrotracer off, the background stars blur due to their own motion, and you get still foreground and midground.

The setup in photoshop is very simple. I use the astrotrace-on layer as the base layer/background. Stars are still, everything in front is blurry.
Immediately on top of that, I add the layer with astrotracer off. With a stable tripod, everything is stacked exactly where it should be without resorting to computer layer alignment.

For the mid/foreground layer, I add a quick mask to this layer and dump it black. If you're not familiar with layer masks, it's a digital shortcut to make parts of a layer visible or invisible. When the layer mask is painted black, it's invisible. When it's painted white, it's totally opaque. Any value of gray between them is a corresponding value of partial opacity/transparency.

So you dump the layer mask to black. This makes the layer invisible. Then I paint using a white brush into the layer mask for all the foreground/midground objects that I want visible. If you paint too much, switch to a black brush, and you can make your layer mask neater or try it again. You can change your brush shapes for different sizes or hardness of edges.

The reason it's important to expose the non-astrotracer layer exactly the same (or barely different) than the astrotracer layer is that you want the images to be identical in terms of brightness. So when you're painting in the edge of where a mountain or horizon meets the skies, you can paint a little past the mountains without having to be pixel-perfect. The sky (as long as there aren't stars) will be exactly the same regardless of which layer is showing through.

Obviously where you have stars hitting the horizon lines, you need to be a little more careful.

When you're using a really, really long shutter speed with the astrotracer or a mid to telephoto lens, I guess you could get a scene where a perfectly level horizon tilts somewhat due to the sensor movement. In that case, paint in your mask on the still midground/foreground layer, then using the Nudge tool, just shift it up a few pixels.

It gets more complex if you've got tree branches close to the camera, because they may blur a little more. This is where (again) the value of matching scene exposures exactly between frames is very handy. You can over-paint a little beyond the limits of the still tree. As long as there aren't stars, you're painting in a few pixels of the moving sky to cover up the moving tree below, but it's dark sky. Without a star, you don't know if it's moving or not.

I sort of buzzed through that explanation and wasn't very succinct at all. If this isn't intuitive, I can take a screenshot at home of what I'm talking about, and hopefully that will illustrate where plain words come up short.

Thank you for your clear and very complete answer!

[jtkzoe]

OK. So I settled on the Rokinon 24mm f1.4. It's MUCH better than what I had before, and with stitching, I can get a decent panorama. Just started playing around with it in the backyard tonight, which is way too bright, but it's practice. My issue now is that the camera isn't taking single exposures. I'm in bulb mode, playing around with ISO. I set the aperture for 24mm. Noise reduction on (and some with it off). Long exposure NR off. Single frame shooting. Manual focus. Using a remote, set so one push opens, second shuts. But it's taking multiple pictures. How can I get it to just take a single shot? Not sure what I'm doing wrong here.....