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by Tim Zurowski on Thu Sep 13, 2018 1:28 pm
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Following up on my post about night sky photography, I would like to start a new thread on the stacking process and stacking software choices. I have researched this a bit online, but I am not really finding what is required when taking the images for stacking. I have found examples of final images from 2 to 80 stacked RAW files, but nothing about what they did when taking these images. So let's say I go out and take 20 images of Andromeda M31. Is there anything I need to do in camera that is different for each frame taken, and if so . . . . .  what? From what I am reading, you just take 20 images and load them into special stacking software and voila. Is that it . . . . . . nothing different with regards to exposure, ISO, focus, etc. for each of those frames?

Next is the stacking software. I have found references to RegiStar, Deep Sky Stacker, ImagePlus, Maxim, CCDStack, Pixinsight and more. Some are free  while others require purchasing. I am not opposed to purchasing the best software I can get for this purpose. Can anyone with experience in this area offer me some suggestions for the best software for astro photo stacking?

I have on order an Irix 15mm lens and a Sky Adventurer clock drive. My plan is to start with wide angle lenses for Milky Way and general night sky images. However, I do plan on moving up to closer subjects using my 300 f4 PF. With most of the images I am finding, the longest lens they are using is a 200mm. Since my 300 PF is very small and light, I am hoping it will work fine with the clock drive. I have seen reports of people using the 200-500 f5.6 with this drive and having some success.
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Tim Zurowski
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by SantaFeJoe on Fri Sep 14, 2018 3:16 pm
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This guy uses Starry Landscape Stacker. He has a bit if info on shooting foregrounds here, as well.

Adam Woodworth

More

The link I gave you on the other thread had info on stacking, too.

Joe
Learn the rules like a pro, so you can break them like an artist.  -Pablo Picasso
 

by E.J. Peiker on Fri Sep 14, 2018 3:58 pm
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SLS is Mac only and Tim is a Windows guy...

I use SLS as well and love it. It's the only image related thing I do on a Mac.
 

by Tim Zurowski on Fri Sep 14, 2018 4:45 pm
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As EJ says, I need Windows software. I will start with Deep Sky Stacker (DSS) and then continue to research other options as I learn. The DSS website has some info about the Light, Dark, Bias and Flat frames, but I still do not fully understand what they are or how or why this works.  :?  I guess I just need to accept that it does and try it for myself. I was hoping to get a thread going here about this process and learn from it myself, as well as help others that might be interested in getting into astro photography.
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Tim Zurowski
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by Scott Fairbairn on Sun Sep 16, 2018 7:45 pm
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Tim Zurowski wrote:
As EJ says, I need Windows software. I will start with Deep Sky Stacker (DSS) and then continue to research other options as I learn. The DSS website has some info about the Light, Dark, Bias and Flat frames, but I still do not fully understand what they are or how or why this works.  :?  I guess I just need to accept that it does and try it for myself. I was hoping to get a thread going here about this process and learn from it myself, as well as help others that might be interested in getting into astro photography.




Hi Tim, here are some suggestions. I use Pixinsight for Astro stacking and development. It's a bit expensive, but it is terrific and will do almost everything you need from stacking to development of the stacked image. I have also used ImagesPlus and Photoshop in the past.
I'll try to keep my reply reasonably brief, but there is a lot to talk about. 
Light frames are merely the images that you take of the object, whether it's the Milky Way or Andromeda galaxy. Unlike normal daytime shooting, you do not expose to the right side of the histogram. If you do, it will blow out the brighter areas of the image(M31/Andromeda has a very bright core as does M42/Orion Nebula) and over saturate stars. Typically you want the peak of your histogram around the 1/4-1/3 mark from the left side. However, you want to make sure that you don't clip the shadows. 
Dark frames are images taken with the exact same exposure settings as your light frames, AND at the same temperature, but with the lens cap on so no light hits the sensor. Using a DSLR versus a cooled Astro camera makes this tough to do. The further you deviate from the temperature the Lights were taken at, the less accurate the Dark frame subtraction will be. You can set most cameras to do subtraction in camera, but it wastes time in the field. The purpose of the dark frames is to subtract the noise that is caused by temperature and long exposures. You can build a library of dark frames taken at a series of temperatures and reuse them. 
Bias frames are images taken with the lens cap on at the highest shutter speed your camera can be set at. Their purpose is to capture the noise created by the electronics of the sensor and image pipeline. They can be taken at any time and temperature isn't an issue due to the short exposure times. Again, the purpose is to remove sources of noise from the stacked images.
Flat frames are images taken to eliminate the effects of optical problems such as vignetting and crud on your sensor. Exposure time and temperature aren't an issue, but it is imperative to take them at the same focus setting as the light frames. Stretching a white T-shirt over the lens(no wrinkles) works well. As long as the histogram isn't clipped on either side, it will be fine.
The darks, flats and bias frames can be combined to give a "Master" frame, so you only need one file for future work. It is essential to keep in mind that the darks, flats and bias frames are specific to the camera and/or lens used.
It might be helpful to think of stacking images in this manner; Night time subjects are dim, but the noise in the image is high, so the purpose of stacking is to maximise the signal in the mess. You do this by subtracting the garbage(noise from electronics, heat and long exposures) and combining many photos to increase the amount of signal. So for argument's sake, let's say that each pixel captures a couple thousand photons from your subject for each frame. Stacking attempts to take those photons and add them together into a single frame. Therefore, as a general statement, the more images to stack, the better.
As far as taking specific images goes, the general rule of thumb beside my histogram suggestion above is to make lots of shorter duration images in light polluted skies and longer exposures in dark skies. But, you will be limited by the tracking ability of your drive and how good your polar alignment is. You should have NO star trailing using a wide angle lens even for exposures of several minutes if your polar alignment is good. 
To use longer focal lengths like 300mm, proper polar alignment essential to avoid star trailing, even at relatively short exposures. I would think the 300mmPF lens would be excellent as the Fresnel optics usually correct coma well(coma is your biggest enemy with camera lenses) and are light and compact.
Clip in light pollution filters are available for many camera models if you are imaging in areas with light pollution. They sit in front of the mirror in the camera so you can use them with any lens. However, they will rob you of a stop or two of light.
Hope this helps!
Scott
 

by Tim Zurowski on Mon Sep 17, 2018 1:44 am
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Scott, thank you very much for that in depth reply. That is exactly the kind of info I was hoping to get.  Now it has raised more questions :) I know I will have to get out there and try this stuff myself to get a better handle on it and to better understanding of it, but I am still not clear on the Flat frames. How will this be any different than using the lens cap for the Dark frames, if the T shirt is covering the lens? Do you have to shine a light through it or something? Won't the exposure be really dark with the shirt over it? Should I use the same exposure time that I used for the Light frames?

So if I stack say 30 frames, do I take 30 frames of each four types? This could be time consuming with a LOT of frames taken.

With regards to the polar alignment, is this a tough thing to do?  I believe the drive I am getting has a "built in illuminated polar scope". Will this not help with getting an accurate Polaris alignment? I suppose there will be tutorials online to accomplish this, but I guess I can;t see why it would be very tough to achieve?

When you say "areas with light pollution", are you talking about being right in town, or still out away from a city? I plan to drive 50+ km north of town for this, but if I can't do it closer with a light pollution filter I would consider getting one.  Any idea how much they cost?
Cheers
Tim Zurowski
www.timzphotography.com
 

by Scott Fairbairn on Mon Sep 17, 2018 10:08 am
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Tim Zurowski wrote:
Scott, thank you very much for that in depth reply. That is exactly the kind of info I was hoping to get.  Now it has raised more questions :) I know I will have to get out there and try this stuff myself to get a better handle on it and to better understanding of it, but I am still not clear on the Flat frames. How will this be any different than using the lens cap for the Dark frames, if the T shirt is covering the lens? Do you have to shine a light through it or something? Won't the exposure be really dark with the shirt over it? Should I use the same exposure time that I used for the Light frames?

So if I stack say 30 frames, do I take 30 frames of each four types? This could be time consuming with a LOT of frames taken.

With regards to the polar alignment, is this a tough thing to do?  I believe the drive I am getting has a "built in illuminated polar scope". Will this not help with getting an accurate Polaris alignment? I suppose there will be tutorials online to accomplish this, but I guess I can;t see why it would be very tough to achieve?

When you say "areas with light pollution", are you talking about being right in town, or still out away from a city? I plan to drive 50+ km north of town for this, but if I can't do it closer with a light pollution filter I would consider getting one.  Any idea how much they cost?



Sorry, I wasn't very clear in the procedure for flats. The point of using a T-shirt is to make sure the light is evenly distributed so any flaws such as vignetting will be imaged. It is easier to do with a long lens than a wide one. To take the image, cover the front element by placing a white cloth over it and use elastic bands to stretch and hold it wrinkle free. Point the lens at uniform light source such as the sky on an overcast sky, the sky shortly after sunset opposite to the sun, or even your computer screen with a white background(no desktop icons). With a 15mm lens, pointing at the sky doesn't work well due to light gradients across such a large area, so I use a computer and put the lens right against the screen(white cloth in place). 
Exposure is straightforward, just put the histogram bump in the middle with no clipping at either end. The point of flats is to correct for optical defects by using the flats images to compensate for corner shading(it will also remove dust blobs on the sensor). The exposure does not have to be the same as the lights, and in fact, I would not take them at a high ISO either; otherwise the noise may influence the flats. Note that the focus must be the same as when you made your night images(i.e. infinity focus) because optics perform differently at different focus points.
The number of frames is a bit subjective, and you will find many different opinions on the figure. Generally, around twenty flats each of darks and bias will be sufficient. Some people take the same number of support frames as lights. For darks, once I am finished imaging, I put the lens cap on, place the camera in a light-tight container or bag and using a computer or intervalometer, take the images while I pack up. As I alluded to before, you can build up a library of darks for different temperatures(say within 5 degrees of each other) and save them for the next session. You only need to take bias frames once, take them and keep them for the next session.
Polar aligning with the builtin polar scope should work well, especially at shorter focal lengths. I would get an app that tells you where to place Polaris on the alignment circle, I use PS Align Pro to say where to put it. A couple of suggestions. Make sure you do the polar aligning after checking tripod stability(hang a weight from the centre to increase stability) and getting your gear balanced and so on. I believe your tracker allows you to view through a plate? The reason is that lightweight units tend to shift a bit if you load it up after aligning. It's probably not that big a deal with wide angle lenses, but at 300mm and exposures of a few minutes, it will be. There are other solutions for alignment, but I'll leave that topic alone for now.
Light pollution is the bane of night sky photography. I'll put a dark sky link here, and you can check your locations with it. I'm not sure where you live, but imaging directly over a large city, even from fifty km away will show it. Ideally, you want to adjust your location, so you aren't shooting straight over the top of a city. Btw, the moon is considered light pollution too so ideally time your night sky shooting for nights when it isn't in the sky. A light pollution filter will typically cost a couple hundred dollars or so. If the light pollution isn't severe, I will try it without it.
One thing I forgot to mention is the problem with dew. Unless you are in a bone-dry area, dew will form on the front element in a relatively short time. Once it's on the lens, your shooting is over. The solution is a dew strap. It is a heated strap that you wrap around the barrel near or around the front element and runs on a 12v battery. You set it as low as possible(otherwise the heat will cause atmospheric effects), but it is essential if you plan to shoot outside for any length of time.
The proper focus is another critical item, and it's worth it to spend the time nailing it at the beginning, and double checking as the night progresses. Do not rely on the infinity mark on your lenses, even manual lenses. Use magnified live view and once you have it set, use a piece of painters tape to keep the focus ring from moving(turn off autofocus).
 

by Tim Zurowski on Mon Sep 17, 2018 12:08 pm
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Thanks very much again Scott, all good info BTW, my location is in my upper right posting info. :)

So another question: If I am using a star tracker and shooting 30+ frames, do you still need to take long exposures? I thought the reason for shooting all these frames was to reduce the noise, but if I shoot longer exposures with a tracking device, doesn't this mean I can shoot with lower ISO and thereby eliminate noise that way?

I know I will need to experiment with it, but how long are your typical exposures for M31?
Cheers
Tim Zurowski
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by Scott Fairbairn on Mon Sep 17, 2018 2:07 pm
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Tim Zurowski wrote:
Thanks very much again Scott, all good info BTW, my location is in my upper right posting info. :)

So another question: If I am using a star tracker and shooting 30+ frames, do you still need to take long exposures? I thought the reason for shooting all these frames was to reduce the noise, but if I shoot longer exposures with a tracking device, doesn't this mean I can shoot with lower ISO and thereby eliminate noise that way?

I know I will need to experiment with it, but how long are your typical exposures for M31?


Here's the link to the dark sky map. 
https://darksitefinder.com/maps/world.html#3/32.10/-84.20

Your exposure will be determined by the histogram. You have to get the left side off the left border otherwise you are clipping. So long as there is a clear separation of the base of the histogram from the edge, you are exposing correctly. My rule of thumb is the peak of the histogram should be around the 1/4 to 1/3 mark. Unfortunately, noise is a fact of life with astrophotography.
The shutter duration will be determined by the speed of your lens and ISO. There is a trade-off going to higher and higher ISO's just like there is with daytime shooting. It's nice to get the shutter speed low so noise is reduced, but increasing ISO increases the noise so a little experimentation is needed to find your comfort level. I rarely go over ISO 1600 for my modified DSLR(Canon 60D), it's performance is poor above that. 
M31 despite being big and bright is challenging due to the bright core and dim outer regions. The last time I shot it, I used a wide range of exposures. I shot it on a German equatorial mount with a telescope(slow f-stop) and guiding. But the exposures I used were across the board. As low as 30 sec to hold the bright core and all the way out to 900 seconds to lift the outer regions. I can post a link to it if you like.
It's hard to say what exposures you should try since it depends on the speed of the lenses you use, whether or not a light pollution filter is needed(I almost always use one), how good your alignment is, etc. 
I would suggest two sets of exposures, one set, with maybe ten images to expose the core properly, and then another, longer set. 
I'd try 15-30 seconds and see if it is blown. Then follow the histogram suggestions to determine your next series, and make it the longest series. 
I'd recommend staying wide field until you get comfortable setting up the mount and camera. Gradually work up in focal length until you get to 300mm. It can be a challenge just framing your intended target with long lenses, let alone tracking.
 

by Tim Zurowski on Mon Sep 17, 2018 2:23 pm
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Thanks for that link. Really displays how we humans have ruined this planet. One thing it doesn't say is which of the shaded areas we need to get to, to be in dark enough skies for Astro photography. Should I assume the grey to black areas?

I don't even have the tracking gear yet (shipped from B&H today) so at this point I think I am at a stage where I need to get out and do some trial and error experimentation, which I am sure will raise more questions.
Cheers
Tim Zurowski
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by Scott Fairbairn on Mon Sep 17, 2018 3:03 pm
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The grey to black area is better for sure. I have to deal with yellow areas for my home shooting, that's why I rarely take off the light pollution filter. Depending on how wide angle you are, you can still image with less than truly dark skies, but nothing beats good skies.
I would suggest a planetarium app like Sky Safari as well. It is helpful for planning an outing.
Good luck and let me know if you have other questions.
 

by Scott Fairbairn on Tue Sep 18, 2018 11:43 am
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Tim Zurowski wrote:
Thanks for that link. Really displays how we humans have ruined this planet. One thing it doesn't say is which of the shaded areas we need to get to, to be in dark enough skies for Astro photography. Should I assume the grey to black areas?

I don't even have the tracking gear yet (shipped from B&H today) so at this point I think I am at a stage where I need to get out and do some trial and error experimentation, which I am sure will raise more questions.




Here’s a link to a wonderful resource for learning astrophotography. Jerry Lodriguss has a wealth of knowledge and several inexpensive books that will make learning much easier. He has lens reviews as well. 
http://www.astropix.com/
 

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