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Newbie Question: Photo Stacking and One Shot Color (OSC) Camera's?


Jaime
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Hello all,

 

As a newbie I'm trying hard to embrace the whole notion of photo stacking. But as we all know, all camera's have a shutter lifespan. My camera for example has a rated lifespan of 150000 actuations.

 

In all of the research I've done on Photo Stacking, the one common denominator is to take "lots and lots" of shots. One well-known Astrophotographer said he typically captures 100 Light Frames, 75 Dark Frames, and 75 Flat frames. That's 250 shutter actuations per target. And let's say you're shooting 3-5 targets that night. Now multiply each target by 250 shutter actuations, and it becomes easy to see how in Astrophotography, your shutter count will grow rather rapidly. 

 

While I do understand that my camera isn't going to last for ever, I also don't want to expedite it's demise into the camera graveyard.       

 

With that in mind:

 

1. Is Photo Stacking still needed for images captured with a One Shot Color (OSC) camera?

2. If so, will less shots (shutter actuations) still produce great results? 

3. Do OSC's even have shutter's / shutter lifespan's? 

4. Pros and Cons of OSC's?

 

Thanks for your input. 

 

Regards,

Jaime

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Hi Jaime,

I rarely aim for more than one target in a session unless it goes behind a tree and another target comes into view. Then the next clear night I will go back to target 1 again.

The average number of shots I get per session is around 80 x 6 minute long subs but this really is dependant upon the time of the year in my latitude (53 degrees North)

 

On targets like Orion Nebula I would take 30 second subs as it is quite bright and you can easily blow out the core or the bright stars can blow out the entire image. So in that case I would do maybe a few hundred 30 second subs. When stacking that you still get much of the faint detail such as the stacking process improves the signal to noise and still brings out the detail but the bright stars are easily controlled.

 

Below is an example of such a shot. The bright star towards the bottom left (you can actually see the companion star right next to it which is normally missing due to being over exposed) is notorious for blowing out and spoiling the resultant picture. In my case it didn't due to the short exposures. This was produced with a mono camera and 2 filters (Ha and O3) to produce an image called an HOO which is Ha mapped to Red, O3 mapped to Green and Blue

 

I only had a short time on this due to it setting in the sky and it comprises a total of 189 Ha subs and 70 O3 subs. But I could have quite easily spent 500 shots on each filter, time permitting, to capture much more detail and improve the signal to noise ratio.

image.thumb.jpeg.4dc3112169beedeb518565dda8526ce3.jpeg

 

Here are a few answers

 

1. Is Photo Stacking still needed for images captured with a One Shot Color (OSC) camera? Yes amongst many other things it improves signal to noise ratio

2. If so, will less shots (shutter actuations) still produce great results? Good results can be obtained but DSLR cameras in general are not as good as dedicated cooled astronomy cameras (see below)

3. Do OSC's even have shutter's / shutter lifespan's? Your Canon is a OSC camera and uses exactly the same technology as many dedicated astronomy cameras (CMOS sensor). The main difference is that yours has a mechanical shutter whereas the vast majority of astronomy cameras (like cell phones) use an electronic shutter. 

4. Pros and Cons of OSC's? There are many highly debated pros and cons and this has been mentioned many times on various forums so it may be best to Google rather than me regurgitate.

 

Dark frames.

There is one major thing that a dedicated cooled astronomy camera, whether OSC or mono, can bring to the party. You can take a sequence of darks at the start of a season and use the same darks all year. You just have to make sure that the lights match the temperature and exposure length/offset. You can then refresh the darks the next season. This is because the sensor itself does age and may develop more hot or cold pixels just through usage. Generally a year is ok to reshoot darks. So you do not have to take darks after every session.

 

Flat frames.

You do have to take flats after every session unless you have not altered anything in the optics path (rotated the camera relative to the scope for instance). However flats only take a few minutes to do, don't need to be cooled and don't have to be done at night time. Just make sure you don't break down the optics until after you have shot the flats.

 

Bias frames.

Some people don't even bother with bias frames. I do but again these only have to be done once at the start of the season.

 

Dark flats.

Some people shoot dark flats. I personally don't and there are some pros and cons to that process. Google that or search in this forum as I know there are many advocates of it.

 

IR Cut filters in DSLR cameras

The disadvantage that DSLRs generally have is that they have an IR cut filter in place that seriously limits the amount of data coming from emission nebulas which glow in Ha (Hydrogen Alpha). Most people either have the filter professionally removed but that means that terrestrial photos are compromised. They will then get a pink glow on the pictures and autofocus might not work.

 

A very long answer and other people will chime in but hope this helps.

 

 

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I was going to come in here and offer my sage wisdom but Terry's pretty much covered it all. 😄

 

My only add would be I've never used as many as 75 flats or dark/Flats, I max out at about 30 each but have gone as low as 20 each with no obvious difference in effect.

 

Terrys also right about the dedicated cooled camera and taking darks for a whole season. However, when I shot DSLR (I still do sometimes) I would build a dark library and use it for months. My camera was a canon and holds a temperature record in the exif data. Now, I expect i'll be told that this is wrong because its not a sensor temperature, but I worked on the basis that even if that temp was not the actual sensor temperature, it still had to be giving me a relative camera and sensor temperature record. So in order the to deal with the darks & temp thing I would build a darks library as follows,

 

1. I looked at the exif data from various sessions to see what the normal exif temperature showed. I found that on most winters nights the temp would range between 12 and 15 C. On the very coldest nights (not so often in my location) it could go down to 5C. On a warm summer night it could go to 35C. I figured if I had the same exif temp on a dark as alight then the sensor would be close enough in terms of temperature.

2. I rigged up an insulated cool box with some ice packs, stuck it out in the shed on a cool day and found that, with judicious placement of the camera,  it would deliver camera exif data in the range of 12-15C. If I stuck it in the freezer (very understanding wife here!) under a bag of frozen peas (extremely understanding wife!!), I'd get the 5C +/-2. and for the hot summer night variety i just did it on the kitchen table.

3. I picked a range of times and ISO (On my canon 700D I almost invariably used ISO 800 and 120 0r 180 second) set up a sequence in backyard EOS (better than APT cause it shows a realtime exif temperature) and let it run away. I have a small temp/humidity sensor I place in the box that i can monitor from my phone so that when the temp starts to rise in the box I can stop the sequence and replenish the ice packs. But I could usually get 6-8 hours on images which was well enough.

4. I'd throw in a few extra images at the start of each exposure time series because the temp would increase with the new longer exposure but would stabilise after maybe 4/5 images.

 

I've been told that that is way too much work, but for me its better than trying to shoot flats at the same ambient temp of the camera every session. But what else would you do on a cold cloudy night? And it seemed to work!

 

I hope this helps and good luck

 

David

 

 

 

Edited by Dmack1
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  • 2 weeks later...

Flats you need 15 tops, Adam Block did a video on it.  The point of a flat is to capture the anomolies, not try to reduce any noise.  If you capture Dark Flats (Flat Darks.. potato tomato) these do need to reduce noise so the more the merrier.

my standard are:

flats 15

dark flats 35

 

I use a masterBias with my OSC rather than a masterDark; never CMOS chips have very little to no thermal noise or 'amp glow' and as such no need for Dark Frames.  This is where I use a MasterBias instead.

 

OSC is a big jump from DSLR.. I moved over about 3 years ago.  The pro of a DSLR is you are pretty much set up ready to go.  OSC you need a PC and software AT the scope to control the camera.  If you go the Tec cooling route, you need 12v power available for it as well.

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