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How do you determine the total time needed on a given target?

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#1 mjh410

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Posted 23 January 2021 - 10:32 PM

I have been searching here and elsewhere to find an answer to this question and I can't seem to come up with anything more than a rabbit hole of complex mathematical equations. That or my searches are not leading me to the correct answers. I have only ever taken untracked short 10-15 second exposure subs with a wide angle lens and DSLR for Milky Way images. I have been looking at many different options and after a lot of research and deciding to stick with my own camera gear for a while I decided to get a Star Adventure 2i Pro with a ZWO 30mm Guide Scope and ZWO ASI 120mm-S guide camera. I already have a good tripod for bird photography that I use with a gimbal head and my 150-600mm lens. I had debated on whether to get an EQ6-R Pro and use my camera gear until a later time when I can add a scope, but I decided that the Star Adventurer is quite capable and will give me a lot to do while I learn. So that is where I am and where I am getting started on soon once everything arrives. So I have been watching lots of videos and reading lots of thread. 

 

I have already purchased Charles Bracken's Deep Sky Imaging Primer and Allan Hall's Getting Started: Long Exposure Astrophotography books. I have not yet read them cover to cover, I am working on Allan Halls right now as it seems easier to digest and written in a manner well suited to a beginner.

 

With that in mind here is my question.

 

I understand this is always going to depend on what gear is being used and where in the world you are and what the light pollution is in that area. With that said is there any easy rule of thumb for how long the total exposure time should be for any given target?

 

I have seen examples tossed around various threads here with images of with a total time of 10 minutes, 30 minutes, 1 hour, 4 hours, and many more. Is there such a thing as too much time?

 

I am aware that the individual subs would be better if they were longer. So 10 x 1 minute subs are better than 40 x 15 second subs.



#2 jonnybravo0311

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Posted 23 January 2021 - 10:51 PM

OK... here are some rules of thumb for you :)

  1. Total integration time is king. How you get there is less relevant.
  2. There is no such thing as too much integration time.
  3. If your individual exposure is long enough to swamp the noise, it's long enough.
  4. Find the ISO value at which your camera gets the best performance. For example, the Nikon D5300 likes to be low ISO. My own G9 performs best at ISO 800. Some Canons work best at ISO 1600.
  5. Once you've figured out the ISO, then your exposure time should get the histogram to about 1/3 from the left. Far enough that you aren't losing faint details in the background noise, but not so far as to clip the bright stars.

Please note, these are very generalized. Your mileage may vary, these rules will not guarantee you great results, etc, etc, etc.

 

PS - don't listen to anyone who tells you 10 minutes of total integration time on a DSO is enough.


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#3 mjh410

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Posted 23 January 2021 - 11:04 PM

Thank you, that sounds like a great set of general rules to start with. I have not tested my camera yet for the best ISO but according to this website and an explanation I saw in a video it's likely ISO 400 is good for my camera, Nikon D850. So, that's where I was going to start.

 

https://www.photonst...Charts/RN_e.htm


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#4 jonnybravo0311

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Posted 23 January 2021 - 11:35 PM

Precisely the site I was going to refer you to smile.gif

 

Edit:

 

A few more things for you to chew on..

  • The heavier and sturdier the tripod, the better. If you can, hang some weight from the center.
  • The trackers (like the Star Adventurer and Sky Guider) perform best with focal lengths 200mm and under. With guiding, you'll get better results; however, that 600mm birder will likely be completely unusable.
  • Polar alignment is key to getting usable data. I highly suggest buying SharpCap Pro. It's $15 a year. Since you're going to need a laptop for guiding, it's a very worthwhile investment. Makes polar alignment extremely easy.

Edited by jonnybravo0311, 23 January 2021 - 11:45 PM.


#5 bobzeq25

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Posted 23 January 2021 - 11:39 PM

It's just a personal judgment.  More total imaging time is better, but the way it goes up is as the square root.  If you want an factor of 2 improvement, you need a factor of 4 more total imaging time.

 

Light pollution puts you in a hole, more total imaging time is a way to improve things.  IE in light polluted skies you need more total imaging time.

 

Optical speed (lower F number) helps a lot.  More photons is better.

 

My rough rule of thumb.  One hour minimum, two is better, four good.

 

This is an excellent discussion (the upfront part about data acquisition).

 

https://jonrista.com...duction-part-1/

 

"I am aware that the individual subs would be better if they were longer. So 10 x 1 minute subs are better than 40 x 15 second subs."

 

You'd think so, wouldn't you?  Seems intuitive.  One thing that this is not, is intuitive, so that's wrong.  Common misconception.

 

For a given setup, and given skies, there's a range of optimum subexposures.  So long as you're near that range, total imaging time is the big deal.  How you break it up into subs is far less important. 

 

There are lots of discussions here about how to determine that range of optimal subexposure times.

 

Bottom line.  You can argue about whether 30X2 minutes is better than 60X1 minute.  But the important thing is that _either_ 60X2minutes _or_ 120X1 minute will beat either one of the first two alternatives.  Big time.


Edited by bobzeq25, 23 January 2021 - 11:47 PM.

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#6 mjh410

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Posted 24 January 2021 - 12:28 AM

 

Precisely the site I was going to refer you to smile.gif

 

Edit:

 

A few more things for you to chew on..

  • The heavier and sturdier the tripod, the better. If you can, hang some weight from the center.
  • The trackers (like the Star Adventurer and Sky Guider) perform best with focal lengths 200mm and under. With guiding, you'll get better results; however, that 600mm birder will likely be completely unusable.
  • Polar alignment is key to getting usable data. I highly suggest buying SharpCap Pro. It's $15 a year. Since you're going to need a laptop for guiding, it's a very worthwhile investment. Makes polar alignment extremely 

Thank you again. I will look into that software. I have what is referred to as the holy trinity of lenses. They are all the Tamron brand. So I have the 70-200mm f/2.8 and the 15-30mm f/2.8 that I will be using for wide field and DSO. I have seen some people be successful with the 150-600 but I figured I'd start with the two others mentioned for now. As far as weight goes, my tripod is rated for 35 pounds, the Star Adventurer for 11, and with all the gear I listed as well as an L bracket on my camera for mounting the guide scope, and a red dot finder on the shoe of my camera I think it comes to just under 8 pounds with the heaviest lens I have the 150-600. I'm confident as far as weight and my tripod goes I think I'll be good.

 

This is the tripod I have and it has held my camera and the 150-600 with a gimbal head just fine for birds in flight and air shows.

 

https://www.amazon.c...e?ie=UTF8&psc=1



#7 mjh410

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Posted 24 January 2021 - 12:35 AM

"I am aware that the individual subs would be better if they were longer. So 10 x 1 minute subs are better than 40 x 15 second subs."

 

You'd think so, wouldn't you?  Seems intuitive.  One thing that this is not, is intuitive, so that's wrong.  Common misconception.

 

For a given setup, and given skies, there's a range of optimum subexposures.  So long as you're near that range, total imaging time is the big deal.  How you break it up into subs is far less important. 

 

There are lots of discussions here about how to determine that range of optimal subexposure times.

 

Bottom line.  You can argue about whether 30X2 minutes is better than 60X1 minute.  But the important thing is that _either_ 60X2minutes _or_ 120X1 minute will beat either one of the first two alternatives.  Big time.

I was aware that more total time is always more important. Perhaps what I had read was a question regarding stacking hundreds and hundreds of 1-2 second exposures vs a less quantity of longer duration exposures. The idea I got from it was that the longer the exposure the higher likelihood that the sensor is capturing that fainter data. In the short exposures there is that faint light that may not get captured simply because not enough photons hitting the sensor. Once you start opening up that exposure time I imagine that gap starts to narrow to the point that the difference may be negligible. 

 

The more that I think about it, I think that particular article or thread I had read was in fact comparing longer subs with those short 1-2 second untracked subs. So, as you said once you are near or exceeding that optimal range of exposure time the differences would indeed be negligible and total time is indeed king.

 

In any case I appreciate your feedback and I am looking forward to my new mount arriving and hopefully the skies clearing out so I can start practicing.


Edited by mjh410, 24 January 2021 - 12:35 AM.

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#8 fewayne

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Posted 24 January 2021 - 12:36 AM

Welcome Matt!

 

I'm not familiar with those particular lenses, but if you notice problems with coma or other aberrations, you should know that usually prime lenses work better than zooms for astro. In fact quite old prime lenses can work, and they can be manual-everything (can't use autofocus or exposure anyway), which makes them less expensive.

 

Astrophotography happens to be like a torture test for lenses -- very high contrast, very high spatial frequency, problems at the edges or corners much more obvious than with terrestrial photography. If you're getting good results with your current glass, great!


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#9 dswtan

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Posted 24 January 2021 - 12:51 AM

The idea I got from it was that the longer the exposure the higher likelihood that the sensor is capturing that fainter data. In the short exposures there is that faint light that may not get captured simply because not enough photons hitting the sensor. Once you start opening up that exposure time I imagine that gap starts to narrow to the point that the difference may be negligible.

Sure, but the longer the exposure, the more chance you're clipping the bright objects in the image (typically stars), so it's a tradeoff. What is counterintuitive is how far to the left of the histogram you can be and still get good images. Bob has a nice post on this where the subject is completely invisible until the stretch.


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#10 mjh410

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Posted 24 January 2021 - 12:52 AM

Here is an example of my 150-600mm on the moon in April of last year. I can't remember if I used my tripod or if it was handheld. I know the moon is not a DSO, but I was happy with what I could get with this camera and lens with a single image no stacking and mild post processing.

 

_MHF1907 April 07, 2020 For Web.jpg

 

 


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#11 mjh410

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Posted 24 January 2021 - 12:52 AM

One more

 

MHF_2077 April 08, 2020 For Web.jpg


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#12 jonnybravo0311

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Posted 24 January 2021 - 01:15 AM

The moon is absurdly bright. It's easy to snap off a photo, even handheld. We're talking many orders of magnitude brighter than even the brightest DSO.

 

I use my G9 with the 50-200mm lens and a 2x teleconverter to shoot the moon. I'm typically stopped down to f/11 and still need to be around 1/100s exposure time at ISO 200. Point your camera to the Andromeda galaxy and take that same shot... you'll end up with a very black image smile.gif.

 

Very nice, shots of the moon, by the way!


Edited by jonnybravo0311, 24 January 2021 - 02:25 PM.

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#13 Huangdi

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Posted 24 January 2021 - 01:32 AM

Sure, but the longer the exposure, the more chance you're clipping the bright objects in the image (typically stars), so it's a tradeoff. What is counterintuitive is how far to the left of the histogram you can be and still get good images. Bob has a nice post on this where the subject is completely invisible until the stretch.


There's a difference between showing a linear stack and a non-linear DDP stretch your DSLR does on a subexposure. Because if the DDP of your camera doesn't show much, if anything at all, you have severely underexposed your image.

#14 GR-Amateur

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Posted 24 January 2021 - 05:45 AM

https://youtu.be/3RH93UvP358

A great insight. This video helps a lot, not just to get a hint on the main principals of exposure time but on many other aspects.

#15 bobzeq25

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Posted 24 January 2021 - 10:59 AM

I was aware that more total time is always more important. Perhaps what I had read was a question regarding stacking hundreds and hundreds of 1-2 second exposures vs a less quantity of longer duration exposures. The idea I got from it was that the longer the exposure the higher likelihood that the sensor is capturing that fainter data. In the short exposures there is that faint light that may not get captured simply because not enough photons hitting the sensor. Once you start opening up that exposure time I imagine that gap starts to narrow to the point that the difference may be negligible. 

 

The more that I think about it, I think that particular article or thread I had read was in fact comparing longer subs with those short 1-2 second untracked subs. So, as you said once you are near or exceeding that optimal range of exposure time the differences would indeed be negligible and total time is indeed king.

 

In any case I appreciate your feedback and I am looking forward to my new mount arriving and hopefully the skies clearing out so I can start practicing.

I'm not talking about 1-2 second subs.  You need to be "in the ballpark".  With a DSLR it's pretty easy, if you can get the histogram on the back of the camera.  Expose enough that the obvious peak (which is light pollution) is about 1/3 over from the left.

 

What other people do has little or no relevance for you.  You need to set subexposure based on your equipment and your skies.

 

You talked a lot about weight.  Weight is just one factor.  It knocks out really bad setups.  It doesn't qualify good ones.

 

Just as important is focal length, which magnifies tracking errors.  With my iOptron Skytracker camera tracker, the limit is about 200mm.  Otherwise, I can't track well enough to do proper subexposures.

 

DSO AP has little in common with terrestrial photography.  You are trying to tease out a tiny signal from a sea of all kinds of noise.   While tracking a moving target.   Here's an excellent book to help you start, and it's downloadable.

 

https://www.astropix...bgda/index.html


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