Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Formula to Calculate max exposure time.

  • Please log in to reply
15 replies to this topic

#1 niktnowy

niktnowy

    Lift Off

  • *****
  • topic starter
  • Posts: 22
  • Joined: 13 Feb 2020

Posted 24 May 2020 - 07:25 PM

Hello everyone.

 

I just got my first new ZWO ASI 294 MC Pro camera. 

 

My current setup is:

Mount: Sky-Watcher HEQ5- Pro

Imaging Camera: ZWO ASI294MC Pro 

Guide Camera: ZWO ASI120MM-Mini

Telescope: Explore Scientific ED102 FCD100 CF

Guide Scope: ZWO 30F4 Miniscope

Field Flattener  Hotech 2" SCA-FFT58

 

Software

Image Acquisition: APT

Guiding: PHD2

 

I have two questions:

 

1. Is there a formula or calculator that will allow me to find optimal time exposure /lenght for different objects- that will take into consideration - location, equipment, sky condition?

 

2. ZWO ASI 294 came with sets of different thickness  washers-spacers.  I can't find any  info about them? Are they supposed to be used  for fine focus adjusting? 

 

Thank you



#2 ks__observer

ks__observer

    Apollo

  • *****
  • Posts: 1,324
  • Joined: 28 Sep 2016
  • Loc: Long Island, New York

Posted 24 May 2020 - 08:20 PM

For exposure time:

Look up on Cloudy Nights about swamping read noise.


  • niktnowy likes this

#3 maxsid

maxsid

    Vostok 1

  • *****
  • Posts: 160
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 24 May 2020 - 08:24 PM

Hello.

I also have an ASI294.

1. Exposure time will depend on many factors. Darkness of the site. Moon on/off. Gain of the camera, etc.

Just take a test shot and look at the histogram. You want the histogram peaks to be in the first half of the range.

My exposure times with AT72EDII and ASI294 (Gain 200, offset 20) with a LPro filter vary from 60 to 360 sec for a single frame.

With long exposures you might have stars over-saturated but if you don't care about stars (but nebulae and galaxies) then it's OK.

The total time is more important. You can shoot with 60 sec always but you would have hundreds of exposures to process.

With 360 sec sub-frames you'll have 5 times less frames to process.

Lower exposure time for sub-frames give you better dynamic range but strain your computer when stacking.

In a dark site you'll normally need longer exposure sub-frames.

2. I don't use those. I guess they are used  for fine focus adjusting.


  • niktnowy likes this

#4 niktnowy

niktnowy

    Lift Off

  • *****
  • topic starter
  • Posts: 22
  • Joined: 13 Feb 2020

Posted 24 May 2020 - 10:47 PM

Thank you. I am in North East US - Tonight I had my first chance to try this camera for the first time.  For some reason - ZWO ASI 294 - live view was very choppy.  From what I've read there are some adjustment to be made but unfortunately clouds rolled in and I will have to try it tomorrow again.  

Thank you again for your help



#5 maxsid

maxsid

    Vostok 1

  • *****
  • Posts: 160
  • Joined: 11 Sep 2018
  • Loc: Sunnyvale, CA

Posted 25 May 2020 - 08:02 PM

What software do you use for capture?

"Live View" will be choppy. The frame rate of this camera is pretty low.

You should take single exposures.

At gain 200 you should be able to see stars (when it's dark) with 1 sec exposure.

Higher gain - more signal, less noise but also lower dynamic range (bad).

Lower gain - less signal, more noise but also higher dynamic range (good).

 

For ASI249 the ideal Gain is 120 (I use 200 for some historical reasons - should switch to 120 - will require slightly longer exposure times).



#6 the Elf

the Elf

    Vanguard

  • *****
  • Posts: 2,447
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 26 May 2020 - 12:52 PM

This might answer parts of your question:

https://www.youtube....h?v=3RH93UvP358


  • JPKellysr likes this

#7 Noah4x4

Noah4x4

    Soyuz

  • *****
  • Posts: 3,525
  • Joined: 07 Apr 2016
  • Loc: Colchester UK

Posted 26 May 2020 - 01:22 PM

I have an ASI294 and struggled with optimum exposures and Gain until I turned to Sharpcap's Sensor Analysis and Smart Histogram, plus Brain tool.

 

I did a local sensor analysis, but this isn't strictly necessary as the data is already in Sharpcap. These are seriously good tools.


  • niktnowy likes this

#8 Phil Sherman

Phil Sherman

    Gemini

  • *****
  • Posts: 3,063
  • Joined: 07 Dec 2010
  • Loc: Cleveland, Ohio

Posted 26 May 2020 - 11:52 PM

For imaging at any site except the very darkest, the maximum exposure time you should use is one that has the background sky peak in the histogram separated from the left edge. Longer exposures significantly decrease the dynamic range of the image but can gain a reduction in noise. If you're imaging bright objects, then the longest exposure you should use is one that does not saturate the pixels containing the brightest object. This is most likely to have the background sky peak buried in the left edge of the histogram. Objects with a large range of brightness may require imaging using HDR (High Dynamic Range) techniques to avoid blowing out the bright areas while keeping good detail with low noise in the dim areas. HDR requires additional processing steps.

 

In summary: it's a compromise depending on what you're imaging.


  • niktnowy likes this

#9 niktnowy

niktnowy

    Lift Off

  • *****
  • topic starter
  • Posts: 22
  • Joined: 13 Feb 2020

Posted 27 May 2020 - 02:38 PM

Thank you. Love your input.

#10 Peregrinatum

Peregrinatum

    Apollo

  • *****
  • Posts: 1,170
  • Joined: 27 Dec 2018
  • Loc: South Central Valley, Ca

Posted 27 May 2020 - 03:27 PM

great video, if you don't want to watch the whole thing your answer for exp time is at the 49' mark

 

https://www.youtube....h?v=3RH93UvP358



#11 cuivienor

cuivienor

    Vanguard

  • -----
  • Posts: 2,022
  • Joined: 06 Oct 2010
  • Loc: Tokyo, Japan

Posted 28 May 2020 - 04:16 AM

I have a simplified version of that video that explain from the start gain, offset, well depth, and dynamic range, and how that affects exposure time. Then I go into actually choosing a proper exposure time, using intuition, or using an optimal exposure time calculator (available for free in N.I.N.A. for example):

 

https://www.youtube...._VIds1DIap2zx3f

 

I have gotten very good feedback on these videos - hopefully it can be useful. Also, it will help understand why for the 294, you will typically want to use gain 120 (or if you are paranoid like me, gain 121).

 

I have another ongoing series about sources of noise in astrophotography. In an upcoming episode, I do the same optimal exposure calculation based on a calibrated frame, and my camera specs.



#12 Wileyglance

Wileyglance

    Lift Off

  • -----
  • Posts: 13
  • Joined: 23 May 2020

Posted 28 May 2020 - 05:48 PM

So I have followed those videos and others and for say a zwo 1600mm under my sky conditions an exposure of say 120 sec at 139/21 is optimal. My question is do many of those 120 sec stacked yield fine detail (nebulae and galaxies) and less noise or would 4 minute (overexposed) subs give more detail but blow out some bright parts/starsand be “less efficient “. Personally I’m not very concerned with efficiency but best final image.

Edited by Wileyglance, 28 May 2020 - 05:52 PM.


#13 ks__observer

ks__observer

    Apollo

  • *****
  • Posts: 1,324
  • Joined: 28 Sep 2016
  • Loc: Long Island, New York

Posted 28 May 2020 - 06:50 PM

What scope, aperture, and f-ratio do you use?



#14 the Elf

the Elf

    Vanguard

  • *****
  • Posts: 2,447
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 28 May 2020 - 10:52 PM

The impact of sub exposure time is very low compared to a) going to a darker place and b) adding more over all exposure time. If you listen to Dr. Glover's video that is the message. The case he makes up is somewhat academic: if you have a given over all time there is some optimal exposure time that is just a little better than another one. My real life situation is this: if the image is poor after one night I put up the scope again and add another night. And/or try again when there is less moon. I was asked the fine detail question recently. I just copy my answer below. Hope it helps to put image quality in a wider context.

 

====================================

Resolution has a lot of limiting factors.

 

1) the physical limits given by aperture. Light is this famous particle wave dualism. In it's wave nature it is diffracted whenever there is an obstacle in the way like waves on a pond are when there is something in the water. The circular opening of the scope causes diffraction and limits the largest thing you can see. This depends on the wavelength. It is worst for red and best for blue. What you see in the technical data is for green unless otherwise stated.

https://astronomy.to...pe_capabilities

Dawes and Rayleigh are different ratings, basically two stars are fully divided (i.e. it is black between) or just can be distinguished (i.e. two maxima).

 

2) Seeing. Air is not vacuum and it has got an optical density that varies with it's physical density which varies by height and temperature. Here the red light is slightly less affected than the blue light. Seeing depends mostly on weather, mainly the jet stream high in the atmosphere but on near ground effects like hot areas of concrete and cold areas of water and wind going up or down a mountain. As far as optical resolution is concerned this is often the limit for ambitious amateurs. Seeing can change within seconds that is why planetary has got a higher resolution when doing lucky imaging.

 

3) Optical quality of the lenses and mirrors. If a mirror is not as it supposed to be the resolution is limited. This happened to the Hubble Space Telescope and it needed a corrector. A telescope is called "diffraction limited" when the mirror or lens offers more resolution than the diffraction.

 

4) Chromatic aberration, lateral and longitudinal. The colors to not come to focus in the same place in refractors and so a detail in one color is not resolved as well as in another color. Mirrors do not suffer from it and good triplets also work well.

 

5) Imaging scale. This is how many arcsecs you have on one pixel. If you use a 300mm focal lenght telescope and 6 micron pixels seeing is not your problem. Drizzle can shift the border a bit further but in general you should have a good sampling. Nyquist and Shannon found you need twice the sampling rate than the highest frequency in the signal. In spatial sampling where you have rows, columns and diagonals 3 times higher is better. So, if your seeing and diffraction limit allows for 2 arcsecs you want 0.66 arcsec/pixel in a monochrome camera, even a lower value in a color camera.

 

6) The Bayer pattern if you do OSC. As only 2 out of 4 pixels are green and only 1 out of 4 is blue or red a color sensor resolves less than a monochrome sensor. There often is a spatial low pass (blur in other words) on color filters to avoid that a point light source like a star can illuminate just one physical pixel. The light is intentionally scattered to a few pixels. Some cameras have it, some don't.

 

7) Noise. Noise eats up the fine structure first. Even if your seeing is perfect, your scope can resolve what you want to image, the imaging scale is well chosen and you have no trouble with CA fine detail is just invisible because the signal to noise ratio (SNR) was too low. You need very long over all time to push the noise level down and find the fine detail. Denoise strategies do not work here. Noise reduction is always sacrificing resolution.

 

8) Tracking. For planetary the exposure times are so short that tracking is not important. For long DSO images it is pretty obvious that the image shows motion blur if the tracking does not follow with the accuracy you want to resolve overt the whole time of one exposure. Auto guiding comes into play here as well as your mounts capabilities. There are two reasons why your stars drift: polar alignment error and periodic error.  Recently I posted this unguided experiment:

https://www.cloudyni...experiment-m13/

It happened to have a drift of 0.75 pixels per minute. One minute shots were just ok but not good. (I have near 2 arcsec FWHM in perfect moments, 3 on average. Adding 0.75 is quite a loss of resolution.)

Without guiding you need perfect polar alignment and the periodic error will kill you (unless you go for a $8k+ mount with precision encoders like 10 micron). When guiding the guider takes care of that. In theory. If your mount does not react on a command because of backlash, sticktion or poor balancing or if it overshoots guiding may not succeed or make it even worse. That is why just guiding a poor mount does not lead to good resolution. It is primarily a mechanical system that need to be well engineered, well crafted and well adjusted. All guide program parameter cannot heal what is wrong in the mechanics. If the mount reacts soon and moves smooth and does not overshoot you can guide with low aggression settings because over a period of many seconds only fractions of a pixels need to be corrected. If you have a poor mount that just comes with bumpy or loose bearings huge errors occur within second and you need to use short guide exposures. If that is combined with poor seeing you are chasing the seeing.

 

9) Flex in the telescope. The telescope is long and heavy. It rotates during the night and gravity pulls all parts down. In my RC6 the rear cell tilted in relation to the tube and the secondary mirror because the tube is weak and the rear cell is a (quote) "brain dead" design. You can have the best guide scope attached bomb save to the tube and fail if the inner parts move. This is one reason to use an OAG. It is behind the scope and can correct the internal flex as well. The second reason to go for OAG is saving weight especially if you are at very long focal lengths and need a long guide scope as well.

 

10) perhaps, not sure if I want to count it here. You have to do deconvolution in the post processing to correct for the unavoidable error like diffraction. This only works well if you have low noise data, good tracking and a reasonable resolution. In my images it makes about half an arcsecond.


  • JPKellysr likes this

#15 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 20,137
  • Joined: 27 Oct 2014

Posted 28 May 2020 - 11:25 PM

Hello everyone.

 

I just got my first new ZWO ASI 294 MC Pro camera. 

 

My current setup is:

Mount: Sky-Watcher HEQ5- Pro

Imaging Camera: ZWO ASI294MC Pro 

Guide Camera: ZWO ASI120MM-Mini

Telescope: Explore Scientific ED102 FCD100 CF

Guide Scope: ZWO 30F4 Miniscope

Field Flattener  Hotech 2" SCA-FFT58

 

Software

Image Acquisition: APT

Guiding: PHD2

 

I have two questions:

 

1. Is there a formula or calculator that will allow me to find optimal time exposure /lenght for different objects- that will take into consideration - location, equipment, sky condition?

 

2. ZWO ASI 294 came with sets of different thickness  washers-spacers.  I can't find any  info about them? Are they supposed to be used  for fine focus adjusting? 

 

Thank you

Here's a relatively easy formula that's good enough for starters.

 

Shoot a light and a bias (or dark flat).  Look at the histogram of the light.  Several imaging/processing programs will display it.  There'll be an obvious skyfog peak.  Note the ADU value on the x-axis.  Subtract the ADU value of a bias.

 

Using the second graph down on this page, convert ADU to electrons.

 

https://astronomy-im...294mc-pro-color

 

You ave a number.  Then, using the 4th graph down, get the read noise of the camera.  Square it.

 

You want the first number to be 5-10 X the second, anywhere in between is "in the ballpark".  Once you've done the calculation once or twice, it's pretty easy. 

 

Once you're there subexposure time is not very important.  Somewhat contrary to your intuition, you capture dim detail with long total imaging times, not long subexposures.  Shoot more subs.  <smile>

 

If you want to know the "why" of all that, there's a chapter in this book.  But, fair warning.  It's not an easy read, until you have substantially more experience.  It finishes up by discussing when skynoise equals 5-10 times read noise squared is not optimal, and what to do about that.  Not something you need to worry about now.  Shoot more subs. 

 

https://www.amazon.c...h/dp/1138055360

 

The spacers are used when you use a flattener on your telescope.  In order for that to work, the flattener to camera sensor spacing must be precisely adjusted to within 1-2mm.  Most of us have a drawer full of spacers.  <smile>

 

Welcome to astrophotography.  The good news is that you'll never, ever run out of things to learn.  <grin>  This book is an excellent place to start.  It does not require experience, although you'll reread it as you get more.  My copy is well worn.  <smile>  Books are much more useful than short posts here, things are just too complicated.

 

https://www.amazon.c.../dp/0999470906/


Edited by bobzeq25, 28 May 2020 - 11:29 PM.

  • Seaquel47 likes this

#16 niktnowy

niktnowy

    Lift Off

  • *****
  • topic starter
  • Posts: 22
  • Joined: 13 Feb 2020

Posted 30 May 2020 - 02:02 AM

Thank to all for your amazing input. 


  • bobzeq25 likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics