Exposure Time

[Pete75 (Pete)]

So im starting to work my way through this maze of information. I have the SVBONY 503 80mm with flatenner. So acodring to my calculations my F number is 5.6. So Is there a difference between a 3 minute exposure at lets say 800iso or 3x1minute exposures. Im shooting with the dslr and I realize that the longer exposure will introduce noise and lets say tracking is perfect. I hope this is making sense, basically why would I chose a 2 minute exposure over 2x1minute exposures.

Cheers Legends

[AstroViking (Steve)]

Hi Peter,

Before the experts get in here, these are my two cents worth.

Longer exposures allow you to take fewer (overall) images for a quicker processing (stacking) time, but at the cost of potentially higher thermal noise in your camera. Your guiding also has to be better than the arc-seconds per pixel size of your camera. You also open up the possibility of capturing more light pollution and washing out the contrast between your target and the background.

Shorter exposures give less thermal noise per image but at the expense of longer processing (stacking) times. It also allows for slightly worse tracking, and doesn't capture as much light pollution.

The general consensus is that overall imaging / exposure time is what counts, not the number of individual images.

Given you're using a DSLR (presumably without any filters) in a reasonably dark location, then I would go for shorter exposures in the summer to try to keep thermal noise down, and longer exposures in the winter because heat issues aren't as great. Assuming you keep ISO and everything else the same...

Cheers,
V

[Pete75 (Pete)]

Originally Posted by AstroViking

Hi Peter,

Before the experts get in here, these are my two cents worth.

Longer exposures allow you to take fewer (overall) images for a quicker processing (stacking) time, but at the cost of potentially higher thermal noise in your camera. Your guiding also has to be better than the arc-seconds per pixel size of your camera. You also open up the possibility of capturing more light pollution and washing out the contrast between your target and the background.

Shorter exposures give less thermal noise per image but at the expense of longer processing (stacking) times. It also allows for slightly worse tracking, and doesn't capture as much light pollution.

The general consensus is that overall imaging / exposure time is what counts, not the number of individual images.

Given you're using a DSLR (presumably without any filters) in a reasonably dark location, then I would go for shorter exposures in the summer to try to keep thermal noise down, and longer exposures in the winter because heat issues aren't as great. Assuming you keep ISO and everything else the same...

Cheers,
V

Ok cheers that makes sense.

[Startrek (Martin)]

Years ago I used a Canon DSLR in both 6” and 8” Newts for over 3 years under City suburban Bortle 8 heavy light pollution and rural Bortle 3 dark skies

First find your cameras recommended ISO sweet spot from the attached link
The ISO sweet spot is generally mean value to use which provides good dynamic range across various exposure times.

http://dslr-astrophotography.com/iso...rophotography/

Exposure times with DSLR’s are tricky but the best way to get a suitable exposure time on an object is look at your Histogram . Generally the peak should be no more than a third or around 25 to 30% along the graph.
Obviously your imaging system has limitations to what object you can image in regard to FOV and magnitude or brightness.
For example: take an image of say M42 with 1min , 2 min and 3 min exposures and look at where your Histogram sits. This will give you a good starting point. If your Histogram peak ends up at 60% or 70% along with say 3 min subs then reduce down until the peak sits around 25 to 30%.

Unless you have a dedicated Astro DSLR like the Ra , most DSLR’s were designed for general photography, not deep space astrophotography, but we learnt how to use them effectively for Astro.

DSLR’s are obviously uncooled and exhibit high thermal noise from the sensor electronics. So Signal to Noise ratio is severely affected. We are always trying to increase Signal and decrease Noise, this is difficult with a non cooled camera like a DSLR but the following can improve your outcomes -

1/ Find ISO sweet spot
2/ Use your Histogram to ensure exposures peak at no more than 25 to 30%
3/ Use separate Dark frames or cameras internal long exposure noise reduction setting ( this will reduce thermal noise ) But not both
4/ Use other Calibration frames like Flats , Dark Flats and Bias to improve quality and evenness of image background
5/ Stacking your frames ( stacking improves SNR and is the best way to reduce noise ) Generally the more you stack , the more improvement in SNR. Stacking hundreds of subframes is not uncommon with DSLR’s. Free stacking software like Deep Sky Stacker and ASTAP are excellent stacking programs for DSLR raw frames (CR files)
6/ Dithering your subframes ( most imaging acquisition software has a dithering command which usually is performed by your guiding software eg: PHD2 ) Dithering reduces and can eliminate fixed pattern type noise in your images.
7// Processing techniques . Most Astro processing software have tools for noise and gradient mitigation.
8/ Try to image during the New Moon period ( The Dark ) This will greatly reduce your Skyglow background and noise.

If you have a Canon DSLR below are the recommended settings for Deep Sky Astrophotography ( by Jerry Lodriguss, one of the best DSLR Astrophotographers in the US )

Canon EOS DSLR Astro settings for prime focus with telescopes at focal lengths +500mm ( Canon EOS camera’s manufactured from 2004 to 2014)

Exposure - set to Manual
Autofocus - off or Manual
Grid display - off
Aspect ratio - 3:2
Screen colour - 1 or preferred
ISO - sweet spot is 800 for Canon 600D
Aperture - aperture is fixed through telescope
Shutter speed - Bulb with remote shutter release
White Balance - set to auto white balance AWB ( if Camera has been Astro modded you have to set a “custom” white balance )
Drive - set to single shooting
Colour Space - set to RGB
Image file - set to Raw + JPEG
Image review - turn off
Metering mode - set to Evaluative
Exposure compensation - set to 0
LCD Auto off - set to disable
Live View Shoot Function - enable
Live View Exposure Sim - enable

Custom functions -
ISO expansion - set to off
Exposure level increments - set to 1/3 stop
Long Exposure Noise reduction - off if shooting darks
High ISO speed noise reduction - off if shooting darks
Highlight Tone Priority - disable
Auto Lighting Optimiser - disable
Auto focus assist beam firing - disable
Auto power off - set to 2 minutes
Auto rotate - off
Built in flash - turn off
Red eye reduction - turn off
Dust delete data - set to none or turn off

Picture Style -
Sharpness 0 or halfway on slider
Contrast 0 or halfway on slider
Saturation 0 or halfway on slider
Colour tone 0 or halfway on slider
Mirror lock up - disable

Note : Some older Canon cameras may not have some of the above settings so ignore them

Steve’s post is excellent, just thought I would add some more info and my experiences in my earlier days of Astrophotography with my DSLR

Jerry Lodriguss DSLR Astrophotographer supremo website is astropix.com ( worth a visit , I bought all of his ebooks when I first started using my DSLR )

Hope others chime in as well. Theres quite a lot of information online too !!

All the best
Martin

[Luis]

I don't know the technicalities well, but if you separate the different shots and then stack them with Photoshop or something similar, the noise will be much less. Normal rules of photography do not apply in astrophotography; everything is more complicated. Initially, you'll be dazed, but when you learn, it is a breeze."

[ChrisV (Chris)]

All the below, plus

Every time you take a shot a certain amount of read noise is generated. This read noise depends on the camera and the gain being used. For most DSLRs this can be quite considerable compared to CMOS astrocams. Some websites have this listed, or sharpcap has a routine to calculate this and more.

So, for a certain total exposure, the total read noise will be less if you take longer (and therefore fewer) sub-exposures as the read noise adds up. But one of the downsides is that longer exposures will lead to saturation of bright stuff (stars).

What many try to do is have a long enough exposure so that the background level 'swamps' the read noise. One such criterion is background noise > 3x read-noise. And note that a shorter exposure time is required to swamp the read noise in worse light pollution. Or you can look at as the overall signal-to-noise ratio - the improvement in this reduces as you increase the sub-exposure length.

And it also depends on the speed of your scope, etc etc. I made some spreadsheets to calculate it all (basically stole the info from Shiraz, CloudyNights topic 536809). Its fun to do. But I'd ignore everything I've said and use the longest sub-exposure where you only get saturation of the larger stars.

Chris

[Pete75 (Pete)]

Wow have so much information to go on with now. thank you