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Old 04-07-2021, 10:31 AM
bluesilver (Peter)
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Working out how long to image for?

Hi, I am hoping someone might be able to offer a bit of advise on this if possible please.
I am working on imaging the Cats Paw Nebular, NGC 6334

Basically i am trying to work out if there is a general rule to work out how long to image a particular target for?

I have tried to image this with 120 second exposures, but i am thinking that it needs more to get in more detail and a better image in general.

Would something like 300 second exposure be a better option for this nebular?
I realize that apertures play a part in this also.
I am using a skywatcher 150 Evostar ED scope, not ideal, but it is working well for me.
Camera is a Canon EOS 600D

I am using PHD2 multi star guiding and APT as the imaging software.
I get a good polar alignment and so far haven't had any issues with star trailing.

Any advise would be greatly appreciated.
Thanks.
Peter.
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Old 04-07-2021, 11:03 AM
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Here are a couple of articles that go into signal to noise ratio - they are old but explain the considerations quite well..

http://www.stark-labs.com/craig/reso...SNR-Part-1.pdf

http://www.stark-labs.com/craig/reso.../CCD-SNR-2.pdf

http://www.stark-labs.com/craig/reso...s/CCD_SNR3.pdf

http://www.stark-labs.com/craig/reso...s/CCD-SNR4.pdf

http://www.stark-labs.com/craig/reso..._Sampling2.pdf
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Old 04-07-2021, 11:22 AM
bluesilver (Peter)
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Thanks for the links, some detailed reading in there.
After reading through it, it seams to me and i could be wrong here.
But it basically comes down to take a bunch of different exposure times, have a look at each one and then select one from them that appears to give the best results and then set you imaging plan up based on the best image.
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Old 04-07-2021, 11:28 AM
Startrek (Martin)
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Peter,
I used my Canon 600D for 4 years in 6Ē and 8Ē newts in Sydney Bortle 8 heavy light pollution and South Coast NSW Bortle 3/4 nice dark skies
My ISO setting was predominately ISO800 which provides the best dynamic range at the lowest read noise ( which is high anyway being a DSLR , the drawback with using a DSLR that damn noise )
Sub exposure lengths vary due to the quality of sky conditions, moon phase , light pollution levels etc...
Unfortunately not one shoe fits all
In Sydney Bortle 8 , I never used a NB filter ( and should have ) but generally used the following sub exposures -
No Moon
Emission Nebula 90 sec to 120 sec
Planetary Nebula 120 sec
Galaxies 90 sec
Globular Clusters 60 sec

During Moon
Emission Nebula 90 sec
Planetary Nebula 90 sec
Galaxies 90 sec
Globular Clusters 60 sec

On the South Cost NSW Bortle 3/4 I used the following -
No Moon
Emission Nebula 120 sec to 300 sec
Planetary Nebula 120 sec to 300 sec
Galaxies 120 sec to 300 sec
Globular Clusters 60 sec to 90 sec
Note: I only used 5min subs on nights of good seeing

During Moon
Emission Nebula 120 sec
Planetary Nebula 120 sec
Galaxies 90 sec to 120 sec
Globular Clusters 60 sec to 90 sec

The Cats Paw is a very challenging object for a DSLR as itís not a very bright object , I certainly didnít attempt it with my DSLR. Thereís a risk with DSLRs running longer subs as you introduce more noise not more detail into your images and we are all trying to get a better SNR
Good Sky conditions will allow you to push longer subs and capture a bit more quality detail but a DSLR is a DSLR
When I jumped from my old 600D to my current ZWOASI2600MC it like trading in a 64 Holden to an E class Merc , absolutely chalk and cheese in performance
My recommendation for the Cats Paw is to keep your 600D at ISO800 and maybe increase to 150 sec subs but capture at least 3 to 4 hours worth of data with darks etc.. I presume your in fairly darks skies ??
Hope the above is helpful

PS I still use my 600D for dabbling in lunar and planetary imaging during planetary season ( July to September)

Best
Martin
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Old 04-07-2021, 11:31 AM
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If you don't have issues with guiding or wonky stars due to tracking, one approach might be to take the longest sub-exposure you can without saturating stars....ie take a look at the histogram and ensure you arn't clipping the white point.
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Old 04-07-2021, 12:27 PM
AdamJL
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Whilst total integration time is the goal (so lots of short subs vs few long subs is the secondary point), what you're trying to do is
1. swamp the read noise of your camera. Each camera sensor model is different, but you want to ensure you get enough signal to fight against read noise. Remember that when you stack images, you're stacking signal, but also stacking noise (until you remove it).
2. do it to the point (as Martin suggests) that is appropriate for your light pollution level. The general rule is shorter subs in light polluted areas, longer subs in dark skies. I would never shoot 10min broadband subs in the middle of Sydney but would love to in B1-2 skies.
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Old 04-07-2021, 12:57 PM
bluesilver (Peter)
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Thanks heaps for the replies and advise.
All very much appreciated.
Yes i should of added that i am on a farm, so light pollution is not an issue for me so much apart from the moon.

I will check the histogram graph on the DSLR screen, never thought to check on that.
I am looking at upgrading to a ASI107MC a tad latter.

I did post post this also on CN just to get some ideas on what they had to say also.

I did get a reply that mentioned on SharpCap there is a Sensor Analysis option that help to determine the correct exposure time and ISO settings.

But do appreciate all advice, it has helped me out heaps already, hoping to give it a try tonight if the clouds stay away.
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Old 08-07-2021, 05:17 AM
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A little clarification on the histogram. The back of camera histogram will not show you that the white point is clipped. It would show that on total overexposure like a white image. The camera histogram does not resolve blown out stars. But on camera is via display mode button a view available showing clipping in the image. It blinks over and under exposed images on and off. So that way you could try find the point just prior to blowing out stars. I will try this next time, too. Checking for overexposed stars in the field to determine exposure is a good tip. I’d absolutely love to have some colour in the stars but I shoot dual narrowband on DSLR in Bortle 7. while nebulae come out really great, the stars are white.
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Old 10-07-2021, 12:13 PM
ChrisD
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This video from the author of sharpcap helped me understand how to select imaging time. Quite technical by worth the effort.

https://www.youtube.com/watch?v=3RH93UvP358
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Old 11-07-2021, 08:47 AM
bluesilver (Peter)
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That is a very video, pretty much explains what is going on and how to measure for exposures plus many other things.
Great find, if i can find more videos like this it would be great.
Appreciated.
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Old 11-07-2021, 01:26 PM
ChrisD
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The original lecture was cut short because of time, however, Dr Glover did a followup to discuss gain selection.

https://www.youtube.com/watch?v=ub1HjvlCJ5Y
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Old 11-07-2021, 02:53 PM
bluesilver (Peter)
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Thanks for that extra link, lots of good information in both of those.
Well worth watching if anyone else is curious about this.
Appreciated.
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Old 11-07-2021, 06:49 PM
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After you have done a few images you will get a feel for this.

If they are too short in total exposure you won't be able to get the noise completely under control.

You almost basically can't expose too long except for the opportunity cost of missing out on imaging other targets.

Generally speaking and as a very rough figure most images require about 3-8 hours. Galaxies longer.

There are things that speed that up. Wider aperture. Faster optics. More sensitive cameras like the latest CMOS cameras that are very sensitive with low noise. They for example require 50% less time to get the same type of image that a typical CCD would require.

Of course there are practical considerations that limit exposure time. Weather, work and willingness. Then gear problems that prevent a proper image from being taken.


Greg.
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