TheSkySearchers

Good day all!

I'm not sure where to post this but I've heard several times now that My camera (ZWO ASI 071 MC Pro) would give the best results with short (35 sec) exposures...

This seems to go against popular thought but one guy said it has to do with the SNR. He provided me with this chart.

My sky is "20.16 mag./arc sec2" and my scope is F4 so that comes to 35 sec exposures...

Any thoughts?

That's interesting. I wonder if it is to do with light poliution over exposure?

How do you find out your Sky Brightness?

Edit: https://en.wikipedia.org/wiki/Bortle_scale

Regards

Graeme

I use this map for sky brightness.

https://www.lightpollutionmap.info/#zoo ... TFFFFFFFFF

So for 150 gain, that's about 3 or 4 minute exposure on my 9.25.
I was contemplating a ZWO ASI294MC-PRO, I wonder if the same figures apply?

Regards

Graeme

I'm not sure. I think the chart is camera specific.

Have you tried 35 second exposures?

I dont buy it. There is no way you can collect as much depth of data with 100 x 30 second exposures vs 10 x 300 second exposures.

I have the 294MC. I had been taking 60 second exposures with my Canon D70 and the 294. Recently I've been getting less than 1/2 pixel movements of my mount using PHD2. I live in Bortle 5 skies and according to signal/noise ratios 1 minute is supposedly optimal. However, I tried doing 2 minute exposures and the results are breath taking. I am also thinking that I am probably in Bortle 4.5 skies. I cannot see the Milkway, but a Bortle 4 sky should give 3 minutes, so I reckon I must be in between. I have not seen a noticeable increase in noise during processing.

Graeme1858 wrote: ↑Thu Jun 18, 2020 4:51 pm Have you tried 35 second exposures?

I have not. I just came across this and, like I said, this is the third time someone has given this advice.
I had asked the previous owner of my camera what settings he used and he replied with this.

"You are quite welcome Larry!



I'm also in Bortle 6 skies, sort of a medium sized city suburban backyard.



A lot depends on the f-ratio of your scope. I used unity gain (90) for almost all of my images. In my f/7 refractor, 60 second exposures worked out pretty well. You can go longer, but you will start to saturate more stars. Although you do get more signal in a galaxy for example, so it's a bit of a trade off. If you want to keep the number of subs down and use 3 minutes, I would reduce the gain to 50 or maybe even zero. You get more dynamic range that way as well, but also a little more noise. I also always dither every other sub. It really helps reduce stacking noise and I highly recommend it if you are guiding. In my f/5.3 mak-newt, I was using 30 to 45 second subs at unity gain for a lot of objects. Ithink the main thing to get used to with CMOS cameras, is you will tend to work with 30 second to maybe 90 second subs a lot more than you will 4 minute to 5 minute subs. You will probably only need 3 to 5 minute subs if you decide to use one of the neat duo band filters or if you can get to a very dark sky, then you can expose lot longer."

UlteriorModem wrote: ↑Thu Jun 18, 2020 4:57 pm I dont buy it. There is no way you can collect as much depth of data with 100 x 30 second exposures vs 10 x 300 second exposures.

That appears to be the popular thought.

I don't shoot for more than 180" because it seems to just make the image brighter without adding any more detail.
I assumed this was due to my bortle 6 skies...

UlteriorModem wrote: ↑Thu Jun 18, 2020 4:57 pm I dont buy it. There is no way you can collect as much depth of data with 100 x 30 second exposures vs 10 x 300 second exposures.

It has to do with the gain - sure, if you use the same gain you'd be absolutely correct (there are YouTube videos demonstrating this), but if you change the gain and especially in these newer CMOS cameras you can push up the gain with little consequence (in terms of noises, etc.) then you can capture the same details. So if you ever do these experiments where you compare the two, you need to consider different gain. Still in the end even with a higher gain, I think you can get very close but it will not be the same unless you do MORE. If you do 100+ 30 second exposures (the + would depend on your conditions/environment, rig, and target) then you could match or even exceed the the 10x300 second exposures. You may argue that's not a fair comparison since the total exposure length changes but the people looking to do shorter exposures have some reason that might involve tracking/guiding/etc. so they may not have a choice but it is possible.

In the end, the correct exposure length/gain/other parametres is based on a combination of factors that includes your setup, your environment, and your target (which is related to the environment, I suppose but different places in the sky have different brightness and also whether you're doing NB or RGB or OSC). SharpCap has this "brain" histogram function that ostensibly do this for you - it takes you through a series of steps and figures out what the best sky limited exposure for that night would be that includes your setup and your environment but it doesn't always work though for some older cameras this has been figured out similar to the table posted by the OP so you can just look it up.

It's something I've been spending a lot of time figuring out and I still have little idea of how it all works but I do enough to know it can all be worked out in terms of math and it corresponds to real life pretty well provided we have people of the same expertise taking the images (the human factor obviously isn't taken into account).

--Ram

PS: Yes, the chart is camera specific and also different for different filters, etc. It's all about how much background light gets in relative to how much of your target you see and for modern CMOS cameras it seems longer exposures aren't necessarily useful in places with a lot of light pollution especially.

Thanks ram!

I was thinking that if I'm not really collecting much more data beyond 35 sec then I would stand to gain MUCH more by shortening my time and stacking more images...

In the end, if there is no advantage, I would rather stick to my usual 180" exposures and only have to work with 80 or 90 of them versus several hundred...

Larry

Larry,

What does your histogram look like? Basically, you use the placement of the spike on your histogram to decide how long your exposures should be. This takes into account all the factors already discussed in this post.

Also, it may be helpful to refer to the articles in this post specifically the ones covering camera gain.
viewtopic.php?f=24&t=10445&p=89141#p89141

Cheers,
JT

Thanks JayTee!

That should keep me busy for now!

Larry

Also, this page about your camera and its different gain settings may also be helpful.
https://astronomy-imaging-camera.com/tu ... river.html

JT

JayTee wrote: ↑Fri Jun 19, 2020 12:59 am Also, this page about your camera and its different gain settings may also be helpful.
https://astronomy-imaging-camera.com/tu ... river.html

JT

Thanks again JT but that page is for the 071 MC Cool... The Pro is a bit different. Something about the 256 Mb buffer???

Larry

Larry 1969 wrote: ↑Thu Jun 18, 2020 6:11 pm Thanks ram!

I was thinking that if I'm not really collecting much more data beyond 35 sec then I would stand to gain MUCH more by shortening my time and stacking more images...

In the end, if there is no advantage, I would rather stick to my usual 180" exposures and only have to work with 80 or 90 of them versus several hundred...

Larry

As usual the correct answer is "it depends". There are a lot of factors in the short vs long exposure theories. If you have great sky conditions at a dark site go for the long exposures. But, most of us (I think) image from not so ideal conditions. Remember, the more subs the higher the SNR. So, sometimes lots of short exposures give better results than a few long ones. I have done some experiments on this myself, and there have been many instances where the large number of short subs is ideal.

My bottom line: go for the longest exposure you can get away with considering all the various parameters you need to be concerned with. The chart was made to give you a best 'short' exposure for a given sensor and sky background. Start from there and work up.

I altered my approach last night, thinking of this thread in the pre-planning. I've been gettin' my butt kicked by Momma Nature (Clouds, wind or BOTH) and my recently purchased camera (ASI533MC Pro). The sky was marginally better last night than when I was last attempting AP on June 15th.

My images of a like magnitude DSO on the 15th were shot at 5 minute exposures. Last night I dropped the exposure to 2 minutes, saw a lot more detail and it was easier to coax 'em out in post....

One regret I had was I didn't lower the exposure time even more!!!!! There may be some merit to Larry's findings, given sky conditions.

OhNo wrote: ↑Fri Jun 19, 2020 11:13 pm I altered my approach last night, thinking of this thread in the pre-planning. I've been gettin' my butt kicked by Momma Nature (Clouds, wind or BOTH) and my recently purchased camera (ASI533MC Pro). The sky was marginally better last night than when I was last attempting AP on June 15th.

My images of a like magnitude DSO on the 15th were shot at 5 minute exposures. Last night I dropped the exposure to 2 minutes, saw a lot more detail and it was easier to coax 'em out in post....

One regret I had was I didn't lower the exposure time even more!!!!! There may be some merit to Larry's findings, given sky conditions.

That's what I'm wondering...
Do read the reply from JayTee to this thread though... Some VERY good information...

Larry

OhNo wrote: ↑Fri Jun 19, 2020 11:13 pm One regret I had was I didn't lower the exposure time even more!!!!! There may be some merit to Larry's findings, given sky conditions.

Yes, unfortunately, many of us (myself included) are so happy with a clear night sky that we'll image come h3!! or high water without regard to the seeing conditions. As you noted, it can be a major factor in how long an exposure should be.

I have no rule of thumb yet on determining optimum exposure based on the seeing conditions. I think the rule of thumb would basically tell you which scope/optical accessories (FR) (ie, imaging scale) you should use, not how long your exposure should be.

Cheers,
JT