Minimum exposure time?

Hi Petro,

I image from a balcony in the center of Geneva, mag 17.80.

I usually need around 10hrs of usable exposures to have a decent result whether LRGB or Narrowband + RGB. Of course some brighter targets allow you to do it in less time, but there are not so many of them (globular or open clusters, a couple of planetary nebulas or galaxies).

Narrowband + RGB is definitely more rewarding, but needs the same time. I use 3 nanometers filters.

So that makes easily 2 to 3 nights per target in the summer, counting overheads and lost exposures.

I see no spectacular improvement (if any) with broadband light pollution filters (I used CLS-CCD, UHC and IDAS QRO).

Now I use none of them in order to get more signal, have the non scientific impression that it eventually works better in terms of SNR and color balance.

Kind Regards,
Stevan

Yeah I am still learning all of this and without an eq mount I usually don’t take more than 8-10sec pictures. Still trying to learn the optimal settings for the subs. Like how bright should they be before the data starts to get drowned out.  Should I go darker and just get a lot more time overall?  I am sure every situation is different but there are just so many variables to play with and so little time lol.

Hi Petro,

I see you are in Chicago. Can you please tell us your Bortle scale? In a Bortle 8 or 9 is pretty illusory to think you could obtain a decent image in RGB with a DSLR. 5sec exposures are not adapted for your system it is too slow. You can use Sharp Cap for example to establish the brightness of your sky (photon flux/electron flux). Pixinsight also has some estimates based on your sky background flux. Anyway, probably you have to stay under 60s and use an LP filter. The LP will cut through the galaxy signal since they are broadband sources. And integrate a lot of hours 15 to 20h depending on the brightness of your object. Then you could use Pixinsight to extract the gradients.

Clear skies

Bogdan

Thanks all for the replies.  Yeah I am stuck in a bortle 8-9.  I am not in the heart of the city but straight across from my deck is a hospital with a very bright sign.  I have been able to get results that get me excited to keep going, but I am just trying to learn more that I can do to get better quality without going to dark area and/or spending a lot more money on a new tracking system.  I know that will need to happen eventually but for now I just want to try and make due with what I have.  I will have to look into SharpCap.   I don't currently have a laptop or anything hooked up to my system, but I have need to for the future.

Thanks all for the replies.  Yeah I am stuck in a bortle 8-9.  I am not in the heart of the city but straight across from my deck is a hospital with a very bright sign.  I have been able to get results that get me excited to keep going, but I am just trying to learn more that I can do to get better quality without going to dark area and/or spending a lot more money on a new tracking system.  I know that will need to happen eventually but for now I just want to try and make due with what I have.  I will have to look into SharpCap.   I don't currently have a laptop or anything hooked up to my system, but I have need to for the future.

If you are not looking to get a filter, the next best thing is to image high in the horizon (~25 degrees or higher) and do as much as you can to block stray light. Put up clothes / light barriers, and use  a sun shade/dew shield on your lense if you have one.

There is a scientific method to determine an optimal exposure time. The best explanation I have seen is from  Blair McDonald in JRASC feb 2021. It is easy to use and you can measure the parameters yourself for your system. With one sub exposure, a bias frame and a dark. Your camera converts photons to electrons --the ADU  (analog-to-digital units, if your camera takes 16-bit images the maximum signal is 65,536 ADU). Your camera sensor  responds to photons the same way no matter how long the sub exposure. Your final signal ADU depends on the total time of exposure, not the length of the subs themselves.  Think of a bucket in the rain--the rain representing photons, the bucket your sensor. The bucket fills depending on how long you leave the bucket uncovered. It doesn't matter if you cover it occasionally, as long as you leave it open the same length of time. The only thing you lose is the time the bucket is covered, when your sensor is closed. With a digital camera downloading an image takes 2 seconds at minimum, so this is lost time. Note though longer exposures are more prone to problems like clouds, or winds, or satellites, so you have more discarded subs. I think this then is a wash. So, why take longer exposures at all? The only reason is read noise. Every time you take an image (a sub), the camera will add read noise. This depends on your gain--for many cameras higher gain gives less read noise. Fewer subs, the less total read noise. So how long should a sub exposure be? There is a good way to decide. The key point is that if your background sky is bright, in light polluted skies, most of your noise (shot noise) comes from the sky. It overwhelms read noise. So if you sky is bright the read noise rapidly becomes insignificant compared to the sky shot noise. If your sky is very dark, then read noise is a more important contribution. Mr. McDonald provides this estimate for your optimal time for your sub, Tsub, considering your sky  brightness, Esky. This equation comes from that fact that for this length of sub, about 95% of the noise is from the sky noise, only 5% is camera read noise. Longer subs reduce the percentage of read noise, but with severely diminishing returns. Tsub = 10 (R^2/Esky)  Where Esky=(ADUsky-ADUdark)/Ts - R is read noise in ADU. Measure the standard deviation in the background ADU from a single bias frame taken at the ISO/gain that you will use for your sub. Or use a literature value in electrons for R and divide by the gain to get R in ADU.  You can find the R and gain for your camera on-line if you search. But really easy to do it yourself. -ESky is the sky signal in ADU for an arbitrary sub length Ts. Doesn't matter how long, as long as your sky gives you a reasonable signal in ADU and is not saturated. You should subtract the ADU for a dark, though generally this is pretty small. Divide by the length of the sub exposure you used, Ts. Now you have all you need to calculate your optimal subexposure Tsub. You can measure R and ADUsky in many programs. Nebulosity free trial version does it, if you don't haven't purchased software that can do it. The key here for short exposures is find your lowest read noise ISO/gain, generally it is at higher ISO and gain. Generally with short exposures dynamic range is not an issue, but dial back the ISO if you are blowing out bright regions. I don't have experience with filters. The theory above says a filter will reduce Esky. But it will also reduce your signal. So now you are cutting down on your photons.  If you have a Ha only target, and an Ha sensitive camera (most DSLRs are not), then an Ha filter would certainly make sense. Other cases would depend on the target and your skies, I suspect difficult to come up with general rules. But due to less light from stars especially with a narrowband filter, you likely need to increase exposure, in some star sparse areas of the sky you might not get enough stars to stack in your subs.  I am in a big city, nearly full moon bright, operating at F6.3, my DSLR cameras are low read noise at high ISO--so  my optimal exposure is <10 s, more doesn't buy me anything. The darker the site, the longer your subs need to be to be optimal. But remember, in the end the same total exposure will give you the same number of photons, and averaging the subs will improve your S/N, no matter what your sub length was.

I use short exposures in my work: @rveregin My early work was with a 6SE and digital camera, just as you have.

Regards
Rick

It depends on the target and the camera sensitivity, etc. When I image with d780 in my Bortle 7/8 skies, my single sub exposure time is anywhere between 3 - 10 mins. Galaxies are usually on the shorter side. Emission nebula are on the longer side, because I use the L-extreme filter, and one needs to collect the faint H-alpha, etc.

You have to try a couple exposures and then compare for details to make sure you are getting what you need. Once you nail down the exposure of a single sub, then you should plan on taking many of them. In light polluted skies, that number is really important. If you want a good final picture, then take lots of them. In my experience, only after 30-35 subs is when I start seeing a decent signal to noise ratio. The higher number you can shoot for, the better your final image is. 

Lastly, I have compared images with and without LP filters. In my backyard, LP filters help significantly. I use the Optolong L-pro for galaxies and planets. For emission nebula, I use L-extreme. Both work really well.

Hope this helps.

I live in Bortle 8 and use a Canon EOS Ra with the L-Pro and L-Enhance.  I have been using the L-Pro extensively this past winter/spring with good results.  I do 300s ISO800 subs.  Yes, the L-Pro does cut some signal but the colors can be corrected in post.  Sometimes I go to a Bortle 4 dark site to image galaxies without a filter.  The L-Enhance, however, I can do 600s ISO800 from my yard.  I have imaged nebulae like the East and West Veils, Lagoon, Eagle, Pacman, Crescent, and Wizard.