Eta Carinae

[Allan3026 (Allan)]

So I was wondering what exposure times people are using. Is it better to do 100 5 second subs or 5 100 second subs? Or longer??
This image is 70 5 second subs. It was live stacked in sharpcap. No dark frames or processing, just live stacked.
AZ EQ5
Saxon 102 triplet APO
ASI 294MC Pro
Antilla triband filter
No guiding

[Mickoid (Michael)]

Allan, this really depends if your rig is set up to handle long exposures or if you don't mind that processing your data may take some time to stack. Providing you're using the same Gain in each case, the 100 x 5 sec subs should give you the same result as 5 x 100 sec subs. However, because you are not auto guiding, the preference would be to keep your exposures shorter due to the limitations of your mount or inaccurate polar alignment or both. The only way to really know is to do a series of test subs to see how long you can expose for before star trailing is noticed. Do a few exposures as periodic error of your mount can give you a false impression that your exposure was too long. I tend to favour longer exposures with lower Gain than shorter exposures with higher Gain. You just want to avoid stretching your image too much in post processing as it will introduce more noise, gradients and colour degradation to a certain extent. Also, keep in mind that your sub file size will be the same, regardless of your exposure, so do you want to process 100 subs or just 5? It is definitely advantageous in shooting more subs than less in either case because both will help reduce noise and increase the signal which will give you more detail. As an example to your question, one of the previous posts using the ZWO Seestar included a stack of 1000 x 10 second subs and the result is very good. Hopefully someone will chime in on this post with a better response than mine but this is my understanding of which exposure method is better than the other. Keep up the good work and take notes of what you've done, experiment and see what works best with the gear you have at hand.

[Allan3026 (Allan)]

Originally Posted by Mickoid

Allan, this really depends if your rig is set up to handle long exposures or if you don't mind that processing your data may take some time to stack. Providing you're using the same Gain in each case, the 100 x 5 sec subs should give you the same result as 5 x 100 sec subs. However, because you are not auto guiding, the preference would be to keep your exposures shorter due to the limitations of your mount or inaccurate polar alignment or both. The only way to really know is to do a series of test subs to see how long you can expose for before star trailing is noticed. Do a few exposures as periodic error of your mount can give you a false impression that your exposure was too long. I tend to favour longer exposures with lower Gain than shorter exposures with higher Gain. You just want to avoid stretching your image too much in post processing as it will introduce more noise, gradients and colour degradation to a certain extent. Also, keep in mind that your sub file size will be the same, regardless of your exposure, so do you want to process 100 subs or just 5? It is definitely advantageous in shooting more subs than less in either case because both will help reduce noise and increase the signal which will give you more detail. As an example to your question, one of the previous posts using the ZWO Seestar included a stack of 1000 x 10 second subs and the result is very good. Hopefully someone will chime in on this post with a better response than mine but this is my understanding of which exposure method is better than the other. Keep up the good work and take notes of what you've done, experiment and see what works best with the gear you have at hand.

Thanks Mick. Without guiding I've been sticking to short subs but will try longer with lower gain as you suggest. My mount is not top of the line but I think does OK. I am looking at getting an OAG and using my ASI482 as guide cam.
Also, does livestacking in sharpcap automatically stretch the images? Never really understood how it works.

[Mickoid (Michael)]

I think you have a choice in your live stacking setup where you can adjust the histogram. You may have had a stretch applied already. Check your histogram settings. I'd be inclined to leave it linear and apply a stretch if required using Photoshop or some other image processing software. That way, your saved stacked Raw data can be tinkered with without ruining the original captured data.