SIRIL Short vs long exposures , computer power and auto stack option? Siril Team Siril · Astroplantfish · ... · 28 · 626 · 2

Gary JONES:

Mark Worthington:
*Thanks Gary, for your really helpful reply! All these details are brilliant. Picking out a couple in particular, for external storage, would you recommend going for any cheap large ssd, so focus more on size than write speed, if all the hard work will be on the internal ssd? Now, whether the ssd in the refurbished T430 is up to the job in terms of writing speed, is another matter... it certainly isn't an M.2 or nvme type. And your comments on exposure times and use of AP, very useful. I guess I could have both to start with.

Hi Mark,
Many thanks for the compliment

In terms of your computer gear, I'd recommend the following :-

1. Assuming you use your AAP in local mode, buy a large SSD with reasonably fast write speed.
The RAM inside the AAP spools images to the disk while the next image is being exposed, so write speed is not the main consideration.
But if you copy your files to another drive for processing, you want something with a very fast read speed.
Of course, if you get one, you usually get the other.
Also have TWO SSDs in case one becomes faulty, particularly if you image in the field.

2. Try performing a read/write speed test on your internal RAM, then match the R/W speed of your SD card to that, plus a 50% margin.
No point paying for extra speed on the SSD if your PC can't make the most of it.

3. The AAP has a 256 GB internal SSD which is pretty fast (RW = 121/95 MB/sec).
That's more than enough to store all your images for a night (256 GB will store about 2,500 APS-C RGB images), so you don't really need an SD card.
But it does make things easier if you want to copy images to your PC without using a network.
Of course, you can copy images direct from the AAP to your PC via WiFi, which can take a very long time - or ethernet, which is much faster.
But it's still a good idea to have an SD card handy just in case you run out of memory ...
It happened to me once on my AAP after I forgot to erase the images from the previous night's imaging.

4. For storing all your image files, it is *really* risky to use an SD card or USB stick.
I highly recommend against this.
USB sticks have a finite failure rate, so if the drive fails, you could lose everything.

Much better to invest in an inexpensive SSD RAID drive for storing all your RAW and processed images.
RAID gives you redundancy, so one SSD can fail completely and you won't lose any data.

Considering the cost of your mount, and the cost of all your other gear, IMHO it's definitely worth investing in a good RAID - just part of the overall package.

Just one other thing to follow on my my earlier comment ...
In terms of the ideal exposure time - that depends on many factors - the best thing is to experiment, but around the 3-4 minute mark is a good place to start - an AZEQ6 is certainly up to the task. It totally depends on your equipment, the target and atmospheric conditions.

By 'atmospheric conditions' of course I'm referring to seeing. Seeing is composed of a number of elements - fast turbulence, slow turbulence and short-term density changes.

One way to reduce the effects of fast turbulence is to use really short exposures - in the order of milliseconds. Anything longer than that makes little difference in terms of fast turbulence.

To reduce the effects of slow turbulence you can use short exposures - in the order of seconds. Anything longer than that makes little difference in terms of slow turbulence.

To reduce the effects of short-term density changes you can use limit exposures to less than a few tens of seconds. Anything longer than that makes little difference in terms of short-term density changes.

See as example of seeing in the short movie I posted on Astrobin here.
You can see the fast and slow effects, both of which will affect any exposure of more than a second or so.

So, although better seeing and shorter exposures will always give you sharper images, any exposure exceeding a few seconds will still suffer from the effects of seeing. But shorter exposures will also suffer from more noise.

As I said before, the best thing to do is experiment. Try imaging a single target on the same night using different exposures - 1/2/3/4/5 minutes, and make sure to record the seeing. Measure the noise and star size in each set of RAW images and in the stacked result - that will be your best guide.

Cheers,

Gary