Quote:
Originally Posted by Shiraz
Hi Scott. Hope some of this is helpful.
As Colin points out, if you image for an hour you will collect the same number of photons regardless of gain settings or sub length - you don't get more photons by using higher gain and with the 1600 you actually get less noise at higher gain.
If you use short subs/high gain, you will collect relatively few photons in each sub. When you stack the short subs, you will get back all of the signal, but will have read noise from the larger number of subs (each one contributes read noise). To keep the total read noise manageable, you will need a low-read-noise chip if you want to use short subs - the 1600 is suitable.
If you use high gain with the 1600, you will have limited well depth. However, you will have a lot of full wells to add up in stacking (one for each sub), so you get back the dynamic range over most of the gain region, particularly since the read noise drops with higher gain. With broadband, you could use any gain from 0 to about 150 with appropriate sub lengths and get essentially the same results in the same total time. The big advantage is that you can use very short subs at high gain - makes guiding etc easier. suggested gain/sub table attached, along with graph of total well depth.
With narrowband, the basic rule is always use the longest practical sub length to get the lowest read noise. However, balance that against the need to keep from saturating bright stars, so choose the longest sub that will give you acceptable stars. FWIW, I have found that 5 minutes under moon is OK with an f4 system at gain 100 - under dark sky, I am currently testing 10 minutes at gain 200.
edit: also, the "unity" gain setting is actually very high gain cf other chips - the 1600 behaves much like a normal CCD when it is used at gain 0, so using it at gain 139 is really pushing it into a region that is not reachable by conventional CCDs.
|
Quote:
Originally Posted by glend
We don't aim for 30 minute narrowband sub with the ASI1600, they are not necessary. With the ASI1600, you get high sensitivity and very low noise. Narrowband does not require very long subs with this camera, 300 seconds is enough at Unity Gain settings. There are plenty of examples in the Astrobin ASI1600 group it is worth having a look through those images. Shiraz here on IIS has produced a suggested optimal broadband chart for this camera, and Beta testers on Cloudy Nights did a lot of work on settings and exposures.
|
Quote:
Originally Posted by Atmos
As long as you are over the read noise then 60x1 will be the same as 2x30. If you had a camera with higher read noise (KAF-8300) then 60x60s would not be equal to 6x600s in LRGB but 2x1800s would be overkill.
|
Alright. I think I am getting it. Thanks everyone.
SO because this 1600 sensor has such low read noise, the signal is appearing sooner than with an 11000 sensor for example. And because of that you can get away with shorter subs. Is that correct?
My next quandary is about the "collection" of photons. I recall, somewhere (and I believe it was Mike and Trish, and Colin I think you were in this discussion) that the other reason you do longer subs is that you increase the probability of a photon striking the sensor for the reeeeeally dim stuff. The discussion, if I remember correctly was around hour long 2x2 binning of the helix and having areas where only one photon per hour (statistically speaking of course) would strike.
Once again, I probably misunderstood this, but what I took away from it was that by taking multiple shorter subs you run the risk of missing that elusive photon in the download and dithering gaps between subs. Am I really off the track there?