Best number of subs

[NGC7789]

I am preparing an imaging session at one of the remote observatories in Australia. To stay in budget I have to choose between 6 ten minute subs or 12 five minute subs. Is it better to take more but shorter subs to take advantage of the better rejection algorithms in ImageIntegration or fewer longer subs to perhaps capture fainter details? The target does have faint details to capture.

Any opinions would be appreciated.

[RickS]

Ideally, your subs will be long enough that the noise contribution from camera read noise will be small by comparison to shot noise from the object and sky glow.  How long that is I can't even hazard a guess without knowing some basic details of the scope and camera (scope type, aperture, focal length, type of camera/sensor.)

Cheers,
Rick.

[NGC7789]

The telescope is a Planewave 27" (0.7m) CDK700WF Corrected Dall-Kirkham Astrograph with a 700mm aperture and 4531mm focal length. The camera is an FLI PLO09000. The specs say QE: 68% Peak, Full Well:  > 100X Saturation Exposure (ABG), Dark Current:   <0.1 e-/pixel/sec. @ -35º C. With this setup the FOV is 27.1 arcminutes square and 0.53 arc-secs/pixel.

 

[jkmorse]

If you can do the math and have the image train details there are a number of calculators on the web that you can use to get a good idea of an "ideal" subimage time.

Jim

[RickS]

The telescope is a Planewave 27" (0.7m) CDK700WF Corrected Dall-Kirkham Astrograph with a 700mm aperture and 4531mm focal length. The camera is an FLI PLO09000. The specs say QE: 68% Peak, Full Well:  > 100X Saturation Exposure (ABG), Dark Current:   <0.1 e-/pixel/sec. @ -35º C. With this setup the FOV is 27.1 arcminutes square and 0.53 arc-secs/pixel.

That's a pretty fast combination but the camera has a read noise spec of 10e- and your skies will presumably be pretty dark, so I think that 10 mins would give the best results.

Cheers,
Rick.

[jerryyyyy]

Believe it or not, I am familiar with that set up, I think.  iTelescope?  If so, would look at the default parameters for whatever you are planning to image (they have canned sequences) and talk to the Techs there.  My bias is that this is a very dark site (easier to get more signal than noise) and I think they suggest the 10 minute subs... especially if it is some sorta darkish nebula... 

Appropriate disclaimers....

PS   Look around for other images using the scope and see what others did...



If this is the scope, shot was 3x600s.  I got fed up trying to image M83 from up my backyard.  I did this before I joined in with another dark site. 

[NGC7789]

You pegged it Jerry. Your picture is worth 1000 words. Thanks to all. 10 minute subs it will be.

[Russ]

As you might know, PixInsight has a script called "CalculateSkyLimitedExposure."  I would like to use the script to calculate exposure times for T-27 at Siding Springs. However, I lack some data  the script requires. I recently asked the staff at iTelescope to supply me that data. They didn't know the answers and have forwarded my request to Finger Lakes Instruments. If I get the data, and if the results generated by the script make sense, I plan to post them on this forum.

In the meantime, here is a link to a PixInsight thread commenting on the script. http://pixinsight.com/forum/index.php?topic=2584.0

The link will make you smile. It's the usual story: the experts do not agree with each other's calculations. In fact, they can't even agree on how to ask to question properly. But it's a fascinating question, because exposure time is confronted every time an astrophotograph is taken.

These days, all my images on T-27 use only three frames and 30 minutes of imaging time, divided equally among R, G and B. I use a synthetic luminance. I enjoy this minimalistic approach (it's a lot like fly fishing), but look forward to the day when I might have a more holistic understanding of the relationships among system characteristics, site conditions, subframe exposures, total exposures, and the signal to noise strategies of stacking.

[mschuster]

Sometimes it is not clear what the online calculators are calculating, so if you know a few numbers, a quick sanity check is possible:

(3 * ReadNoise)^2 / Gain + Offset

ReadNoise is sensor read noise in e- RMS. Gain is sensor gain in e-/DN. Offset is median of a bias frame in DN.

For my binned QSI, ReadNoise is 12.8 e- RMS, Gain is 1.06 e-/DN, Offset is 400 DN.

( 3 * 12.8 )^2 / 1.06 + 400 = 1800 DN.

To be sky limited, the dimmest areas in my binned uncalibrated light frames need to be measure at least 1800 DN. I can measure frames from each filter to double check and adjust exposure time up or down to meet this requirement.

Basically you want noise in the background signal to be at least 3 times read noise.

If you bin some channels and not others, you need to do calculations for both binned and unbinned cases.

You can tweak the constant 3 up or down a bit if you want, to be more or less conservative and more or less sky limited. But IMHO 3 works as a good rule of thumb. Using 3, read noise contributes about 5% of the total noise in the background.

I like having a fixed DN target to check my exposures against, independent of optics and sky conditions.

Mike

[NGC7789]

These days, all my images on T-27 use only three frames and 30 minutes of imaging time, divided equally among R, G and B. I use a synthetic luminance. I enjoy this minimalistic approach (it's a lot like fly fishing).

This is up my alley. Sounds like I'll not only use 10 minute subs but cut back from 6 to 4 so I can add 4 Ha also.

[Russ]

Hi Mike,

An extremely intriguing reply. Thank you.

Offset and gain are the two numbers I don't have for T-27 at Siding Springs. The staff at iTelescope is very helpful, so I hope to have those numbers soon. I find this topic extremely interesting. Compared to you, I'm starting at a primitive level, but with diligence, I hope to inch my way up.

Russ

[mschuster]

Hi Russ,

Note: If you can get two good bias frames and two good flat frames, the read noise, gain, and offset numbers for the camera can be determined, from which the sky limited exposure target DN can be calculated. One procedure is described in the Berry/Burnell Handbook of Astronomical Image Processing. The BasicCCDParameters script does similar calculations but I have had trouble getting this script to work on my frames. So I wrote my own easier to use scripts that basically do these calculations and provide the numbers (DarkBiasNoiseEstimator and FlatSNREstimator).

Mike

[Physicist13]

Always work reading Craig Starks notes on signal to noise! huge amounts of confusion abound. The main point is whether you are Read noise dominated or not. The number of subs needed to get a particular SNR level changes...the scaling is different. But crucially even if Read noise dominated it still improves....it just takes longer! (ie the total exp time needs to increase).
Patrick

[Russ]

Hello Mike and Patrick,

I was able to download bias and flat frames from the iTelescope site and to use Mike's scripts. Life is good!

Russ

[NGC7789]

was able to download bias and flat frames from the iTelescope site and to use Mike's scripts.

And what was the result?