Smart histogram short exposures

[RobertH]

Dear Robin,

When i use the smart histogram it gives me very short exposures (1.5 sec).
According to clear outside i have a Est. Sky quality of 19.13 magnitude, Bortle 6, 2.41 mcd/m2 brightness and 2239.02 ucd/m2 Artificial Brightness.
I use a RASA-8 (F2) and Risingcam/touptek Sony IMX571 camera (see analysis below).

Thanks,

Robert

[oopfan]

Ultra-fast scope, sensitive camera, bright skies, 1.5s sounds about right. What do your stacks look like?

Brian

[admin]

Hi Robert,

yes, Brian is spot on - all of those ingredients combine to not needing long exposures. You can happily use longer exposures than those suggested without any loss of quality (until you hit an exposure length when your guiding becomes an issue). In your case, you probably will *want* longer exposures just to keep the amount of data to be processed down.

The rule of thumb for minimum exposure time is 10 * (read noise squared) / (light pollution level in e/pix/s)

Based on your situation, I would assume a read noise of 1.7 to 2.0 (depending on gain) and a light pollution of 15-20, so that fits right in with 1.5s

cheers,

Robin

[RobertH]

Hi Robin,

I'm puzzled about how 1.5 sec exposures can get enough data from a faint nebula for example.
It looks like the target is not included in this equation.
Can you explain that some more?

Thanks,

Robert

[oopfan]

Hi Robert,

It is your sky brightness that limits how deep you can see. You can't "force it" by using long exposures. It's better to integrate 30 two-second exposures than take only one 60-second exposure. Why? On the one extreme with a long exposure the sky might saturate your sensor and you'll see nothing. On the another the sky will rob you of dynamic range. Robin's algorithm recommends the minimum exposure where the sky begins to play a significant role. You gain little by increasing your exposure further. Your friend is total integration time. In my experience the recommended exposure offered by SharpCap is the minimum. You can safely double it but there is little to gain by increasing it further. If your target is faint then your only option is to increase total integration time.

Brian

[RobertH]

Thanks for the explanation Brian!

Can you show me some pictures with short exposures?

[oopfan]

Hi Robert,

Sure but this image of M13 isn't a nebula, however, it does show stars down to 20th magnitude which is pretty impressive given the short exposures I used:

M13_2022-04-18_R-600x6s_G-600x6s_B-R-0.1_ST-0.00045_BA-0.01875_HL-0_SAT-0.22_TH-0.0_SH-1.5_PRO-5.jpg (421.24 KiB) Viewed 1707 times

I have a 71mm f/5.9 refractor so it is considerably slower that your RASA-8. I used a monochrome planetary CMOS camera: Altair 290M. Read noise is 1.5e-. Air-cooled but the ambient temperature was in the 40's Fahrenheit.

600x 6-second exposures in red.
600x 6-second exposures in green.
Total Integration Time: 2 hours.

I also have a CCD camera. Read Noise 5.3e-. It requires a minimum exposure of 60 seconds due to the high Read Noise. One of the great advantages of CMOS is the ultra-low Read Noise. That's the reason why you can get by with such short exposures.

Why did I choose CMOS for M13? The sensor's pixel size is small, 2.9um. My CCD is 4.65um. The small pixel size was best for a dense star field in the globular cluster. My CCD could not have resolved the tightly packed stars.

Plus, at the time Tim (@timh) was doing some fine work using "lucky imaging" techniques on star clusters in an attempt to freeze the turbulent atmosphere. I wanted to give it a try.

Here is another image that I took recently. Same exposure and integration time as M13:

Melotte-14-G200-BL50-R600x6s-G600x6s-Dark50x6s-Flat100x-Bias100x.jpg (521.36 KiB) Viewed 1706 times

Brian

[RobertH]

Impressive!

I'm totally convinced now!

Thanks!

[oopfan]

Hi Robert,

You made a wise choice buying a fast RASA-8. Everyone would agree that 2-second exposures have multiple advantages over slower instruments:
1. Shorter recovery from passing aircraft and satellites.
2. Little need for the complexities of auto-guiding including the extra cost of a guide camera and scope.
3. Greater resilience to imperfect polar alignment.

If you want long exposures then:
a. Try narrowband imaging in Hydrogen Alpha (Ha), Ionized Oxygen (OIII), and Ionized Sulphur (SII). The narrow pass-band of these filters (typically 6nm) blocks almost all light pollution, so it lends itself to much longer exposures. For me I need a minimum of 10 minute exposures.
b. Travel with your instrument to darker skies. SharpCap will advise you to take a longer exposure.

However, like I mentioned earlier, once you opt for longer exposures then you need a guide scope, camera, and associated software.

Here are some photos I've taken in narrowband:
viewtopic.php?t=4235
viewtopic.php?t=4238
viewtopic.php?t=4202
viewtopic.php?t=4101

Watch how you consume my posts. Generally you need to scroll to the end to find my best work. I like to engage other forum members so as to integrate their feedback to improve my images.

Brian