Sh2-132

[AstroGS]

Finally! After 7 weeks, last weekend we managed to get a couple of clear nights.

I had the Lion Nebula in my list for some time now but, I wanted to wait until it was high enough in the night sky to capture as much data as possible. Lately I am using the WO Redcat61 in combination with the ZWO ASI2600MM + Antlia 2.8nm NB filters and I am super happy with the result, with almost pin-point stars end-to-end.

Here is the result of 14 hours total integration time:

• Ha: 30 x 600sec
• OII: 27 x 600sec
• SII: 21 x 600sec
• RGB: 20 mins each (for stars)
• Calibration frames: Darks, Bias and Flats

 

p.s. The ZWO AM5 is an absolute gem - offering very good guiding at 10-mins subs consistently. 

Edited October 23, 2023 by AstroGS

[Phillyo]

Beautiful image! Curious as to why you're shooting 10min subs with the Redcat61 and the 2600mm? Or was this just an experiment to see if the AM5 could manage 10min subs?

[AstroGS]

1 hour ago, Phillyo said:

Beautiful image! Curious as to why you're shooting 10min subs with the Redcat61 and the 2600mm? Or was this just an experiment to see if the AM5 could manage 10min subs?

I have found that 10-mins subs collect more detail than e.g. 2x 5-mins subs. It just works for me, I have a library of darks and bias calibration files and I have never bothered since my old OSC days to experiment with different exposure times. Probably for OIII, longer exposure times seem to collect more data/ detail. Do, you suggest that 5-mins might be better? 

[Phillyo]

With newer CMOS sensors, more shorter subs are generally better than less longer subs. Gone are the days of old CCD 20-30min sub lengths due to the read noise. You only really need to do enough to swamp the read noise by a certain amount, then anything more is just collecting light pollution. There are other benefits of shorter subs like less requirement on the guiding/mount, more subs to integrate means better rejection of satellite trails and if rogue clouds pass over you only lose 5 min rather than 10 min of data for example.

There is the downside of more subs requiring a more powerful PC or longer preprocessing times. However, all that being said...astrophotography is a hobby (for most of us) and is there for our enjoyment. If you are getting the results you like and are enjoying what you're doing, then I would suggest carrying on with what you're doing as that's the moist important thing

[AstroGS]

50 minutes ago, Phillyo said:

With newer CMOS sensors, more shorter subs are generally better than less longer subs. Gone are the days of old CCD 20-30min sub lengths due to the read noise. You only really need to do enough to swamp the read noise by a certain amount, then anything more is just collecting light pollution. There are other benefits of shorter subs like less requirement on the guiding/mount, more subs to integrate means better rejection of satellite trails and if rogue clouds pass over you only lose 5 min rather than 10 min of data for example.

There is the downside of more subs requiring a more powerful PC or longer preprocessing times. However, all that being said...astrophotography is a hobby (for most of us) and is there for our enjoyment. If you are getting the results you like and are enjoying what you're doing, then I would suggest carrying on with what you're doing as that's the moist important thing

I see - is there any tool to “calculate” optimal exposure time for a specific focal ratio, taking under consideration the camera chip?

[symmetal]

The optimum exposure time is reached when the camera read noise is swamped by the sky background noise. A sky background noise of 5 x the read noise makes the read noise contribution to your image negligible and it can be ignored. Once the read noise contribution is negligible then there is no advantage in exposing for any longer, and it's best to start another sub. Exposing longer than necessary makes any tracking errors more noticeable, along with more possible star bloating.

I've previously made a chart for the ASI2600 which I assume you're using as you say ASI200 in your first post. 🙂 You just need to expose until the sky background ADU level as shown by most capture programs when you hover over the image background, is at least the ADU value indicated.

I've made it for different gain settings, though modern ASI cameras where their HCG mode is only enabled above certain gain settings, means fixed gains are more generally used. If you're using default gain 100 and offset 50 then a sky background of around 717 ADU is how long you need to expose for to swamp the read noise by a factor of 5.

This ADU value is independant of the focal ratio or filter used, as a slower scope will need to expose for longer anyway to reach this sky background ADU value, likewise with narrowband filters.

In theory separate optimum exposures are needed for R, G and B filters that's too much hassle so choose one exposure for all, perhaps based on blue. Then make the luminance exposure about 1/3 of the colour exposures.

With a OSC you can only use one exposure anyway, and the median value of the image, usually displayed in the image parameters, is generally a good figure to use as the sky background ADU value unless you want to examine the separate RGB pixels.

For doing a test exposure choose a sky with little nebulocity and no Moon, to determine your optimum exposure, and use this for all your future imaging, with this camera and gain setting. 🙂

I forgot to mention that higher light pollution will mean shorter exposures before this sky ADU is reached, compared to a dark site, so if you move to a site with different light pollution you'll need to redo your test exposures.

Oh and great image by the way. 😊

Alan

Edited October 22, 2023 by symmetal

[AstroGS]

11 hours ago, symmetal said:

The optimum exposure time is reached when the camera read noise is swamped by the sky background noise. A sky background noise of 5 x the read noise makes the read noise contribution to your image negligible and it can be ignored. Once the read noise contribution is negligible then there is no advantage in exposing for any longer, and it's best to start another sub. Exposing longer than necessary makes any tracking errors more noticeable, along with more possible star bloating.

I've previously made a chart for the ASI2600 which I assume you're using as you say ASI200 in your first post. 🙂 You just need to expose until the sky background ADU level as shown by most capture programs when you hover over the image background, is at least the ADU value indicated.

I've made it for different gain settings, though modern ASI cameras where their HCG mode is only enabled above certain gain settings, means fixed gains are more generally used. If you're using default gain 100 and offset 50 then a sky background of around 717 ADU is how long you need to expose for to swamp the read noise by a factor of 5.

This ADU value is independant of the focal ratio or filter used, as a slower scope will need to expose for longer anyway to reach this sky background ADU value, likewise with narrowband filters.

In theory separate optimum exposures are needed for R, G and B filters that's too much hassle so choose one exposure for all, perhaps based on blue. Then make the luminance exposure about 1/3 of the colour exposures.

With a OSC you can only use one exposure anyway, and the median value of the image, usually displayed in the image parameters, is generally a good figure to use as the sky background ADU value unless you want to examine the separate RGB pixels.

For doing a test exposure choose a sky with little nebulocity and no Moon, to determine your optimum exposure, and use this for all your future imaging, with this camera and gain setting. 🙂

I forgot to mention that higher light pollution will mean shorter exposures before this sky ADU is reached, compared to a dark site, so if you move to a site with different light pollution you'll need to redo your test exposures.

Oh and great image by the way. 😊

Alan

Many thanks Alan - i will most definitely read through this properly tonight. I am very interested indeed!

p.s. I have corrected the camera model to ASI2600MM 🙂

[symmetal]

8 hours ago, AstroGS said:

Many thanks Alan - i will most definitely read through this properly tonight. I am very interested indeed!

It looks more complicated than it is. If you want to read the theory behind it then these two extensive threads should explain all. And it's vlaiv approved. 🙂

https://stargazerslounge.com/topic/393694-swamp-read-noise/

https://stargazerslounge.com/topic/391776-gain-on-533mc-pro-with-fast-lens/

If you're using default offset 50 then the table as shown is fine, and you're most likely using gain 100 all the time, to use the benefit of HCG, so that one line entry is all you need be concerned with.

The ADU 717 is just a general figure to aim for, (too avoid too much under or over exposure), it needn't be exact. When the Moon's around, the sky background will be brighter, hence more sky background noise, and you will reach these read noise swamping ADU values at much shorter exposures, but it's best to keep using the same exposure you determined under optimum conditions to save keeping track of different exposures all the time.

You can quickly determine your optimum exposures, rather than trying test exposures, by examing one of your raw  subs from each filter. I initially thought you had a OSC ASI2600, which is why I mentioned the OSC methods, but as you have a mono camera it's easier.

First look at your master bias and read off its average value. The camera offset contributes to most of this bias and you want to remove this bias, containing the offset, from your calculations and just determine the ADU values added by your sky background. On my camera this is 503 ADU. Note than 500 of this figure is purely the offset as adding 1 offset value adds 10 ADU to your image. Just using 10 times your offset would be pretty close anyway to this bias figure.

Just read out the average sky background ADU value displayed when moving your mouse over the image, as any capture program should display. 

Say 10 mins exposure gives 1000 ADU sky background

Subtract the bias signal,  1000 - 503 = 497

Subtract the bias from optimum swamping ADU value, 717 - 503 = 214.

Optimum exposure is then  214 / 497 * 10 mins = 4.3 mins

These calculations are mainly useful for LRGB imaging. For narrowband you will likely not achieve these optimum exposure ADU values, particularly with Oiii and Sii, with 10 mins, unless you're in a light polluted sky so 10 mins is possibly worth sticking to.

For LRGB much shorter exposures can be used. As I mentioned each RG and B will have an optimum exposure but just choose one exposure for all 3. Perhaps use Blue to determine what that is. Optimum Luminance exposure will be around 1/3 of this length.

If you wish I can give you a copy of the Excel spreadsheet I posted above, which also includes the calculations to do the above sums. You can tailor it to any camera by filling in the table from the ZWO data sheets. 🙂

Alan

Edited October 23, 2023 by symmetal

[Phillyo]

There's also the option of using Sharpcap Pro to do a sensor analysis, then using that data you can plug it it into NINA and use the optimum exposure calculator and it'll tell you what to aim for. I think mine was around 289 seconds so I just go with 300 sec (5min) subs. YMMV.

Phil