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ZWO ASI 294MC-PRO sensitivity?


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I’ve recently purchased a ZWO ASI294MC-PRO and last night was first light for this on my SW 120ED.

Conditions were not ideal for imaging, but to familiarize mysef with the camera I setup a comparison with the SW 50mm guidescope / ASI 120MM Mini using Sharpcap and APT. I had great hopes for the 294, especially in terms of sensitivity, but I have found that the 294 appears far less sensitive than the 120MM.

As an example, M57 using the 120MM with a gain of 54 and an 8 second exposure clearly showed the object and a multitude of stars. Whilst the 294 on the 120ED showed a much fainter M57 with only a few stars present. Only by increasing the gain on the 294 to >500 were the images comparable, but this then resulted in much greater noise.
It was a similar situation with M51. Using APT with a 2 minute exposure, there was no sign of the object using the 294, but it could be seen with the guidescope/120MM.

I’d be grateful if anyone could provide advice on this or indeed guidance or a link for initial settings to use with a 294. I realise that the migration from DSLR to a CMOS/OSC is a steep learning curve and so I’d like to eliminate user error before contacting FLO about a potentially defective camera.

Many thanks
Adrian

Edited by microbe
typo error
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Probably mostly  because guidescopes are usually around f2 and your 120ED is f7.5. You should use SharpCap Pro to do an analysis of your imaging train and suggest an exposure, gain and offset. I have a 294 and for my sky (which is quite dark when the streetlamps go out at 11pm) it recommends E250s G121 (unity - my choice) Offset 4 (yes 4) and I get very nice images (mostly - as you may see on my site). This is with an f4 scope so a good bit 'faster' than your 120ED. I love my 294 but it took me a year to get the best out of it, happy snapping!

Hope that helps, 

Duncan

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Not sure if it's the "speed" of the imaging scope vs guide scope that can explain this.

SW guide scope is F/4.8, or has focal length of 242mm, while imaging scope is F/7.5, or 900mm FL.

With ASI120 guide scope gives resolution of 3.2"/px, while imaging resolution is 1.06"/px.

Gathering surface of 120mm scope is 14400mm2 while that of 50mm scope is 2500mm2, ratio of these two is 5.76.

Ratio of pixel surfaces is 9.11.

Guide camera will gather x1.6 more light per exposure - that is not significantly faster to explain what's been observed (8s exposure from ASI120 should match 12.8s exposure from ASI294 - provided same QE).

If we assume that no extraordinary conditions arose - like dew on scope, or fogged up / iced up camera, only explanation that I could offer is matter of histogram stretch for preview.

ASI120 is probably working in 8bit mode, so ADUs are directly mapped to 0-255 monitor output. ASI294 is however working in 14 bit mode, that means 6 bits of difference, or x64 in intensity if we assume 14bit -> 8bit straight mapping. What is obvious in image from ASI120 in unstretched image will be very faint, almost invisible in x64 less bright unstretched image from ASI294.

Someone with more knowledge of APT could advise how to manipulate histogram for preview (is there auto stretch functionality or alike?)?

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Try using Sharpcap for testing as suggested. It gives you much more control over the camera 'in session' whereas with APT you have to temporarily disconnect to access the driver settings to change gain and offset.

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1 hour ago, vlaiv said:

 

SW guide scope is F/4.8, or has focal length of 242mm, while imaging scope is F/7.5, or 900mm 

I always thought the SW guide was @190mm . At least that's what I've always used for settings on mine when I used it with phd2. 

Not sure how much difference that makes on your calls. 

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5 hours ago, dunc said:

You should use SharpCap Pro to do an analysis of your imaging train and suggest an exposure, gain and offset. I have a 294 and for my sky (which is quite dark when the streetlamps go out at 11pm) it recommends E250s G121 (unity - my choice) Offset 4 (yes 4) and I get very nice images

Duncan

Thanks. I only use the ASCOM derived Unity settings (G120, O30) last night, so optimising this would be a good start

1 hour ago, vlaiv said:

Not sure if it's the "speed" of the imaging scope vs guide scope that can explain this.

SW guide scope is F/4.8, or has focal length of 242mm, while imaging scope is F/7.5, or 900mm FL.

With ASI120 guide scope gives resolution of 3.2"/px, while imaging resolution is 1.06"/px.

Gathering surface of 120mm scope is 14400mm2 while that of 50mm scope is 2500mm2, ratio of these two is 5.76.

Ratio of pixel surfaces is 9.11.

Guide camera will gather x1.6 more light per exposure - that is not significantly faster to explain what's been observed (8s exposure from ASI120 should match 12.8s exposure from ASI294 - provided same QE).

If we assume that no extraordinary conditions arose - like dew on scope, or fogged up / iced up camera, only explanation that I could offer is matter of histogram stretch for preview.

ASI120 is probably working in 8bit mode, so ADUs are directly mapped to 0-255 monitor output. ASI294 is however working in 14 bit mode, that means 6 bits of difference, or x64 in intensity if we assume 14bit -> 8bit straight mapping. What is obvious in image from ASI120 in unstretched image will be very faint, almost invisible in x64 less bright unstretched image from ASI294.

Someone with more knowledge of APT could advise how to manipulate histogram for preview (is there auto stretch functionality or alike?)?

Many thanks for your detailed reply. I agree, I'd have expected the additional light gather to have compensated for the slower speed of the ED120, which is why I was surprised. There was no dew on the lens and yes the ASI120 was in 8 bit mode.

I'm new to APT and it's complex compared with my previous BackyardEOS. I need to do some more reading 🙂

1 hour ago, david_taurus83 said:

Try using Sharpcap for testing as suggested. It gives you much more control over the camera 'in session' whereas with APT you have to temporarily disconnect to access the driver settings to change gain and offset.

Thanks, I will. Only just noticed your location. You can't be too far away from me, we probably share the same light pollution!

Edited by microbe
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9 minutes ago, Anthonyexmouth said:

I always thought the SW guide was @190mm . At least that's what I've always used for settings on mine when I used it with phd2. 

Not sure how much difference that makes on your calls. 

Mine is 242mm, F/4.8 

https://www.firstlightoptics.com/guide-cameras/sky-watcher-evoguide-50ed-guidescope-zwo-asi120mm-bundle.html

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13 minutes ago, microbe said:

Thanks, I will. Only just noticed your location. You can't be too far away from me, we probably share the same light pollution!

I get off M42 J2 almost every night on way home! Think my LP is probably worse lol

 

Yes, in Sharpcap you can adjust exposure, gain, offset (brightness) with sliders on the right. It continuously loops frames as well.

 

Just a thought about what's been said above, did you perform a stretch to the preview in APT? The default preview is no stretch?

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As you are moving from the DSLR, why aren't you comparing it to that? 

sorry but surely the 120mm on a different scope cannot give you any kind of meaningful comparison really..

interested in the discussion however, as i had been considering the 294c also

Good luck

Mike

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15 minutes ago, david_taurus83 said:

I get off M42 J2 almost every night on way home! Think my LP is probably worse lol

 

Yes, in Sharpcap you can adjust exposure, gain, offset (brightness) with sliders on the right. It continuously loops frames as well.

 

Just a thought about what's been said above, did you perform a stretch to the preview in APT? The default preview is no stretch?

Hope you don't have to go via M42 J3 as the roadworks are causing big delays 😞

I didn't perform a stretch, so I'm wondering if that was part of the problem

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18 minutes ago, mikeyj1 said:

As you are moving from the DSLR, why aren't you comparing it to that? 

sorry but surely the 120mm on a different scope cannot give you any kind of meaningful comparison really..

interested in the discussion however, as i had been considering the 294c also

Good luck

Mike

Thanks Mike. It was all a bit "last minute" and the battery on the Canon 600D needed charging, but you're right I need to do a comparison. On the whole the 600D works well, but is noisy when the sensor gets warm.

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12 minutes ago, microbe said:

Hope you don't have to go via M42 J3 as the roadworks are causing big delays 😞

I didn't perform a stretch, so I'm wondering if that was part of the problem

I come down from Longbridge and past King Norton golf club in the mornings to J3 to avoid J2 at that time. I got caught up in it Friday! 😓 Looks like its Barnt Green and J2 again for the next 6 months!

 

The good news with APT then is it saves all your images. Look on your C drive and APT_Images. There should be a dated folder with all your captures. Depending on what software you use you can load them up and perform a stretch. Or, you can run APT again and the Img tab on the right let's you navigate to the same folder and load the images up again. Then you can perform a preview stretch.

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Here you go! Run APT and under the tools tab make sure the Histogram button is on.

 

Capture1.thumb.PNG.73ecb8b108de03ec32e25bffbdc50f44.PNG

 

Then under the Img tab navigate to your folder. Clicking on one of the small previews on the right loads it onto the main screen.Capture2.thumb.PNG.f4518661c50165c63d5da817c42bffcb.PNG

 

On the Histogram bar press Auto-Str L and voila!

 

Capture3.thumb.PNG.503776c38472ab1492e62592fb8dbf1a.PNG

 

 

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20 hours ago, david_taurus83 said:

Here you go! Run APT and under the tools tab make sure the Histogram button is on.

 

Capture1.thumb.PNG.73ecb8b108de03ec32e25bffbdc50f44.PNG

 

Then under the Img tab navigate to your folder. Clicking on one of the small previews on the right loads it onto the main screen.Capture2.thumb.PNG.f4518661c50165c63d5da817c42bffcb.PNG

 

On the Histogram bar press Auto-Str L and voila!

 

Capture3.thumb.PNG.503776c38472ab1492e62592fb8dbf1a.PNG

 

 

That's great, many thanks

Adrian

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I recently bought an ASI224MC and found that it appeared to be less sensitive than my ASI120MC - basically it seemed to need longer exposures.

This was the case both in the kitchen with the fisheye lenses, and on Jupiter and Saturn when I had the IR-cut filter (which is supposed to have 90%+ transmission) on the ASI224MC.

I am aware that there are Sharpcap settings that might affect the exposure and might be defaulting to different things.  Despite this mystery, the ASI224MC seems to give better images.

Still  awaiting an informed explanation.

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What doesn't seem to have been discussed earlier in this thread is that the ASI294MC is a colour sensor, whereas the ASI120MM is mono. Hope I'm not missing something here, but here is a stab at an explanation (I'm just starting with understanding the differences between mono and colour imaging).

Due to the colour filters on the 294MC, photon capture rate will be reduced by a factor of 3. This is equivalent to the difference between stacking a single sub and three subs or alternatively the difference extending the overall imaging time by a factor of 3.

Taking into account the relative f#s of the two scopes and the different pixel sizes results in a 1.6x higher arrival photon count per pixel for the 120MM compared to the 294MC (as noted above by @vlaiv). The end result is the 120MM captures 3x1.6 = 4.8 times more photons per pixel than the 294MC (assuming same QE). For the same overall exposure time the SNR will be higher for the 120MM due to more photons being collected.

@microbe increased the gain of the 294MC image to try and match the target brightness of the two scope images taken with the same imaging time. This will increase the apparent on screen noise of the image. Part of this noise will include read noise and thermal noise, which will be a higher portion of the 294MC overall noise due to the lower photon capture rate. 

 

Edited by bobro
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On 13/05/2019 at 21:51, Anthonyexmouth said:

I always thought the SW guide was @190mm . At least that's what I've always used for settings on mine when I used it with phd2. 

Not sure how much difference that makes on your calls. 

I have the ED version of SW guide scope and that is 242mm, as he kindly answered a quest for me recently where I quoted the F/L it may have stuck in his mind, I must agree though the normal SW finder is smaller, maybe this is what is being used.

Alan

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