Iris Nebula

[Bobby1970]

Captured over a couple of nights. Approx 60 subs of 180s at 270 gain. with the 178mc and 72mm Semi apo refractor mounted on my AZGTi and using ASiAir Pro.

I used some darks and flats. but as i don't have a cooled cam i used darks from both sessions, not sure this is right or not tbh. 

Not as good as i hoped for tbh. Must try harder. lol

Thanks for looking anyway 🙂

Edited September 28, 2020 by Bobby1970

[tooth_dr]

Seems pretty decent to me Bobby, a job well done. 

[MarkAR]

Not too bad, I think somewhere along the processing you've pushed it too far and ended up with some black blobs. You have to be very careful with processing, little steps instead of bug jumps are best. Sometimes re-appyling the same process a few times in smaller steps.

[Bobby1970]

Can anyone shed any light on gain settings for the 178MC at all? 

I read something that said 270 gives lowest read noise so have generally been using this setting. Although i also read something that said the "scale" in sharpcap is different to perhaps ASIAir??

Surely there is an optimal gain setting which should give reasonable results most of the time. tbh i am unsure of the relationship between gain, dynamic range and offset (whatever that is). 

 

[vlaiv]

2 minutes ago, Bobby1970 said:

Can anyone shed any light on gain settings for the 178MC at all? 

Sure

4 minutes ago, Bobby1970 said:

I read something that said 270 gives lowest read noise so have generally been using this setting. Although i also read something that said the "scale" in sharpcap is different to perhaps ASIAir??

ZWO website has specs for ASI178 and there you can see relationship between e/ADU and read noise:

If we consult this diagram - about gain about 400 has lowest read noise, but differences are so small.

You shouldn't really concern yourself with read noise if you don't have cooled camera. For any sensible sub duration - thermal noise is going to swamp read noise.

Also note that ASI178 does not have sweet spot - all gain values are below unity since camera is 14bit and gain 0 is e/ADU of about 0.9.

Relationship between gain and read noise is given with above graph. Relationship between dynamic range and gain is a bit more complicated.

First - note that gain with ASI cameras is in 0.1dB units. Which means that gain 270 is actually 27dB increase over gain 0. If gain 0 is e/ADU of 0.9, then gain 270 will be ~22.387 smaller so about 0.04 e/ADU.

You can calculate this by decibel formula - value in dB is equal to 20 * log_base_10 (ratio_of_intensities). Now we can see what is maximum number of electrons that can be recorded with this gain factor - 14 bit max number is 16384 - that is our ADU, lets convert to electrons - 16384 * 0.04 = 655.36

We need to divide with read noise to get dynamic range. 655.36 / 1.38e = ~475. How much is that in bits? ~8.9bits. Dynamic range at gain 270 is about 8.9 bits. But you can check the graph:

If you are using sharp cap - it might be sensible to use ASCOM driver instead of native drivers for long exposure.

My ASI178 requires very high offset - last time I used it, I decided on offset 256.

[Bobby1970]

thanks @vlaiv

Seems to me that the lowest gain would give the best dynamic range?

Guess its a trade off of the two?

interesting point you make regards read noise and non cooled cameras, thank you. 

Im considering making my own cooler for the camera, i realise it will not be as good as a proper cooled cam but got to be worth a punt i reckon.

I use my ASiAir these days so am still unsure of how the "gain" setting in sharpcap equates to the one in AsiAir. Might enquire on the ZWO forums.

 

[vlaiv]

51 minutes ago, Bobby1970 said:

Seems to me that the lowest gain would give the best dynamic range?

Guess its a trade off of the two?

Problem with dynamic range is that it is completely useless thing in astrophotography

This is because we do stacking. Dynamic range might be useful metric for single exposure. It tells you how much exposure compensation you can get in post processing if you miss your exposure in regular photography.

In astrophotography we do stacking and each new stacked image increases dynamic range of the stack. Want better dynamic range? Just simply stack more images.

Another thing that dynamic range tells us is how likely it is that we will saturate bright parts of the image. Again - not a problem in astrophotography. With regular photography and single exposure - any saturation is simply lost data - no way of recovering actual pixel values.

Same thing happens in AP, but in AP we are used to taking multiple exposures and we can simply do "exposure bracketing" - take bunch of long exposures and in the end - take a few short exposures that we will use to recover signal that is saturated in long exposures.

56 minutes ago, Bobby1970 said:

interesting point you make regards read noise and non cooled cameras, thank you. 

Im considering making my own cooler for the camera, i realise it will not be as good as a proper cooled cam but got to be worth a punt i reckon.

I use my ASiAir these days so am still unsure of how the "gain" setting in sharpcap equates to the one in AsiAir. Might enquire on the ZWO forums.

Cooling in itself is not very important. Yes, it lowers the noise, but if ambient temperature is low enough - passive cooling these cameras have, especially in winter - does a good job.

Problem with passive cooling and even active cooling with fan is that it does not have set point regulation. You can't say - cool the camera to N degrees and keep it there. This is rather important if you want to match darks and lights.

If you go for some sort of DIY solution to camera cooling - maybe pay more attention to getting stable sensor temperature, by creating some sort of feedback loop, then to cooling efficiency. Again - this will be ambient temperature dependent, but if you can keep sensor at set temperature - that is a big plus.

With TEC - you need to be careful of icing and dewing issues. Maybe keep camera temperature above 0 and do what it takes to prevent dewing once camera body temperature drops below ambient.

[Bobby1970]

Today i came across some software called Topaz Denoise and thought i would have another crack at processing the Iris Nebula i captured a couple of nights back.

I know its a long way from perfect but i much prefer this one lol.