exposure tables for ASI1600 (and possibly QHY163?)

[Shiraz (Ray)]

Hi

The ASI1600 allows the user to choose almost any combination of settings -
question is, which is best.

To answer the basic question of how to choose a sub-length, an astrograph model was used to generate tables of suggested sub-lengths for the ZWO1600 under a wide variety of conditions – broadband and narrowband. These could be helpful in navigating between different combinations of camera gain, fNo and sky brightness. Should be good enough to find a starting point if new to the camera, or to see what might happen if you try something different – attempted to keep assumptions reasonable, but your system optics/sky/filter etc may differ, so don't expect extreme accuracy.

BROADBAND GREEN AND YELLOW - 21.5 and 21 sky

http://astrob.in/291972/0/

BROADBAND ORANGE AND RED - 20 and 19 sky

http://astrob.in/291973/0/

NARROWBAND - 21 and 18 sky

http://astrob.in/291974/0/

To use the tables, first determine your sky brightness. If possible, measure with an SQM, but otherwise, find your colour zone from a sky brightness atlas – maybe try this as a starting point http://www.skyandtelescope.com/astro...-your-skyglow/

When you know your sky brightness/zone, go to the appropriate attached table and identify the column headed by your scope fNo. Then choose a gain from the first column – lower gain for dynamic range, higher gain for low noise short subs or narrowband. If you are new to the camera, suggest that you start out at gain 50 or 75 and that you use the original ZWO intermediate offset default of 21. Now look directly across the row for your chosen gain setting and find the sub-length that is also in the fNo column for your scope. If you cannot find the exact fNo for your scope, or want to use a gain value that is not listed, look at the surrounding data and guess – that will be good enough.

The value in the table is the shortest sub length that you can use (for that scope, sky and gain) and still be shot noise limited (5%RN criterion) - this is the most efficient way to image. The suggestion is definitely not a hard and fast rule though - you can use longer subs with no loss of signal to noise ratio, but more stars will be saturated. You can use shorter subs, but the total exposure time to get to a given image quality will be longer.

To refine the sub length, first take a sub and use the cursor of your acquisition software to look at the ADU values in a background sky region of the scene (no stars and no hot pixels, preferably away from vignetting). Dither the cursor about slightly to estimate the average ADU background value in the clear region. From this background ADU value, subtract the bias value for your camera (again - use the cursor to estimate the average ADU signal in a bias frame taken at the same gain and offset as the light sub). Then compare your sub-minus-bias value with the “expected ADU value above bias” in the final column of the table. If your sub-minus-bias is lower than expected, increase the sub length to compensate – if you get much a higher value, consider reducing the sub length.

The Narrowband tables should be used in the same way as the Broadband ones, although there are more unknown variables to upset the accuracy – so use as a guide only. Because of the wide possible variability in parameters, I chose to generate tables only for the extremes of dark sky and full moonlit conditions.

You could use the same gain setting as for broadband, but you may get slightly better results using higher gain – 200 would be a good value to try at some stage. If you do change the gain, you will need new dark and bias calibration and could also set the offset higher (maybe use 50).

I assumed 7nm filters, so if you have narrower filters, you could use longer subs – (or vice versa).
The estimates should be reasonably applicable to commonly used Ha, O3 and S2 filters.


Some comments:
Don't go by the visual appearance of a sub – with the 1600, they can look very thin, but when stacked, the final result will be fine – so ignore the fact that subs from the 1600 can look weak.

The modelling assumes an average optical efficiency. If you use a high quality refractor, you could possibly get away with shorter subs and if you use a highly obstructed scope, or one with standard Al mirrors, you will need longer subs.

The tables may possibly also be applicable to the QHY163. I don't have access to one, but found a couple of posts on other forums/threads suggesting that 139 gain on the 1600 corresponds to gain 12 on the 163. If so, and if the mapping is the same, maybe divide the values in the 1600 gain columns by 12 and go from there – eg, assume that gain 75 on the 1600 corresponds roughly to gain 6 on the 163.

If you change any of temperature, gain or offset, you will need new bias and dark calibration data.

your bias value should be at least a couple of hundred ADU - if you measure less, then increase the offset value.

If you decide to use very short subs, be aware of the processing load – it can take hours to process a few hundred 16mpix subs.



Thanks for looking, hope the above is useful. Regards Ray

[glend (Glen)]

Great stuff Ray! Your tables are spot on for what i am using for broadband and narrowband in my mag20 and full moon sky. Obviously this builds on the chart you created last year for broadband sub lengths.
Very useful contibution for people just starting out with the ASI1600MM-C and for old hand to check their mode of operation. I agree absolutely with you comment about not getting fooled by single sub appearances. I have played around with various gain settings and different targets but i always seem to come back to Unity (139).
I have your old chart on the wall in the observatory over my laptop, this will go up now.
Thanks for all your valuable work on this topic.

[Atmos (Colin)]

Great work Ray, coincides well with what I see (albeit with 3nm and not 7nm filters).
What I really take out of it though is that I should never be imaging from my red zone back yard

[Slawomir (Suavi)]

Thanks Ray. Actually I also coincides very well with my experience with ICX814 (gain 0 and narrowband). Colin, don't complain, I image from an infra-red zone;best SQM I measured was about 18. Usually it is worse than that.

[lazjen (Chris)]

Nice tables - wish I had them last night as it looks like I was overdoing it.

What's interesting is that some of these exposure times are less than dithering times. I think I'm going to have to look at optimising the dithering more to not lose as much time.

For LRGB imaging I've started doing LLRGB then dither, repeat. I might have to extend that to something like LLLRRGGBB then dither, repeat.

[Slawomir (Suavi)]

Originally Posted by lazjen

Nice tables - wish I had them last night as it looks like I was overdoing it.

What's interesting is that some of these exposure times are less than dithering times. I think I'm going to have to look at optimising the dithering more to not lose as much time.

For LRGB imaging I've started doing LLRGB then dither, repeat. I might have to extend that to something like LLLRRGGBB then dither, repeat.

Interesting approach to collecting the entire range of subs and repeating the whole cycle Chris. I did very little RGB imaging, but used to aim to collect blue channel when the object was at its highest in the sky.

[lazjen (Chris)]

I do it so in theory I have at least something to work with when a few cycles go through - the weather is so variable I've had very limited time for imaging.

Another reason is that I'm concerned about the dithering time when taking a lot of short images - as I mentioned previously. I know I need to dither to stop any potential fixed pattern noise appearing and with a large number of subs that could be an issue.

Tonight I'm going to put these tables to the test (for broadband) before the moon comes up. I think I'm in an orange zone, maybe red, but I think it's safe enough to start working off orange and go from there. I was using gain 75, but I'm either going to go to gain 0 or 200 - haven't decided yet.

I have to think about the narrowband values more because I wonder how much amp glow will affect things with such long exposures?

[lazjen (Chris)]

I thought I might as well try gain 200 (offset 50) last night and jump in the the mega-number of images processing...

If I was reading the stats data correctly last night, I think it showed I am probably closer to Red zone than Orange, however I compromised and went with 15 sec images for my F6.5 scope. I also switched to LLLRRGGBB then dither, repeat.

I managed to get 783 light frames across LRGB before the clouds came over and stopped everything again. I know there were a few problems with some of the frames when the guiding, which was overall very good, went whacko in DEC a few times.

To add to it, I discovered the temperature adjustment with the focuser is not as good as I had hoped (I haven't calibrated it at all, just used default settings), so I'll have some softer focused images to deal with. I think I'll change to include a refocus periodically.

Anyway, I'm starting to process the images and at the rate the machine is chugging along, it's going to take sometime. Hopefully I can get at least a basic result before the end of the day.

[glend (Glen)]

Ray, looking at your exposure suggestions for f10 (where my unreduced Edge Hd sits), it occurs to me that the guide problems I presently have with DSOs at f10 may not be an issue, provided I shoot at normally unguided sub durations (but just do alot of them). Am I correct about that? Seems like a modified DSO Lucky Imaging strategy.
For example, look at the Orange Sky chart, 20mag, Gain 200, f10, I can shoot 35s subs. Given my precise PA, I don't expect any noticable drift at that sub length.
Are there other factors at play in this?
Even though my location is in your Green zone chart, I see no reason that I could not shoot using the Orange zone durations but doing many more subs.

[lazjen (Chris)]

If I understand correctly, you'd just need more total exposure time to get the image quality. There's a about 4:1 difference in the times from the Orange to the Green, but I don't know if you'd need > 4 times the number of subs?

If you're doing it unguided and you believe you've got good PA, are you going to dither at all? Are you concerned about possible fixed pattern noise if you don't, especially given the number of images you'll be taking?

[Atmos (Colin)]

You can dither without guiding

[lazjen (Chris)]

Yes, I'm aware of that, but Glen didn't mention if he was going to do that or not.

[glend (Glen)]

Normally i don't dither with this camera. I used to dither everything when using my mono cooled DSLR, as it had a bad column. I have not noticed a pattern noise problem in my 1600 images and gave had no comments about it from those looking at them. To me dithering wastes a significant amount of time, as you need to allow settle time as well, so your probably looking at 10-12s. On long sub narrowband thats not an issue but if your shooting short subs, dithering after every one is a big additive to overall time required. I could see dithering after maybe every 10 to 20 subs, and there was a thread about that on CN. I can set that interval up in SGP. Also consider this, if i am shooting short unguided subs, there is likely to be some 'natural drift', despite near perfect PA, which could likely replace dithering as pixels shift over a significant series of subs. These are things to explore, but first i want to try f10 unguided subs. If my stars are good then i will worry about these other things. First i need to clear storage space and get the new TB drive connected.

[lazjen (Chris)]

Yes, I think that's a fair enough approach. It's something I might consider doing later, although the dither after 9 images seemed to be ok. I guess that would take up the time for another image though, so that would be a 10% loss of imaging time overall.

As for storage, yes, I can relate. Part of my morning has been making enough space on the processing machine to handle the different stages. I'm just trying to integrate the L channel now - 261 images is taking a while. I can't remember exactly when I started it, but it's 1.5hr so far.

[Shiraz (Ray)]

Originally Posted by glend

Great stuff Ray! Your tables are spot on for what i am using for broadband and narrowband in my mag20 and full moon sky. Obviously this builds on the chart you created last year for broadband sub lengths.
Very useful contibution for people just starting out with the ASI1600MM-C and for old hand to check their mode of operation. I agree absolutely with you comment about not getting fooled by single sub appearances. I have played around with various gain settings and different targets but i always seem to come back to Unity (139).
I have your old chart on the wall in the observatory over my laptop, this will go up now.
Thanks for all your valuable work on this topic.

thanks Glen. I have pretty much settled on 100 gain for broadband.

Originally Posted by Atmos

Great work Ray, coincides well with what I see (albeit with 3nm and not 7nm filters).
What I really take out of it though is that I should never be imaging from my red zone back yard

I Have not tried imaging in anything approaching red - must be soul destroying.

Originally Posted by Slawomir

Thanks Ray. Actually I also coincides very well with my experience with ICX814 (gain 0 and narrowband). Colin, don't complain, I image from an infra-red zone;best SQM I measured was about 18. Usually it is worse than that.

thanks Suavi - blimey 18!!

Originally Posted by lazjen

Nice tables - wish I had them last night as it looks like I was overdoing it.

What's interesting is that some of these exposure times are less than dithering times. I think I'm going to have to look at optimising the dithering more to not lose as much time.

For LRGB imaging I've started doing LLRGB then dither, repeat. I might have to extend that to something like LLLRRGGBB then dither, repeat.

Originally Posted by lazjen

I do it so in theory I have at least something to work with when a few cycles go through - the weather is so variable I've had very limited time for imaging.

Another reason is that I'm concerned about the dithering time when taking a lot of short images - as I mentioned previously. I know I need to dither to stop any potential fixed pattern noise appearing and with a large number of subs that could be an issue.

Tonight I'm going to put these tables to the test (for broadband) before the moon comes up. I think I'm in an orange zone, maybe red, but I think it's safe enough to start working off orange and go from there. I was using gain 75, but I'm either going to go to gain 0 or 200 - haven't decided yet.

I have to think about the narrowband values more because I wonder how much amp glow will affect things with such long exposures?

yep, dithering is an annoying overhead - as is refocus. I haven't tried alternative dither concepts - be interested in what you find. Your idea of cycling through the filters has a lot going for it - thanks for posting. I don't think that the amp glow issue is as significant as it seems - the amount of glow signal is really very low and it can be easily calibrated out.

Originally Posted by glend

Ray, looking at your exposure suggestions for f10 (where my unreduced Edge Hd sits), it occurs to me that the guide problems I presently have with DSOs at f10 may not be an issue, provided I shoot at normally unguided sub durations (but just do alot of them). Am I correct about that? Seems like a modified DSO Lucky Imaging strategy.
For example, look at the Orange Sky chart, 20mag, Gain 200, f10, I can shoot 35s subs. Given my precise PA, I don't expect any noticable drift at that sub length.
Are there other factors at play in this?
Even though my location is in your Green zone chart, I see no reason that I could not shoot using the Orange zone durations but doing many more subs.

I would think that you can shoot very short subs with some loss in sensitivity. FWIW, I tried 20 second subs one good night and got some blindingly good resolution, so it definitely might be worth seeing what you can get at f10 - provided your target is bright.

Originally Posted by lazjen

If I understand correctly, you'd just need more total exposure time to get the image quality. There's a about 4:1 difference in the times from the Orange to the Green, but I don't know if you'd need > 4 times the number of subs?

If you're doing it unguided and you believe you've got good PA, are you going to dither at all? Are you concerned about possible fixed pattern noise if you don't, especially given the number of images you'll be taking?

I think that you roughly need 4x the number of subs if they are 1/4 as long - will probably be a bit more, since the shorter subs may not be strictly shot noise limited and you could have additional read noise to deal with.

[lazjen (Chris)]

I can give you one bit of feedback so far - processing is taking longer than image collection. Just doing the basic integration (no drizzle integration) of the last channel now - so probably about 5 hrs+ integrating.

Since I generally have no real problem with guiding, I'm considering switching to gain 0 and doing 90 - 120 sec subs just to cut down on the number of images and speed up the integration process. And if I keep with the same dithering process I mentioned previously, the time lost to dithering will be a lot less overall. Refocus time applies either way, so no changes there.

[Atmos (Colin)]

Originally Posted by lazjen

I can give you one bit of feedback so far - processing is taking longer than image collection. Just doing the basic integration (no drizzle integration) of the last channel now - so probably about 5 hrs+ integrating.

Since I generally have no real problem with guiding, I'm considering switching to gain 0 and doing 90 - 120 sec subs just to cut down on the number of images and speed up the integration process. And if I keep with the same dithering process I mentioned previously, the time lost to dithering will be a lot less overall. Refocus time applies either way, so no changes there.

Probably wouldn't drop below Gain 75 otherwise you'll start running into some other issues. The reason for this is that the chip only outputs 12-bits while at Gain 0 it is approaching 13-bits of dynamic range. At Gain ~75 it is capturing 12-bits of information.

[lazjen (Chris)]

Thanks for the tip - and now it looks like I'll be going full circle - I was doing Gain 75 the other night, but for far too long according to the tables.

I just had Pixinsight crash as I was trying to make a rough assembled image to post. Fortunately I've got the base data saved so I can have another go soon. I wasn't too impressed with the results I was seeing though - it seemed very noisy.

Edit: no, something has gone wrong somewhere in the process as I'm getting banding (I think that's from the master dark which I've reprocessed), but weirdly I'm getting a deeper black near perfect circle area that's not apparent in any subs I can see. If I can't work it out, I'll post it elsewhere to troubleshoot.

[glend (Glen)]

Just bumping this back up as i think it should be readily accessible. I am about to head out to the obs to start up a sequence and had to hunt this thread down to get a sub time reference.

Good work Ray.

[rustigsmed (Russell)]

Originally Posted by Shiraz

Hi


The tables may possibly also be applicable to the QHY163. I don't have access to one, but found a couple of posts on other forums/threads suggesting that 139 gain on the 1600 corresponds to gain 12 on the 163. If so, and if the mapping is the same, maybe divide the values in the 1600 gain columns by 12 and go from there – eg, assume that gain 75 on the 1600 corresponds roughly to gain 6 on the 163.

Just to update QHY users who may use this info there was a driver update around May I believe, and at that time qhy changed the gain scale on the qhy163.

The gain range is x10 to get more smooth adjustment. (before it is 0-58
now it is 0-580.So the system gain =11 will changed to system gain
=110)

might avoid some confusion