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sshenke

Poor image quality- ? cause

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Tthis might be a daft question,but would appreciate your thoughts  please.

on the last few imaging sessions, i am having to use longer exposure times to see DSO's and even with that, image quality is much much poorer. For example, 3 weeks ago, i only had to do a 15 second long exposure to see the elephant trunk nebula and last night, when the sky was much more clearer, it took me a 300 sec long exposure to see it and the quality was very disappointing. the optical train, filter , time of viewing -( at least 2 hours post sunset ) etc were exactly the same on both occasions. only difference was the temperature - about 4 degrees compared to  14 degrees previously. camera was not cooled on either of the session. the other thing i found out as an afterthought when i looked at the telescope about 10 to 15 minutes after bringing it indoors was the presence of dew/ condensation occupying almost the entire face of the primary mirror and the backing of the mirror was soaking wet. No dew bands or other such stuff were used. Would be very helpful if someone could shed light on what i am doing wrong. sorry for the long post, but i thought it would be helpful to give as much information as possible.

heq5 pro mount, asi1600 mono camera, good guiding ( similar to my previous standards, that is)

thanks

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also just wanted to add that the image quality was poor right from the start as it was at the end of 3 hours of imaging, which i think might be relevant when considering the possibility of dew/ condensation being the cause

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It would be very helpful if you could describe in which way was the image poor compared to previous attempt. Maybe even post images for comparison?

If all things equal, I would say that 300s image can be poorer because of cumulative guide error / seeing so it can be blurrier than 15s one. Although you say that your guiding was good (any chance of guide graph screen shot, or at least RMS figures? Maybe guide log from your session? What do you guide with?) it could be the case that 300s is true picture of what your guiding is like, while in 15s you don't really see effects of guiding/seeing- too short exposure.

If image quality suffered in signal department - not enough detail visible in 300s exposure vs 15s exposure, it can be due to number of reasons - level of stretch (how do you examine your images, is there auto stretch applied?) - in this case 300s is in fact better sub but you are not seeing that on your screen, or it can in fact be due to dew - it will very much kill any signal to the lowest possible level (only very bright stars) and hence SNR.

You can examine if dew is the case by looking at subs across your session - there should be evidence of things getting worse (unless the scope managed to dew up before you even started taking subs).

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thanks vlaiv, the guiding is done through phd2. RMS error about 0.4 seconds on both the occasions. i think the main issue with the images is poor light gathering. unstretched 15 sec sub taken previously looks absolutely fantastic compared to the unstretched 300 sec one. i will try to post the images here later. like you said, it's only the bright stars that standout

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just to add, i started imaging within about 20 minutes of getting the scope outdoors, not sure if this is enough time for dew to build up. have had this scope and mount only since this summer, so don't have any previous winter experiences to compare with. 

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Could be ice on the sensor. Really you need to post some images. 

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9 minutes ago, sshenke said:

just to add, i started imaging within about 20 minutes of getting the scope outdoors, not sure if this is enough time for dew to build up. have had this scope and mount only since this summer, so don't have any previous winter experiences to compare with. 

Don't think 20 minutes is enough for dew to start forming. It does depend on how your scope was stored, but if it was not already close to ambient temperature (practically being outside, or at least in a wooden shed / garage that provides little to none warmth compared to outside temperature) it will take at least that long to bring scope close to ambient, and I'm sure it will not have time to cool down more than ambient (that is needed for dew to really start building up).

 

9 minutes ago, Adam J said:

Could be ice on the sensor. Really you need to post some images. 

I would say that ice on sensor is ruled out by this:

1 hour ago, sshenke said:

camera was not cooled on either of the session

 

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Sounds like dew, or dust in the atmosphere... you might have experienced a number of nights of hazey seeing?? is/was there a bush/forest fire within a few hundred km of you location recently?

Did you perhaps leave a filter inline by accident??

Did you leave the focusing bask on the objective?

 

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as far as my subjective assessment goes, on the recent occasions when we had clear sky, there was no haze or dust and i was so positive that i was going to get fantastic images. Focussing which i do on every session was fine as well.

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So, I have attached the images here. the top one was taken on 22 Sep, - exp Time- 240 secs _ Luminance filter

the middle one was on 2 Oct= exp time- 306 secs- Luminance filter

Sorry I have deleted the image with a much shorter exposure time, so cant post it here for comparison.

The other issue I had last night was that I could not even centre the image before starting to take the subs, as the image was too faint and reducing the exposure time (even to around 30 seconds!) to see which way the stars (at least the brightest one) were moving meant that they completely faded away.

also attached at the very bottom is a 300 second Luminance sub of Andromeda galaxy, again taken last night.  pretty unimpressive for an exposure this long. This was the first time I imaged it, so no prev images for comparison.

All the above are unstretched images.

Thanks for all your advice.

 

2019-09-17-2236_8-L-CapObj_0000.thumb.PNG.18fa0c3fc064a9c9ae219ac3cc7c2eb9.PNGCapObj_004939_0001.thumb.PNG.fa10eda4656da06494c30ff80ff0c526.PNG

 

CapObj_230539_0001.thumb.PNG.c5b770a61621f30bf7c2c638ef0a83c2.PNG

Edited by sshenke

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when i was targetting the Andromeda galaxy, I could see a better image of it in my guidescope feed ( 3 second exposure loop) on phd2, than i could see on the main camera feed which was on about 30 to 40 second exposure loops.

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Could you by any chance upload fits straight from camera of that M31?

That looks rather "ok" if it is unstretched.

Quick stretch of 8bit data from the image you attached looks quite ok:

image.png.59c71c88e52bf6be6723192918439a9d.png

Could you give more info on the kit used to image this?

What is the focal length of your scope? This was ASI1600MM but without cooling (I mean camera is cooled but cooling was turned off or is it model without cooling?), if so what settings did you use (gain, offset).

Just for comparison - here is 60s sub I've taken with 80mm scope and ASI1600 cooled at -20C also unstretched:

image.png.5f749b3c152c8c0ce010a73df9e0e0ae.png

Not much to be seen either. Even when stretched it is not much better, and certainly not as good as your 5 minute sub even at 8bit:

image.png.aca18ba27525e0028352131ad42079ee.png

I'm just wondering how much of what you see in the image is due to way image is stretched, and if you don't have any issue at all with scope and camera and just need to bring images to equal footing to compare them.

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Hi vlaiv, those images posted are straight from the camera, not stretched or processed in any way. the kit i use is heq5 pro mount, asi 1600 mm pro cooled, but cooler was turned off, guidescope is asi120 mini.  

while i can still process the andromeda galaxy, i can see a dramatic reduction in light collection compared to what i have seen before. as i said, the biggest and fundamental issue is that once the mount is set to a new target using the goto function, i can't even confirm if it has reached the intended target without taking 100 second long exposure or centre the target in the fielf of view. All dso's i had imaged previously ( during the summer nights!) would be easily viewable within 20 to 30 second exposure, but that is almost impossible now.also, as you can see from my sub of M31, it is off centre and i left it as it is as attempts at centring failed because of the long exposure needed to see where star has moved to with each manual movement of the scope

main scope is skywatcher 130 pds, focal length of 650mm

Edited by sshenke

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Can you check following: What is bit precision on that .png file that you are downloading? Is it 8bit or 16 bit?

I can't tell if SGL website engine is doing anything with attached image (like optimization for web display). If it is not - ones that are attached above are 8bit, which is very very bad thing. You want 16bit format for your subs.

Also, what sort of gain / offset are you using when capturing?

If you have issue with attaching .png file so that it stays the same (unaltered by SGL engine) - take one sub of m31, put it in zip archive and upload it.

Btw, it is better if you capture your subs in fits format. It is format meant for that, so it will contain all the capture parameters (provided capture application writes those along with the image, and it should).

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Thanks so much for yourhelp vlaiv  really appreciate your time.

i will check the bit type and post thatinfo later. in terms of gain, i always keep it at zero. offset- i didn't even know this functionality existed. also i willfind out howto change the capture format to .fit. thanks. will try and post theimage in zip format as per your suggestion.

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