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Old 28-05-2021, 06:13 PM
DRCORTEX (Lance)
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What shoul darks look like ?

Finally decided to go for guided shots. I've taken nurerous flats, just a plain white image, so I guess that is correct, but when I look at the dark frames I have taken ( at night, scope cover on ), they seem to be what I would call noise.

Is this what is expected ?

Many thanks,

Lance
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Old 28-05-2021, 06:46 PM
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Yep.
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Old 28-05-2021, 07:26 PM
DRCORTEX (Lance)
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Thanks John,

My first venture into this.

I usually use hyperstar, but want to move into F/6.3 as it opens up more possibilities, and the images, etc, are more demanding.

Many Thanks,

Lance
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Old 28-05-2021, 07:46 PM
DRCORTEX (Lance)
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lol, I've also seen spikes on one side, all I can put it to is the moon, which is full. just shows you how much light, or lack of, effects things.

Might take the camera inside, cover the lens, and try again.

Cheers,

Lance
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Old 28-05-2021, 07:58 PM
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A normal image will have signal and noise - hopefully a favourable ratio towards the signal (SNR) but a dark has no signal, just the noise, so they will look like a frame of noise. Normal

Spikes on one side of the frame may be amp glow. This can be calibrated out with darks.
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Old 28-05-2021, 08:22 PM
DRCORTEX (Lance)
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Quote:
Originally Posted by Craig_ View Post
A normal image will have signal and noise - hopefully a favourable ratio towards the signal (SNR) but a dark has no signal, just the noise, so they will look like a frame of noise. Normal

Spikes on one side of the frame may be amp glow. This can be calibrated out with darks.
Was the moon, must be a slight gap in the scope cover. Threw a black jacket over, and all good. I actually thought my ZWO filter drawer may have been the culprit, all a learning experience

Cheers,

Lance
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Old 28-05-2021, 08:27 PM
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By your equipment listed I assume you are talking about the 294MC?


Firstly darks will show a fair bit of noise, stacking software that auto stretches will stretch a dark really hard so it looks like a mess.


The ASI294 has a characteristic "Starburst" pattern on the top right, a dimmer one top left and a dimmer one again bottom right. They look horrible under a heavy stretch but if you look at the pixel values they are not really very much.
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Old 28-05-2021, 08:40 PM
DRCORTEX (Lance)
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Quote:
Originally Posted by Craig_ View Post
A normal image will have signal and noise - hopefully a favourable ratio towards the signal (SNR) but a dark has no signal, just the noise, so they will look like a frame of noise. Normal

Spikes on one side of the frame may be amp glow. This can be calibrated out with darks.
I think in the case, it was simply light getting in, via a tiny gap between the corrector plate and cover, all good.

I would like to know how many flats/darks make a decent sample.

I have taken flats ( tshirt ) at 30/60/180secs at LOW, MEDIUN/HIGH with the ASI during the day, cooler on, trying to get close as possible to the lights I have already taken, and camera exactly the same orientation.

Now working on the darks, as I thought, not much use imaging with a near full moon., with same parameters.

Am I wasteing my time in some ways, it takes a lot of time.


Cheers,

Lance
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Old 29-05-2021, 12:27 PM
srmnm (Will)
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See attached for examples of Flat, Dark and Bias.
Hope this helps.
Attached Thumbnails
Click for full-size image (Master_Flat_GreenFilter.jpg)
24.7 KB23 views
Click for full-size image (Master_Bias.jpg)
148.7 KB24 views
Click for full-size image (Master_Dark.jpg)
147.6 KB23 views
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Old 29-05-2021, 06:27 PM
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Quote:
Originally Posted by srmnm View Post
See attached for examples of Flat, Dark and Bias.
Hope this helps.
Firstly don't use bias frames with this camera, use flat darks which are the same exposure as your flats but with the cap on.
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Old 29-05-2021, 06:32 PM
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Quote:
Originally Posted by DRCORTEX View Post
I think in the case, it was simply light getting in, via a tiny gap between the corrector plate and cover, all good.

I would like to know how many flats/darks make a decent sample.

I have taken flats ( tshirt ) at 30/60/180secs at LOW, MEDIUN/HIGH with the ASI during the day, cooler on, trying to get close as possible to the lights I have already taken, and camera exactly the same orientation.

Now working on the darks, as I thought, not much use imaging with a near full moon., with same parameters.

Am I wasteing my time in some ways, it takes a lot of time.


Cheers,

Lance
without showing us an image it's hard to tell, if you did not use darks you have amp glow, you only need to do darks once a year if you use the same gain and temperature. It's not a waste of time.
The 294mc has amp glow and it's there as a light in the sides, the moon would give gradients
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Old 29-05-2021, 08:41 PM
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As an example, this is a 300 second master dark shot with my old ASI294MC Pro. This is stretched really hard as Astro Pixel Processor does that with darks. This is normal for a 294.
Attached Thumbnails
Click for full-size image (MD-ISO_gain_120.0-exp_300.0s-85subs-ZWO_ASI294MC_Pro_Cam1_-4144x2822-AR_8.0_4.0-avg-St.jpg)
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Old 29-05-2021, 10:48 PM
srmnm (Will)
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Firstly don't use bias frames with this camera, use flat darks which are the same exposure as your flats but with the cap on.
You bothered to share any science here? 'Secondly' or otherwise?
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Old 30-05-2021, 05:42 AM
gregmc (Greg)
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Originally Posted by srmnm View Post
You bothered to share any science here? 'Secondly' or otherwise?
I get the impression that people, when describing a process, just carry over processes from CCD to CMOS without knowing why. It just becomes what everybody does.
I do remember seeing some explanations of why no bias but I also remember ZWO mentioning it so I did a quick google search on ZWO seeing you mentioned a ZWO camera.

Check out this from ZWO. In particular, I think the second last paragraph mentioning no bias frame.
https://astronomy-imaging-camera.com...-amp-glow.html

i.e. "Unlike CCD cameras, to ensure you properly remove glows, you should only subtract just the master dark. Do not use a master bias. A master flat may still be used."
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Old 30-05-2021, 08:47 AM
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Originally Posted by srmnm View Post
You bothered to share any science here? 'Secondly' or otherwise?
Sure
ZWO recommends Dark Flats or Flat Darks over bias as their sensors don't work too well with very small exposures Hence bias frames are useless and tend to stuff up the flats when combined, the best flats and dark flats for these cameras are in the average range of 25000 to 30000 adu at 2-3 second exposures for flats and the same for the flat darks (with the cap on) the flat darks are more or less bias frames better matched to the flats.

These cameras work better this way then the old CCD or dslr's
If you follow cloudy nights or the ZWO cameras facebook page it's all over it. as for scientific explanations I'll leave that to the nerds
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Old 30-05-2021, 11:17 AM
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Regardless of if you meant it that way or not, having a crack at people is not going to help anyone.

The advice changes from camera to camera, but for the ASI294MC (And possibly the new ASI294MM) bias frames are not useful. They are used to subtract the repeatable read noise from other frames. The ASI294MC sensor is a SOC (System On a Chip) and for exposures below a certain threshold value (Which no one seems 100% sure on, somewhere less than 5 seconds apparently) the exposure timing is done on the sensor chip. Below that the external circuitry takes over. That means that the read noise for a bias frame (Taken at the shortest possible exposure) is different to the lights, darks, flats etc. Because of that bias frames are not useful with the ASI294 camera.


For flats I used a panel (before I had a panel I used a screen with a dim grey image on it) and an exposure time of 10 seconds with the camera cooled, and dark flats under the same conditions.


My ASI2600 is a different kettle of fish and bias frames work properly with it.

Last edited by The_bluester; 30-05-2021 at 11:28 AM.
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Old 30-05-2021, 12:50 PM
srmnm (Will)
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Quote:
Originally Posted by gregmc View Post

Check out this from ZWO. In particular, I think the second last paragraph mentioning no bias frame.
https://astronomy-imaging-camera.com...-amp-glow.html

i.e. "Unlike CCD cameras, to ensure you properly remove glows, you should only subtract just the master dark. Do not use a master bias. A master flat may still be used."
Dont think anyone is having a crack at anyone here. I'd rather have someone back up their comment with an explanation rather than a 'do this one-liner'.

Note - comment from ZWO in same article:

Support@ZWO March 1, 2021 at 10:05 am
Bias,Dark,Flat all needed.

Clearly there are confusing messages coming from ZWO on the topic.
But, I find that using the bias (even though my dark-flat's might also be picking up the bias) helps out out with calibration.

OP sorry this got off track.
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