mgutierrez

18 minutes ago, han59 said:

I will phrase it differently.  It should be measurable higher then the pedestal. Assume your pedestal value is 1000 ± noise. For a short exposure the sky background introduces maybe 1 extra resulting in 1001 ± noise. This  could be difficult to measure. The pedestal could drift or the sky background signal could drown in the noise or being significant less then the  flat correction . A sky background addition of 100 will likely be measurable resulting in 1100 ± noise.

So for short exposures the SQM can be measured but could become unreliable. 

Calibration is best for accuracy.  You can apply the routine on calibrated lights and enter for the pedestal 0.  Or  provide darks (and flats & flat darks) in corresponding tabs for automatic calibration .  Else type in the pedestal value manually based on a dark. How better the routine can measure the sky glow influence the more accurate the SQM value will be.

Han

 

 

Thanks for the reply Han. That's much clearer for me. Regards

Han, thanks for such a nice feature.

I have a question. Point #2 requirement states that background value must be higher than pedestal; if not, expose longer. The provided light must be uncalibrated, right? So, given a particular pedestal; how is possible that the corresponding light could have a lower background even with very short exposures since that light has already the pedestal?

For example. Let's say my pedestal is 400, as measured from the corresponding dark. The corresponding light should have always a background value higher than 400, since the pedestal is included in the light. Am I missing something?

m

https://www.cloudynights.com/topic/864002-siril-tutorials/?p=12498709

I also noticed this. Site is down since a couple of days ago. Any news?

hi all,

just a quick update. Switching to Astronomik MaxFR version of OIII filter, has led to a HUGE improvement:

27min of an uncalibrated OIII stack (same object). Left, the maxfr version. Right, the older one. Note the brigther corners on the right image.

Watch the corresponding calibrated stacks (on the left the maxfr):

The difference is evident. Note, however, that maxfr calibrated stack has still some gradient. But that's a completely different history. I expect that kind of gradient: bad flats (OIII flats are specially tricky), light pollution (OIII filter is more sensitive to lp compared to Ha or SII), etc. I consider that's completely normal. In fact, a standard ABE or DBE gets rid of it completely. Not the same with the other stack. Is a pain to deal with it in post-processing.

Well, I just wanted to share with you my findings.

m

23 minutes ago, teoria_del_big_bang said:

Another great image Miguel, this is a target I have tried recently but weather was not good and much clouds passing stopped me getting any decent detail.
I take it this was with the Esprit 100 and the QHY268M ?

Like the others above I love the image and you have capured lots of the dark stuff 🙂 

Steve

Thanks Steve. Yes, esprit and the 268. Great combo 😄

14 minutes ago, Ouroboros said:

That really is a nice image, @mgutierrez.  I like that blue against the brown dust. It really is a rather special object the Iris isn’t it?  My ED80 doesn’t really do it justice because as an object it’s quite small.    It would be interesting to know how many subs/filters etc you used if you’d care to tell us. 

yes, sorry, I missed the details (pasted from astrobin):

Frames:

Astronomik Deep-Sky Blue 36 mm: 15×120″(30′) (gain: 56.00) f/5.5 -5°C bin 1×1
Astronomik Deep-Sky Green 36mm: 15×120″(30′) (gain: 56.00) f/5.5 -5°C bin 1×1
Astronomik Deep-Sky Red 36mm: 15×120″(30′) (gain: 56.00) f/5.5 -5°C bin 1×1
Astronomik L-3 Luminance UV/IR Block 36 mm: 178×60″(2h 58′) (gain: 56.00) f/5.5 -5°C bin 1×1

Integration:

4h 28′

Darks:

50

Flats:

20

Bias:

500

36 minutes ago, carastro said:

Excellent result and details.   Love the dust and Iris core detail. 

I presume you mean you first image upload on here and not your first image ever.

Yes, sure, first image I upload here, not my first image ever 😄

Thanks for the comment

Hi all,

This is my first image I upload. One of my favorite targets. Hope you like the rendition.

7 minutes ago, scotty38 said:

Just to be clear I wasn't suggesting you had reflections and the blackened edges won't stop light getting past if there is a small gap. That's what the Buckeye masks prevent. If this is a problem for you I guess it's not unreasonable that one filter/slot in the filter wheel could be affected more than another. Like I say just a thought.

yep, sure, I got your point, thanks!

1 hour ago, scotty38 said:

Is there a possibility it's a light leak past the filters which is "different" for real images versus flats hence the effect when stacked? I use the Buckeye filter masks with my filters and it made a big difference to the reflections I was seeing.

May be different for you but just wondered if it was worth a thought that's all.

yes, I thought about reflections. But this is the second filter I try. I also guess that it should affect to other filters as well; at least not only to OIII. Furthermore, this filters have already a blackening edge. I'm in contact with another user and starting to think that this is the "normal" behaviour of this filter and the trick is to take the proper flats. Honestly, I don't know how a light with white corners can be considered normal, but now I'm considering it.

1 minute ago, Adam J said:

No its not normal for that to happen i have a Esprit 100 and I have never seen anything like that. If that is not calibrated at all then I would think that its an issue with the filter, I don't have first hand experience as I use mounted filters but I know that people used to get something like that with the older V1 ZWO 36mm filters that did not have edge blackening applied. Maybe take a look at the filter in question and see if there is any chipping of discontinuity in the edge coating. 

Adam

Thanks for the reply. I also think is not normal. These filters have already blackening edge. Also, this filter was replaced by astronomik because of a similar issue (see 1st post). My guess is that for some reason I cannot understand, I'm meeting the requirements for a perfect storm.

8 minutes ago, Adam J said:

Over correction is normally caused by poorly calibrated flat frames. Did you correctly match dark flats to the flats in temperature offset gain and exposure? 

I will summarize the question: do you think is normal to have bright corners instead darker ones in a single light?

1 minute ago, teoria_del_big_bang said:

You say obviously the same is that definite ?
Are we sure the master dark is not affecting anything other than hot pixels etc ?
Light leak whilst taking darks could be an issue , just a thought.
 

Steve

absolutely sure steve. I was going, in fact, to upload the non-dark version but I saw some hot pixels and I didn't want you to think I'm a bad boy ;-D

2 minutes ago, Stuart1971 said:

So the single light with the reverse vignetting is straight from the camera with no calibration at all…? Is that correct….?

That's it. Well, only calibrated with darks to remove hot pixels (without darks, the result is obviously the same)

8 minutes ago, teoria_del_big_bang said:

Hi Miguel, Is this an issue you think you have had all along because lately you have had some magnificent images and not mentioned any issues.
Or is it just this target because as I say I am no expert but this is not an easy target and is a very faint object.
When I look at the single frame in PI there is actually very little signal there.
So I could be wrong here but if I were to attempt this target with NB filters I think I would be going for more than 180 seconds exposures so that the signal is far greater than the noise. Not that that should affect any vignetting (I wouldn't think). So I know you have a fair number of frames to stack (40) and usually 180 secs * 40 would be good for this camera maybe this target on NB requires longer exposures.
I really cannot say why the reverse vignetting but if you look at the actual pixel values of background  in the corners compared to the pixels of background closer to middle there is very little difference I think it is just the low signal means that PI is really boosting the stretch to see the nebulosity and so any changes in pixel values no matter how small look bad to the eyes.
Sorry I cannot help further but probably I am not experienced enough on this.
 

Steve

I'm having this issue from the beginning. I deal with it with post processing, but I would like to get rid off it. Many times makes the post processing quite difficult. It has nothing to do with the target. Longer exposures makes more evident the issue. In any, case, being well exposed, the final stack will reveal the gradient independently of the individual exposure time.

21 minutes ago, newbie alert said:

I'm pretty sure on the flats, do a stack without them and the edges where they show now as light will be dark

So for me that means your flats are over correcting

its a 16 bit camera so what's the adu that you're aiming for?

 

No. That's the problem. The stack without flats shows lighter corners. Download the file ending in NOFLATS from the dropbox link

1 minute ago, newbie alert said:

It's a slightly over correcting flats...

Also your stars look abit strange, are you using the flatener?

forget about the flats. Do you think the "reverse vignetting" on the single light is normal? (the gradient is more prominent in the stack)

PS: thanks for pointing out about the stars. Yes, I do use the flattener. I usually have a quite flat field. This time I rotated the cam so I may messed it up; no worries

Hi Steve,

Thanks for the reply mate

You can find here some files: https://www.dropbox.com/sh/6ajza48ha3vkcpg/AAD-F6PtCeUilew4x189jBPRa?dl=0

There is a single light, the master flat, the non flat calibrated master light and also the calibrated one. The gradient on the single light is not so evident. After stacking, it is revealed completely.

I always have some kind of vignetting on all of my lights with different filters. What makes this vignetting special, is that is reversed. That is, corners are lighter, not darker as usual. My flats, as you can see, look like a normal flat: corners are darker. But since the lights have an opposite gradient, what the calibration does is to exaggerate these gradients.

I want to focus on the lights. If that gradient is normal, then I would focus on taking proper flats (sky-flats, a different flat panel, etc.). But to me, those gradient present on the single lights should not be there. That's basically my question.

m

Hi all,

I'm searching someone who shares this setup with me: QHY268M camera, Astronomik OIII 6nm 36mm filter (non-MaxFR) and a Esprit100.

I'm experiencing a weird issue that I cannot fix

Here is a only dark calibrated master OIII:

Note the white gradient towards the corners. My master flat looks like a regular one: darker in the corners and lighter towards the center. But since the gradients are reversed between them, the flat calibrated OIII master is even worse, with more white gradient.

My master flat may be bad or not. I don't know and honestly I think it's not important: the white gradient is present on my uncalibrated master, and I think it's not normal (or it is?!)

This filter was replaced by astronomik a couple of months ago for a similar issue. Similar, cause the gradient was mostly present on one lateral; my guess is that the other lateral had some spot on the filter. So I think it's not a filter issue.

I also swapped the OIII position within the fw, just in case. Same result.

Shooting different objects at different altitude and completely different position, makes no difference.

What do you think?

m

PS: sorry if I posted in the wrong forum

5 hours ago, vlaiv said:

You are swamping it by factor of 5.07 / 1.61 = ~ x3.152

that matches very exactly with the stddev practical aproach (to have a stddev > 3xbias_stddev on the background) among other calculations I'm playing with.

I was aware I was using ADUs, and noticed as well that converting to e- led to more correct results, but I thought it also should work with ADUs. Clearly is not

Thanks a lot for your help and clarifying, Vlaiv. Very useful

Thanks for the prompt reply@vlaiv

I need some time to digest your info 😅 and make some tests.

Just wanted to clarify your doubts. Read mode is effectively 1. At gain 56 the camera enters high gain mode and read noise decreases drastically, as you can see in the graphs.

Furthermore, read noise, gain and other values, have been calculated with basic ccd parameters pixinsight script, and also compared with other colleagues which have characterized the qhy268m. I would trust the results.

Regarding the dark noise, yes, I though about it but since this cam has a very low dark noise and the sub is not too long, I ignored it for this example. Maybe I shouldn't do that.

I will come back to you with the proposed tests.

Thanks a lot for your time

Hi @vlaiv,

I'm trying to make the same exercise with one of my lights, just out of curiosity. Let me explain, cause I cannot figure out what's going on.

I have a 60sec light (M33): https://www.dropbox.com/s/j9bglelhgmkh4v1/M33_Light_L_60_secs_035.fits?dl=0

And the corresponding bias: https://www.dropbox.com/s/qh0ah4ix1g5shnn/M1_G56_OFF30_Bias_300.fits?dl=0

Note the following parameters:

Gain: 0.333 e-/ADU

Bias mean value: 486.9 ADU

Bias stddev: ~4.9

Read noise: 1.61 e-   ---> 1.6 / 0.333 = 4.83 ADU

ADC depth: 16bit -> no need to convert from DN to ADU

I've read multiple times that a more practical approach to make sure I'm swamping the read noise by x3 (read noise, not LP signal, as you correctly clarified) is to get a clean region of my light, with no stars nor nebulosity (only background) and measure its stddev. I want this stddev to be at least x3 higher than my bias stddev. This makes completely sense to me. But cannot match this approach with yours. I'm doing something wrong.

** editing to complete post. Pressed sent accidentally **

Using ImageJ and defining a clean region, I get a stddev of ~17, which is > 3x bias stdev (4.9 x 3). That means is sufficient swamped. Furthermore, using Rista's formula, I get a very close DN compared with the current DN background of my image using a swamp factor of 10 over the background signal (don't want to extend explanation with this calculations). I assume is right

Using the same region, I get a mean of ~565 ADU. Background is, hence, 565-486.9 = ~78. Read noise is 4.83 ADU (I guess I need to use ADUs here...). Then, I need a signal of (4.83*3)^2 = ~209 ADU.

78 / 209 indicates I'm clearly underexposed (~0.37 times).

What am I doing wrong here? Why the results are so different. I'm clearly missing something quite trivial here...

Thanks

Nice shot Steve!

LP does not deteriorate the photo at all. Color and nebolisity process is outstanding. Looking forward to see your progress!

m

No worries Dave; thanks for the feedback!