pixel rain

[alcol]

Advice please.

I have been taking Ha images of the Monkey Nebula. ASI1600 mono camera cooled to -25C using ZWO filters, 240s subs at gain 200 ofset 50. With ED120 scope used Darks of same exposure and Flat darks applied to Flats. 44 subs were stacked.

Never seen this before on other subs taken, but when calibrated, registered and integrated I get a downward pattern on the image. I have integrated the subs without using the darks, so it doesn't seem to be the darks that are causing the problem. The pattern can't be seen on individual images

I have attached the image along with the expanded section of the image which shows the lines down the image. I subsequently took another evening of subs and the lines appeared on these as well. Took new darks and flats an applied them but the lines were still there.

Can anyone offer any advice as to what it might be.

Thanks for looking

[blinky]

Did you use Bias frames?  If so try without them

[alcol]

Hi blinky, thanks for input. No I didn't use bias frames as the advice seems to be that they are inconsistent when using ZWO ASI1600MM CMOS cameras. The strange thing is that I have taken many HA images in the past and this effect hasn't been seen.

[alexxw24]

Strange pattern.
Did you used darkframe scaling? The subframes are dithered?

Try to stack the subframes uncalibrated, then only dark calibrated and so one to find the source of the pattern.

Please send some feedback as curious to get to know where it comes from.

[alcol]

Hi Alex
My dark frames were the same length and camera temperature was the same (as far as I can recall!) so no scaling was used.
Now stacking the subframes uncalibrated and then only dark calibrated. Hope to report back this evening

[alcol]

Alex  Should also have said that the light frames were dithered, one in two frames

[alcol]

Back quicker than I thought.
Stacked the 44 HA subs (240s 200g 50 ofset SGPro settings) UNCALIBRATED

See attached, image has the same pattern. Conclusion seems to be that the pattern was developed in the taking of the subs. In the past these have been fine, so either something has affected the equipment, lens, filters or camera.

OR I have inadvertently changed one of the SGPro settings which controls the taking of images.

Any further comments appreciated

[alcol]

Further info. I have attached a single, un-calibrated 240s sub which doesn't show up the pattern Maybe it wouldn't on 1 sub?

[bulrichl]

I'm quite sure that the subframes that resulted in the integration were not dithered, even though unintentionally. The integration looks as if drift between the subframes produced these streaks from warm pixels ("walking noise"). You can take a look at the SGP logfile in order to find out whether the frames were actually dithered or not.

Bernd

P.S.: It is easy to prove whether missing dithering is the culprit:

Just integrate your not registered light frames. I bet that the warm pixels will coincide then.

[alcol]

Thanks Bernd. Stupid question, but could you point me at the SGPro log file?

I will now stack the unregistered subs

[alcol]

The image of the same Ha subs stacked with no registration or calibration is attached

[alcol]

Further bit of investigating done:

Couldn't find the SGPro logs. However, checked the PHD logs and found that approx 5s after every other sub was recorded there was an entry:
""4720 Mount: notify guiding dither settle done success=1""

Looking into SGPro's Control Panel under the "Auto Guider" settings, dither was ticked, Medium Dither was set and every 2 frames was set.

It looks as if Dither was set to run??

Views anyone?

[bulrichl]

Usually the SGP logfiles are stored at
C:\Users\<username>\AppData\Local\SequenceGenerator\
as "sg_logfile_YYYYMMDDHHMMSS.txt" or (latest versions) as "sg_logfile_YYYYMMDDHHMMSS.log".

Is it possible that in PHD2, dithering was set up only for RA, not for Dec (see appended screen section)?

Bernd

[alcol]

Thanks Bernd
My PHD settings have always been as attached and they seem to have worked in the past.

Found the log file folder as you suggested but only have 2 recent logs listed and they are not relevant to this issue. Is it possible that logs are turned off somewhere?
Many thanks for your continuing interest,

[bulrichl]

In the shown orientation of your NGC 2174 integration, north is at the left hand side. The dithering was applied in RA only which corresponds to movements in east - west (= up and down in the image).

A similar case was discussed here: https://www.cloudynights.com/topic/631276-is-ra-only-dither-enough/ . In this case the orientation was north at the top, dither in RA only, which resulted in horizontal streaks.

I guess the implication is clear: sufficient dithering in both RA and Dec will generally avoid this kind of artifact.


However, additionally it might be that your image calibration is not appropriate. Please show the used settings in ImageCalibration (append a screen section or the process icon).

Bernd