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Author Topic: Sky Background value in DSS/Baader 3.5nm  (Read 3432 times)

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Offline pomerol

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Sky Background value in DSS/Baader 3.5nm
« on: 12:02:31, 11 February, 2016 »
I have been using a Baader 7nm Ha for a couple of years and recently acquired the 3.5nm from FLO who have a 30 day "no quible"on their sales.
I had 3 hours of IC417 though each filter with same sub length, same calibration, on a different night so sky conditions different.
The tif image from th 3.5nm is MUCH noisier than the 7nm.The sky background reading in DSS is 0.06 for the 3.5nm and 0.37 for the 7nm.I have looked for some understanding of what these readings mean but cannot find an answer.
Any information on the Sky background or experience with he 3.5nm most welcome.
Ian S.
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Offline chris.bailey

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #1 on: 15:02:08, 11 February, 2016 »
Ian

The 3.5nmHa image may well just "look" noisier and DSS is not really the best thing to be making any sort of judgment on.

Because of the narrower bandpass, less of the sky background makes it through giving very low background values so the noise is going to be more obvious. Thats a GOOD thing as faint Ha signal is better separated from the background allow it to be teased out a little more in processing.

HTH

Chris
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline pomerol

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #2 on: 12:06:17, 12 February, 2016 »
Thanks Chris, that is useful but would still like to know what the DSS Sky Background readings actual mean (if anything)?May need to go to the DSS Forum.
Rgds
Ian S.
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Offline chris.bailey

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #3 on: 12:48:37, 12 February, 2016 »
... but would still like to know what the DSS Sky Background readings actual mean (if anything)?May need to go to the DSS Forum.

In the DSS manual it says - "Sky background value for registered image in %" but does not enlighten as to what its a percentage of - Average, Saturation, Image Maximum, Median etc. It also does not say what is used to compute the value of sky background - Values below a certain sigma of median or what. The DSS forum is probably the best bet. What I normally do is open the stacked image and sample around the darkest regions. I then aim for that to be around 10% of Saturation after stretching.
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline pomerol

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #4 on: 23:52:19, 14 February, 2016 »
Chris, had a few comments on the DSS forum on the sky background  but none explained the meaning of the numbers or the how calculated.

Just did 3 hours of 20 min subs Bin 1x1 on sh2-264 and again extremely noisy.Did some basic post processing and worked on the noise with Neatimage  and Astronomy Tools but looks to me very poor.I have put it in Dropbox at:https://www.dropbox.com/sh/ajz11za7g3ynwk8/AABcriCNr8D3_hUDZy4O2dFaa?dl=0.
I am using an Atik 383l on a 10inch R-C.
My question is, is this what I should expect from 9 subs of 20 minutes and would another 3 hours make a big difference?
Rgds
Ian S.
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Online tomhow

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #5 on: 09:34:56, 15 February, 2016 »
Dropbox says "this folder is empty" when I click on the link.

Are you using the RC in your sig? Is it operating at F8 or is there a reducer involved?

Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

The Curdridge Observatory

Offline pomerol

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #6 on: 10:38:50, 15 February, 2016 »
This link should be better....
https://www.dropbox.com/sc/t5tgji51hdzo584/AABRWhn__FmOOuiEXbn-5dmza
I have a .66 reducer so running at F5.3
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Offline chris.bailey

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #7 on: 10:52:12, 15 February, 2016 »
Ian

Its quite hard to tell from a .jpg, especially one that has been processed already but a few comments -

There is quite a lot of pattern noise in the image. If you are not dithering subs, then that would really help. I have not seen pattern noise in my 383 so this might be a result of using a heavy application of NeatImage.

I tried the 383 on my 10" RC and it was not a good combination. Pixel scale is under 0.5"/pixel which is very much oversampling. You can try binning to get a better image scale but the 383 does not bin nicely as the readout registers have the same depth as the pixels. When 4 are summed together it is easy to overwhelm the readout registers so the stars can get 'blobby' and in extreme cases you get small horizontal blooms. Being heavily oversampled at f8.0 makes for sloooooow imaging. I now run a TV CCD67 reducer on mine to get about f6.2 which not only gives a speed benefit but a better field of view and image scale with the 7um pixels on my H16 (circa 1 "/pxl)

Based only on the jpg you have captured pretty much what I would expect for 3 hours of integration time. I aim for 5 hours on these sort of targets and am happier with 8.

You don't mention doing Bias, Darks and Flats. My experience of the 383 is that good calibration is essential and with Ha frames I do Bias, Darks, Flat Darks and Flats. Flats need to have a good signal to noise ratio to avoid injecting noise into the light frames but overexposing them can cause issues so tend to stick to about 1/3rd full well or about 20k ADU. I routinely do 200 Bias, 50 Darks and 36 Flats.

The final issue I have seen with the 383 is supply voltage. If it drops below about 12.5 volts the noise floor rockets. I run my off a 13.8volt bench supply and it really does make a huge difference to feed it a bit of extra juice.

Chris
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Online tomhow

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #8 on: 10:52:47, 15 February, 2016 »
The data is not unreasonable for the combination. Maybe trying binned 2x on some of these fainter ones?

There does seem to be some curious mottling artefacts on the image which I don't quite understand. Or am I am seeing things?
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

The Curdridge Observatory

Offline pomerol

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #9 on: 14:12:47, 15 February, 2016 »
Haven't used Dropbox much so didn't realize you could send big files so converted to jpg.
I had calibrated with darks only and dithered (using Tom's Maxim phd-dither).
The pattern noise/artefacts I have never had using the Ha 7nm but always get it when using the O3 8nm filter binned 2x2?
I am using a mains converter so hopefully voltage is not a problem.
Will try binning 2x2 tonight and see what I get with a full calibration, if not good, will have to thing whether  3.5nm with the R-C is right for me.
Thanks for input.
Rgds
Ian S.
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Offline pomerol

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #10 on: 11:17:13, 16 February, 2016 »
I did 3 hours last night on the same target with slightly different positioning and have calibrated with flats/bias and darks.If anything it is more noisy than the 1x1 with again a mottling which I cannot explain.
See https://www.dropbox.com/sc/xm3oblo7jtyel7n/AAC5ppU-HFhHpmYnzrXPe8Xna.
Applied  only one iteration of Neatimage on this version but did more noise reduction on other versions but mottling remained a problem.
Starting to think, that Chris's comment about 383/R-C  compatibility is at play here and may abandon ship on the 3.5nm.
Rgds
Ian S.
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Offline chris.bailey

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #11 on: 12:43:02, 16 February, 2016 »
Ian

If you have the space (and time) can you put the uncalibrated subs, master Bias, Dark and Flat up on Dropbox and I'll try and diagnose. Just post a like to the folder.

Chris
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline pomerol

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #12 on: 14:51:22, 16 February, 2016 »
Chris, think I finally got yhere with Dropbox...as requested.
Rgds
Ian S.https://www.dropbox.com/sh/3jr39lwwuqd2kyd/AADcLTCMrZcd87dYx5iuxWGxa?dl=0
Skywatcher Quattro 12 inch F4 on NEQ6 Pro
Skyshed Pod and Pier
Atik383+L
EQMOD / CS5 / Maxim DL5
Baader Ha /O111/S11 Filters
Altair  80mm Guidescope
QHY5 Guider Camera/PHD

Online tomhow

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Re: Sky Background value in DSS/Baader 3.5nm
« Reply #13 on: 15:04:29, 16 February, 2016 »
do you have an fits from targets other than 264? That is a particularly low contrast object, even by Sharpless standards!
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

The Curdridge Observatory

Online tomhow

  • Tom How
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  • Making equipment is far more fun than buying it.
    • The Curdridge Observatory
Re: Sky Background value in DSS/Baader 3.5nm
« Reply #14 on: 15:05:21, 16 February, 2016 »
i.e. can you upload the IC417 fits - comparing the two filters?
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

The Curdridge Observatory

 

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