Jump to content

Narrowband

Help with settings please


Recommended Posts

Hi Hopefully a easy question for your experts. 

I'm using a ZWO385MC camera settings gain 135 exp 60s using a L-eNhance filter with a 102mmAPO refractor.

Looking at the fits file which is 20 mins in total. I have saturated red areas. Does this suggest i should reduce exposure or gain or take it down from 16bit to 8 bit? 

Thanks for looking, Oh this is supposed to be the Crescent nebula  😁

Cheers

Dean 

Stack_16bits_20frames_1200s02.fits

Link to comment
Share on other sites

Looking at your fits file there is almost no red signal present. I converted it to 16 bit tiff and loaded it into Photoshop and gave it a levels stretch. As the histogram shows, green is dominant as would be expected but the red is only showing very faintly in the brighter nebula area.

Untitled-1.thumb.png.a80ad30a64f97e18ce07bd51d4438cd8.png

If I do a rough background equalization you can see the red histogram content is negligible.

Untitled-2.thumb.png.23bfe8a9fb4f8a3c425d95ce3f8bfca4.png

You need to have much longer exposures to get a better red narrowband Ha signal. The filter's green and blue OIII bandpass width is about three times wider than the red Ha bandpass width so your red signal will be the weakest of the three. If possible, then 3 to 5 mins exposures will be better and plenty of them. I assume you're in a light polluted area so too long exposures may overexpose the green too much though. I'd still have thought you would have got more red than you have though, looking at sample results from 60s exposures with this filter

L-eNhance.png.d95ca8b02377d9cf34585361fd30e273.png

Definately don't select 8 bit instead of 16 bit. It will make things far worse with heavy posterization of the stretched data. 🙂

You can increase the camera gain if you wish to reduce the exposure duration. I'll have to check what the electron/ADU gain settings are on the ASI385MC first.

Alan

 

Edited by symmetal
  • Thanks 1
Link to comment
Share on other sites

Abig thankyou for looking into the data for me very helpful. My sky bortle is 4/5.  The electron/ADU gain settings for this camera is 135. My camera is uncooled also may be noisy?  I may struggle with 5mins duration on the AVX mount. 

I guess i will have these issues with other  nebula's such as lagoon 

Would it work if I went back to the IR/UV cut filter and tried the gain at 135 and 60s exposures? 

 

Cheers

Dean

Link to comment
Share on other sites

Bortle 4/5 isn't too bad for light pollution so I assume you got the filter to try narrowband imaging. I think it's a bit misleading to call the l-eNhance a tri-band filter as it's really a dual band filter with one band larger in width than the other, giving lower Ha signal. You won't get much difference between the blue and green signals whether you're going for H-beta or OIII. so you can't tell three bands apart

A better filter for dual band imaging with OSC in my opinion would be the IDAS NBZ nebula boost filter as it has similar pass band widths in Ha and OIII so the ratio of red signal compared to green/blue will be much higher. This is over twice the price of the Optolong filter though.

Your 135 camera gain setting of 135 is the optimum to use, as it is the unity gain setting of 1 electron/ADU and is also above the gain setting where the HCG kicks in giving lower read noise, so I'd leave it at 135.

The crescent nebula is generally brighter in OIII than Ha I believe, which doesn't help here. The Lagoon Nebula and those nearby are much higher in Ha, compared to OIII so they should give you significantly better results. Worth giving them a try.

Your IR/UV cut filter would be standard RGB imaging and not narrowband, so the lagoon etc. will still show up quite nicely with long enough total integration time, though the colours will be rather muted compared to narrowband.

Being uncooled your camera will be more noisy, particularly in the summer nights, but temperature dependant dark current noise isn't such a problem with CMOS compared to CCD. Taking more exposures to stack will mitigate the extra noise due to being non-cooled.

Alan

  • Like 2
Link to comment
Share on other sites

Gave it a quick go. Increased exposure to 180s, but need dark frames as some amp glow in top corner. Increasing exposure and keeping gain at 135 brought out a little  more Ha data.

Guess will need to save up for IDAS NBZ nebula, sounds more productive then the l-eNhance filter. 

Again thanks for your feedback and suggestions Alan

Dean 

  • Like 1
Link to comment
Share on other sites

Dean,

Looking at your original image posted the red signal is actually quite heavily black clipped which is why there's little or no red background signal. Is there some stacking feature causing this. Could you post one of your single subs which should give a better indication of how your filter is actually performing.

Also, what does your master flat frame histogram look like with this filter. Are there three peaks corresponding to the RGB outputs visible and none are getting clipped. Your green and blue peaks will likely be a lot further to the right on the histogram than the red.

Alan

Edited by symmetal
  • Like 1
Link to comment
Share on other sites

14 hours ago, symmetal said:

Dean,

Looking at your original image posted the red signal is actually quite heavily black clipped which is why there's little or no red background signal. Is there some stacking feature causing this. Could you post one of your single subs which should give a better indication of how your filter is actually performing.

Also, what does your master flat frame histogram look like with this filter. Are there three peaks corresponding to the RGB outputs visible and none are getting clipped. Your green and blue peaks will likely be a lot further to the right on the histogram than the red.

Alan

Thanks Alan for feedback, raw file from original image 60s sub gain 135 correct dark incorrect flat as i changed filter from previous imaging filter from a previous target

 frame_00012.tif

This was master flat i used in initial image

MasterFlat_l-enhance 030623initial 2023-06-03T19_35_24.fits

New master flat that i used last night

MasterFlat_040623 l-enhance 2023-06-04T19_12_19.fits

Also attached FIT  file from yesterday session. (not correct darks used) This is todays task :) Quite blustery  so 3min subs where a challenge with this mount today

Stack_16bits_4frames_720s02.fits

But I think data looks better then initial FIT file that  I posted

Thanks Alan  for taking time to have a look will be interested with your opinion

Cheers

Dean

 

Link to comment
Share on other sites

Dean,

Good news.  🤗 Your latest postings look fine. I converted the fits to 16 bit tiffs to show in Photoshop as it's easier to visualize the results.

The 4 frame stack unstretched. The red signal is present now and not clipped. It's the lowest level of the three as expected and looks good overall.

Newstackunstretched.thumb.png.84e56cad7b41242aab1e580499ae0e20.png

The 4 frame stack with a quick stretch and background neutralized. All colours good and quite reasonable for just 4 frames. Amp glow present but easily fixed with darks. For uncooled the noise is reasonable too. Both the master flats look fine too. No clipping which is main thing. Some bad pixels but a bad pixel map and/or dithering will fix them.

Newstackstretched.thumb.png.3feafc2b0b5f3e92208a840dcf6d7bad.png

For information, here's your single sub, which looked fine, stretched and enlarged greatly so that the single pixels are visible. The red pixels are the darker ones in each group of 4.

subpixels.png.ef90eac7b82d69cc2a8b83f18d273e13.png

Everything looking good now. 🙂 Not sure what happened with your first posting where the red was black clipped. You may as well continue with the L-eNhance as the red output level is better than I would have thought given the relative widths of the bandpass segments and is quite useable.

You just need plenty of subs now on each target and you should have some good results. 😊

Alan

  • Thanks 1
Link to comment
Share on other sites

6 minutes ago, gonzostar said:

Thats a relief, Thankyou Alan for time and helping me out with these issues. Fingers crossed for plenty of subs. How did you neutralise in Photoshop? 

You're welcome. 🙂In Photoshop you can use 'levels' or 'curves'.

First display the 3 channel histogram so you can see what you're doing.

Using levels, select each colour channel in turn and move the 'Input levels' left most slider to the right until all three coloured histograms are on top of each other in the top histogram. Select 'colors' as the histogram's operating mode so you can see the individual colours. You'll probably leave the red channel alone and bring the green and blue channels down to match the red.

If the histograms are all bunched up to the left so you can't separate them initially, select channel RGB in levels and move the 'input levels' centre slider to the left (stretching the image) until the RGB histograms are away from the left edge.

Using curves, again select each channel in turn and move the left hand (black) input slider to the right to achieve the same results in the histograms as above. 

Don't move any of the sliders too far to the right, (which moves the histograms to the left) such that the left edge of the histogram starts getting clipped. Always leave a bit of space to the left of the histogram while processing. As a final move you can start clipping the left of the RGB histogram to give the most visually pleasing result.

Alan

  • Thanks 1
Link to comment
Share on other sites

You just need more time on the target. At one point I had all three main optolongs at the time and each one went progressively better for narrowband signal except the lpro which is more a general LP filter. I thought the enhance was capable, but having the extreme negated the need to have both, and I'm sure the ultimate will be better. A lot of the Idas get decent reviews, but I wouldn't spend more on one at the moment, just image for longer. I use a 485 and usually do 60s subs and repeat for hours, over 120s and it does start to amp glow a bit.

  • Thanks 1
Link to comment
Share on other sites

Hi Dean,

Your dark frame has dealt with the amp glow well, but your focus looks a little off as it's softer than your last 120s stack you posted. Maybe your guiding is contributing so 120s may be a better exposure to use. Your hot pixels seem to have turned into black pixels and 'walking noise' is quite evident by the black pixel diagonal streaks.

A bad pixel map created from your darks should replace the bad pixels with the average value of the surrounding good pixels which will help with the bad/black pixels. When creating the master dark there should be a stacking option to create a bad pixel map too.

Dithering is the best way to randomize the walking noise making it less noticeable, if you have an option to dither in your capture software.

Good luck! 🙂

6x180s.png.0be034ba2819fb656212abde736c38f5.png

Alan

  • Thanks 1
Link to comment
Share on other sites

It would be boring if it was easy 😁 Yes those black pixels look bad. Is this worse due to longer exposures

Didnt see your message till later, so did another hour of 3minute subs. Agree that these exposures maybe to long with my guiding on the AVX mount. Also brighter  stars looking "blotted"  Also tried dithering 

Initial frames where 60s in duration so will give that another go. May risk 120s? 

 

I'm using sharpcap and PHD2 for guiding. Although PHD2 did dither however took a long time to settle back. Not sure how i alter setting in livestack for guiding. 

Anyway will try again 

Cheers

Dean

Link to comment
Share on other sites

Although unity gain is fine with UV/IR filters, I tend to use higher gain when I am using the L-Enhance filter, since there is less chance of the stars saturating, and the increased gain also means lower noise. I took a photograph of the Crescent Nebula last year, through an f/8 Takahashi TSA 102 mm refractor, using 3 minute subs, with a ZWO 071 camera with the gain set to 200 (on this camera unity gain is 90 and maximum gain is 240) and the sensor cooled to 0 deg C. Photo below is result from 3 hours of imaging (stack of 60 three minute images) from this session. I have a higher resolution photo of this if you want to see it (drop me a message). 

Crescentnebulav69thAug2022.thumb.jpg.fa9809f788006442bd0e62d51bee9b99.jpg

  • Thanks 1
Link to comment
Share on other sites

Wow that's a amazing picture thanks for showing. 

Unity gain for the ZWO385MC is 135 will try also to increase gain to around 200? and see what happens. May reduce exposures to 2 mins as my mount is quite susceptible to wind and guiding errors. Although i did 5 mins subs with my 70d DSLR! 

Do you use sharpcap? Trying also to figure out dithering using sharpcap, and after how many exposures do you tend to set the dithering?

 

So many questions 😁 but thanks again

Dean

 

Link to comment
Share on other sites

Dean,

The longer exposures as such won't make the walking noise worse. Slight polar alignment errors is a main cause of this, not enough to give you elongated stars in one sub, but over an hour or so of imaging the image will drift over the sensor, and as you align on the stars during stacking, the camera fixed pattern noise ends up appearing as streaks on the stacked result. This isn't random in its appearance, (it's called correlated noise), so isn't removed effectively by stacking. Dithering in a random direction will break up this correlation making it more random, and so more effectively removed by stacking. 

It can take a while to settle after dithering, especially if you dither a large amount, though you have to dither by a enough pixels for it to make a difference, at least around 6 pixels or more at least by looking at your image. You don't have to dither every frame, (though the more often the better),  just as long as you dither around 10 times at least in your imaging run. If you aim to take 40 subs then dithering every 4 frames should be ok for the stacking to help remove the bad pixels. You also need to enable sigma reject in your stacking options to make this work effectively.

Alan

Edited by symmetal
  • Like 1
Link to comment
Share on other sites

2 hours ago, iantaylor2uk said:

Yes sorry, I meant that I personally don't use sharpcap

Amazing photo's on your link.  Did look at ASI pro, maybe one day however need to save up! Like the idea that it's a all in one. At the moment using polemaster for polar alignment, Which i think is fine for the DSLR but not less forgiving with the OSC camera

Link to comment
Share on other sites

1 hour ago, symmetal said:

Dean,

The longer exposures as such won't make the walking noise worse. Slight polar alignment errors is a main cause of this, not enough to give you elongated stars in one sub, but over an hour or so of imaging the image will drift over the sensor, and as you align on the stars during stacking, the camera fixed pattern noise ends up appearing as streaks on the stacked result. This isn't random in its appearance, (it's called correlated noise), so isn't removed effectively by stacking. Dithering in a random direction will break up this correlation making it more random, and so more effectively removed by stacking. 

It can take a while to settle after dithering, especially if you dither a large amount, though you have to dither by a enough pixels for it to make a difference, at least around 6 pixels or more at least by looking at your image. You don't have to dither every frame, (though the more often the better),  just as long as you dither around 10 times at least in your imaging run. If you aim to take 40 subs then dithering every 4 frames should be ok for the stacking to help remove the bad pixels. You also need to enable sigma reject in your stacking options to make this work effectively.

Alan

Thanks Alan.  Also thinking that i should find then a alternative for PA and not polemaster.  Polemaster seemed adequate for 5min subs with the DSLR, The ZWO385MC seems less forgiving! Will figure out Dither setting in sharpcap. 

Cheers

Dean

  • Like 1
Link to comment
Share on other sites

Dean,

I use Sharpcap's Polar Alignment routine on my portable mount and you can get it better than 1 arcmin very quickly with instant readouts on how far off you are. It uses the guide camera for measurements and there's no need for the guide camera to be well aligned with the main scope either. I looked at Polemaster's setup procedure and it does require quite a few time consuming steps to complete, so it's worth seeing if Sharpcap may be easier for you.

Alan

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.