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Advice for monochrome imaging

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#1 AstroPharma

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Posted 28 September 2021 - 02:29 PM

Hello everyone,

 

I'm happy to inform you that I've been bit by the bug, and my wallet is currently despairing at my latest fit of passion. I've been a visual observer for years but I’ve been saving up to buy into the hobby for a good while, leaning heavy on the ‘buy once , cry once’ philosophy. Anyway here is what I went with:

  • WO GT81 + small guide scope
  • WO Flat6A III 0.8x Reducer / Flattener 
  • Ioptron GEM28 
  • ASI120MM-S for guiding
  • ASI2600mm pro 
  • ZWO electronic auto focouser
  • ZWO EFW 7x36mm
  • 36mm LRGB set 
  • 36mm NB set (Ha, OIII & SII)
  • Pegasus powerbox advance 
  • A 500w battery with a 12VDC/10A output 
  • A stellermate Plus 

 

I wanted to start with monochrome astrophotography straight away because it seems like a respectable challenge and a mentally stimulating learning curve. Durring my research, I learned all about the process, the calibration, the softwares available to me (Ekos or otherwise), some fundamental image processing skills but I could never figure out the ratios for LRGB or NB imaging. My question is, given my equipment and my frankly nonexistent time management skills, how does one logically structure a data capturing session in monochrome?

Let’s say I was to image a target and I had, say 7 hours to do so, would I divide that time equally for each wavelength? Do I use long (5 or 10 minutes) exposures or more shorter exposures? Perhaps in the future, I’d be willing to do multiple night projects but starting out I’d love to see some results from one full night sessions. I know that trial and error is the best enlightener in most cases, but what is a good starting point? 

 

Thank you all, Ali.


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#2 OldManSky

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Posted 28 September 2021 - 02:42 PM

Aw, shoot -- you already bought the stuff?  I was hoping I was going to get to help you spend some money!  smirk.gif

 

I think you've made some really good choices, and will have a very nice setup.

For a "starting point," try to get one-night shots done.  For ideally-placed objects, that usually means you can start when they're 40-deg. or so above the eastern horizon, go across the meridian, and image until they're 40-degrees or so above the western horizon.  How long that is depends on Declination, but somewhere between 8 and 4 hours in a night.  That's enough time to get good LRGB or decent HSO for narrow-band.  Get used to all the processing, etc. and then you can start doing multi-night collections.

 

For "ratios," try starting with 3x as much L as R,G, or B.  Adjust as you see fit depending on results.  For narrowband, do equal amounts HSO to start.  

Look at lots of videos, read lots of books and forum posts.  In a year you'll be helping out all the even newer newbies and will know everything to help them get started!

 

:)


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#3 AstroPharma

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Posted 28 September 2021 - 03:07 PM

Aw, shoot -- you already bought the stuff?  I was hoping I was going to get to help you spend some money!  smirk.gif

 

I think you've made some really good choices, and will have a very nice setup.

For a "starting point," try to get one-night shots done.  For ideally-placed objects, that usually means you can start when they're 40-deg. or so above the eastern horizon, go across the meridian, and image until they're 40-degrees or so above the western horizon.  How long that is depends on Declination, but somewhere between 8 and 4 hours in a night.  That's enough time to get good LRGB or decent HSO for narrow-band.  Get used to all the processing, etc. and then you can start doing multi-night collections.

 

For "ratios," try starting with 3x as much L as R,G, or B.  Adjust as you see fit depending on results.  For narrowband, do equal amounts HSO to start.  

Look at lots of videos, read lots of books and forum posts.  In a year you'll be helping out all the even newer newbies and will know everything to help them get started!

 

smile.gif

Thanks for the advice and the feedback on the setup, I was shamelessly fishing for it as I was in desperate need of validation after burning a hole into my savings :lol: .

 

I'm planning a trip to a dark site once I test my connections at home and I'll find a target that would be visible for 6 hours based on your recommendation, I'll do 12*300s per color and 36*300s luminance. Hopefully, nothing will spontaneously combusts.

 

Thanks again,

 

Ali.



#4 DRK73

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Posted 28 September 2021 - 03:11 PM

Part of your time management really depends on where you're imaging from. Are you imaging from home or are you having to travel to (at least) get useable horizons? For me, from my house I can only image to the north and even there the light pollution is pretty bad so I'm really limited to narrowband. But - if I do that I can image the entire night. If I want to image to the south or do LRGB, then I need to travel, which means I'm really not going to want to stay out and freeze all night or drive home at 3AM and have to turn around go to work in the morning...

 

I also have been trying to manage my imaging sessions based on what the moon is doing, and usually only shoot channel per night. For example, if the moon is kind of full-ish, I'll only shoot H-alpha or (only more recently) SII. I'll only do OIII if a bright moon isn't going to be as of a factor. I'm trying to get to where, per target, I get one night each of HII, SII, and OIII. 

 

That used to seem like a LOT to me and serious over-kill, but that was when I had to drive somewhere to image and the idea of multiple-nights seemed foolishly unrealistic. 

 

Other than travel, the advantage of having that much/many sub-exposures is that if stuff happens and you have to toss out, say, two dozen subs, it matters a lot less than if your total number of subs you had in the first place was three dozen :)

 

Other equipment you haven't listed but will probably want to get include: 

 

A flat panel for taking flats. Something like this works great.

How are you powering your set-up? Do you have 12V male to male cables? AC adapters? Are you traveling and need portable power? 

My experience with ZWO and iOptron cables is that they are...poo, and was getting a lot of dropped/disconnected devices. I started using Amazon Basics cables and they've been MUCH more reliable. 

 

Good luck!


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#5 AstroPharma

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Posted 28 September 2021 - 03:25 PM

Part of your time management really depends on where you're imaging from. Are you imaging from home or are you having to travel to (at least) get useable horizons? For me, from my house I can only image to the north and even there the light pollution is pretty bad so I'm really limited to narrowband. But - if I do that I can image the entire night. If I want to image to the south or do LRGB, then I need to travel, which means I'm really not going to want to stay out and freeze all night or drive home at 3AM and have to turn around go to work in the morning...

I also have been trying to manage my imaging sessions based on what the moon is doing, and usually only shoot channel per night. For example, if the moon is kind of full-ish, I'll only shoot H-alpha or (only more recently) SII. I'll only do OIII if a bright moon isn't going to be as of a factor. I'm trying to get to where, per target, I get one night each of HII, SII, and OIII.

That used to seem like a LOT to me and serious over-kill, but that was when I had to drive somewhere to image and the idea of multiple-nights seemed foolishly unrealistic.

Other than travel, the advantage of having that much/many sub-exposures is that if stuff happens and you have to toss out, say, two dozen subs, it matters a lot less than if your total number of subs you had in the first place was three dozen :)

Other equipment you haven't listed but will probably want to get include:

A flat panel for taking flats. Something like this works great.
How are you powering your set-up? Do you have 12V male to male cables? AC adapters? Are you traveling and need portable power?
My experience with ZWO and iOptron cables is that they are...poo, and was getting a lot of dropped/disconnected devices. I started using Amazon Basics cables and they've been MUCH more reliable.

Good luck!


I think we face similar issues, I live in a light pollution red zone so color imaging from home is out of the question and driving out into the desert cold nightly sounds like it will take a precious part of my soul with it each time. I don’t mind multiple nights of NB capturing from home but If i were to capture in LRGB I’d have to either camp the weekend using the force to dim the light of the moon or choose a new moon night to do a one night capture session and gut it out at work the next day (Unless the new moon falls on a weekend).

I worry that I would never get a good image in one night but I guess it’s possible with experience and adaptability…

#6 bobzeq25

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Posted 28 September 2021 - 03:39 PM

This is pretty non critical, people do things differently.  Two common approaches.

 

For LRGB devote half the time to L, and 1/6 each to R, G, and B.

 

For NB, devote somewhat more time than Ha to (O(III) and even more to S(II).  This is very target dependent, targets have very different emissions of those three.  But Ha generally hhas the most si9gnal, followed by smaller amounts of O(III) and even smaller amounts of S(II).

 

I think black and white Ha images are nice, you can see examples on my astrobin.


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#7 AstroBrett

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Posted 28 September 2021 - 03:49 PM

One thing I'll throw out is you might want to consider whether you need to get all your data in one night. With plate solving software, your rig can reposition itself on the same piece of sky in a consistent and repeatable manner, and you don't need to acquire all of your frames in one setting.  Most of my data is acquired over multiple nights, which allows me to work around the moon and poor atmospheric conditions. It also allows me to take only the best frames for integration into my final image, as well as gathering sufficient integration time for dimmer targets. The downside, of course, is you don't get that instant gratification of being able to process an image or two after each session.  I generally collect data from 4-6 targets each night, with data collection overlapping from night to night, so as I complete a data set I can process it while I am still collecting data on other targets, and starting a new target each time I finish up an old one. This also allows me to collect data with the targets close to the zenith, and as they drop down in the sky I can switch over to the next target climbing up toward the zenith. But none of that would be possible without plate solving software and the ability to relocate the scope within a few tens of pixels of the same location in a consistent and repeatable manner. 

 

Good luck!

 

Brett  



#8 klaussius

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Posted 28 September 2021 - 04:09 PM

For SHO, I usually start with equal amounts of everything. I will rarely finish a NB project in one night, so equal amounts gives me data to figure out the ratios. I'll inspect each filter's resulting stack and decide based on SNR levels (visually).

 

Ie: if my OIII turns out too noisy, I'll schedule more OIII time. Etc.

 

I tend to use H as luminance quite often, so I will devote more time to H when possible (ie: when the other filters are good enough for accurate color).

 

For LRGB it's a similar process, but I will start the way Bob described (50% L, 1/6th each RGB) initially, and adjust after I get an initial stack. Sometimes I will need better color data for faint colorful details, so I will lean less heavily on L. Sometimes I can tell that from the start (by looking at telescopius or other images of the target), and I will start with less L straightaway, but I'm always adjusting ratios based on the data I get.

 

I never stop collecting RGB, but I've heard people sometimes collect only a set amount of RGB and then pile up on L. It can work, but I prefer improving everything each session, not just L. No justification really, it's just the way I prefer it.

 

As for how to manage the session's time, I will usually program the filter sequence in the automation software, and the software will handle running it. I interleave all filters in a loop, like LLLLLLRRGGBB, some people like collecting all of each filter in a single run, or at different times in the night. That makes sense in some situations, but it's hard to plan properly. Interleaving the filters is a simple way to avoid mishaps.

 

Each filter will have different exposure times, mostly L will be shorter than RGB and NB longer (say if I'm doing LRGBH). Again, that's all usually automated in software.

 

So, the TLDR is, start NB with equal amounts of each, LRGB with 50% L and equal RGB, then adjust based on the results you get.

 

I would recommend you do your first run at home. Figuring out things at a dark site can be extra challenging and frustrating. Iron out all the kinks at home, and then when you have everything figured out plan your first dark site trip, not before.

 

As for subexposure lengths... my numbers will not match yours, and yours at home will not match yours at a dark site. It really depends on sky conditions a lot. Me, in my sky, I've settled on 60s L, 120s RGB for dim targets, 30s L, 60s RGB for brighter ones, 300s NB in most cases (only a handful targets required shorter NB). Darker sites can support longer LRGB subs in general, and your camera has a deep well so it will probably let you increase exposure without clipping stars a lot. But that's the benchmark, check how many stars have clipped at a particular gain/exposure setting, and pick exposure settings that clip only a handful of stars. You'll quickly figure out what works for your skies, but at first it's going to be trial and error.

 

Just my 2¢


Edited by klaussius, 28 September 2021 - 04:16 PM.


#9 Oort Cloud

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Posted 28 September 2021 - 04:10 PM

It helps to shoot R, G, B, L, B, G, R...red is affected least by the atmosphere, then green, then blue is affected most. And you want your luminance to be taken as close to the meridian as possible since that's where you'll get the best data.
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#10 jonnybravo0311

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Posted 28 September 2021 - 04:25 PM

Thanks for the advice and the feedback on the setup, I was shamelessly fishing for it as I was in desperate need of validation after burning a hole into my savings lol.gif .

 

I'm planning a trip to a dark site once I test my connections at home and I'll find a target that would be visible for 6 hours based on your recommendation, I'll do 12*300s per color and 36*300s luminance. Hopefully, nothing will spontaneously combusts.

 

Thanks again,

 

Ali.

The ratios are fine, but don't lock yourself into that exposure time. Take some test images to figure out the best value. You may very well find that 300" luminance exposures are too much and you blow out all the stars. In fact, since I have the same scope/reducer, I'd be willing to bet 300" with a luminance filter will be far too long, even if you're shooting at gain 0, even under dark skies. Only way to know for sure is by taking some test exposures and looking at the stats... min/mean/max ADU, number of pixels clipped at the high end.

 

You certainly picked up a heck of a nice kit. You'll probably eventually want to upgrade those filters... and then you're going to get aperture fever and realize that GEM28 won't really work too well with the 127mm refractor you've got your eyes on... :p


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#11 photobiker

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Posted 28 September 2021 - 07:40 PM

If you don't mind I'd like to ask a question that can help both of us, new to mono as well.  What should the histograms look like with a mono camera?  Shooting a DLSR I knew what I was looking for.  I we trying to get the RGB at the same point on the histogram?  How about SHO?  I'm out shooting when I can and trying  new things but I'm not sure I'm the right path.  So far I haven't gotten anything to really work with.  At the moment I'm working with about 3 hours of exposure time.

Ralph



#12 idclimber

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Posted 28 September 2021 - 07:52 PM

If you don't mind I'd like to ask a question that can help both of us, new to mono as well.  What should the histograms look like with a mono camera?  Shooting a DLSR I knew what I was looking for.  I we trying to get the RGB at the same point on the histogram?  How about SHO?  I'm out shooting when I can and trying  new things but I'm not sure I'm the right path.  So far I haven't gotten anything to really work with.  At the moment I'm working with about 3 hours of exposure time.

Ralph

I do not look at the histogram. I look at average ADU. This is probably the most common metric to evaluate. I also look at how many stars are visible in a un-stretched sub. I also have a metric that shows me how many pixels are clipped white (Voyager). But I am never sky pollution limited and my mount is pretty decent. 


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#13 jonnybravo0311

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Posted 28 September 2021 - 08:04 PM

I do not look at the histogram. I look at average ADU. This is probably the most common metric to evaluate. I also look at how many stars are visible in a un-stretched sub. I also have a metric that shows me how many pixels are clipped white (Voyager). But I am never sky pollution limited and my mount is pretty decent. 

You're probably the only guy ever to refer to the MX+ as "pretty decent" lol.gif.

 

If you don't mind I'd like to ask a question that can help both of us, new to mono as well.  What should the histograms look like with a mono camera?  Shooting a DLSR I knew what I was looking for.  I we trying to get the RGB at the same point on the histogram?  How about SHO?  I'm out shooting when I can and trying  new things but I'm not sure I'm the right path.  So far I haven't gotten anything to really work with.  At the moment I'm working with about 3 hours of exposure time.

Ralph

As Dave, and his "pretty decent" mount wrote, a better metric to use for evaluation is ADU. I use NINA for my image capture and it shows the stats of each sub you take including min, mean and max ADU along with how many pixels you've clipped on the high end. I've found that under my skies, using my equipment, 300" at unity gain works pretty well for narrowband imaging. For LRGB, I will usually image at gain 0. Luminance subs are 60" at most. RGB, 120". The best advice? Take test exposures and determine what works best for you, your equipment and your skies :)


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#14 Oort Cloud

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Posted 28 September 2021 - 08:07 PM

Or just get SharpCap Pro and let it do all the maths 4 u...

#15 bobzeq25

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Posted 28 September 2021 - 08:10 PM

If you don't mind I'd like to ask a question that can help both of us, new to mono as well.  What should the histograms look like with a mono camera?  Shooting a DLSR I knew what I was looking for.  I we trying to get the RGB at the same point on the histogram?  How about SHO?  I'm out shooting when I can and trying  new things but I'm not sure I'm the right path.  So far I haven't gotten anything to really work with.  At the moment I'm working with about 3 hours of exposure time.

Ralph

There are two kinds of histograms. 

 

What shows up on the back of the DSLR is stretched.

 

What is downloaded by a mono astro camera is linear (unstretched).  An example is shown below.  Click on it to enlarge, it makes things clearer.  Yes, that's correct.  That's also what a linear sub should look like.  Your eyes don't see linear.

 

Imagers don't generally use the histogram to set subexposure.  There are various methods used instead.  Often discussed here. 

 

One is to set the average value of a sub (corrected for bias) to 5-10 X the read noise of the camera squared.  That "swamps the read noise", it makes read noise insignificant compared to sky noise.

 

Another is to look at how many pixels are clipped high.  Zero is bad, you could be very underexposed.  Thousands are bad, you're seriously overexposed which causes indistinct highlights and poor star color.  Overexposure here is more common than underexposure, with modern astro cameras.  People wrongly think longer subexposures must be better.  They think they should see more detail in a sub than what is correct.

 

Warning.  The subexposures others use are often not correct for your equipment and your skies.

 

This video is excellent.  The method he's discussing is swamping read noise.  Look at the subexposure chart about 50 minutes in, see how much good subexposure changes with circumstances,

 

https://www.youtube....h?v=3RH93UvP358

 

a linear histogram and sub.jpg


Edited by bobzeq25, 28 September 2021 - 08:20 PM.

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#16 idclimber

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Posted 28 September 2021 - 08:11 PM

You're probably the only guy ever to refer to the MX+ as "pretty decent" lol.gif.

Well, I said the same thing when I sat in my cousins Mclaren. grin.gif

 

On the 2600mm I believe most are using a gain of 0 or 100. I use the lower gain on LRGB and the higher on NB. I do -10c and offset of 50 as well. 


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#17 jonnybravo0311

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Posted 28 September 2021 - 08:20 PM

Well, I said the same thing when I sat in my cousins Mclaren. grin.gif

 

On the 2600mm I believe most are using a gain of 0 or 100. I use the lower gain on LRGB and the higher on NB. I do -10c and offset of 50 as well. 

You told your cousin that his McLaren was "pretty decent"? HAHAHA... Dave, you are a master of over-the-top enthusiasm.

 

On topic, I do the same with my 294... unity gain for NB and 0 gain for LRGB.


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#18 CorralesRay

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Posted 28 September 2021 - 08:20 PM

AstroPharma - That is a great set up and an amazing place to start! Congrats!!

 

JonnyBravo - I'm surprised, but this is the 2nd post I've seen today where a "gain of 0" is used. I'm asking the obvious, do you mean 0 at the fair left of the Gain (e-/ADU) plots from the ASI manuals (I've attached a snip)? I avoided lower gains because of the read noise. Is the trade off the additional sub exposure time? Thanks.

 

Screen Shot 2021-09-28 at 7.18.31 PM.png

 

 

Ray


Edited by CorralesRay, 28 September 2021 - 08:21 PM.


#19 CorralesRay

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Posted 28 September 2021 - 08:27 PM

I'll put the entire plot here.....is 0 gain to the far left or the far right of this plot? I can't believe how ignorant that question sounds. But thanks for a reply to calibrate my brain.

 

Ray

 

 

Attached Thumbnails

  • Screen Shot 2021-09-28 at 7.25.07 PM.png


#20 jonnybravo0311

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Posted 28 September 2021 - 08:28 PM

AstroPharma - That is a great set up and an amazing place to start! Congrats!!

 

JonnyBravo - I'm surprised, but this is the 2nd post I've seen today where a "gain of 0" is used. I'm asking the obvious, do you mean 0 at the fair left of the Gain (e-/ADU) plots from the ASI manuals (I've attached a snip)? I avoided lower gains because of the read noise. Is the trade off the additional sub exposure time? Thanks.

 

attachicon.gifScreen Shot 2021-09-28 at 7.18.31 PM.png

 

 

Ray

Yup, I mean the far left. With a camera like my 294, and especially with Dave's 2600, the read noise of the sensor is already pretty low. For example, my read noise at gain 0 is just shy of 4e-. The 2600 is even lower at about 3.25e-. The value of shooting at gain 0 is you have the full dynamic range of the camera as well as the full well depth. The tradeoff is that you take longer exposures.


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#21 OldManSky

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Posted 28 September 2021 - 08:34 PM

I'll put the entire plot here.....is 0 gain to the far left or the far right of this plot? I can't believe how ignorant that question sounds. But thanks for a reply to calibrate my brain.

 

Ray

Ray, yes — at the far left.

Yes, there is more read noise than at gain 100. But “more” is relative, and read noise is still quite low at gain 0. Yes, you have to expose longer, but you get a much larger well depth and higher dynamic range.

I have done images with mine at both gain 0 and 100. Both are great. But I really like the visibly better star color and dynamic range of the gain 0 shots.


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#22 idclimber

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Posted 28 September 2021 - 08:38 PM

You told your cousin that his McLaren was "pretty decent"? HAHAHA... Dave, you are a master of over-the-top enthusiasm.

Yep, an 95 F1 to boot.... I think that might just blow by enthusiasm. But then I like the Porsche more. 

 

Yup, I mean the far left. With a camera like my 294, and especially with Dave's 2600, the read noise of the sensor is already pretty low. For example, my read noise at gain 0 is just shy of 4e-. The 2600 is even lower at about 3.25e-. The value of shooting at gain 0 is you have the full dynamic range of the camera as well as the full well depth. The tradeoff is that you take longer exposures.

 

With the 2600 there is less read noise at 100 than 0 and the dynamic range is almost the same. I would start there and keep things simple. If you have a colder climate go with -10 on temperature, if you are down south and have warmer nights try 0. Offset can be anything from 30 to about 50 and is a lot less important. 


Edited by idclimber, 28 September 2021 - 08:41 PM.

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#23 CorralesRay

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Posted 28 September 2021 - 09:20 PM

AstroPharma, as a beginner at the mono game, I started with RGB objects, just to simplify things. If you picked a nice target like M13, you could start with an image that really doesn't need the Lum or Ha filters. That could help you get a "stack" to process in one night (weather, clouds and moon willing).  Ray



#24 photobiker

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Posted 28 September 2021 - 09:35 PM

Thanks guys.  The histogram I was looking at was in APT.  I turn off the screen on the back of the camera, unnecessary power drain.  I need to get back on learning NINA, sounds like it more useful than APT in a lot of ways.



#25 AstroPharma

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Posted 29 September 2021 - 04:37 AM

Thanks everyone for the great feedback!


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CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


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