Question: Preparing flats etc in Nebulosity

[mldee (Mike)]

I am unable to find a clear procedure on how to prepare darks, flats and bias frames in Nebulosity 2.2.8. I have read the 2.2 manual but still a little unclear.

When preparing them, the manual says to treat them like 'light frames'. This would mean pre-proc including any subtractions, then debayer, stack, then post proc.

What is needed to make master Dark, Flat and Bias frames? Bias subtraction on darks and flats, grading? normalising? Debayering? and then the stacked frame is the master? Or is it simply bias subtraction on flats then stack without debayer? For darks, what steps? etc, etc. Specifically how they're done for Neb. The manual seems to credit me with more intelligence than I actually possess.

I understand Darks should be temp and exposure-time related to lights, but with the QHY8 at my level of quality, it's more the flats and bias I am trying to fathom out, as they're not directly related to those lights parameters and the absence of inherent dark noise is pretty impressive compared to my old 1000d or DSI III.

Finally, what are folks' experiences on Bad Pixel Mapping compared to Dark subtraction? Needed? I would prefer to leave out the Darks altogether, and so far I haven't seen any hot pixels either, but then again, there's 6 million of the little buggers

[peter_4059 (Peter)]

Mike,

For what it's worth I haven't bothered with darks or bias' - just flats. I'm using Nebulosity to capture images and Deep Sky Stacker for calibration and stacking. I capture the flats in the same way I get the lights, only much shorter exposure. Load them into DSS and it takes care of the rest.

Peter

[mldee (Mike)]

Thanks Peter, I'm torn between DSS and Astra Image, the price of DSS is very attractive, but the AI user interface is pretty cool and it does deconvolution etc as well.

I've just spent two really productive nights imaging right through to M83, so now I have about a week of cloudless nights to do the processing

If the nights stay this clear I might just put the WO triplet on and try some with the FF4 flattener. Yet to try it.

Anyway, thanks for the response, guess you're setting up for another night with the QHY8 Pro!

[peter_4059 (Peter)]

Mike,

I use both Astraimage and Deep Sky Stacker however I think they offer different things and you can't beat DSS for calibration and stacking (and it is free!). The only thing I use Astraimage for these days is deconvolution.

Tonight looks completely clouded over so I'm not even set up and it looks like it's going to stay that way for a few days.

Peter

[mldee (Mike)]

Great! Gives me more chances to pick your brain I'd like to be able to remove DSS from the loop and do my stacking in Neb. I just tried to stack some pics in DSS (same ones I'd already done in neb) and when I tried to use neb-debayered pics, it gave the "wrong size" error. Sure enough, the RGB pics are 2 pixels less on both dimensions compared to the unbayered bias and flats, etc.

So...Question: Do I Debayer my flats and bias frames before using them in DSS? I'll try it and see what happens.

[troypiggo (Troy)]

Originally Posted by mldee

I am unable to find a clear procedure on how to prepare darks, flats and bias frames in Nebulosity 2.2.8. I have read the 2.2 manual but still a little unclear.

When preparing them, the manual says to treat them like 'light frames'. This would mean pre-proc including any subtractions, then debayer, stack, then post proc.

What is needed to make master Dark, Flat and Bias frames? Bias subtraction on darks and flats, grading? normalising? Debayering? and then the stacked frame is the master? Or is it simply bias subtraction on flats then stack without debayer? For darks, what steps? etc, etc. Specifically how they're done for Neb. The manual seems to credit me with more intelligence than I actually possess.

I understand Darks should be temp and exposure-time related to lights, but with the QHY8 at my level of quality, it's more the flats and bias I am trying to fathom out, as they're not directly related to those lights parameters and the absence of inherent dark noise is pretty impressive compared to my old 1000d or DSI III.

Finally, what are folks' experiences on Bad Pixel Mapping compared to Dark subtraction? Needed? I would prefer to leave out the Darks altogether, and so far I haven't seen any hot pixels either, but then again, there's 6 million of the little buggers

G'day Mike, I use Neb v2 and can briefly describe how I do them. You don't need to do Bias frames if you're doing Darks and Flats.

I prepare Darks first. Choose Batch, Align and Combine. In that window I pick Save Stack output method, None (fixed) alignment method, Std Dev 1.25 stacking function. Save as master_dark_ISO800_4min.fit or something like that. The None alignment method is the most important part there.

Next, Flats. Exactly same as Darks, but save as master_flat.fit.

Lights I Pre-process first, applying the master darks and flats. Then normalise, sometimes I use grade, then demosaic. Finally align and combine and save the stack as NGC1234_ISO800_20x4m.fit or something like that.

Hope that makes sense.

I haven't used the Bad Pixel Mapping side of things, so can't help you there. Hope this answers most of your questions.

[peter_4059 (Peter)]

Originally Posted by mldee

Great! Gives me more chances to pick your brain I'd like to be able to remove DSS from the loop and do my stacking in Neb. I just tried to stack some pics in DSS (same ones I'd already done in neb) and when I tried to use neb-debayered pics, it gave the "wrong size" error. Sure enough, the RGB pics are 2 pixels less on both dimensions compared to the unbayered bias and flats, etc.

So...Question: Do I Debayer my flats and bias frames before using them in DSS? I'll try it and see what happens.

Mike,

You can debayer in DSS so no need to do anything in Nebulosity apart from capture the image. There is a settings section in DSS where you tell DSS what camera your raw files were taken with.

Peter

[mldee (Mike)]

Peter,

OK, I'll take a look at that too. I got a reply from Craig Stark overnight on the same questions I had asked him last night! Really great service. So I have two routes to investigate! I Like DSS, but I prefer Neb's interface and the possibility of only needing two programs, including AI, to keep track of.

BTW, Neb v2.2.5 onwards has a new "multi pre-proc" feature, which Craig kindly suggested I try, and sent me a pdf on. I've attached it for general interest. It's also on his website under "pre-release" section.

Basic problem is me, I just can never leave well enough alone

I gave up last night and went to bed instead.

[mldee (Mike)]

Hi Peter and Troy,

Just a final update on progress with Neb, after another day of emails with Craig Stark and IIS.

Thanks for your inputs Troy, that's basically the final procedure I've come to as well. It turns out there are some very good tutorial docs on Craig's website in the help section (Duh!), which answer pretty well all the questions and show a walkthrough approach, same as yours Troy.

One small gotcha we finally figured out this morning is that if you use the ASCOM driver for Neb, you also lose the ability of Neb to automatically read the Fits camera info, and so it gives the popup box asking for pixel and offset. If you use the WinUSB driver and QHY8 camera in Neb, all goes auto according to plan. Not rocket science, but there's no warning to tell you of this. I suggested to Craig it would be good to include such a caveat in the next version of docs.

Surprisingly, he's also going to implement a rather trite suggestion I made to him to rearrange the Batch menu choices to reflect the actual processing flow, as it's a little easy to forget the debayer step before aligning. He agreed and said it will be in the next update version!

So, I've now got Neb working fine all the way to stretch & levels plus some basic post processing, ready for PS or AI. The Batch Pre-proc system works well with darks and flats, and the drizzle in align and stack is also good. All I need now are better imaging skills. That's purely a time and effort investment.

Peter, DSS works just fine too but has a quite restricted repertoire. For the time being, I've decided to go with Neb as it seems to be capable of doing all my entry-level imaging and processing tasks in a very user-friendly environment, for a reasonable price and backed with great support. No doubt Maxim et al are far superior, but I would like to expend my next learning efforts to PS, when my skills can do justice to it.

Last week's clear skies gave me a wealth of new QHY8 FITS to play with during this week's clouds Galaxies can be a bit tough, can't they?