Newbie HaRGB exposures

[Jezphil]

Newbie here getting hooked. I've imaged only a couple of DSOs so far (Hurcules cluster and Western Veil Nebular) and have managed to grapple with the basics of PixInsight for processing. Still lots to learn. Heading out tonight aiming to capture the North American  Nebula with Skywatcher Evostar 120ED Pro on Skywatcher HEQ5 pro mount with 0.8 flattened/reducer, so f7.5 reduced to f6.

I have an Astronomik Hydrogen-Alpha 12nm Clip filter to use for this. I will try out a few exposures before committing  to taking subs. With this filter should I expect to add a fair amount of length to my subs as it reduces light intake? Should I be looking at 300 seconds exposure maybe?  What parameters would you expect?

I plan to take the clip filter out and run off some straight RGB subs unfiltered of the same image to combine with the filtered ones in Pixinsight for HaRGB.  Presumably the two sets of subs can be different exposure lengths from each other but are stacked separately in two batches before combining in Pixinsight? 

Any pitfalls her here?

All thoughts and advice welcome and thanks in advance. 
 

Jez

[jiberjaber]

Wow - a lot to unpack in that!

Have you looked at the NAN in any software relative to your FoV ? I know on my 80ED it's a 4 panel mosaic to fit it all in, 120 will be less FoV

 

You don't mention if you are guided or not, if you are guided you might be able to try for a longer exposure time but if 300s is working for you with the filter already then just take lots of exposures  Best best is to try it out and see what it looks like I tend to run 600s subs others a successful with 900s subs.

Key thing is to remember you will benefit greatly from a suitable master dark and flats when it comes to processing them.  You can do the Darks later but don't forget to do the flats at the same focus and camera orientation - so often easier to do either straight after the imaging run or before if you are happy with focus and camera orientation. 

Your assumption about how to combine in PI is correct, stack the two sets of image types separately then combine  

You don't mention which camera but I am guessing it might be a DSLR? This might be worth a look to confirm FoV:

http://astronomy.tools/calculators/field_of_view/?fov[]=136||104||0.8|1|0&messier=20

(You'll need to search for NGC7000 in the objects to add it as it wouldn't share the URL with it in )

Hope you get clear skies  

[Jezphil]

Thanks Jason. Really useful information. I'm just tracking, not guiding. Camera is a Canon EOS Ra. I've checked the FoV on SkySafairi and realise I won't get all the nebula but this is fine by me. I'm planning darks, flats, bias and I've done these once before... am slightly fearful though that I only understand the basics of Pixisight so combining Ha and RGB might will be a challenge but I've got to try to nail it. 

600s is a lot more than I was going to try, but will give it a go and see how it looks so thanks for the advice. Cheers.

[jiberjaber]

4 hours ago, Jezphil said:

Thanks Jason. Really useful information. I'm just tracking, not guiding. Camera is a Canon EOS Ra. I've checked the FoV on SkySafairi and realise I won't get all the nebula but this is fine by me. I'm planning darks, flats, bias and I've done these once before... am slightly fearful though that I only understand the basics of Pixisight so combining Ha and RGB might will be a challenge but I've got to try to nail it. 

600s is a lot more than I was going to try, but will give it a go and see how it looks so thanks for the advice. Cheers.

I'm not familiar with that camera, but if you are not guiding 600s might be a step too far! It's such a long time ago since I did imaging just tracked with the scope.  I have recently been doing 2min subs on an iOptron SkyGuider without a guide camera. Best some test exposures and see how far you can stretch the exposure time before you suffer from star elongation - you can do these tests from a Bortel 8 location I expect as its just about the exposure time and stars rather than capturing actual content which might save you a trip. 

In PI the sub frame selector process is quite good for seeing how the eccentricity of the stars in an image is.....

You'll only need the one set of bias frames I'd expect and you can reuse them - I think there's probably a trick here about maximising imaging time and avoiding using it up with the calibration frames

 

[jiberjaber]

How did it go Jez? 

[ollypenrice]

Imaging with narrowband filters is looooong exposure territory or it may not be worth it. Remember that a narrowband filter cannot bring in new light. All it can do is exclude light not of its selected wavelength. Your chip gets just as much Ha signal without the filter as with it. The difference is that it ONLY gets the Ha signal with it. Why bother? Because, if you have enough of it, you can drag out structures and details which only exist in Ha. But you need a lot of signal...

Olly

[DaveS]

I would be wary of using Bias and Dark. With modern CMOS cameras I think Dark frames only as they already contain the Bias signal. Otherwise you may find that you are over correcting the dark signal.

[Jezphil]

Thanks guys for all the above advice. I tried five minute exposures, unguided. First go, but it seemed to go okay and I was brutal about dumping subs that weren't sharp for stacking. I've only used darks, flats, lights (thanks Dave S).  I'm stuck with the Ha image for now as it's been too cloudy to get RGB subs so far. I've created two layers of the same Ha image in processing, one version rich in colour with lots of middle tones, and without stars. The other layer of the same image for highlights and I combined the two, altering the mix until it looked okay and gave me some structure. The Ha filter meant I could get this image in my Bortel 8 south London back garden and with a moon, so it's definitely a good friend. Just got hold of a guide scope and Lodestar x2 guide camera to help these long exposures. Baffled now as I try to figure out the right settings and hook it up with PHD2, HEQ5 and SynScan which is going badly so far. I think this game is all about pushing through the bafflement and being bloody minded until you get to where you want to be. Anyway, my first nebula image...