NGC 7635 in H-alpha with DSLR

[tooth_dr]

I tried 12 times in DSS to get a decent stacked image, ended up not using my flats or darks, just stacked the 23 x 600s subs ISO1600, Canon 1000d, EOS insert filter Ha, ED80.

 

Are people still using darks or is there another method now?

 

There are loads of streaks in the image, stacking artefact?  Opinions of the guiding, does it look ok?

 

Thanks

Adam.

[MikeODay]

Nice image with great detail and tones in the nebula.

I'm not sure what the streaks are if you are not using darks ( sometimes this rain type efffect can be seen with darks ).  Sorry for the silly question, but did you check each sub to make sure you did not average in a bad sub?  You also seem to have stars that have a bright central part surrounded by a diffuse cloud.  I have gotten this in the past when I have included all subs without excluding the ones with larger stars due to poor seeing / shift in focus.

[DaveS]

Are you dithering? Back when I was using a DLSR I had streaks running through my images but dithering got rid of them.

[tooth_dr]

5 minutes ago, DaveS said:

Are you dithering? Back when I was using a DLSR I had streaks running through my images but dithering got rid of them.

I'm not familiar with that? Does PHD do it for you? I'm using PHD1.14

[DaveS]

I've not used PHD1.4 for a while, the last version I used was 2.0.

I think that when I was using a DSLR I used APT as my capture program, which will dither through PHD

[tooth_dr]

3 minutes ago, DaveS said:

I've not used PHD1.4 for a while, the last version I used was 2.0.

I think that when I was using a DSLR I used APT as my capture program, which will dither through PHD

Thanks. Just getting the hang of it all. My IS DMK doesn't seem to work with the latest phd so I'm using the last of the old version. 

[Adam J]

Yes the streaks are caused because you failed to dither between subs, dithering is the most important thing you can do with DSLR astrophotography much more so than with a CCD. Dithering will 100% cure this issue in my experience. Just make sure its a large dither...about 25 pixels is what I use. Darks are not a great idea with a DSLR, I have never bothered myself.

[wimvb]

Nice bubble wide field.

The streaks are caused by movement between subs. This can be either insufficient PA, unbalanced setup, differential flexure, etc. As others mentioned, dithering in both RA and DEC will solve this. Have a look here:

https://www.youtube.com/watch?v=PZoCJBLAYEs

Guided dithering is more difficult to set up, since you have to allow the guiding software to settle after dithering (moving the mount).

Darks can be tricky for DSLR imaging. Before I started guiding, my exposures were limited to a few minutes at most (with lens), and darks didn't help. But since I started guiding and my exposure time has increased to up to 15 minutes, darks improve my images. My advice: experiment.

As an alternative to darks, you can use a master bias (and flats of course), and cosmetic correction (available in DSS and PixInsight), as well as aggressive pixel rejection during stacking.

[Adam J]

On 25/04/2017 at 17:24, brrttpaul said:

12 minutes ago, wimvb said:

Nice bubble wide field.

The streaks are caused by movement between subs. This can be either insufficient PA, unbalanced setup, differential flexure, etc. As others mentioned, dithering in both RA and DEC will solve this. Have a look here:

https://www.youtube.com/watch?v=PZoCJBLAYEs

Guided dithering is more difficult to set up, since you have to allow the guiding software to settle after dithering (moving the mount).

Darks can be tricky for DSLR imaging. Before I started guiding, my exposures were limited to a few minutes at most (with lens), and darks didn't help. But since I started guiding and my exposure time has increased to up to 15 minutes, darks improve my images. My advice: experiment.

As an alternative to darks, you can use a master bias (and flats of course), and cosmetic correction (available in DSS and PixInsight), as well as aggressive pixel rejection during stacking.

will do cheers

He is guiding so there will be no movement between subs mate and that is actually the problem.

So given that he is guiding the problem is actually exactly the opposite as his pattern noise is fixed relative to the image he is enhancing it as you would the signal by stacking. Dithering will mean that the pattern noise position (commonly diagonal in a CMOS sensor due to the way the RGB matrix works) changes between frames and cancels out while only the actual signal is enhanced by stacking.

[tooth_dr]

1 hour ago, Adam J said:

He is guiding so there will be no movement between subs mate and that is actually the problem.

So given that he is guiding the problem is actually exactly the opposite as his pattern noise is fixed relative to the image he is enhancing it as you would the signal by stacking. Dithering will mean that the pattern noise position (commonly diagonal in a CMOS sensor due to the way the RGB matrix works) changes between frames and cancels out while only the actual signal is enhanced by stacking.

 

So in a practical sense how do I approach this process? I understand the principle, and normally I allow 30s - 60s between subs anyway. Are there free programs that I can use?

[wimvb]

1 hour ago, Adam J said:

He is guiding so there will be no movement between subs mate and that is actually the problem.

There can be; it is called differential flexure. While not as common with a refractor as with a reflector (where the mirror can shift), it is still possible. One cause is the connection between the guide scope/camera and the imaging scope/camera. If there is any movement, the guide camera will move relative to the imaging camera. There will be no star movement in the guide camera, but there will be in the imaging camera.

There are two very easy ways to check for movement:

1. look at the offset in x and y between subs, especially between the first and last

2. take the first and last sub of the stack (not registered) and overlay them in PS with some transparency. Any shift will show up.

[tooth_dr]

7 hours ago, wimvb said:

There can be; it is called differential flexure. While not as common with a refractor as with a reflector (where the mirror can shift), it is still possible. One cause is the connection between the guide scope/camera and the imaging scope/camera. If there is any movement, the guide camera will move relative to the imaging camera. There will be no star movement in the guide camera, but there will be in the imaging camera.

There are two very easy ways to check for movement:

1. look at the offset in x and y between subs, especially between the first and last

2. take the first and last sub of the stack (not registered) and overlay them in PS with some transparency. Any shift will show up.

As i flick through the on screen camera images the fov very very slightly moves downwards, maybe this is an polar alignment issue? As for flexure, I'm not sure, ST80 is bolted onto dovetail on top of ED80.  Maybe the diagonal would be a possible weak point?

 

thanks again

Adam

[Adam J]

3 hours ago, tooth_dr said:

As i flick through the on screen camera images the fov very very slightly moves downwards, maybe this is an polar alignment issue? As for flexure, I'm not sure, ST80 is bolted onto dovetail on top of ED80.  Maybe the diagonal would be a possible weak point?

 

thanks again

Adam

That would indicate a small amount of flexture however, to produce that pattern it would need a reasonably significant diagonal motion not just a small downwards motion, the appearance looks like the pattern noise I got in my 1000D. Its actually worst at 1600 as its not a native ISO (boost) and the camera kicks in an additional amplifyer thats allot more noisy, use ISO800 with a 1000D for best results on HA.

You can either spend lots of time trying to tune the coupling between your two scopes (could actually be sag in the focus tube as those are not the best) and this may or may not help not solve it, or you can just dither between subs which will solve your issue in either case.  

[tooth_dr]

24 minutes ago, Adam J said:

That would indicate a small amount of flexture however, to produce that pattern it would need a reasonably significant diagonal motion not just a small downwards motion, the appearance looks like the pattern noise I got in my 1000D. Its actually worst at 1600 as its not a native ISO (boost) and the camera kicks in an additional amplifyer thats allot more noisy, use ISO800 with a 1000D for best results on HA.

You can either spend lots of time trying to tune the coupling between your two scopes (could actually be sag in the focus tube as those are not the best) and this may or may not help not solve it, or you can just dither between subs which will solve your issue in either case.  

I agreed with that, could you point me in the direct of how to get started on dithering Adam?

 

Adam.

[Adam J]

33 minutes ago, tooth_dr said:

I agreed with that, could you point me in the direct of how to get started on dithering Adam?

 

Adam.

I use BYEOS and its very easy you just clock the dither button on the top right and then make sure that server is enabled in PHD2 (you probably need PHD2) only other thing to do is to set the dither scale in PHD2 to about 25 and you are away (though this will depend on your guide scope focal length just play with it until you are getting a reasonable movement between frames.

I am unable to advise you with APT or other image capture programs. You tube is a good place to start as I found quite a few tutorials, just search for Dithering with PHD. 

All else fails then you can manually move the aim point between subs by turning guiding off then moving then turning guiding back on. I used to do it that way before I got BYEOS and it works just fine for longer subs (as in you dont need to be doing it ever couple of mins). 

[tooth_dr]

17 minutes ago, Adam J said:

I use BYEOS and its very easy you just clock the dither button on the top right and then make sure that server is enabled in PHD2 (you probably need PHD2) only other thing to do is to set the dither scale in PHD2 to about 25 and you are away (though this will depend on your guide scope focal length just play with it until you are getting a reasonable movement between frames.

I am unable to advise you with APT or other image capture programs. You tube is a good place to start as I found quite a few tutorials, just search for Dithering with PHD. 

Thanks Adam.  Back Yard EOS - I have it downloaded but never used it.  Will try it tonight if it stays clear.  Is it easy to use?

[Adam J]

5 hours ago, tooth_dr said:

Thanks Adam.  Back Yard EOS - I have it downloaded but never used it.  Will try it tonight if it stays clear.  Is it easy to use?

I find it very easy to use. 

[tooth_dr]

1 hour ago, Adam J said:

I find it very easy to use. 

Thanks Adam. Appreciate all the help. 

[tooth_dr]

Making progress!

[tooth_dr]

I added another 20 subs to this, from  next night after I had taken the first ones.  I didnt use dithering, but did get bias and flats added to it this time, and it stacked ok.  Just a rough comparison below of 3.75 versus 7.5 hours, the latter with bias and flats, I didnt apply the exact same processing to each as one was done last week, but it's pretty simple stuff like just levels and curves so should be similar.  I've been using dithering on data capture since then and even just 2 hours of data looks as good as this.

 

 

[PatrickGilliland]

Nice work and field of view.

[tooth_dr]

With nothing else to process I started watching a few tutorials on the net about masks and tried to reduce the stars and make them less red after running Noels Actions on it to produce false colour Ha.  Few weird halos, you can really appreciate the amount of work that goes into processing the amazing images on here.  It looks nice enough I think

Off to enjoy a glass of red wine.

 

Thanks everyone,

Adam