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First time imaging in Ha (small refractor + Star Adventurer)


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As the title says

I've finally, after owning a 7nm Baader Ha filter for more than a half year, did my first imaging session in Ha.  My target was the Lagoon Nebula because the weekend before I captured 4.5 hours on it and wanted to see what difference it would make. It's also very bright so easy to locate and it helped my getting focus as well.

Focus was kind of a challenge as I had to locate the nebula without the filter first ( the Star Adventurer has no GoTo), screw on the filter and take numerous amounts of test shots (max iso at 30sec) to finally get the focus right. Though I still think the focus was a bit off. For some reasons I had problems with the B-mask and focusing will have to do more tests in the future on this.

That said and done I was ready to start clicking, used settings of iso 1600 and 180sec subs. This was what I saw on the back of the camera after 3 minutes.

1.jpg.cbdcd7478f46fb31ee05100a0f37b50b.jpg

Pretty exited! Just the fact that I had something showing up amazed me. Decided to keep the exposure at 3minutes and planned on getting 1 hour of data at least before calling it a night.

 

Quick info on the gear used here: 

-Camera: self modded Nikon d90(Ha data), Nikon d610(RGb data)

-Optics: TS-Optics Photoline 72mm f/6 FPL53+TSflat72

-Mount: Skywatcher Star Adventurer

-Guidescope: TS-Optics Optics 50 mm DeLuxe Mini 

-Guidecamera:  ZWO ASI120MC-S

-Filters: Baader 7nm 2" Ha 

 

Back on imaging. After 1hour and 18min I stopped the session, took 10 flats, 10 darks and 20 bias frames and called it a night.

The day after loaded everything in DSS, imported the stack in Photoshop and did a little stretch on the red channel. This was the result (1hr 18min at iso 1600).

2.jpg.c5eabb8de713486a7b2156687e7b593d.jpg

First there is so so much more data than I'd captured the weekend before with my unmodded Nikon d610 and almost x4 as much integration. That was a real excitement!

Second is a question(s), the diagonal pattern you see is this walking noise? Will dithering remove these lines? and is it even possible to dither with the SA? I've seen the dithering option in PHD2 but not sure if the combination with the Sa works.

So I've left the noise ,to be hopefully resolved in the future, and tried to combine the Ha with the previous captured RGB.

The RGB data ,as said before, was captured using a full frame Nikon d610. Aligning the two was kind of a challenge but eventually with all the twisting and turning managed to get it almost perfect.

This is the fully processed image from ONLY the RGB data the week before. 4.5hrs of data with the unmodded d610.

3.jpg.a01f92f3e0ff888528a0980d02ae2ef5.jpg 

 

Combining the datasets I decided to re-edit the RGB set with just a curves and levels stretch and some minor tweaks in Adobe raw. Followed a simple tutorial of changing the red channel of the RGB set with the red channel of the Ha set and adding another Ha layer on top to use as a luminance layer. The result was a bit weird to be honest, green in the background and pretty ugly colors in the nebula. ( I didn't had the original to show so just now I made a quick alignment just to show more or less the results. 

4.jpg.07d884a76effba0b243327d046147d90.jpg

Should it look like this? Hope to find out what caused this.

Made some tweaks in the channel color mixer in Ps to get rid of the odd colors(in my eyes at least) and did more editing to get a final image. Besides some tweaks here and there I added an extra layer from the RGB set and used 'color' as blending mode.

This is the result I came up with.

5.jpg.25c3b341186ab1a1436a93eef1ae9319.jpg

Apart from perfect alignment of both data sets and maybe a better color balance I'd have to say the image itself looks a lot cleaner and more pleasant to the eye.

Though it seems the image is not as sharp as just the RGB image, maybe that's because it has less stars and it only appears to be more fuzzy?

Hope to get some input and cc on my workflow and/or the images to improve my results.

 

Thank you,

Ken Mitchell

 

 

PS. For those interested I've also did some imaging with the unmodded d610 + Ha filter to show the differences and if it is worth it to use a Ha filter with an unmodded dslr. 

I'll see if I can make a separate thread for this.

 

 

 

 

Edited by Ken Mitchell
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Looks like all the effort you've put in has paid off for a pleasing final image. 

To answer your first question, yes it definitely looks like walking noise to me. 

You can always manually dither with the SA by using the fire tion buttons on the side of it. Just remember to wait a short while after pressing them to remove backlash. 

Well done

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6 hours ago, geordie85 said:

Looks like all the effort you've put in has paid off for a pleasing final image. 

To answer your first question, yes it definitely looks like walking noise to me. 

You can always manually dither with the SA by using the fire tion buttons on the side of it. Just remember to wait a short while after pressing them to remove backlash. 

Well done

Thanks Geordie!

That sounds like a lot of work if I understand correct what you mean. Every sub push the Ra buttons wait a few seconds and take another sub? 

Do you know if the dither function in phd2 works together with the SA? I have no knowledge how dithering really works in terms of communication between software and mount/guiding camera.

Never looked into it as I've never encountered this type of noise with the full frame camera.

Ken

 

 

Edited by Ken Mitchell
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43 minutes ago, Ken Mitchell said:

Thanks Geordie!

That sounds like a lot of work if I understand correct what you mean. Every sub push the Ra buttons wait a few seconds and take another sub? 

Do you know if the dither function in phd2 works together with the SA? I have no knowledge how dithering really works in terms of communication between software and mount/guiding camera.

Never looked into it as I've never encountered this type of noise with the full frame camera.

Ken

 

 

It is alot of work but worth it for a clean final image. I'm not sure about the dither function with the star adventurer as I've never tried. 

You could always manually dither every few subs and still get reasonable results

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2 hours ago, geordie85 said:

It is alot of work but worth it for a clean final image. I'm not sure about the dither function with the star adventurer as I've never tried. 

You could always manually dither every few subs and still get reasonable results

Thank you! I'll definitely look into that!

Ken

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Lovely image well executed with the SA. Wish this nebula was higher in the sky at my location.

On 11/06/2019 at 13:20, Ken Mitchell said:

Second is a question(s), the diagonal pattern you see is this walking noise? Will dithering remove these lines? and is it even possible to dither with the SA? I've seen the dithering option in PHD2 but not sure if the combination with the Sa works.

Yes, as @geordie85 says, looks like walking noise - resulting from pattern noise appearing to move across the final image due to drift in the subs. 

A couple of things you could try with the subs already taken to see if the final image improves:

1) If you are using Average stacking in DSS, try Median (or perhaps Kappa-Sigma) stacking to help reject too high/low pixel values.

2) Try stacking with many more dark, flat and bias (at least 30) to see if cleaner calibration frames help.

For future narrowband imaging longer sub exposure and a higher ISO would help reduce the read noise component. Better polar alignment would also help reduce sub drift and so the appearance of walking noise, although a longer exposure makes drift more likely. Dithering together with Kappa Sigma stacking is a very good method of dealing with walking noise.

To dither automatically a control program (such as Astro Photography Tool) tells the camera when to expose and then, following the exposure end, tells a guiding program to dither, with dithering complete before the next sub starts. It's not clear from your post but I think you are not using a program for exposures but just the camera (or perhaps intervalometer). 

As you are using guiding, if you wish to dither manually you would need to stop guiding, press the SA buttons to move the mount a little in RA and DEC, and then restart guiding. This doesn't need to be carried out after every frame - every 5 frames should be ok for a 30 frame total. Use Median or Kappa Sigma stacking - this will reject pixels too far from the norm, resulting in a reduction of various types of image noise in the final image.

The SA only supports RA guiding/dithering. Dithering in RA alone can make walking noise appear as it may produce a similar effect to drift. Therefore the best solution is likely to be manual dithering in RA and DEC.

Hope this helps.

 

Edited by bobro
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Hmm. Focusing is a bit of a dilemma without goto. Either you find your target without the filter and then have to focus without a bright star in Ha, or you have to focus on a bright star and then find your target with the Ha in place.

I assume that you can't swap out the filter without disturbing your focus -- after all we're talking microns here! I would recommend that you exploit plate solving regardless. Get pointed at a really bright star like Vega, and you can install your Ha and use pretty short exposures (1-5 sec, depending on gain) to really dial in those Bahtinov spikes. Then point as well as you can at your target, shoot an image, and plate-solve it. That will tell you where you're pointing, and you can home in on your target by iteration.

Bit of a PITA, I admit.

And that wound up a really gorgeous Lagoon, man.

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22 hours ago, bobro said:

Lovely image well executed with the SA. Wish this nebula was higher in the sky at my location.

Yes, as @geordie85 says, looks like walking noise - resulting from pattern noise appearing to move across the final image due to drift in the subs. 

A couple of things you could try with the subs already taken to see if the final image improves:

1) If you are using Average stacking in DSS, try Median (or perhaps Kappa-Sigma) stacking to help reject too high/low pixel values.

2) Try stacking with many more dark, flat and bias (at least 30) to see if cleaner calibration frames help.

For future narrowband imaging longer sub exposure and a higher ISO would help reduce the read noise component. Better polar alignment would also help reduce sub drift and so the appearance of walking noise, although a longer exposure makes drift more likely. Dithering together with Kappa Sigma stacking is a very good method of dealing with walking noise.

To dither automatically a control program (such as Astro Photography Tool) tells the camera when to expose and then, following the exposure end, tells a guiding program to dither, with dithering complete before the next sub starts. It's not clear from your post but I think you are not using a program for exposures but just the camera (or perhaps intervalometer). 

As you are using guiding, if you wish to dither manually you would need to stop guiding, press the SA buttons to move the mount a little in RA and DEC, and then restart guiding. This doesn't need to be carried out after every frame - every 5 frames should be ok for a 30 frame total. Use Median or Kappa Sigma stacking - this will reject pixels too far from the norm, resulting in a reduction of various types of image noise in the final image.

The SA only supports RA guiding/dithering. Dithering in RA alone can make walking noise appear as it may produce a similar effect to drift. Therefore the best solution is likely to be manual dithering in RA and DEC.

Hope this helps.

 

I've tried different stacking modes but all are giving the same result on the noise. 

The reason I just took 10 darks was that I just had over 1hr data and thought that would be enough. In the future I will definitely take more darks. Is there a some sort of % between the subs and how much calibration frames needed? Should you take more(or less) darks for example when shooting 5 hours vs 1 hour? Does the stacking of subs remove some noise by itself?

With the full frame camera I could not get more than 7min before any drift showed, most of my subs are 5 minutes at iso800. I haven't imaged much with the apsc sensor but 5 minutes exposures should be possible(hopefully). I'll also experiment with higher iso, although I'm not a fan of the iso performance on the d90 and not just in terms of noise but in terms of loss of detail.

Yes indeed I used an intervalometer. Just had a quick look at 'Astro Photography Tool', interesting to say the least. It would be a nice upgrade to control exposure from the laptop! 

Yes it has and will help! Thanks so much for all the info!

Ken

 

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20 hours ago, rickwayne said:

Hmm. Focusing is a bit of a dilemma without goto. Either you find your target without the filter and then have to focus without a bright star in Ha, or you have to focus on a bright star and then find your target with the Ha in place.

I assume that you can't swap out the filter without disturbing your focus -- after all we're talking microns here! I would recommend that you exploit plate solving regardless. Get pointed at a really bright star like Vega, and you can install your Ha and use pretty short exposures (1-5 sec, depending on gain) to really dial in those Bahtinov spikes. Then point as well as you can at your target, shoot an image, and plate-solve it. That will tell you where you're pointing, and you can home in on your target by iteration.

Bit of a PITA, I admit.

And that wound up a really gorgeous Lagoon, man.

The focus difference is roughly 10mm. Without the filter focus is at +-55mm and with the filter is +-45mm.

Indeed I'll have to make the step to start controlling the camera through the laptop( what bobro suggests) so I can platesolve directly on the laptop outside. Now I remove the sd card and load it on the desktop(inside) to platesolve. Laptop's card reader doesn't work as it should so it can be time consuming at moments. It seems APT has platesolving function right?

Thanks a lot for commenting and helping, much appreciated!

Ken

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2 hours ago, Ken Mitchell said:

Is there a some sort of % between the subs and how much calibration frames needed? Should you take more(or less) darks for example when shooting 5 hours vs 1 hour? Does the stacking of subs remove some noise by itself?

Although darks are intended to reduce pattern noise, a master dark will add some noise to each target sub before it stacked. A sufficient number of stacked darks will reduce the added noise - hopefully to a low level. However, whether the amount of noise added is significant depends on such things as the target signal strength (brightness) and the noise level of the camera. Narrowband imaging reduces the target signal and DSLRs typically have higher noise than dedicated astrophotography cameras.

The aim with longer overall exposure time is to further reduce random noise in the final image. If an insufficient number of calibration frames have been taken, the calibration frames could end up introducing so much noise that the benefit of longer overall imaging time is lost. The overall exposure time is typically made up of stacked subs. It's not the stacking that reduces the noise, it's the length of the overall exposure - stacking is just a convenient method of increasing overall exposure time.

I don't have much experience with using darks (I used dithering as noise reduction with my DSLR) - perhaps someone else can suggest rules of thumb or a way of determining how many to take, especially with narrowband? However, as a general finding, stacked exposures start to become smoother once about 25-30 subs have been taken and stacked.

You have done really well with the Ha image. I briefly tried Ha with my Canon 1000D but didn't go further due to the long exposure times required and noise in subs.

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Love the last image very good colours.

You don't need bias frames if you subtract darks, as the bias is in the darks it gets subtracted anyway.

Dark noise is reduced by the square root of the number of exposures, personally I usually use 36 which
should be enough but do as many as you like but 16 most likely a minimum.

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On 14/06/2019 at 02:55, bobro said:

Although darks are intended to reduce pattern noise, a master dark will add some noise to each target sub before it stacked. A sufficient number of stacked darks will reduce the added noise - hopefully to a low level. However, whether the amount of noise added is significant depends on such things as the target signal strength (brightness) and the noise level of the camera. Narrowband imaging reduces the target signal and DSLRs typically have higher noise than dedicated astrophotography cameras.

The aim with longer overall exposure time is to further reduce random noise in the final image. If an insufficient number of calibration frames have been taken, the calibration frames could end up introducing so much noise that the benefit of longer overall imaging time is lost. The overall exposure time is typically made up of stacked subs. It's not the stacking that reduces the noise, it's the length of the overall exposure - stacking is just a convenient method of increasing overall exposure time.

 

Thank you!

I will definitely remember that for the future. Take more calibration frames! :) 

The reason I asked if 'only stacking the subs' eliminate a certain amount of noise was when I shoot wide field milky way with landscape, I take multiple shots and later stack them in Photoshop with a median filter applied to reduce the noise. There is definitely a difference with the final image on the noise level and no calibration frames were added. 

Ken

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On 17/06/2019 at 07:26, wxsatuser said:

Love the last image very good colours.

You don't need bias frames if you subtract darks, as the bias is in the darks it gets subtracted anyway.

Dark noise is reduced by the square root of the number of exposures, personally I usually use 36 which
should be enough but do as many as you like but 16 most likely a minimum.

Thanks for that.

With my unmodded d610 I normally take 10 or 20 darks per session/night. I imagine that number applies for each session, no?

I never really encountered any problems with noise before(with the d610). Most of my images have low to almost no noise in them. Most of them have an integration time of minimum 4 hrs, would that have something to do with it?

On the bias frames, indeed I've read that what you say but from the beginning I started using all the calibration frames and still do with every image. Never really tried to stack without bias, could it give you bad or undesired results when using both?

Ken

Edited by Ken Mitchell
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You have everything you need for a great result. However, you can't just substitute Ha for red. They are not equivalent, most obviously for the stars which will be tiny in Ha and much larger in red, meaning you'll get insufficient red in the outer parts of the stars. Nor can you simply use Ha for luminance. If you do, you'll light the entire image in deep red light and diminish all the blues.

Personally I'd suggest splitting the channels and adding the Ha to red in Blend Mode Lighten. Once it is added in this mode as a layer you are free to stretch it further while, probably, holding down the bottom of the curve. In blend mode lighten the Ha will only affect the red channel where it is lighter than red so you can ignore noise in the darkest parts of the Ha. They won't influence the red.

Olly

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1 hour ago, Ken Mitchell said:

The reason I asked if 'only stacking the subs' eliminate a certain amount of noise was when I shoot wide field milky way with landscape, I take multiple shots and later stack them in Photoshop with a median filter applied to reduce the noise. There is definitely a difference with the final image on the noise level and no calibration frames were added.

Of course - that's stacking with pixel rejection, which further reduces apparent noise and is also useful in removing satellite and aircraft trails. A bit of magic!

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Hi Ken, 

Very nice photos and processing, hope you keep us updated with coming images.

You said that you made images using your unmodified dslr and Ha filter. That is a very interesting topic for me, could tell me how huge the difference is using Ha filter on modified or unmodified DSLR? I'll really appreciate it if you put some pictures also.

Best regards, 

Alan

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6 hours ago, OJ87 said:

Hi Ken, 

Very nice photos and processing, hope you keep us updated with coming images.

You said that you made images using your unmodified dslr and Ha filter. That is a very interesting topic for me, could tell me how huge the difference is using Ha filter on modified or unmodified DSLR? I'll really appreciate it if you put some pictures also.

Best regards, 

Alan

Yes indeed. I'll see if I can find some time this weekend or next week and will post it in a new topic (with pics). 

Ken

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22 hours ago, ollypenrice said:

 

Personally I'd suggest splitting the channels and adding the Ha to red in Blend Mode Lighten. Once it is added in this mode as a layer you are free to stretch it further while, probably, holding down the bottom of the curve. In blend mode lighten the Ha will only affect the red channel where it is lighter than red so you can ignore noise in the darkest parts of the Ha. They won't influence the red.

 

I tried your method but didn't get a good result. Probably did it wrong though. Could you help me with a link or tutorial for this method?

Thanks

Ken

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On 21/06/2019 at 21:35, Ken Mitchell said:

I tried your method but didn't get a good result. Probably did it wrong though. Could you help me with a link or tutorial for this method?

Thanks

Ken

I don't have one, I'm afraid. I demo this method here from scratch each time. I'll try to put something together, though.

Olly

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