NGC 6914 - Seeing Red

 

My emphasis on the Hydrogen Alpha data and its use in the luminance
had the intended effect on the detail, but it cost me a lot of the 
reflection nebulosity that's in this nebula complex.

Over the last few months as I've been working to get all of the new equipment from the summer to work together.  I have to say, it wasn't difficult.  I had it all lined out and behaving very quickly. The SBIG imaging setup has performed without a hitch in all that time.  With my implementation of CCDAutoPilot I have been able to massively increase the amount of time that I spend imaging.  Much narrowband data gets collected from my Arizona backyard while I sleep.  During weekends at dark sky observing sites, CCDAutoPilot takes over the equipment and I just forget about it.  It's allowed me to return to visual observing when I'm out on those new moon weekends. 

NGC 6914 is a huge nebular complex in Cygnus.  It's one of those great star filled areas of sky that feature all three types of nebulosity (emission, reflection and dark) in one field of view.  Having seen images of it previously, I wanted to focus specifically on the hydrogen alpha data and the contrast between the emission and dark nebulae in the area.  

Gathering the Image

The imaging rig all setup for the
magnitude 7.5 skies of Portal, AZ.



I started data collection on NGC 6914 in late August.  I cut my teeth on CCDAutoPilot with this object, ironing out the small learning curve and getting all of the software to play nice together.  A handful of subs were lost early on, but it only took a couple of sessions to get everything running like clockwork. 

I live on the far outskirts of the Phoenix metro area, so there is moderate light pollution at my home.  This was the real reason that I wanted narrowband filters - to collect that data from my backyard where it's relatively unaffected by light pollution.  All of the hydrogen alpha data was collected there.  It was this data collection that revealed one shortcoming of my SBIG ST-8300 camera. 

In August and September where I live, temperatures even at sunset are still usually well north of 100ºF (38ºC).  Overnight lows rarely drop below 85ºF (29ºC) during that time.  As a result, it was absolutely impossible to cool the chip of the ST-8300 to anything below 0ºC.  Thermal noise was a huge factor in the 30 minute subexposures that I captured for the hydrogen alpha data. Thankfully, a good set of 30 minute dark frames captured at the same temperature did a good job of minimizing the noise.  Still, I have to believe that it has an impact on the final outcome.

A September trip to a dark sky site in the shadow of the Discovery Channel telescope at 6800' elevation was used to capture the RGB detail.  I began processing the image fully intending that I would use just the H-Alpha data for the luminance channel.  I couldn't achieve an image that I felt was even vaguely good because of the big difference in star size between the H-Alpha and RGB data.  During the October trip to Portal, Arizona I supplemented the H-Alpha with some quality luminance data. 

Over the course of many nights, and three different locations in Arizona I collected just shy of 20 hours of data for this image.  Here's the breakdown:

• 28 x 1800s - 1x1 bin - H-Alpha 3nm bandpass
• 10 x 600s   - 1x1 bin - Luminance
• 16 x 300s   - 2x2 bin - Red, Green and Blue (each) 

Image Processing

I made use of a couple of tools specific to PixInsight to great advantage when blending the H-Alpha data with the rest of the image.  I used the HaRGB combination script to combine the H-Alpha with the rest of the color data.  I wanted also to use the H-Alpha as part of the luminance, so I ran the HaRGB script again with slightly different parameters:

• I created a new "RGB" image, but used the luminance data for the R channel.  Essentially, I created an LGB.
• I ran the HaRGB combination script to blend the Ha with the "red" channel of the LGB image.  This accomplished my task of blending the Ha and L channels.
• Using the Channel Extraction module, I extracted the R channel and threw away the other two.  This gave me a luminance channel that had all of the H-Alpha detail and star sizes from the luminance data.

 

SBIG's All Up In Your Grill - First Light

NGC 6888 - Crescent Nebula



NGC 6888 as captured with the new imaging equipment.  SBIG cameras, filter & off axis guider utilizing Explore Scientific optics.

So the Arizona monsoon let up enough over last weekend long enough to allow me to setup the equipment and take some serious first light images.  Once I got the mount polar aligned I left it setup for the next four days - just taking the optics and cameras off each day as the sun rose above the eastern horizon. As you may remember from my recent discussions, I have a lot of hardware and software changes that I've been trying to incorporate all at once.  As a result, this session didn't go completely trouble free, but it was darn close all things considered.  Even since writing those articles I went ahead and pulled the trigger on the SBIG OAG-8300 off-axis guider and the SBIG ST-i mono guide camera.  So far I'm happy with the two of them but there are a couple of minor annoyances that I didn't expect at this price point.  That's another topic for another time though.  Over the course of the four nights I was able to get the entire system integrated and working the way that I want it to work.  All I'm left with now is minor tweaks here and there.  

Gathering the Image

When I saw Manuel Jimenez' image of NGC 6888 last year I knew that it would be my first image if I ever got into narrowband imaging.  It was that simple selecting my first light target.  I didn't expect that I'd get anything near that awesome considering I'm not rocking $100,000 of equipment that includes a 17" Planewave CDK.  I was curious if I could capture some of the same detail with a 5" refractor.  All things considered, I'm really happy with the performance of the Explore Scientific triplet.  I'm working on a full review of it - but I'll just say that it doesn't get enough love in the astronomy community.  It's been worth every penny I paid for it - and it wasn't very many pennies comparatively.

Over the course of the four nights (and days) I managed to pile up 16.5 hours of light frame data using two filters - Hydrogen Alpha and Oxygen III.  Here's the breakdown of all the data that went into the image:

• 17 x 1800s Light Frames - 1x1 bin - Astrodon 3nm H-Alpha filter
• 16 x 1800s Light Frames - 1x1 bin - Astrodon 3nm OIII filter

With the exception of a noise reduction plug in (Neat Image) that requires the use of Photoshop CS5, I did everything after the capture using PixInsight 1.7.

• Created master calibration frames - Bias, Dark, Flat & Flat Darks
• Calibrated, registered & stacked the light frames with Winsorized Sigma Clipping
• Using Pixel Math I generated a synthetic green channel that is a mixture of 25% H-Alpha and 75% OIII data.
• I combined the images as a traditional RGB and processed as normal.

What I Like - and What I Don't

I've come to accept that there will always be things I don't like about one of my images.  This one is no exception.  But overall, I enjoyed putting this one together as much as anything I've ever done in astronomy.  The data was excellent and I couldn't be happier with how it came out in the end.  Okay, yes I could.  But considering the factors involved in gathering the data I'm as happy as can be.  I'm also a firm believer that we don't learn from our successes, we learn from our mistakes.  And thus, a constructive critique is the best way to really improve - so on to what I don't like.

Field Curvature - If you click on the above image and look at the larger size, it's pretty apparent looking toward the edges that there's a good amount of field curvature.  This is self inflicted.  I had hoped to be able to use the ST-8300 without a flattener and my calculations told me that it would be close but acceptable.  I was wrong.  Because the cameras, filter wheel and OAG combined weigh 4.5 lbs (2.1 kg) I don't feel comfortable depending on three thumbscrews and a compression ring to hold the equipment.  Right now the imaging rig is all threaded connections from telescope to camera and there's no flexure.  So I guess I'll start looking for a solution that incorporates a field flattener into the mix while keeping my 100% threaded solution.

Noise Reduction - I overshot it a bit on the noise reduction.  The image is "too smooth" to me.  In my defense, I was shooting in the lower deserts of Arizona from my backyard.  Daytime temperatures were approaching 110º F (43º C) and the lowest night time temperature I saw was in the upper 80's (about 30º C).  This limited the amount of cooling that I was able to get from the camera and the entire image was shot with the TEC sat at 0º C.  The images had huge amounts of thermal noise in a 30 minute sub.  A good set of darks certainly helped but the starting point for post-processing was a pretty noisy image to begin with.  I expect I'll have a much better result when I get the camera in some cooler ambient air - hopefully soon.

Taking Stock – The Software

Last time out I covered the hardware, which looks all pretty and imposing when it’s all setup – but it’s exactly half of the equation.  It’s the software that makes modern astrophotography possible.  In my own setup, the software plays a critical role in every step of the process from setting up to publishing a finished image.  In this second installment I’ll finish detailing the pieces of my imaging rig that all contribute to the end result and how it all fits together.  From this point forward it’s my hope that I can start tackling some specific tasks and how I handle them when it comes to deep sky imaging. 

PEMPro-V2

PEMPro is designed to a do a handful of things – chief among them is to measure and program periodic error correction on a telescope mount.  It excels at this task and I had excellent results using it on both of my previous mounts.  On my original Celestron CGEM I had a native periodic error of 17.6” peak to peak.  A couple of hours with PEMPro and I was able to program the mount with a curve that reduced it to 3.1”.  It helped me similarly in taming my Celestron CGE (52.1” down to 4.3”).  As I detailed in the last post, my Astro-Physics AP900 has a native periodic error of 1.45” peak to peak.  I haven’t bothered to even try and improve that yet.  Interestingly, Astro-Physics uses a specialized version of PEMPro to measure and program the periodic error in their mounts prior to shipping them out. 

For me though, PEMPro’s greatest strength is in the Polar Align Wizard that is another of its functions.  I don’t have a permanent observatory so I’m setting up and aligning the equipment every time I go out to image.  I’ve managed to arrange my imaging rig and the setup workflow in such a way that I can be polar aligned well enough for 30 minute sub-exposures before the end of twilight.  PEMPro’s Polar Align Wizard is a huge part of that.  Essentially, it’s a computerized version of drift alignment that works as follows:

• A quick routine is used to determine the camera’s orientation (position angle) and the calibration of North/South/East/West to the X/Y coordinates of the camera chip.  Typically this takes me about 2 minutes. 
• PEMPro will measure a star near 0º declination and near the meridian by taking repeated subexposures and measuring the star’s drift over a period of time.  Typically, I do this for 7 minutes.
• After selecting a star in the star field, PEMPro will draw an arrow and a circle over the frame and begin taking repeated images of the field.  Using the azimuth adjustment you simply adjust the mount east/west until the star is in the circle drawn. 
• Altitude is then set by moving 45º relative (east or west) to the meridian and repeating the 7 minute measuring phase. 
• After selecting a star in the star field, center it in the superimposed circle using only the altitude adjustment on the mount.
• Repeat if necessary.

It’s that simple and takes about 20 minutes.  Typically I’m starting this process as soon as any star bright enough to see naked eye appears near the meridian and 0º declination.  As I mentioned, I’ll write a post on my entire process some time in the future.  My method gets my polar alignment to about 1 arc minute from the celestial pole before the end of astronomical twilight and a big part of that is due to PEMPro. 

Maxim DL

Maxim DL is one of those suite software appllications that tries to be all things to all people and it does a pretty good job of it.  Nearly everything that has anything to do with data acquisition goes through Maxim DL in my setup.  I use Maxim DL to control the telescope and the camera through every phase of capturing data.  I use Maxim DL to:

• Align the telescope at the beginning of the evening. Using the built-in PinPoint LE astrometry software after polar alignment I can sync the telescope to a plate solved field and I’m done.  Ready to slew anywhere in the sky.
• Slew the telescope.  Using the built in catalogs I’ll slew the telescope to the chosen object.  An automated exposure is taken after a slew and plate solved to ensure that I’m exactly centered on the field.
• Focus the telescope.  Sort of.  I use Maxim DL to tweak the Moonlite focuser during a filter change to the appropriate offset for the filter being selected.  Real focusing chores are handled by FocusMax – which uses the camera control in Maxim DL to do it's thing.
• Guide the telescope.  I use Maxim’s built in guiding capabilities to autoguide during subexposures. 
• Full camera control including temperature regulation, switching filters, taking exposures and moving the telescope slightly to dither between exposures. 
• Capture of calibration frames.  Dark, bias and flat frames are all taken with Maxim DL.  Soon I’ll be working to configure Maxim to automate flats through each filter using an Optec FlatMan.  Stay tuned for more on that.
• Maxim DL does an excellent job of controlling the telescope to correctly capture the multiple frames needed for a mosaic. 
• I use DDP in Maxim DL during an evening just to preview what kind of data I’m capturing.
• Maxim DL has many other built in features for calibrating, stacking and processing images.  While they’re effective I have found that there are other tools that I like better for those tasks. 

PixInsight

PixInsight completely changed the way that I process images.  It’s software developed by astrophotographers specifically for the purpose of processing astrophotos.  It has gotten a bad rap by people who haven’t thoroughly investigated the product saying that the learning curve is steep and that educational resources are lacking.  I’ll agree that it’s a different approach to processing than most people are used to, but if you take the time to use the resources that are available it’s quite intuitive.  I use it for nearly all of my calibration, stacking and image processing.  The 800 pound gorilla known as PhotoShop still figures in to my image processing scheme for some final tweaks, but 95% of my workflow after I capture an image is done with this application.  I use it to:

• Create master bias, dark and flat frames.  Here’s a tutorial on the creation of master calibration frames.
• Calibrate, Register and Stack a series of light frames.  It takes more effort in PixInsight than in other applications but the end result is a much cleaner master light frame in my experience.  I stopped using Maxim DL’s calibration & stacking routines after the first using PixInsight’s methods just once.  
• All image processing tasks including color calibration, background neutralization, stretching, contrast enhancement and teasing out detail in the image.

Expect to see some more entries into this column detailing various aspects and techniques that I use in PixInsight.  Meanwhile, if you’re interested in learning more there are abundant resources on the web.  Start at the PixInsight website.  Many tutorials are there.  Also, go to Harry's Astro Shed and start there.  I learned to process an image by taking one of my own and just walking through Harry’s tutorials.  I haven’t personally seen them myself – but I know that Warren Keller and Rogelio Bernal Andreo are working on a tutorial series at IP4AP.  I’ve seen the Photoshop series that Warren did and it is an excellent resource.  M81 and M82 shown below was the first image I processed in PixInsight. 

 

PhotoShop CS5

The heavyweight of all image processing packages is without question Adobe’s PhotoShop.  I once used it for everything I did in a stacked image but it has fallen very much by the wayside since I picked up PixInsight.  In all fairness though, I do still use it at the very end of my workflow to do some final background smoothing, perhaps an Unsharp Mask and to resize/save the image for publishing on the web.  There are a lot of resources available to learn how to process an image with PhotoShop.  I found Adam Block’s DVD “Making Every Pixel Count” to be the best among them.  I’ve since met Adam and had an opportunity to pick his brain on a couple of things.  He’s a gracious guy who’s more than willing to share his extensive knowledge.  Check out one of his imaging workshops if you want some hands on experience with him.

What’s Missing?

If I had to go through the rest of my life with just the equipment and software that I have I could still die a happy man.  I don’t think that I’m missing any critical piece of the puzzle any longer.  But let’s face it – astrophotographers are probably the most desperate breed of astro-equipment junkies out there.  Now that I have a narrowband imaging rig and could conceivably image every clear night from my backyard it appears that imaging could become a threat to sleep and my ability to conduct what normal people call a normal life.  I’ll evaluate the situation before long and see if it makes sense to incorporate an automated imaging platform to handle imaging sessions for me.  I’ve been quietly eyeing CCD AutoPilot to fulfill that role but I’m not a point where I’m ready to seriously take on that task just yet.

With new moon coming tomorrow I hope that the Arizona monsoon cooperates enough to allow me a solid first light with the SBIG ST-8300 and filter wheel this weekend.  Once I get that incorporated into the workflow then I’ll see about the last couple of “nice to have” pieces – an off axis guider and an automated imaging platform.  Until then,  I continue to be a happy guy who has it far better than he deserves it.