Dark Site, Dark Site, Dark SiteI cannot emphasize enough what a difference it makes to shoot at the darkest available site you can get to. Driving for an hour or more to get away from city lights should really not be an issue, and for that reason the portability of your equipment becomes an important factor. Bigger is not always better! For these tests I went a little further than my usual spot on Piuma Road in the hills above Malibu: I drove up to 2000 feet in the mountains near the Ventura County line. Being away from Los Angeles, the skies were darker and the core of the Milky Way was clearly visible. The Lagoon Nebula (M8), my target for the evening, could be seen with the naked eye. For this comparative test, I decided to keep all sub-exposures at 60 seconds. I use a Skywatcher EQ-5 mount (much like the Celestron CG-5). After carefully polar aligning the mount, I first did a visual comparison of the three scopes. Right away, it was obvious how much better the image looked in the ED and the EON: not only were stars sharper and more pinpoint, but the background was noticeably darker and the image more contrasty in the two apochromats than in the short tube achromat. Between the 80 ED and the EON, the differences were slighter: the EON has a wider field and is more compact. The dual-speed focuser on the EON is more precise and does not shift upon locking it, but both scopes present a tack-sharp image.
The ShootoutThe first scope to shoot with was my 80mm ED. It's a solid piece of equipment and my only gripe is the focuser: it is very smooth but tends to shift whenever you lock focus. It took a few tries, but using live view – a very important asset for AP - at its 10x magnification on my astro-modified Canon 1000D, I was able to anticipate the shift during focusing on Antares so that after locking, the focus was perfect. I always choose a bright star not too far from my desired target to focus on. In order to get a flatter field and faster ratio, I used a 0.8x William Optics Focal Reducer/Field Flattener (version 3). Thus, the focal length became 480mm and the ratio went down from F/7.5 to F/6. I found this combination works very well: it is fast enough to get by with shorter exposures and still have plenty of detail in a field that is sharp from corner to corner. I also used a clip-in light pollution filter (CLS for Canon EOS) right over the sensor, because even at a fairly dark site it still helps the contrast. The only time I will take it out would be at the perfect dark site, which near Los Angeles probably means Death Valley: a five-hour drive. It takes a lot to avoid light pollution if you live in the big city! With the relatively short 60 sec subs (unguided) at ISO 1600 I was actually surprised to see how much detail was being recorded, visible even on the small display of the camera. Here is one (click on image to see it in higher resolution): A fair amount of nebulosity in the Lagoon Nebula (M8) and beautiful pinpoint stars across the field. This is the entire frame and there is no trace of breakdown in the corners. It is really a testimony to the great optical quality of the 80mm ED, and the lucky combination with the William Optics FR/FF v.3. I shot about seven or eight exposures, followed by five dark frames, of course also at one minute each. Then I programmed my GoTo to slew to the very nearby Trifid Nebula (M20) and quickly shot five subs of the Trifid as well. Here is one: Again, the detail is very good and there is a fair amount of color in the nebula, without any processing. (Note: since I always shoot in RAW mode as opposed to JPEG, the image has been digitally corrected for color and contrast in Camera Raw 6.4 – but no real enhancements yet.) Time to change scopes. Slew back to Antares, take off the 80 ED and set up the EON. Right away, it feels like a very well designed instrument: everything moves smoothly, it is compact and after attaching the camera (again with the WO FR/FF v.3 and the CLS clip-in filter) working the dual-speed focuser in live view is just like a dream, super smooth and zero image shift upon locking it. Hopefully it will be perfect focus – even with the 10x magnification you can sometimes be a hair off. So back to the Lagoon Nebula and start another series of eight subs, which takes less than ten minutes. We are now shooting at F/5 as opposed to F/6 with the 80 ED – so I am expecting the image to be a little brighter and the field a bit wider. Here is one of the sub-exposures: Since the orientation has been corrected, the nebula is better framed and the field is indeed a little wider. Stars again are pinpoint across the field and the image is a hair brighter. Here's the Trifid: Clearly, it's a wider field, now with part of the Lagoon showing below. The ED shot looks a little more contrasty, but that could well be a result of the Camera Raw processing, because the images were not developed identically. Anyway, again plenty of detail to work with. I shot five subs of the Trifid. Why eight of the Lagoon and only five of the Trifid? Don't ask – it was getting late... Finally, testing the 80 mm short tube. As I mentioned earlier, clouds were starting to come in and I had to move fast. I took five subs of each nebula and only one of each turned out to be (borderline) usable. The focal reducer/field flattener could not be used on the 80 ST because it is too big for the 1.25” eyepiece holder, but I left the CLS filter in the camera. Here is the best exposure of the Lagoon: There is simply no comparison. Stars in the corners are horribly bloated and deformed, the field curvature is totally distracting. BUT at F/5 and 60 seconds, the nebula is definitely showing! I did another series of dark frames, just in case the temperature had changed, and we packed up. By the time we left, most of the sky was covered in clouds. I did not get to shoot my flat frames, but I confess I often don't even bother.
Processing the RAW FilesIt is always a wonderful challenge to get the most out of your shots. I have found that at times even a single exposure (without using Deep Sky Stacker, just Photoshop) can be processed and enhanced to a surprising degree as long as you keep the noise under control. Therefore, the first step is to get rid of as much noise as possible without sacrificing sharpness and detail. After opening all your subs in Camera Raw (working from Adobe Bridge) you will see several icons in the top right corner: the third icon from the left has two triangles and if you move your mouse over it, it will say “Detail”. Click on it. You want to see the files at 100% (indicated in the lower left corner) and move the levers for noise reduction (right side) to a point that all the noise is gone. For these particular sub-exposures I set the Luminance bar at 54 and the Color bar at 68. Then I brought the Sharpening up to 61. This gave me a nice balance with an almost noise-free image. The other levers are at their default settings. Make sure to click on “Select All” (top left corner) before you make the adjustments, so you do it to all the exposures at the same time. You may notice that the image (this is from the EON) actually looks better and more detailed here than in the sub-exposure that was shown earlier. That is because I could not help myself: once I brought up the subs up in Camera Raw to take a screen shot I played with them some more! It's a real good idea to work the files as much as you can in before saving the images (as uncompressed TIFF files would be my recommendation). For those who prefer to use Adobe Lightroom (another great software for RAW processing) I'd give the same advice: get rid of the noise and work the files before saving and stacking them in Deep Sky Stacker. Please take a look at the values displayed below which is the first (Basic) template that comes up when you open the subs in Camera Raw: it will give you a good idea how much messing around is needed to get the subs to look their best. A closer look makes it easier to read: What I find most striking is the color correction: temperature all the way down to 3000 K and the tint -84 into the green. That means the original file had a very strong magenta bias and was pretty warm to begin with. Color is usually all over the place in the RAW files and even though I set my color balance on Auto during the shoot (hoping it will correct whatever bias is there), using different filters to reduce light pollution invariably changes the color. Position in the sky also is an important factor: lower in the sky makes the image warmer. I pushed the blacks up to 12, which is quite high – I could have left more definition in there, even though that would have added a bit of graininess (= noise). Personally, I like the background close to black, it just looks more dramatic and appealing that way. The brightness is pretty high as well at 79, which helps to get more definition in the nebulosity. A bit of fill light is good for the same purpose. Finally, some extra vibrance and saturation helps the color come alive. The whole point of showing all this is to emphasize the importance of shooting in RAW and how much can be done to the subs before they are stacked. It makes a huge difference in the final result.
Stacking the Images in Deep Sky StackerThis part of the tutorial can be pretty short, since I do not want to repeat a lot from the previous one. Deep Sky Stacker has its own instructive guidelines on their site (http://deepskystacker.free.fr/english/) and it is totally fine to use their default settings. Dark frames are imperative to shoot, because your sensor will otherwise add its own “stars”. Flat frames really help correcting the fall-off towards the sides and corners of the image. I strongly recommend shooting them even though I don't always do so myself! Dark flat frames and bias frames will add additional quality to the shot and ultimately it is all a matter of how perfect you need the end result to be. In my opinion, first get your tracking and processing skills in place, then you can start working on the finer details. Deep Sky Stacker is a miracle and without it, most amateur astronomers would not be able to get the results they are getting. It is an absolutely must to use when shooting Deep Sky Objects with a DSLR.
Final ProcessingThese days, the first thing I do with the final stacked (but mostly unprocessed) image is go straight to curves in Photoshop (Ctrl – M), grab it in the middle and pull it way up. That will bring out a lot of the nebulosity but at the same time it makes the image look flat and too bright. The background becomes pale and washed out, but that is exactly what we need. The only black left in the shot are the very edges where Deep Sky Stacker had to adjust the frame to make sure that all the sub-exposures were perfectly lined up. In order to bring back the density, we go to Levels (Ctrl-L) and move the left cursor further to the right: We can even do this a few times: first bring up the curves and then bring back the density by going to Levels and moving the left cursor to the right. To make sure that the highlights (in this case the central part of the nebula) don't burn out, we can put a feathered lasso around the bright areas, and hit Inverse (Ctrl-Shift-I). That will leave those areas untouched while we apply the corrections to the general image. Typically, the photo will still have a color shift, in this case magenta – but it can also be green, cyan or red. In order to improve the color, we go to Color Balance (Ctrl-B) and highlight “Shadows” in the lower left corner of the Color Balance window. Then we choose the cursor that corresponds with the color shift and move it in the opposite direction: so in this case towards green. What happens is that the background will go darker and at the same time the color becomes more natural. In order to save time, you can try using the automatic corrections: Auto Tone, Auto Contrast, and Auto Color. In my experience, you get lucky sometimes and other times it is way off. You can always undo it or bring the effect down by going to “Edit” (top left) and then “Fade Auto Tone” (or Contrast or Color). It is by combining different tools and corrections and playing around until it starts looking really good that you end up with a truly nice image.
ConclusionsAll three telescopes have their merit. The 80mm short tube is really not meant for imaging but it makes a great guide scope and a very affordable and portable “grab and go”. I ended up stitching the two shots of the Lagoon and the Trifid Nebulas (the only usable subs) together and the result is not even bad, especially considering the instrument it was shot with: As long as you keep it small, it looks nice and shows both nebulas. The 80mm ED fully lived up to its reputation of great optics. It may even have slightly better contrast than the more expensive EON, but that is a tough call – they both throw up a beautiful and sharp image. If I were to use the 80 ED on a regular basis, I would probably want to change the focuser. Here is the final combined image of the 80 ED: It is clearly an excellent imager, and the fact that it sells for only $300 on the used market makes it a very attractive candidate as a first scope for AP. My personal favorite of the evening was the 80mm EON. It is a little shorter than the ED, beautifully finished and really easy to work with. There is nothing I would change about the design and at some point I'd actually like to play with its bigger brother, the Orion 120mm EON. With its fast optics (F/6.25) and wide field the 80mm EON makes a perfect imaging scope that is still very portable. Here is the final combined image, shot with the EON: Finally, I would like to add a few words about the Celestron 80mm ED, which is optically identical to the Orion 80mm ED. However, it has a lighter and more elegant looking build. The copy I used had an upgraded focuser (a dual-speed Crayford) which made working with it very comfortable. Also, I used a better camera: a modified Canon 5D Mark II. As opposed to the Canon EOS 1000D, it has a full frame sensor. I shot at ISO 1600, taking about ten unguided sub-exposures of 1'42” each. Even though there was a fair amount of noise, due to the warm weather and some vignetting because of the larger sensor, the end result was better than any shots I had taken so far. I used an AT 0.8x reducer/flattener which worked very well in combination with the scope, giving me an effective focal length of 480mm at F/6. I followed the same processing steps as I did with the other images. Here is the final image: it comes from only one frame rather than combining two, since the larger sensor produces a significantly wider field. I highly recommend this excellent little scope as well as the beautiful 5D Mark II. Clear skies!! * * *
View Martin Cohen's astrophotography website here for more of his outstanding work!