"NGC869 and 884, The Double Cluster in Perseus. Open clusters are the best objects for urban imaging. 4x30sec exposure"
I am an urban warrior. Repeat again I am an urban warrior. Yes I am an urban warrior, as in urban imaging (don't worry I am not going out to kill zombies or slay some demons). I have had limited success up until now CCD imaging from my white zone playground/observation spot. Not sure if this is due to me having more experience under my belt imaging and processing, or owning a slightly better camera, or just having the knowledge to select more suitable objects, but I have been getting very encouraging results of late, even with dimmer fare. Imaging from my white zone observing site makes heaps of sense since I do most of my observing from home anyways, seeing I only get the chance to get out to dark skies once a month! That way I can add to my tally of imaged objects and still have a rollicking good time under less than ideal skies. An added plus is I save on fuel and I don't need to pack the car! Its amazing what you can pull out of the muck when you use the right gear.
Battle Gear for the Urban Warrior
"M76, The Little Dumbbell Planetary from my light drenched playground. 10x30sec exposure"
The "gear" is not all that different from what I would employ under darker skies. My imaging arsenal is composed of a GOTO telescope, SkyGlow light pollution filter, 0.5x focal reducer (if I am using the longer focal length OTA for imaging), CCD camera and a laptop. I use the 70mm Travelscope these days as it just offers a much wider canvass to paint on, plus I can expose for longer without trailing (as mentioned in my previous blog I can expose for up to 42 seconds). The mount is still the same old Celestron NexStar GOTO mount that used to hold the 102 mm f/10 refractor (which I still use for smaller objects such a globulars and planetaries). My camera is now the next CCD cam up from my previous cam. I now use a Meade DSI II OSC with a larger CCD chip (752x582 pixels-437,664 pixels almost double the previous one), and it has heaps lower, if any amp glow. As such I have had an intense urge to re-image most of the objects I did with the smaller CCD cam. In short the final product is just more prettier! I employ a Orion SkyGlow Broadband filter when I do my imaging from suburbia. This visual filter knocks down light pollution sufficiently for me to get good images even of galaxies! Another plus is it seems to remove that annoying purple halo around brighter stars from chromatic aberration.
Choices Choices.....
"NGC891, The Outer Limits Galaxy in Andromeda from a white zone no less. Yes it is imageable. 15x30sec exposure"
This is one of the most critical components for success if one wants to get good deep sky images from suburbia, especially if you, like me have a limited amount of equipment. Astronomers with deeper wallets have been getting excellent results even from the city. With accurate tracking mounts, large image sensor CCD camera's (mostly black and white as those are more sensitive!), APO scopes (to banish chromatic aberration) and the use of various narrowband filters, some of these astronomers have been turning up world class images of all manner of deep sky objects rivaling even large observatories!
"M37, Open Cluster in Auriga, 6x30sec. Open Clusters are easy from light polluted skies"
In my case, I stick to mostly brighter objects that can be captured with very short exposures. The trick is to get multiple subs. These days I collect the subs and save them separately (in the past I let the Meade capture software Envisage do the work...but most times this does not work out too well after 6 of more subs). The objects I usually target are open clusters, brighter nebulae (and smaller planetary nebulae as they have very high surface brightness). I leave the galaxies and fainter nebulae for my dark sky jaunts. That said, there are exceptions to the rule. Some of the fainter fare can be imaged with a little patience. The only problem I face with these is the grainy background. The images are almost exclusively better from dark sky sites so whenever possible, I still reserve most of the fainter stuff for my dark sky jaunts!
Post-Processing Magic
"M42, The Orion Nebula in Orion. My best image of this huge nebula complex so far...and this came from white zone imaging. 15x30sec"
The title says it all. This is where the real magic happens. Most of the dimmer fare will not be visible in single frames. That's where good GOTO comes into play, since some of these objects might not even be visible in the scope (Especially from suburban sites. From dark sky sites this is not so much of a problem as you can check to see if the object is in frame!). The secret here is to stack multiple subs. As mentioned in the previous paragraph, I used to let Envisage handle this, prior to discovering MaxIM DL. While it does a decent job, most times the stacks goes wonky after 6 subs. I am currently doing anywhere between 6 and 20 subs at 30 to 42 second exposures each and then stack them afterwards in MaxIM DL. This post-processing software works like a charm! All you have to do is open the images you want to work with, hit the combine button, select the images you want then manually select two stars you want to use as point of alignment. You scroll through the images to mark the alignment stars (alignment star one on all selected images, then alignment star two). You have the option of discarding subpar images. MaxIM DL also has a histogram function to stretch the image but I prefer to use Photoshop (PS) for this. I save the files as 16-bit TIFF's. While not as good as FITS, this image file format has a good enough dynamic range to allow stretching of the histogram to bring out the fainter portions of the object. Besides PS does not work with FITS file format, and this is what I use for all my image manipulation. In summary I apply dark frames, clean up image noise using clone stamp tool, use the curves function to bring up detail and levels dropper tool to adjust the background to noise ratio and color balance.
"NGC2024, The Flame Nebula in Orion, another example of a dim but imageable object from bright skies. 20x30sec"
With this I can get pretty pictures even from my light drenched playground. I let the images here speak for themselves!
