Punching Past Pollution: Target Selection Part 2
Last month I described how to determine whether and when a specific astronomical target, in this case the “Thor’s Helmet” nebula, will be accessible from my yard. This month I will discuss matching up equipment choices against the target. Presently, I have a range of scopes which I can use, with focal lengths ranging from 200 mm to 1600. But when I started this hobby I only had the one Maksutov-Cassegrain scope at the top end of that range, so the question was “which targets are practical for my set-up?” I learned the hard way that my Mak would not be helpful for imaging comets – but what would? So my question became, “what does each new piece of equipment – optics or camera – buy me in terms of capability for additional targets?”
My recent and previous attempts at the Crescent Nebula, a small but bright emission Nebula in Cygnus rich in H-alpha regions, should work well for illustrating the tradeoffs involved here. First, I took a wide shot. I wasn’t even planning on picking up the Crescent but it snuck into the bottom right corner of this take on the extensive nebulosity around the star Sadr:
Next I’ll show two variations on zooming in on the target. First is an older image using my Questar – the Mak with 1600 mm focal length, but tamed by a .5X focal reducer. Note that like many precision components, you could buy a decent “budget” refractor for the price of this lens which was configured for the unusual Questar optical train and is unlikely to ever see any use with other scopes. But it did serve to broaden the targets I could reach with the scope. With an H-alpha filter to bring out nebular detail from the background, I got this:
I got a similar, albeit less detailed, view, with considerably less effort, by placing a 2X Powermate (Barlow lens) on a 400mm lens for an 800mm focal length. Different camera, different filters, different processing, different image. But similar in composition as a close-up centered on the nebula:
And finally, for the Goldilocks in all of us, the 400 mm lens produced this nice shot with the nebula in a starring role, but with plenty of context:
Ok which is the “right” image? Hey I love all my kids (and my dog) so I don’t play that game. I wouldn’t call the first wide shot an image of the Crescent Nebula, though. Otherwise, each has some charm for different reasons, including the rich star fields in the wider shots which always startle me based on my expectations for the skies over Yonkers.
Now we are back to my questions about the best approach for imaging Thor’s Helmet or any other specific target. One great tool for this is an app called CCD Calc. Originally a companion to the seminal astrophotography text, “The New CCD Astronomy” by Ron Wodaski, it is available as freeware here:
The app appears as two small windows; one for inputs and one that displays how selected targets will appear with a given optics/camera configuration.
Let’s take a closer look at the input screen:
For your optics you need to know two of these three numbers: Focal length; Aperture; and/or Focal Ratio (the third can be calculated from the other two). You can enter a multiplier for a Barlow (e.g. 2X) or Focal Reducer (e.g. 0.5X). For the camera you should find these specs: Pixel Size in microns, and the size of the Pixel Array. From these it will calculate the chip size and image scale. Once you enter the attributes for a given product you can save it in the drop down menus for future use.
The graphic on the right side of the input screen shows the size of your camera’s chip relative to a full frame 35 mm chip. This is a helpful reference when first considering cameras. Experienced imagers are very familiar with the limitations of planetary cameras – these less expensive devices work perfectly well for imaging Jupiter or as guide cameras, but the tiny chips will be overwhelmed by the wide image circle of just about any telescope. I have helped a couple of people who were struggling with their first attempts to use a planetary camera in part because they didn’t know what to expect when their high focal length telescope was pointed at the moon. Once they understood that a scope that fills a 35mm frame with the face of the full moon will only fit a few highly magnified craters on a much smaller chip, it was easier to bring those craters into focus for the first time.
The drop down box just above the frame graphic lets you select different targets which will appear in the FOV window with a rectangle which provide a good idea of how a given target will appear with a specific configuration. A large target like the Andromeda Galaxy spills over the edges of all but the lowest focal lengths, while small targets like the Ring appear as little dots at these focal lengths. It is clear that you can’t do both of them justice with the same set-up.
Target sizes can also be found in star maps like Cartes du Ciel. The sun and moon are the same apparent size in the sky (hence eclipses) at half a degree in diameter. Larger targets include the Rosette Nebula (1.3°), the North America Nebula (2°), and the Andromeda Galaxy (3°). The smallest targets include distant galaxies which may appear to be elongated stars at any focal length – there is no lower bound! Smaller deep sky targets have their sizes displayed in arcseconds (1° = 60′ = 3600″), with the Ring Nebula weighing in at 86” and the Owl Nebula at 200”.
So what configuration do I want to use to shoot at Thor’s Helmet? Using Cartes du Ciel, I can see that the dimensions of NGC 2359 are 10’ x 5’. I can answer the question without referring to CCDCalc because we can simply compare this to the Crescent Nebula, displayed above, which is 20’ x 10’. So it’s half the size of the Crescent, which means that I could get an adequate shot in context at 400 mm – similar to Figure 4. A good close-up, however, will require one of my high focal length configurations.
I can use CCDCalc to check this assumption by plugging in the information in the app for the same configuration I show in Figure 3, which is a 400 mm focal length lens doubled with a 2X Barlow. As shown below, the FOV window demonstrates that this will produce a strong composition emphasizing the nebula, and is likely to be the approach I choose when the time comes early next year to work on this image.
Note: This article is appearing simultaneously in Eyepiece, the newsletter of NYC based Amateur Astronomers Association of New York and SkyWAAtch, the newsletter of Westchester Amateur Astronomers. I’m a member and supporter of both organizations.