Appendix a Planning an Astrophotography Imaging Session

Appendix A Planning an Astrophotography Imaging Session One way to obtain a high percentage of successful photographs is to look at imaging a deep space object as a project. The first step of any project is to define the project’s goals and objectives followed by detailed planning and then executing the plan. Perhaps the best way to explain an astrophotography project is by an example. For our example, let’s assume a location at 38° north latitude in a suburban area with moderate levels of light pollution with a skyglow of magnitude 19 per arcsecond squared. The imaging kit is an Orion ST-80A 80 mm f/5 refractor, a Canon EOS T3/1100D camera with an interval timer and a Celestron 4 SE mount on a wedge. The current time of the year is early summer and we see an image of the galaxy NGC 2403 posted by someone on an internet forum, in an astronomy maga- zine, etc. The galaxy looks interesting and we decide that we would like to make our own image of it. Before getting heavily involved in planning we need answers to the following questions: • Is the galaxy visible at our viewing location? • When is the optimum time to photograph the galaxy? • Is our equipment capable of capturing a decent image of NGC 2403? The literature tells us that NGC 2403 has an apparent magnitude 8.4, an apparent size of 21.4 × 10.7 arcminutes, and is located in the Constellation Camelopardalis. It is object number 7 in the Caldwell list for small telescopes and is often observed with a pair of binoculars. At 38° North latitude, Camelopardalis is a circumpolar constellation and is highest in the sky during January and February. For photographing galaxies and nebulae, © Springer International Publishing Switzerland 2015 287 J. Ashley, Astrophotography on the Go, The Patrick Moore Practical Astronomy Series, DOI 10.1007/978-3-319-09831-9 288 Appendix A the moon is an enemy. A quick check on our nearest neighbor and we determine that the nights of 3 December, 1 January, 30 January, and 28 February have no moon. These nights and three or four nights before and after them will be ideal for imaging NGC 2403. We also note that the highest NGC 2403 rises in the sky is 62.5°; thus, either an alt-azimuth or an equatorial mount is usable. (Please note that the above dates are examples only as the actual dates for nights with no moon change continuously.) How did we find the above information? Many sources are available. A simple internet search using the object’s name is all that is typically needed. Several online sites provide lunar calendars. Planetary computer programs are another source and are very popular with amateur astronomers. Some of the more popular ones such as Stellarium are freeware and are capable of controlling telescope mounts. Books and astronomy magazines are other sources. Now we need to verify that our equipment can image the galaxy 2403. In other words: • Is the object bright enough to provide a usable signal to noise ratio with our camera and telescope combination? • Will the apparent size of the object be too large to fit into the camera’s field of view? • Will the size of the image that we obtain of the object be large enough to show interesting details? Determining if the surface brightness of an object is sufficient to produce a usable signal to noise ratio is not a straight forward exercise. Here the best guide is experience with your astrophotography equipment and your location that you will develop over time. Many variables have an impact upon the signal to noise ratio; the major ones being the camera and telescope’s characteristics, exposure duration, and atmospheric conditions that impact seeing, visibility, and our nemesis light pollution as well. NGC 2403 is also on the Caldwell List. This list is a compilation of non-Messier Objects that are visible in small telescopes under dark skies. Since NGC 2403 is visible in binoculars and small telescopes, we know that our equipment is capable of photographing it. However our observation location is in an urban area having artificial skyglow estimated at 19 magnitude per sq arcsec; hardly a dark sky. Objects with a surface brightness three magnitudes dimmer than skyglow can be photographed with some details and contrast but with fainter areas washed out. We know the apparent magnitude of the object as well as its apparent size but do not have its surface brightness. Recall that apparent magnitude is the light from an object concentrated into a point source like a star. Surface brightness is that same light spread evenly over the surface area of the object and is expressed as magnitude per square arcsecond. Since the magnitude measurement is logarithmic, we cannot simply divide the apparent magnitude by the apparent area to get the average surface brightness but use the following equation instead: SM=+25.l´ og A 10 () Appendix A 289 where, S = surface brightness (magnitude per squared arcsecond) M = apparent magnitude A = apparent area (square arcseconds) Doing the arithmetic, we determine that the surface brightness of the galaxy NGC 2403 is approximately 23 magnitude per sq arcsec. Our estimated skyglow is 19 magnitude per sq arcsec. This tells us that photographing NGC 2403 at our location will be tricky. We will need a dark night with no moon and clean air to reduce the artificial skyglow component. We also need to schedule the imaging session when NGC 2403 is high in the sky. The next concern is the apparent size of the object. Our telescope, the ST-80A has a focal length of 400 mm and the camera has an APS-C size sensor. From Table 3.3; notice that the field of view for this combination is 3.2° × 2.1° or 190.8 × 127.2 min. (To convert degrees into minutes multiply the number of degrees by 60.) Recall the apparent size of galaxy NGC 2403 is 21.4 × 10.7 min. If we divide the apparent height and width of the galaxy by the apparent height and width of the field of view produced by our telescope and camera combination and then express the two ratios obtained as a percentage, we will find that the apparent height and width of the galaxy is 8.4 and 11.2 % respectively of the length and width of the field of view. Since both percentages are less than 100 %, the field of view produced by our telescope and camera combination is sufficient to capture the galaxy’s image. Now determine if the size of the galaxy NGC 2403 in the photograph that is produced is sufficient to show any significant details. Table 3.4 shows that the maximum size of a printed photograph at 300 dots per inch using a 12.0 megapixel camera is 9.4 × 14.1 in. Assuming that the photograph contains the field of view produced by our camera and telescope combination with no cropping or vignetting, then the resulting size of the actual image of the galaxy in the photograph will be 8.4 % of the photograph’s height and 11.2 % of the photograph’s width. For a printed photograph at 300 dots per inch, the image of the galaxy will be 0.8 in. high by 1.6 in. wide. A reduction of the image resolution to 150 dots per inch, increases the size of the galaxy in the photograph to 1.6 × 3.2 in. The actual image size of the galaxy is not very large for either case but it is sufficient to display the nature of the galaxy, spiral arms, and other large features, thus, worth imaging. Keep in mind that the above process concerning image size is based upon some assumptions that are close but are not exact and upon a specific DSLR sensor size; thus, the results are approximations. However, they are accurate enough to determine if an object will fit in a camera/telescope field of view and provide an approxi- mate size of an object in a printed photograph. Now we know, our equipment is sufficient to photograph the galaxy NGC2403. We also know that with a little luck with the weather, we can obtain sufficient details to make it worth the effort to image. Now to decide how we want to go about imaging NGC 2403 and what dates we want to do the imaging. One thing to keep 290 Appendix A in mind is “the weather.” Any plan must consider the weather. Weather is what it is and we cannot rely upon it being what we want when we want. The first thing to do is to check our social, business, and astrophotography calendars. We have no other objects scheduled to image or any planned business trips during the nights available. However the social calendar shows that we will spend the Christmas and New Year’s holidays with relatives who are located in a large urban area. Our planned stay, including travel, is from 20 December through 5 January so we eliminate this time period as possible nights to image the galaxy. This leaves the following nights available to schedule the imaging session: • 1–6 December • 28–30 January • 1–2, 26–28 February • 1–3 March We note in our business and social calendars that these nights are scheduled for an astrophotography session. Since we cannot control the weather, the strategy is to schedule the imaging session on the first available night, 1 December, weather permitting.

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